Polar Front

Contents ©Encyclopedia of Life Support Systems (EOLSS) FISHERIES AND AQUACULTURE-Melting of Polar Icecaps: Impact on Marine Biodiversity-David K A Barnes, Stefanie Kaiser current dynamics will influence the deeper communities of animals, even the deep sea by, for example, by carrying less oxygen. Such processes will make polar regions more susceptible to invasive (non-indigenous) species. Antarctica has greater marine biodiversity than would be predicted from its coastline length or shelf area and most species there are endemic. Losing Antarctic species is losses to global biodiversity and some evidence suggests particular sensitivity to environmental variability. The future of polar biodiversity and climate change are hard to predict, but considerable (negative) changes are likely in the Arctic and Antarctic Peninsula.

The variability of Southern Ocean sea-surface temperatures (SST) are important to understanding coastal biology yet are poorly known amongst biologists. We compare sea temperatures at a constant depth (10-20 m) at coastal localities both sides of the Polar Front (PF), around the Scotia-Arc, the West Antarctic Peninsula and at high oceanic latitudes (around the margins of East Antarctica). We assess the wider context of these values by investigating minimum and maximum temperatures and ranges throughout the Southern Ocean using remotely sensed SST data. Data to date show weekly, daily and hourly variation in shallow sea temperature can be one-third of total annual variability (in the summer) but can be very constant (in winter). From comparison across scales in time and space, the strong seasonal signal is the most striking feature of Antarctic shallow sea temperatures even at highest oceanic latitude sites. The winter sea temperatures at localities within the PF are similar (near freezing), but upper temperatures and thus the annual range vary predictably with latitude in a cline. This amounts to 0.2 1C annual range/100 km of latitude between $541S and 671S. The annual range in sea temperatures is little different at SubAntarctic islands, whether they are north or south of the PF. r

Medieval Climate Anomaly (MCA) terrestrial proxy records (speleothem, NW Scotland; tree rings, Morocco) from close to the centres of action of the winter North Atlantic Oscillation (NAO) indicate that the MCA was characterized by a pervasive positive phase of the NAO(1). Spatial gradients across Europe in this MCA synoptic climatological interpretation were analyzed using the Proxy Surrogate Reconstruction approach based

The Cretaceous is a special episode in the history of the Earth named for a unique rock type, chalk. Chalk is similar to modern deep-sea calcareous ooze and its deposition in epicontinental seas occurred as these areas became an integral part of the ocean. The shelf-break fronts that today separate inshore from openocean waters cannot have existed during the Late Cretaceous probably because the higher sea level brought the base of the wind-mixed Ekman layer above the sea floor on the continental margins. A second peculiarity of the Cretaceous is its warm equable climate. Tropical and polar temperatures were warmer than today. Meridional and ocean-continent temperature gradients were lower. The warmer climate was a reflection of higher atmospheric levels of greenhouse gasses, CO 2 and possibly CH 4 , reinforced by higher water vapor content in response to the warmer temperatures. Most of the additional energy involved in the meridional heat transport system was transported as latent heat of vaporization of H 2 0 by the atmosphere. Poleward heat transport may have been as much as 1 Petawatt (20%) greater than it is today. C 3 plants provided for more efficient energy transport into the interior of the continents. Circulation of the Cretaceous ocean may have been very different from that of today. It is impossible for large areas of the modern ocean to become anoxic, but episodes of local anoxia occurred during the earlier Cretaceous and became regional to global during the middle of the Cretaceous. The present ocean structure depends on constant wind systems, which in turn depend on stability of the atmospheric pressure systems forced by polar ice. During most of the Cretaceous the polar regions were ice free. Without polar ice there were seasonal reversals of the high-latitude atmospheric pressure systems, resulting in disruption of the mid-and high latitude wind systems. Without constant mid-latitude westerly winds, there would be no subtropical and polar fronts in the ocean, no well-developed ocean pycnocline, and no tropical subtropical gyres dominating ocean circulation. Instead the ocean circulation would be accomplished through mesoscale eddies which could carry warmth to the polar regions. Greater knowledge and understanding of the Cretaceous is critical for learning how the climate system operates when one or both polar regions are ice free.

Silicon isotopic signatures (delta30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). delta30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the delta30Si alteration of the mixing products and with the exception of AASW, all mixing ...

The ChEss project of the Census of Marine Life (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) helped foster internationally-coordinated studies worldwide focusing on exploration for, and characterization of new deep-sea chemosynthetic ecosystem sites. This work has advanced our understanding of the nature and factors controlling the biogeography and biodiversity of these ecosystems in four geographic locations: the Atlantic Equatorial Belt (AEB), the New Zealand region, the Arctic and Antarctic and the SE Pacific off Chile. In the AEB, major discoveries include hydrothermal seeps on the Costa Rica margin, deepest vents found on the Mid-Cayman Rise and the hottest vents found on the Southern Mid-Atlantic Ridge. It was also shown that the major fracture zones on the MAR do not create barriers for the dispersal but may act as trans-Atlantic conduits for larvae. In New Zealand, investigations of a newly found large cold-seep area suggest that this region may be a new biogeographic province. In the Arctic, the newly discovered sites on the Mohns Ridge (71uN) showed extensive mats of sulfuroxidisng bacteria, but only one gastropod potentially bears chemosynthetic symbionts, while cold seeps on the Haakon Mossby Mud Volcano (72uN) are dominated by siboglinid worms. In the Antarctic region, the first hydrothermal vents south of the Polar Front were located and biological results indicate that they may represent a new biogeographic province. The recent exploration of the South Pacific region has provided evidence for a sediment hosted hydrothermal source near a methane-rich cold-seep area. Based on our 8 years of investigations of deepwater chemosynthetic ecosystems worldwide, we suggest highest priorities for future research: (i) continued exploration of the deepocean ridge-crest; (ii) increased focus on anthropogenic impacts; (iii) concerted effort to coordinate a major investigation of the deep South Pacific Ocean -the largest contiguous habitat for life within Earth's biosphere, but also the world's least investigated deepocean basin.

A watermass-based framework is presented for a quantitative understanding of the processes controlling the cycling of carbon in the Southern Ocean. The approach is developed using a model simulation of the global carbon transports within the ocean and with the atmosphere. It is shown how the watermass framework sheds light on the 5 interplay between biology, air-sea gas exchange, and internal ocean transport including diapycnal processes, and the way in which this interplay controls the large-scale oceanatmosphere carbon exchange.

