Papers by Christopher Hewes
Antarctic Science, 2009
Two contrasting high nutrient/low chlorophyll regions having different conditions that control ph... more Two contrasting high nutrient/low chlorophyll regions having different conditions that control phytoplankton production, and separated by an area of blooming, are found during summer in the vicinity of the South Shetland Islands (Antarctica). Low chlorophyll conditions occur either in Fe-rich, deeply mixed and high salinity Weddell Sea shelf waters, or the Fe-poor, shoaled and low salinity Drake Passage Antarctic Circumpolar Current waters, while phytoplankton blooms are located between in mid salinity water. Contrasting phytoplankton communities were found to populate these different biogeochemical provinces. In data from six field seasons (1999–2007), nanoplankton (2–20 μm) were found to be dominant in the phytoplankton populations from light-controlled coastal waters, including blooms, with most chlorophyll found in the 2–5 μm size class. In contrast, the adjacent and presumably Fe-controlled Drake Passage waters were dominated by the microplankton (> 20 μm) size class. The as...
ABSTRACT Abundance, biomass, primary production and taxonomic composition of the microalgae in th... more ABSTRACT Abundance, biomass, primary production and taxonomic composition of the microalgae in the Venice lagoon are presented in order to describe the interaction between the benthic and the planktonic communities. The surface sediments appeared to be richer in biomass and diversity but the primary production resulted similar when compared with the phytoplanktonic one. Introduzione -Negli ambienti di poco profondi, le microalghe, date le loro caratteristiche morfologiche e funzionali, sono estremamente rappresentative dell'interscambio acqua/sedimento. Sebbene in tali ambienti, a causa dell'elevata risospensione, gli stessi taxa si trovino in entrambi gli habitat tanto che la distinzione tra fitoplancton e microfitobenthos risulta spesso artificiale (MacIntyre et al., 1996), il confronto delle due comunità è comunque in grado di fornire spunti interessanti sulle condizioni ambientali. Ad esempio, nell'ultimo decennio nella laguna di Venezia, a seguito della riduzione della copertura macroalgale, dell'introduzione di Tapes philippinarum Adams & Reeve e del significativo aumento della torbidità, è stato osservato un significativo aumento delle diatomee bentoniche nella colonna d'acqua (Acri et al., 2004). Questo lavoro vuole approfondire il confronto tra queste due comunità ed il loro ruolo ambientale. Materiali e metodi -Campioni d'acqua e sedimento sono stati prelevati
The Tan Brook is a heavily channelized stream that runs through an urbanized watershed in Amherst... more The Tan Brook is a heavily channelized stream that runs through an urbanized watershed in Amherst, MA. It poses a stormwater management problem for the University of Massachusetts Amherst due to flooding of soccer fields and erosion of a drainage ditch. The purpose of this study was to estimate reductions in runoff volume to the Tan Brook based on the hypothetical implementation of permeable pavements in various combinations of parking lots, driveways, roadways, and sidewalks, which cover 26% of the watershed area. A spreadsheet model-based approach utilized the Watershed Treatment Model to estimate runoff volume. The percent imperviousness of various land uses was altered to model permeable pavements. Total replacement of parking lots, roadways, sidewalks, and driveways were found to reduce runoff by 18%, 15%, 12%, and 3%, respectively. Recommendations were made to begin replacing parking lots on the UMass Amherst campus and Town of Amherst property.
The Tan Brook is a heavily channelized stream that runs through an urbanized watershed in Amherst... more The Tan Brook is a heavily channelized stream that runs through an urbanized watershed in Amherst, MA. It poses a stormwater management problem for the University of Massachusetts Amherst due to flooding of soccer fields and erosion of a drainage ditch. The purpose of this study was to estimate reductions in runoff volume to the Tan Brook based on the hypothetical implementation of permeable pavements in various combinations of parking lots, driveways, roadways, and sidewalks, which cover 26% of the watershed area. A spreadsheet model-based approach utilized the Watershed Treatment Model to estimate runoff volume. The percent imperviousness of various land uses was altered to model permeable pavements. Total replacement of parking lots, roadways, sidewalks, and driveways were found to reduce runoff by 18%, 15%, 12%, and 3%, respectively. Recommendations were made to begin replacing parking lots on the UMass Amherst campus and Town of Amherst property.