The variability of Southern Ocean sea-surface temperatures (SST) are important to understanding coastal biology yet are poorly known amongst biologists. We compare sea temperatures at a constant depth (10-20 m) at coastal localities both sides of the Polar Front (PF), around the Scotia-Arc, the West Antarctic Peninsula and at high oceanic latitudes (around the margins of East Antarctica). We assess the wider context of these values by investigating minimum and maximum temperatures and ranges throughout the Southern Ocean using remotely sensed SST data. Data to date show weekly, daily and hourly variation in shallow sea temperature can be one-third of total annual variability (in the summer) but can be very constant (in winter). From comparison across scales in time and space, the strong seasonal signal is the most striking feature of Antarctic shallow sea temperatures even at highest oceanic latitude sites. The winter sea temperatures at localities within the PF are similar (near freezing), but upper temperatures and thus the annual range vary predictably with latitude in a cline. This amounts to 0.2 1C annual range/100 km of latitude between $541S and 671S. The annual range in sea temperatures is little different at SubAntarctic islands, whether they are north or south of the PF. r

We present here the first mercury speciation study in the water column of the Southern Ocean, using a high-resolution south-to-north section (27 stations from 65.50°S to 44.00°S) with up to 15 depths (0-4440 m) between Antarctica and Tasmania (Australia) along the 140°E meridian. In addition, in order to explore the role of sea ice in Hg cycling, a study of mercury speciation in the "snow-sea iceseawater" continuum was conducted at a coastal site, near the Australian Casey station (66.40°S; 101.14°E). In the open ocean waters, total Hg (Hg T ) concentrations varied from 0.63 to 2.76 pmol L −1 with "transient-type" vertical profiles and a latitudinal distribution suggesting an atmospheric mercury source south of the Southern Polar Front (SPF) and a surface removal north of the Subantartic Front (SAF). Slightly higher mean Hg T concentrations (1.35 ± 0.39 pmol L −1 ) were measured in Antarctic Bottom Water (AABW) compared to Antarctic Intermediate water (AAIW) (1.15 ± 0.22 pmol L −1 ). Labile Hg (Hg R ) concentrations varied from 0.01 to 2.28 pmol L −1 , with a distribution showing that the Hg T enrichment south of the SPF consisted mainly of Hg R (67 ± 23%), whereas, in contrast, the percentage was half that in surface waters north of PFZ (33 ± 23%). Methylated mercury species (MeHg T ) concentrations ranged from 0.02 to 0.86 pmol L −1 . All vertical MeHg T profiles exhibited roughly the same pattern, with low concentrations observed in the surface layer and increasing concentrations with depth up to an intermediate depth maximum. As for Hg T , low mean MeHg T concentrations were associated with AAIW, and higher ones with AABW. The maximum of MeHg T concentration at each station was systematically observed within the oxygen minimum zone, with a statistically significant MeHg T vs Apparent Oxygen Utilization (AOU) relationship (p < 0.001). The proportion of Hg T as methylated species was lower than 5% in the surface waters, around 50% in deep waters below 1000 m, reaching a maximum of 78% south of the SPF. At Casey coastal station Hg T and Hg R concentrations found in the "snow-sea ice-seawater" continuum were one order of magnitude higher than those measured in open ocean waters. The distribution of Hg T there suggests an atmospheric Hg deposition with snow and a fractionation process during sea ice formation, which excludes Hg from the ice with a parallel Hg enrichment of brine, probably concurring with the Hg enrichment of AABW observed in the open ocean waters. Contrastingly, MeHg T concentrations in the sea ice environment were in the same range as in the open ocean waters, remaining below 0.45 pmol L −1 . The MeHg T vertical profile through the continuum suggests different sources, including atmosphere, seawater and methylation in basal ice. Whereas Hg T concentrations in the water samples Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site 2 collected between the Antarctic continent and Tasmania are comparable to recent measurements made in the other parts of the World Ocean (e.g., Soerensen et al., 2010), the Hg species distribution suggests distinct features in the Southern Ocean Hg cycle: (i) a net atmospheric Hg deposition on surface water near the ice edge, (ii) the Hg enrichment in brine during sea ice formation, and (iii) a net methylation of Hg south of the SPF.

The occurrence of an early Preboreal climatic cooling/oscillation (PBO) in lacustrine and glacial records from northwest Europe, Iceland and Greenland is reviewed and documented. The often subtle response of the proxy records to this oscillation, in combination with its short duration, make it difficult to detect. Owing to its chronostratigraphic position between the 10 000-9900 and 9600-9500 14 C plateaux (c. 11 300-11 150 calendar yr BP) it is also difficult to 14 C date with precision. We find that the vegetation response to the PBO varies between sites and regions. In contrast to the pioneer vegetation in Iceland and southern Sweden, the expanding birch-pine forest in Germany-Denmark was more susceptible to deteriorating growing conditions. The combined lacustrine, tree-ring and glacial records imply that the PBO was characterised by cool and humid conditions throughout northwestern and central Europe. This is documented by vegetation changes, decreased aquatic production, increased soil erosion, increased 2 H and 13 C content in tree-rings, readvances or stillstands of the ice sheet in Norway and Finland, and ingression of brackish water into the Baltic. Icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous Preboreal period, but not as extreme as during the Younger Dryas. Greenland records suggest that the early Preboreal was characterised by ice readvances, as an effect of cool climate and increased precipitation (in relation to the Younger Dryas). It was not until the end of the PBO that climate was warm enough to melt the land-based ice sheet. This Preboreal oscillation, found on both sides of the Nordic Seas, is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the Nordic Seas, which is implied by a simultaneous and distinct rise in the atmospheric 14 C/ 12 C ratio. A slow-down of the thermohaline circulation may temporarily have pushed the Polar Front further south.

This paper discusses the seasonal evolution of the hydrographic and biogeochemical properties in the Antarctic Circumpolar Current (ACC) during the US Joint Global Ocean Flux (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in 1997-1998. The location of the study region south of New Zealand along B1701W was selected based on the zonal orientation and meridional separation of the physical and chemical fronts found in that region. Here we endeavor to describe the seasonal changes of the macronutrients, fluorescence chlorophyll , particulate organic carbon (POC), and carbon dioxide (CO 2 ) in the upper 400 m of the ACC during the evolution of the seasonal phytoplankton bloom found in this area. While the ACC has extreme variability in the meridional sense (due to fronts, etc.), it appears to be actually quite uniform in the zonal sense. This is reflected by the fact that a good deal of the seasonal zonal changes in nutrients distributions at 1701W follow a pattern that reflects what would be expected if the changes are associated with seasonal biological productivity. Also at 1701W, the productivity of the upper waters does not appear to be limited by availability of phosphate or nitrate. While there is a significant decrease (or uptake) of inorganic nitrogen, phosphate and silicate associated with the seasonal phytoplankton bloom, none of the nutrients, except perhaps silicate (north of the silicate front) are actually depleted within the euphotic zone. At the end of the growing season, nutrient concentrations rapidly approached their pre-bloom levels. Inspection of the ratios of apparent nutrient drawdown near 641S suggests N/P apparent drawdowns to have a ratio of B10 and N/Si apparent drawdowns to have a ratio of >4. These ratios suggest a bloom that was dominated by Fe limited diatoms. In addition, the surface water in the Polar Front (PF) and the Antarctic Zone (AZ) (J.M. Morrison). 0967-0645/01/$ -see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 -0 6 4 5 ( 0 1 ) 0 0 0 7 5 -3 just to the south of the PF take up atmospheric CO 2 at a rate 2-3 times as fast as the mean global ocean rate during the summer season but nearly zero during the rest of year. This represents an important process for the transport of atmospheric CO 2 into the deep ocean interior. Finally, the net CO 2 utilization or the net community production during the 2.5 growing months between the initiation of phytoplankton blooms and mid-January increase southward from 1.5 mol C m À2 at 551S to 2.2 mol C m À2 to 651S across the Polar Frontal Zone (PFZ) into the AZ. r

This study investigates the dynamics of phytoplankton communities and nitrogen uptake in the Indian sector of the Southern Ocean during spring and summer. The study area is oligotrophic (Chl a stocks <50 mg m A2 ); nevertheless, a large spatial variation of phytoplankton biomass and community structure was observed. During both seasons the phytoplankton community in the seasonal ice zone showed higher biomasses and was mainly composed of large diatom cells. However, in the permanently open ocean zone the community had low biomass and was chie¯y composed of nano-and pico¯agellates. In the polar front zone, although biomass was higher, the community structure was similar to the open ocean zone. The results suggest that the variation in phytoplankton community structure on a larger scale resonates with gradients in water column stability and nutrient distribution. However, signi®cant changes in biomass and nutrient stocks but little change in community structure were observed. Absolute nitrogen uptake rates were generally low, but their seasonal variations were highly signi®cant. During spring the communities displayed high speci®c nitrate uptake (mean rate 0.0048 h A1 ), and diatoms (in the seasonal ice zone) as well as nano-and pico¯agellates (in the permanently open ocean zone and polar front zone) were mainly based on new production (mean ¦-ratio 0.69). The transition to summer was accompanied by a signi®cant reduction in nitrate uptake rate (0.0048 h A1 ® 0.0011 h A1 ) and a shift from predominantly new to regenerated production (¦-ratio 0.69 ® 0.39). Ammonium played a major role in the seasonal dynamics of phytoplankton nutrition. The results emphasize that, despite a large contrast in community structure, the seasonal dynamics of the nitrogen uptake regime and phytoplankton community structure in all three subsystems were similar. Additionally, this study supports our previous conclusion that the seasonal shift in nitrogen uptake regime can occur with, as well as without, marked changes in community structure.

The total extent of the atmospheric impacts associated to the aerosol black carbon (BC) emissions from South America is not completed described. This work presents results of BC monitored during three scientific expeditions (2002, 2003 and 2004) on board of a Brazilian oceanographic vessel Ary Rongel that covered the South-West Atlantic coast between 22-62°S. This latitudinal band encloses major urban regions of South America and the outflow region of the SACZ (South Atlantic Convergent Zone), which is an important mechanism of advective transport of heat, moisture, minor gases and aerosols from the South America continental land to the Southern Atlantic Ocean. Our results showed that aerosol BC enhanced concentrations from urban/industrial origin can be transported to the South-West Atlantic Ocean due to the migration of sub-polar fronts that frequently reach tropical/subtropical regions. Despite the decrease of aerosol BC concentrations southwards (from ∼1,200 ng m −3 at latitude 22°S to ∼10 ng m −3 at latitude 62°S), several observed peak events were attributed to regional urban activities. Most of such events could be explained by the use of air mass back trajectories analysis. In addition, a global model simulation is presented (Goddard Institute for Space Studies -GISS GCM BC simulation) to explore the origins of aerosol BC in the South-West Atlantic. The model allowed isolating the biomass emissions from South America and Africa and industrial (nonbiomass) pollution from other regions of the globe. This model suggests that the apportionment of about half of the aerosol BC at the South-West Atlantic may derive from South American biomass burning.

1] Past sea surface water conditions of the western Iberian Margin were reconstructed based on biomarker analyses of a marine deep sea core MD03-2699 from the Estremadura Spur north off Lisbon, providing new insights into orbital and suborbital-scale climate variability between marine isotope stage (MIS) 15 to MIS 9 (580 to 300 ka). We use biomarker-based proxy records such as the alkenone unsaturated index to estimate sea surface temperature (SST), the total alkenone concentration to reconstruct phytoplankton productivity, and terrestrial biomarkers to evaluate the continental input. The results extend the existing biomarker record, namely the SST for the Iberian Margin, back to the sixth climatic cycle (580 ka). A general trend of stable interglacials contrasts with glacial periods and glacial inceptions which are marked by high-frequency variability. Thus, several short-lived climatic coolings were identified by large SST decreases, the occurrence of ice-rafted detritus and high percentages of the tetraunsaturated alkenone C 37:4 . Some of these events were extremely cold and similar in their general trends to the well-known Heinrich events of the last glaciation. We identified eight Heinrich-type events between 580 and 300 ka. The general deglaciation pattern detected between MIS 15 and MIS 9 is similar in their general trends to that characterizing the more recent climatic cycles, i.e., marked by two coolings separated by a short warming episode which may reflect the southward, northward, and southward migration of the Polar Front.

The South Georgia region is characterised by high biomass and productivity of phytoplankton, zooplankton and vertebrate predators. Important commercial fisheries have been based at the island since the late 1700s, initially exploiting seals and whales, and currently taking krill Euphausia superba and finfish. Despite studies dating from the beginning of the last century, the causes of the high productivity remain unclear. The island lies within the Antarctic Zone of the Antarctic Circumpolar Current, to the south of the Polar Front. The offshore waters to its north and east are affected by a northwards deflection of the Southern Antarctic Circumpolar Current Front, together with waters from the Weddell-Scotia Confluence. Despite a retentive circulation over the shelf, the high productivity of phytoplankton and copepods is widespread, occurring far downstream and possibly extending to the Polar Front. High phytoplankton concentrations (>20 mg chlorophyll a m -3 ) may be linked to enhanced supply of iron or reduced forms of nitrogen (up to ~3 mmol ammonium m -3 ). Although macronutrients are generally not limiting in the Antarctic Zone, silicate concentrations of <1 mmol m -3 are growth-limiting at South Georgia in some summers. The growth season is long and blooms of > 2 mg chl a m -3 occur for 4 to 5 mo. Biomass of krill plus net-caught zooplankton in summer is around 15 to 20 g dry mass m -2 , equally dominated by krill and copepods. This greatly exceeds typical values for Antarctica, and is high compared to productive northern shelves. Zooplankton, and in particular krill, appear to have a pivotal role in regulating energy flow in this food web, through selective grazing and possibly also through nutrient regeneration. Abundances of krill and copepods are negatively related across a wide range of scales, suggesting direct interaction through competition or predation. Evidence suggests that when phytoplankton stocks are low, energy flow through krill is maintained by their feeding on the large populations of small copepods. Metazoans and higher predators at South Georgia can feed across several trophic levels according to prey abundance, and they regenerate substantial quantities of reduced nitrogen. Therefore we suggest that these groups have a controlling influence on lower trophic levels, both stabilising population sizes and maintaining high rates of energy flow. Hydrography, nutrient concentrations, phytoplankton, copepod, and krill biomasses fluctuate between years. Periodically (once or twice a decade), shortages of krill in summer result in breeding failures among many of their predators. This appears to be a downstream effect from wider scale, Scotia Sea phenomena, although the processes involved are unclear. The elevated biomass and energy flows at South Georgia appear to be caused by locally enhanced growth rates; there is no evidence so far for any physical concentration mechanism. Even for krill, which do not breed there, local growth rates are probably of a similar order to the biomass removed by their main land-based predators in summer. Thus the transfer of energy to higher predators depends on local enhancement of fluxes through the food web as well as the supply of plankton to the island by the Antarctic Circumpolar Current.