ABSTRACT Abundance, biomass, primary production and taxonomic composition of the microalgae in th... more ABSTRACT Abundance, biomass, primary production and taxonomic composition of the microalgae in the Venice lagoon are presented in order to describe the interaction between the benthic and the planktonic communities. The surface sediments appeared to be richer in biomass and diversity but the primary production resulted similar when compared with the phytoplanktonic one. Introduzione -Negli ambienti di poco profondi, le microalghe, date le loro caratteristiche morfologiche e funzionali, sono estremamente rappresentative dell'interscambio acqua/sedimento. Sebbene in tali ambienti, a causa dell'elevata risospensione, gli stessi taxa si trovino in entrambi gli habitat tanto che la distinzione tra fitoplancton e microfitobenthos risulta spesso artificiale (MacIntyre et al., 1996), il confronto delle due comunità è comunque in grado di fornire spunti interessanti sulle condizioni ambientali. Ad esempio, nell'ultimo decennio nella laguna di Venezia, a seguito della riduzione della copertura macroalgale, dell'introduzione di Tapes philippinarum Adams & Reeve e del significativo aumento della torbidità, è stato osservato un significativo aumento delle diatomee bentoniche nella colonna d'acqua (Acri et al., 2004). Questo lavoro vuole approfondire il confronto tra queste due comunità ed il loro ruolo ambientale. Materiali e metodi -Campioni d'acqua e sedimento sono stati prelevati
Polar Biology, Jun 25, 1997
During January March 1996 the U.S. Antarctic Marine Living Resources program carried out an exten... more During January March 1996 the U.S. Antarctic Marine Living Resources program carried out an extensive multidisciplinary study in a 40,000 km 2 sampling grid around Elephant Island, Antarctica. The physical, chemical, optical, and biological characteristics of the upper water column (0±750 m) were determined at 91 hydrographic stations. Analysis of the temperature and salinity data showed that six dierent hydrographic zones could be dierentiated. The biological (phytoplankton distribution and abundance) and chemical (inorganic nutrient concentrations) data also showed characteristic dierences within each of these six zones. In spite of high concentrations of inorganic N, P, and Si in all six zones, all stations in the northwest portion of the sampling grid (Drake Passage waters) showed very low chlorophyll-a concentrations in surface waters and a sub-surface maximum at increased depth. As stations in this zone have a relatively stable upper mixed layer of 40 m, excess macro-nutrients, and adequate solar radiation for maximal photosynthetic rates, this suggests that rates of primary production in this zone are limited by a micro-nutrient such as Fe. Phytoplankton abundance was much greater in the Brans®eld Strait, in waters in¯uenced by Bellingshausen Sea Water, and in the frontal zones where these water masses mix with Drake Passage waters. Relatively low and deeply distributed phytoplankton abundance was found at all stations in the southeastern portion of our sampling grid, where the upper water column was very weakly strati®ed and showed the characteristics of Weddell Sea water. The areas of enhanced phytoplankton biomass in the AMLR sampling grid roughly correspond to the areas where krill are generally also found in greater abundance. The overall biological productivity of the Elephant Island region would thus appear to be dependent upon the circulation patterns of the major water masses that intrude into this area.
Marine Ecology Progress Series, Aug 29, 2008
Hydrographic, nutrient and trace metal (iron, manganese, and aluminum) concentration data, collec... more Hydrographic, nutrient and trace metal (iron, manganese, and aluminum) concentration data, collected as part of a 2-ship survey during austral summer 2004, were used to examine the influence of upwelling and horizontal mixing on phytoplankton biomass in the region of Elephant Island and South Shetland Islands, Antarctica. Temperature/salinity property analysis and changes in trace metal and nutrient concentrations show that horizontal mixing of shelf waters, not upwelling from depth, is correlated with phytoplankton biomass in the upper mixed layer (UML). The interaction between changing UML depth and nutrient and trace metal concentrations in the UML results in a unimodal distribution of phytoplankton biomass centered at intermediate surface salinities of ~34. Principal component (PC) analysis of hydrographic and chemical observations resolved 3 components that accounted for 99% of the variability in nutrient and trace metal concentrations. The first PC accounted for a conservative loss of nutrients through dilution across a latitudinal salinity gradient. The second and third PCs separated mixed layer depth and nutrient consumption. Although these 2 PCs accounted for just 20% of the variability in the data matrix, they accounted for 65% of the variability in mean phytoplankton biomass, and recreated the unimodal distribution of chlorophyll concentration when modeled across a salinity gradient. We propose that the distribution of phytoplankton biomass is structured by the horizontal mixing of nutrient rich waters, derived from Weddell Sea Shelf Waters, with Antarctic Surface Water that enhances stratification and shoaling of the UML.