January 1990, World Ocean Circulation Experiment Hydrographic Program section S 1) are in close agreement with property sections reported previously. The chlorofiuoromethanes CFM 11 and CFM 12 were measured in Drake Passage for the fast time. CFM concentrations are found to decrease from the surface down into the Upper Circumpolar Deep Water, for which they confinn water renewal from the south. For the Lower Circumpolar Deep Water, in which CFM concentrations were above detection limit only south of the Polar Front, very little water renewal on the CFM time scale is implied. Nonvanishing CFM is again found in the Weddell Sea Deep Water and the Southeast Pacific Deep Water toward the bottom in the south, but recent ventilation for the latter water mass is rejected. CFM 11 and CFM 12 concentrations vary essentially in constant proportion down to very low concentrations, questioning the possibility of using CFM ratios as "age" markers. The observed ratios are shown to be a natural feature of the upwelling regime of the southern ocean. Property concentrations on isopycnal surfaces display large undulations, reaching down into the Upper Circumpolar Deep Water. Their extrema, due to varying contribution of young water of southern origin, are situated at the boundaries of the current bands of the Antarctic Circumpolar Current.

An inverse box model of the Scotia Sea is constructed using hydrographic, tracer, and velocity data collected along the rim of the basin during the Antarctic Large-Scale Box Analysis and the Role of the Scotia Sea (ALBATROSS) cruise. The model provides an estimate of the time-mean three-dimensional circulation as the Antarctic Circumpolar Current (ACC) crosses the region. It concurrently solves for geostrophic and wind-driven Ekman transports across the boundaries of the basin, air-sea-driven diapycnal fluxes, and ''interior'' diapycnal fluxes below the ocean surface. An increase is diagnosed in the ACC volume transport from 143 Ϯ 13 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ) at Drake Passage to 149 Ϯ 16 Sv on leaving the Scotia Sea, supplied by the import of 5.9 Ϯ 1.7 Sv of Weddell Sea Deep Water (WSDW) over the South Scotia Ridge. There is a lateral redistribution of the transport, primarily in response to a topographically induced branching of the 70-80 Sv polar front (PF) jet and an increase in the transport associated with the subantarctic front (SAF) from 31 Ϯ 7 to 48 Ϯ 4 Sv. A vertical rearrangement of the transport also occurs, with differences O(2 Sv) in the transports of intermediate and deep water masses. These volume transport changes are accompanied by a net reduction (increase) in the heat (freshwater) flux associated with the ACC by 0.02 Ϯ 0.020 PW (0.020 Ϯ 0.017 Sv), the main cause of which is the cooling and freshening of the Circumpolar Deep Water (CDW) layer in the Scotia Sea. The model suggests that the Scotia Sea hosts intense diapycnal mixing in the ocean interior extending 1500-2000 m above the rough topography of the basin. Despite these model results, no evidence is found for a significant diapycnal link between the upper and lower classes of CDW (and hence between the ''shallow'' and ''deep'' cells of the Southern Ocean meridional overturning circulation). On the contrary, the boundary between Upper and Lower CDW separates two distinct regimes of diapycnal mixing involving volume fluxes of 1-3 Sv. Whereas in the denser waters topographic mixing is important, in lighter layers air-sea-driven diapycnal volume fluxes are dominant and diapycnal transfers of heat and freshwater are mainly effected by upper-ocean mixing processes. The model indicates that the ventilation of the deep ACC in the Scotia Sea is driven primarily by isopycnal exchanges with the northern Weddell Sea and to a lesser extent by diapycnal mixing with WSDW. The model reveals the existence of a mesoscale eddy-driven overturning circulation across the ACC core involving an isopycnal poleward transport of 8 Ϯ 1 Sv of CDW and an equatorward transport of intermediate water of the same magnitude. This circulation induces a cross-ACC poleward heat flux of 0.022 Ϯ 0.009 PW and an equatorward freshwater flux of 0.02 Ϯ 0.01 Sv. Adequately scaled, the former compares favorably to measurements of the cross-stream eddy heat flux by moored current meters and floats in the ACC and to budget estimates of the circumpolar cross-ACC heat flux.

Marine predators are thought to utilise oceanic features adjusting their foraging strategy in a scaledependent manner. Thus, they are thought to dynamically alter their foraging behaviour in response to environmental conditions encountered. In this study, we examined the foraging behaviour of King Penguins (Aptenodytes patagonicus) breeding at South Georgia in relation to predictable and stable oceanographic features. We studied penguins during their long post-laying foraging trips during December 2005 and January 2006. For this investigation, we undertook a simultaneous analysis of ARGOS satellite-tracking data and Mk 7 WildLife Computers Time Depth Recorder (TDR) dive data. To investigate correlations between foraging behaviour and oceanographic conditions, we used SST data from January 2006 from MODIS satellite AQUA. To determine changes in search effort, first passage time (FPT) was calculated; for analysis of dive behaviour, we used several dive parameters that are thought to be reliable indicators of changes in foraging behaviour. King Penguins appeared to target predictable mesoscale features in the Polar Front Zone (PFZ), either a warm-core eddy in the PFZ or regions of strong temperature gradients at oceanic fronts. Two different trip types could be distinguished: direct trips with a straight path to one foraging area at the edge of an eddy or at a thermal front, and circular trips where birds foraged along strong thermal gradients at the northern limit of the PFZ. It is likely that both trip types were a direct consequence of prey encounter rates and distributions, both of which are likely to be associated with these oceanographic features. Circular trips often included passages across the centre of an eddy where birds made deep foraging dives, but remained only a short time in the eddy, possibly because prey were too deep. All birds showed Area Restricted Search (ARS) at scales of <10 km. The two trip types had different ARS patterns, with clear ARS hotspots for direct trips and several ARS hotspots over the whole duration of the trip for circular trips. Dive behaviour had clear relationships with the changing water temperature and the time of day, presumably in response to different prey distribution. Especially for direct trips, dive behaviour showed significant differences within and outside of ARS hotspots. Thus, King Penguins appear to target predictable mesoscale features in the PFZ. They use ARS in different patterns to exploit the environment and adjust their foraging strategy and diving behaviour depending upon conditions they encountered. Diving behaviour showed correlations to ARS patterns, especially for direct trips, which may represent a favourable foraging strategy. The presence of predictable oceanic features allows King Penguins to focus their foraging effort, presumably allowing them to increase their foraging success and decrease their diving effort.

In two contrasting regions of the ocean, the equatorial Pacific and the southern ocean, the •5•5N of core top sediments were strongly related to [NOs-] in surface waters. With distance from the equator in the equatorial Pacific, •5•5N increased from 7%0 to 16%0 as [NOs-] decreased from 8pM to < 0.1 pM. Going from 60 ø to 30 ø S in the SE Indian Ocean, core top •5•SN increased from 5%0 to 11%o as surface [NOs-] decreased from 25pM to < 0.1 pM. These results are strong evidence that sedimentary •SN in these regions is recording the increasing isotopic enrichment of near-surface NOs-with its depletion by phytoplankton. In the case of the equatorial Pacific, •5•SN values for sinking particles collected at 150 m matched well the core top sediment values, demonstrating little diagenetic alteration of the near-surface generated isotopic signal. These equatorial Pacific data sets have variations with near-surface [NOs-] consistent with Rayleigh fractionation kinetics for a fractionation factor (eu) of 2.5%0.

Individual specialisation is widespread and can affect a population’s ecological and evolutionary dynamics. Whether intra-specific niche differences can influence reproductive investment was examined in a marine mammal, the southern elephant seal (Mirounga leonina), whose females were known to forage in two different areas during the austral winter. The study was conducted at Kerguelen Islands (49°21′S, 70°18′E), southern Indian Ocean, in

During the last 30 years, at-sea studies of seabirds and marine mammals in the oceans south of the Subtropical Front have described an association with major frontal areas. More recently, the advancement in microtechnology has allowed the tracking of individuals and investigations into how these marine predators actually use the frontal zones. In this review, we examine 1) the relative importance to apex predators of the different frontal zones in terms of spatial distribution and carbon flux; 2) the processes that determine their preferential use; and 3) how the mesoscale dynamics of frontal structures drive at-sea foraging strategies of these predators. We review published results from southern waters and place them in a broader context with respect to what has been learned about the importance of fronts in oceans farther north. Some fronts constitute important boundaries for seabird communities in southern waters. At a mesoscale the maximum values of seabird diversity and abundance correspond to the location of the main fronts. At-sea surveys show a strong curvilinear correlation between seabird abundance and sea surface temperatures. High mean species richness and diversity for whales and seabirds are consistently associated with the southern water mass boundary of the Antarctic Circumpolar Current, the Subtropical Front and the Subantarctic Front; in the case of the Polar Front mean seabird densities are more variable. At small-scales, variation in seabird occurrence has been directly related to the processes at fronts in a limited number of cases. A significant positive relation was found between some plankton feeding species and frontal temperature gradient-phytoplankton variables. Telemetric studies have revealed that several apex predators (penguins, albatrosses, seals) perform long, directed foraging trips either to the Subtropical front or Polar Front, depending on locality. Seabirds with low flight costs, such as albatrosses, are able to reach fronts at long distances from colonies, showing variable foraging strategies as a function of the distances involved. Diving birds such as King penguins, that travel at a higher cost and lower speed, rely on the predictable spatial distribution of mesopelagic fish found close to the Polar Front. They may use the currents associated with eddies as oceanographic cues in the active search for frontal zones. Once in these areas they dive preferentially in and below the depth of the thermocline where catches per unit effort are high. Elephant seals concentrate foraging activity principally inside or at the boundary of cyclonic eddies. These mesoscale features appear to offer exceptional productivity favourable for foraging by various diving top predators. The connection between biophysical parameters at fronts and predators is likely to be made through biological enhancement. Top predators appear to forage at locations where prey are advected by physical processes and others where prey are produced locally. Long-term research on at-sea distributions and demographic parameters of top predators are essential to assess the consequences of potential shift in front distributions in relation to global warming. Such environmental changes would add to the impact of fish extraction by the industrial fisheries on the southern food webs.

Sooty (Puffinus griseus) and short-tailed (P. tenuirostris) shearwaters are abundant seabirds that range widely across global oceans. Understanding the foraging ecology of these species in the Southern Ocean is important for monitoring and ecosystem conservation and management. Tracking data from sooty and short-tailed shearwaters from three regions of New Zealand and Australia were combined with at-sea observations of shearwaters in the Southern Ocean, physical oceanography, near-surface copepod distributions, pelagic trawl data, and synoptic near-surface winds. Shearwaters from all three regions foraged in the Polar Front zone, and showed particular overlap in the region around 140 degrees E. Short-tailed shearwaters from South Australia also foraged in Antarctic waters south of the Polar Front. The spatial distribution of shearwater foraging effort in the Polar Front zone was matched by patterns in large-scale upwelling, primary production, and abundances of copepods and myctophi...

A new jet stream description, defined as the geostrophic streamline of maximum average velocity is proposed. An objective algorithm for detecting and tracking the jet has been developed, tested and applied to the NCEP/NCAR 200-hPa geopotential height in the Southern Hemisphere for the period 1958–2002. The results show the variability of the double character of the Southern Hemisphere jet, with a marked seasonality. During the warm season, a single jet can be found around 40°S, while autumn and winter are characterized by a clear double jet structure, with a strong and dominant subtropical jet located around 30°S and a polar front jet, progressively displaced toward southern latitudes and reaching 60°S by the end of the cold season. In general, a trend toward slower subtropical jets and stronger polar front jets has been detected during the study period. The Southern Annular Mode appears as a main modulator of the latitude and strength of the polar front jet, influencing to a minor extent its subtropical counterpart. The ENSO cycle strongly modifies the latitude and specially the strength of the subtropical jet, affecting its preferred wavenumber as well. Nevertheless, the effect of this oscillation seems fairly restricted in the Pacific, thus limiting the ability of this jet to drive the El Niño teleconnections along the Southern Hemisphere. The consistency of the results, when compared with previous jet climatologies, suggests that the new approach is a reliable jet-tracking method, thus providing a new tool to analyze climatic variability at hemispheric scales.