Marine Ecology Progress Series, 2005
A deep chlorophyll a maximum (DCM) at depths between 60 and 90 m in waters south of the Antarctic... more A deep chlorophyll a maximum (DCM) at depths between 60 and 90 m in waters south of the Antarctic Polar Front (APF) occurs only in pelagic waters where the chlorophyll a concentrations in the upper mixed layer (UML) are very low (generally < 0.2 mg m-3). Dissolved Fe concentrations in these waters with DCMs are also very low (generally < 0.2 nM) and are probably a limiting factor for phytoplankton growth and biomass. DCMs occur in the upper portion of the temperature minimum layer (TML), which is the winter residue of the Antarctic Surface Water (AASW). The higher phytoplankton biomass at these depths is thought to result from higher Fe concentrations in the winter remnant of the AASW as compared to that found in the overlying UML. A survey of the literature indicates that DCMs are located predominately over the deep ocean basins where enrichment of surface waters with Fe from either coastal sediments or from upwelling processes would be minimal. DCMs are not found in coastal waters or in pelagic regions where complex bottom topography causes upwelling of deep water with sufficiently high Fe concentrations to enhance surface chlorophyll a concentrations. Such enrichment of surface waters overlying or downstream of topographical seamounts or ridges that rise to within a few thousand meters of the surface usually results in elevated phytoplankton biomass in the UML and no DCM due to decreased solar irradiance in the TML. The effect of such enrichment of Fe in surface pelagic waters that results from upwelling processes is most pronounced in the Scotia Sea, in the Polar Frontal region downstream of South Georgia, over the Southwest Indian Ridge, over the Kerguelen Plateau, and over the Pacific Antarctic and Southeast Indian Ridges.
Marine Ecology Progress Series, 2000
Using an extensive data set acquired in Antarctic waters in February-March of 1998, algorithms ha... more Using an extensive data set acquired in Antarctic waters in February-March of 1998, algorithms have been formulated which permit reliable estimation of chlorophyll a (chl a) concentrations in the upper 100 m of the water column. The algorithms were derived from 105 oceanographic stations where an instrumented CTD-rosette profihng unit was deployed from the surface to 750 m depth. During each upcast 9 water samples were obtained at depths between 5 and 100 m. The values for in situ chl a fluorescence and solar irradiance were recorded s~multaneously with collection of water at each depth. The chl a concentrations at each of the 9 depths were determ~ned by standard laboratory procedures after extraction of the photosynthetic pigments into absolute methanol. Analysis of the data indicated that separate algorithms had to be formulated for coastal ~r a t e r s as contrasted to pelagic, lowbiomass, waters. Each of these 2 algorithms also required 2 equations to compensate for the inhibitory effect of solar radiation on the fluorescence yleld per unit chl a. Water column data obtained during January 1998 were used to test the algorithms. The results showed that the profiles of estimated and measured chl a concentrations were similar throughout the upper 100 m of the water column, but that the estimated values were lower than the actualvalues by an average of 19% (n = 528, r2 = 0.83). It is concluded that the use of a calibrated in situ fluorometer can, with the proper algorithms, provide the investigator the detailed profile of chl a distribution as well as realistic estimations of the actual cN a concentrations in the upper 100 m of the water column.
Deep-sea Research Part I-oceanographic Research Papers, Aug 1, 2009
Eighteen years of summertime hydrographic and chlorophyll-a (Chl-a) data ($2700 stations) from th... more Eighteen years of summertime hydrographic and chlorophyll-a (Chl-a) data ($2700 stations) from the South Shetland Islands (Antarctica) region show that a ''bell-shaped'' (unimodal) distribution of phytoplankton biomass results annually when plotted against the inshore to offshore gradient in surface salinity. The maximum for this unimodal Chl-a distribution corresponds with a shallow upper mixed layer (UML) in iron-rich waters that occurs at salinities $34. Methods of gradient analysis are used to distinguish sources of variability for bloom development among years. The control of phytoplankton biomass is resolved across the salinity gradient that separates the co-limiting conditions of deep UML depths and low-iron concentrations as opposing end-members. Chlorophyll-fluorescence yield data (a proxy for Fe-stress) showed that at salinities $34, phytoplankton biomass was unlikely to be limited by Fe. Instead, blooming at salinities $34 (1.371 mg Chl-a m À3) co-varied with shallow UML depths (41719 m) that occurred as a function of higher UML temperature (1.570.5 1C) among years, and is evidence that atmospheric climate variability impacts summertime phytoplankton biomass and production in this Southern Ocean seascape.
Limnology and Oceanography, Mar 1, 1983
A method has been developed to concentrate nanoplankton in natural water samples for transfer to ... more A method has been developed to concentrate nanoplankton in natural water samples for transfer to solid surfaces for light microscopy. Marine phytoplankton are used to demonstrate the convenience and ease of our filter‐transfer‐freeze (FTF) technique in recovering filtered material, which can be mounted and ready for examination within 30 min. Samples can be observed with standard or inverted microscopes using phase contrast, Nomarski interference, or other optical systems. The micro‐ and nanoplankton mounted on slides prepared by the FTF technique retain their cellular integrity and lifelike appearance.