A qualitative and quantitative study of recent organic-walled dinoflagellate dinocysts recovered in sediments has been undertaken in the coastal zone of the Gulf of Tehuantepec, Mexico. A sediment core was collected in 2004 with a Reineck-type corer using a plastic tube (7 cm i.d); and it was subsampled at 0.3 cm intervals down to 10 cm depth and then at 1 cm intervals at further depths. The 210Pb and 137Cs-derived sedimentation and mass accumulation rates at the site were found to vary from 0.033 to 0.209 cm yr-1, and from 0.05 to 0.29 g cm-2 yr-1., respectively. The cysts concentrations ranged between 477 and 2300 cysts g-1 and the cysts fluxes between 68 a 494 cysts cm-2 yr-1. Twenty-three cyst taxa were identified: Brigantedinium spp., Polysphaeridium zoharyii, Bitectatodinium spongium, Spiniferites delicatus, Quinquecuspis concreta, Echinidinium transparantum, Operculodinium centrocarpum, Selenopemphix quanta, Type Echinidinium granulatum, Echinidinium aculeatum, Protoperidinium americanum, Echinidinium delicatum, Selenopemphix nephroides, Cyst of Protoperidinum stellatum, Lingulodinium machaerophorum, Islandinium spp., Votadinium spinosum, Polykrikos kofoidii, Pentapharsodinium dalei, Tuberculodinium vancampoe, Spiniferites mirabilis, Votadinium calvum and Nematosphaeropsis labyrinthus. The assemblages included cysts of both phototrophic and heterotrophic species, with variation of their respective abundance reflecting changes in the trophic structure of the upper water mass, especially after 1950, with a decrease from ~30% to 15% of photototrophic species, likely as a response to pollution (including cultural eutrophication) created by the industrial development of the adjacent coastal zone. Brigantedinium spp., Polysphaeridium zoharyii and Bitectatodinium spongium, were the dominant species found in the core and are most likely influenced by the seasonal upwelling that characterize the study area, as indicated by the predominance of planktonic foraminiferal species Globigerina bulloides and Globigerinita glutinata throughout the core. Potentially toxic species such as Polysphaeridium zohary (cyst of Pyrodinium bahamense var. compressum) were also observed all along the sediment core and new analysis are to be performed to further investigate their presence in the zone.

The Holocene climate of the Southern Ocean is not well understood, mainly because of the lack of high-resolution reconstructions of ocean surface properties. Here we present a 12,500-yr-long, decadal-scale record of Holocene sea-surface temperatures and sea- ice presence from the Polar Front of the East Atlantic Southern Ocean. The record shows gradual climate change, with no abrupt Neoglacial cooling, and

We identify a universal criterion for the onset of a nonequilibrium Ising-Bloch (NIB) transition, and describe the behavior near the bifurcation by a generic amplitude equation. We found that a NIB transition is caused by an antisymmetric eigenvector passing the translational mode of the system at a critical point. In this context we discuss Hamiltonian and dissipative systems. We report on a NIB in nonlinear optics, manifesting itself in a transition from static to moving polarization fronts.

Multiproxy climate records from Iceland document complex changes in terrestrial climate and glacier fluctuations through the Holocene, revealing some coherent patterns of change as well as significant spatial variability. Most studies on the Last Glacial Maximum and subsequent deglaciation reveal a dynamic Iceland Ice Sheet (IIS) that responded abruptly to changes in ocean currents and sea level. The IIS broke up catastrophically around 15 ka as the Polar Front migrated northward and sea level rose. Indications of regional advance or halt of the glaciers are seen in late Allerö d/early Younger Dryas time and again in PreBoreal time. Due to the apparent rise of relative sea level in Iceland during this time, most sites contain evidence for fluctuating, tidewater glacier termini occupying paleo fjords and bays. The time between the end of the Younger Dryas and the Preboreal was characterized by repeated jö kulhlaups that eroded glacial deposits. By 10.3 ka, the main ice sheet was in rapid retreat across the highlands of Iceland. The Holocene thermal maximum (HTM) was reached after 8 ka with land temperatures estimated to be 3 C higher than the 1961-1990 reference, and net precipitation similar to modern. Such temperatures imply largely ice-free conditions across Iceland in the early to mid-Holocene. Several marine and lacustrine sediment climate proxies record substantial summer temperature depression between 8.5 and 8 ka, but no moraines have been detected from that time. Termination of the HTM and onset of Neoglacial cooling took place sometime after 6 ka with increased glacier activity between 4.5 and 4.0 ka, intensifying between 3.0 and 2.5 ka. Although a distinct warming during the Medieval Warm Period is not dramatically apparent in Icelandic records, the interval from ca AD 0 to 1200 is commonly characterized by relative stability with slow rates of change. The literature most commonly describes Little Ice Age moraines (ca AD 1250-1900) as representing the most extensive ice margins since early Holocene deglaciation, with temperature depressions of 1-2 C compared to the AD 1961-1990 average. Steep north-south and west-east temperature gradients are reconstructed in the Holocene records of Iceland, suggesting a strong maritime influence on the terrestrial climate of Iceland.

A new synthesis of diatom assemblage data from Ocean Drilling Program (ODP) Leg 178 suggests that the Pacific margin of the Antarctic Peninsula underwent a transition from a shallow shelf to an overdeepened shelf in the early Pliocene. This modification of the marine-scape was due to a relatively brief interval of erosion begun at 5.2 Ma. The erosion was caused by high frequency advances of a super-inflated Antarctic Peninsula Ice Sheet (APIS). The frequent advances of the higher elevation ice sheet were a consequence of abundant moisture delivered to the region as the Polar Front migrated southward. By 5.12 Ma, ice streams incised foredeepened glacial troughs into basement on the inner shelf. Sediment eroded from the inner shelf was transported through cross-shelf troughs and deposited in large trough-mouth-fan depocenters on the upper slope. Overdeepened shelf conditions became widespread as troughs widened and intra-trough banks beveled. By 4.25 Ma, trough-mouth-fan construction ceased and subsequent advances of the APIS have been infrequent. We propose that the reduced frequency of grounding events signaled the transition to a modern foredeepened and overdeepened shelf. We hypothesize that a new glacial dynamic emerged in the early Pliocene because overdeepening led to accelerated heat exchange between the ocean and APIS in two ways. Firstly, the overdeepened shelf required that a larger area of the grounded ice sheet's marine terminus be in contact with the ocean. Secondly, erosional deepening of the outer shelf was equivalent to lowering a shelf edge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell in the region. The resultant accelerated melting at the APIS marine terminus, caused the super-inflated APIS to deflate on the mainland, which further decreased the possibility that grounded ice could advance on the overdeepened shelf.► We propose the overdeepening hypothesis. ► The hypothesis states that overdeepening of the shelf altered glacial dynamics. ► Diatom data suggests that the peninsula overdeepened at 4.25 Ma. ► The frequency of ice sheet advance reduced after the margin overdeepened.

1] This paper documents the migration of the Polar Front (PF) over the Iberian margin during some of the cold climatic extremes of the last 45 ka. It is based on a compilation of robust and coherent paleohydrological proxies obtained from eleven cores distributed between 36 and 42°N. Planktonic d 18 O (Globigerina bulloides), ice-rafted detritus concentrations, and the relative abundance of the polar foraminifera Neogloboquadrina pachyderma sinistral were used to track the PF position. These three data sets, compared from core to core, show a consistent evolution of the sea surface paleohydrology along the Iberian margin over the last 45 ka. We focused on five time slices representative of cold periods under distinct paleoenvironmental forcings: the 8.2 ka event and the Younger Dryas (two recent cold events occurring within high values of summer insolation), Heinrich events 1 and 4 (reflecting major episodes of massive iceberg discharges into the North Atlantic), and the Last Glacial Maximum (typifying the highest ice volume accumulated in the Northern Hemisphere). For each event, we generated schematic maps G 3

The analysis presented in this paper suggests that the larger heating over the boreal forest in the spring and summer, as contrasted with weaker heating over the adjacent tundra, results in a preferred position of the polar front along the northern edge of the boreal forest. This positioning is well documented in the literature (see, for example, Bryson, 1966; Barry and Hare, 1974; Kreps and Barry, 1970). This heating results from the lower albedo of the boreal forest which is not compensated by an increase in transpiration, even with the larger leaf area index of the forest. The warmer temperatures are mixed upward by the deep boundary layer over the forest and mesoscale circulations which result from the patchiness of heating associated with the heterogeneous landscapes of the forest. Thus in contrast to previous assumptions in which the arctic front position in the summer determines the northern limit of the boreal tree line, our study suggests the boreal forest itself significantly influences the preferred position of the front. This conclusion reinforces the findings of Bonan et al. (1992) and Foley et al. (1994) on the important role of boreal forest-tundra interactions with climate. 25,758 PIELKE AND VIDALE: THE BOREAL FOREST AND THE POLAR FRONT Luigi Vidale received support from the Fundaqfio Coordenaqfio de Aperfeiqoamento de Pessoal de Nivel Superior (CAPES), Minist6rio da Educaqfio, Brazil, grant 1154/92-2. The authors would like to thank Bryan Critchfield, Dallas McDonald, and Tara Pielke for preparing the manuscript and appreciatively acknowledge Don Lenschow for providing access to preliminary aircraft data from the BOREAS field campaign. Gordan Bonan also provided very effective suggestions on the final draft of our paper.

As global end-member environments, the polar regions are the bellwether of global climate change. Climate modelling suggests that the polar regions are more profoundly impacted by global warming than low latitude regions, and recent warming documented in Arctic and Antarctic environments might be a result of this polar amplification process. It is not yet clear that the short-term records of recent warming represent the start of a long-term trend, however, stratigraphic records of past significant high latitude warm events likely ...

A high resolution record of U 37 kЈ -derived SST estimations has been obtained in a core from the central North Atlantic Ocean (SU90/08, 43°30ЈN, 30°24ЈW) spanning a time period of 280 ky. The general trend of the U 37 kЈ profile parallels closely the ␦ 18 0 signal and represent an independent confirmation that the SST variations in the glacial-interglacial time scales are correlated with the northern ice sheets evolution. In contrast to the CLIMAP (1984) conclusions based on the foraminiferal transfer function technique, the SST during the last interglacial period (LIP, isotopic stage 5.5) are 2-3°C warmer than during the Holocene. This result is in good agreement with many studies based in coastal and continental records that strongly suggest warmer climatic conditions over North America and Europe. Finally, the SST estimates during the last glacial maximum (LGM, 18 kyBP) where 4.5°C cooler than during the previous glacial period (PGP, stage 6). We argue that this difference is caused by a northward position of the polar front during the PGP in the Central North Atlantic.

The 2004 Mid-Atlantic Ridge (MAR)-ECO expedition on the R.V. G.O. Sars provided the first opportunity to correlate oceanic distributions of cetaceans with synoptic acoustic (ADCP to 700 m depth, multi-beam echosounders) measurements of high-resolution, three-dimensional (3D) potential habitat (spatial scaleo100 km). The identified habitat features were tested with independent observations from the Icelandic combined cetacean and redfish cruises in 2001 and 2003 using data from a 3D ocean general circulation model of the MAR region (Regional Oceans Modelling System (ROMS) model 5 km resolution). The spatial autocorrelation of sampled encounter rates of sperm Physeter macrocephalus and sei whales Balaenoptera borealis indicated scale-dependent variability in the distribution of both species. Despite the large area surveyed, the observations of both species exhibited a strong small-scale structure (range parameter 20-50 km), indicating affinities to cross-seamount or cross-frontal structures. Potential cross-seamount and crossfrontal habitat structures were derived from the acoustic transect data by analysing fine-scale gradients in the 3D flow patterns and bathymetry, including interactions between frontal and topographic parameters. PLS regression was used to determine the potential habitat drivers of sperm and sei whales, both during the G.O. Sars cruise and during the Icelandic cruises in 2001 and 2003. The selected parameters, which reflected flow gradients interacting with the steep topography, were finally applied for modelling the habitat suitability of both target species along the northern MAR using Ecological Niche Factor Analysis. The results suggest aggregations of sperm and sei whales along the MAR are primarily associated with fine-scale frontal processes interacting with the topography in the upper 100 m of the water column just north of the Sub-Polar Front (SPF) and the Charlie-Gibbs Fracture Zone (CGFZ). As moderate and high habitat suitabilities were estimated only for areas downstream from the SPF, the findings suggest that the animals capitalise on secondary production maintained by enhanced primary production associated with the frontal processes in the upper part of the water column in the CGFZ and at the Faraday Seamounts. Further studies are encouraged to evaluate the importance of the bio-physical coupling, and the significance of small-scale frontal processes in the surface and subsurface waters north of the SPF for the transfer of energy to higher trophic levels in the North Atlantic. r

The oxygen isotope record combined with radiocarbon dating from two deep-sea cores collected along a transect between New Zealand and the Ross Sea are used to establish a reliable chronostratigraphy for the last 14 kyr. After an integrated geochemical and micropaleontological analysis in this timeframe we detected a cooling interval dated between 12.5 cal kyr BP and 11.4 cal kyr BP. The age control suggests that this event started 1.5 kyr after the onset of the Antarctic Cold Reversal previously observed in several Antarctic ice cores. We infer that the observed cool event corresponds to the Younger Dryas event defined in Northern Europe. This suggests that climate change recorded in this sector of the Southern Hemisphere still shows some synchronicity with Northern Hemisphere variations and that the decoupling of climate change between the two hemispheres likely occurred south of the Polar Front. ß

The large temporal and spatial variability in marine productivity encountered by marine predators may negatively influence breeding success. The Antarctic fur seal Arctocephalus gazella (AFS), a marine predator in the Southern Ocean (SO) ecosystem with a circumpolar distribution, exhibits a short, 4 mo lactation coinciding with increased summer marine productivity. The diet of AFS, and the distance to significant and productive oceanographic features, such as the Antarctic Polar Frontal Zone (PFZ), varies considerably between populations. We studied the foraging activity, foraging efficiency and the pup provisioning strategies of lactating AFS at a key breeding site in the southern Indian Ocean, the Kerguelen Archipelago. Foraging parameters were examined in relation to interannual variability in oceanographic conditions and prey availability in the PFZ over 3 consecutive breeding seasons (1998 to 2000). The location of foraging zones, diving activity, diet and foraging efficiency varied significantly between years, concurrently with annual changes in seasurface temperature (SST) and prey availability. The strongest recorded El Niño Southern Oscillation event in 1997-1998 coincided with anomalously warm waters in the vicinity of the Archipelago. Deeper diving by females, reduced maternal and pup body condition, and minimal pup growth rates and low catch per unit effort of the primary prey species, lanternfishes (Myctophidae) were all recorded in this period. Maternal size was positively related to the growth performance of pups only in this period, indicating the importance of age/size and/or experience in mediating environmental fluctuations. Foraging efficiency over a foraging cycle and variability in mean provisioning rates (trip duration), were identified as proxies of prey availability within the foraging range of seals, emphasising the effectiveness of the use of AFS foraging behaviour as an indicator of both food and oceanographic variability and climatic anomalies. The increasing frequency of anomalously warm SST events in sectors of the SO, however, may elicit specific behavioural responses from 'central place foragers' (i.e. species that return to breeding sites to feed their young) to avoid sustained poor body condition of females and their weaned offspring.

We measured primary productivity in the Pacific Sector of the Southern Ocean as part of the Joint Global Ocean Flux Study. We collected data along 170 W from 54 S to 72 S on four cruises during the austral growing season of 1997-1998. The cruises crossed the Subantarctic Front, the Antarctic Polar Front (APF), the Southern Antarctic Circumpolar Current (ACC) Front, and the Southern Boundary of the ACC. Primary productivity and chlorophyll a increased rapidly in spring, peaked in summer, and decreased rapidly in fall, following the seasonal pattern of irradiance. In early spring (October), primary productivity was 20 mmol C m À2 d À1 and increased to 54 mmol C m À2 d À1 within 3 weeks. During peak irradiance (December), productivity reached its maximum throughout the study area with values ranging from 33 to 93 mmol C m À2 d À1 depending on station location. By February, average productivity dropped to 2071 mmol C m À2 d À1 , and individual station values reached a minimum of 13 mmol C m À2 d À1 . In early spring, chlorophyll was less than 0.5 mg Chl m À3 throughout the study area. In late spring and early summer, chlorophyll values were between 0.15 and 1.5 mg Chl m À3 depending on station location. By late summer, chlorophyll decreased to less than 0.3 mg Chl m À3 across the study region. Highest absolute values of productivity and biomass occurred near the southward-propagating Si gradient (DSi max ). A spatial gradient in photosynthetic performance correlated with DSi max : photosynthetic performance was elevated in low silicic acid waters (less than 10 mM) to the north of DSi max and was depressed in high silicic acid waters (greater than 30 mM) to the south of DSi max . Photosynthetic performance also was correlated with iron-enrichment response: when photosynthetic performance was low, iron-enrichment response was high, and when photosynthetic performance was high, iron-enrichment response was low. These results suggest that phytoplankton were iron sufficient north of DSi max and iron limited south of DSi max . We argue that the southward-traveling DSi max , the APF, and the location of upwelling, iron-rich Upper Circumpolar Deep Water (UCDW) define three regions with differing iron sufficiency. Furthermore, we suggest that a winter recharge of upwelled, iron-rich UCDW within the Antarctic and Southern ACC Zones provides enough iron to support a diatom bloom that annually propagates poleward across the Antarctic and Southern ACC Zones to the Southern

During three cruises, in March and May 1998 and July 1999, seasonal and regional variations in biomass and vertical distribution of mesozooplankton as well as cohort development in Calanus spp. were investigated along a transect across the central Barents Sea and marginal ice zone. There were no considerable changes in zooplankton biomass between the seasons. Throughout the investigation, the average biomass for the entire region approximated to ca. 5 g dry weight (DW) m À 2 while station-to-station variation ranged with an order of magnitude (1 -14 g DW m À 2 ). Biomass of nauplii and small copepods (200 -500 Am in body length) obtained from water bottles samples exceeded that from WP-2 net samples 1.5 -6.6 times. The maximum abundance of this group reached 16 Â 10 5 ind. m À 2 in the upper 100-m layer, suggesting a significant grazing pressure on phytoplankton. Spatial distribution of Calanus species and some selected species suggests that the zooplankton community composition was primarily affected by water mass circulation and bottom topography. Both the depth distribution of mesozooplankton and cohort progress in Calanus finmarchicus and Calanus glacialis revealed two waves of spring events. The first started in the southernmost area of the Barents Sea and the second nearby the Polar Front. Both developed towards the north. D

Vertical distribution and sedimentation of faecal pellets (FPs) as well as the production rates of FPs by larger copepods were studied during three cruises to the Barents Sea in March and May 1998, and July 1999. Three to five 24-h stations were selected during each cruise, where at least one main station was located in Arctic water (ArW), one in the polar front region (PF) and one in Atlantic water (AW). A winter scenario was encountered in March with very low concentrations of FPs in the water column, most of the time well below 0.1 mg faecal pellet carbon (FPC) per cubic meter, and with sedimentation rates below 3 mg FPC m À 2 day À 1 at all depths and stations. Increased concentrations of FPs were observed in May and the maximum biomass of FPs was found in ArW (4.8 mg FPC m À 3 ). This was reflected in high vertical flux of FPs in the ArW, just below the chlorophyll maximum ( f 150 mg FPC m À 2 day À 1 ). FPC sedimentation explained f 40% of the total particulate organic carbon (POC) export at 90 m depth at this station. Copepod FP production was moderate to high in May, reflecting favourable feeding conditions. Large spatial variation in the estimated retention potential of FPs was observed, ranging from 96% in AW to f 40% in the PF region. The July scenario did not differ very much from that observed in May. The lowest suspended concentrations and vertical flux of FPs were again observed in AW, in spite of the high pellet-production rate. FPC explained 34% of the POC export out of the upper layer in ArW, 40% in the PF region, but only 8% in AW. The calculated retention potential of 70% of the produced copepod FPs in AW decreased to 60% and 47% in the PF region and ArW, respectively. Krill FPs comprised a significant fraction of both suspended and sedimented FPC throughout the central Barents Sea. The data show that spatial and temporal variations in the FP ''retention filter'' are extensive and evidently of importance for the patterns of vertical flux of organic matter and the regulation of pelagic -benthic coupling in the Barents Sea. D