Marine Ecology Progress Series, Feb 26, 2009
Deep-sea Research Part Ii-topical Studies in Oceanography, Jun 1, 2004
Chlorophyll-a (Chl-a) concentrations in surface waters were measured at 137 hydrographic stations... more Chlorophyll-a (Chl-a) concentrations in surface waters were measured at 137 hydrographic stations occupied by four research vessels participating in the CCAMLR 2000 Survey and the values were compared to estimates from data acquired by the SeaWiFS satellite. The Chl-a concentrations measured on board ship ranged from 0.06 to 14.6 mg m À3 , a range that includes most surface Chl-a concentrations during midsummer in the Southern Ocean. Owing to persistent cloud cover over much of the Southern Ocean, it was necessary to acquire multi-day composites of satellite data in order to obtain reliable estimates of Chl-a at each of the hydrographic stations. The correlation between the median value for the eight-day composites and the Chl-a concentrations measured on board ship had an R 2 value of 0.82, with the satellite data underestimating the values obtained on board ship at high Chl-a concentrations and slightly overestimating the shipboard data at Chl-a concentrations of o0.2 mg m À3. For Chl-a concentrations of o1.0 mg m À3 , the ratio of the satellite estimates divided by the shipboard values was 0.8970.45 (n=50). As the mean Chl-a concentration in most pelagic Antarctic waters is close to 0.5 mg m À3 , satellite estimates for Chl-a concentrations in surface waters are thus close to shipboard measurements, and offer the advantage of providing synoptic maps of Chl-a distribution over extensive areas of the Southern Ocean. Satellite Chl-a images for the months preceding (December 1999) and following (February 2000) the CCAMLR 2000 Survey cruises showed that the general pattern of Chl-a concentration in the Scotia Sea and adjoining waters was similar in all three months, but that the phytoplankton biomass was generally
Hydrographic, chemical, and biological data relevant to phytoplankton productivity in the South S... more Hydrographic, chemical, and biological data relevant to phytoplankton productivity in the South Shetland Islands region were collected in January and Febrary of 2009. According to the 2008/09 AMLR Survey: • A total of 104 phytoplankton samples were taken at stations on the AMLR Survey grid. • Mean Chl-a concentrations in the Upper Mixed Layer (UML) were lowest in the Elephant Island (~0.7 mg m-3) and West (~0.4 mg m-3) Areas, but comparable to the historical mean. Highest mean Chl-a concentrations were measured in the Joinville Island Area and the South Area. • Two stations in the Joinville Island Area had unexpectedly high Chl-a concentrations. Th is area might need further investigation to understand . Introduction Th e ability to ultimately manage various trophic groups in the Antarctic, as envisioned by CCAMLR, must be based on a thorough understanding of food web dynamics, including the food resources available to Antarctic krill and other zooplankton in the region. It is thus ...
Frontiers in Marine Science, 2019
An oceanographic survey around the South Shetland Islands (SSI) and the South Orkney Islands (SOI... more An oceanographic survey around the South Shetland Islands (SSI) and the South Orkney Islands (SOI) was conducted during January 2007 and February 2008, respectively, as part of the United States Antarctic Marine Living Resources (AMLR) program ecosystems surveys. At 27 stations, concentrations of dissolved labile Fe (DFe) and total acid leachable (unfiltered, TaLFe) iron (Fe) were measured in the upper 200 m (including coastal and oceanic waters) to better resolve the factors limiting primary production in these regions. Northwest of the SSI, a region influenced by Drake Passage (DP) waters, mean DFe (∼0.26 nM) and TaLFe (∼1.02 nM) concentrations were the lowest, whereas intermediate concentrations for both DFe and TaLFe were measured in the Bransfield Strait (BS). Around Elephant Island (EI), over and off the continental shelf, Fe concentrations differed between the west and the east margins. DFe and TaLFe concentrations further support the argument that the effect of the Shackleton Transverse Ridge (STR) is a crucial structure affecting both the Fe and the chlorophyll distributions in this region. The waters around the SOI had DFe concentrations higher than those in the SSI, with the area north of the South Scotia Ridge (SSR) (60 • S), having the highest DFe (0.54 nM) concentrations and the waters in Powell Basin (PB) having the lowest DFe (1.17 nM) and TaLFe (4.51 nM) and concentrations. These spatial patterns of Fe suggest that there are different Fe inputs from shelf waters near the Antarctic Slope Front (ASF). The overall TaLFe:DFe ratios, used as indicator for understanding the relative distance of Fe sources, were lower around the SOI compared to those in the SSI, suggesting that the Fe source for SOI waters was more distant. The spatial patterns between Fe and chlorophyll-a (Chl-a) concentrations in relation to the hydrography highlight the complexity and variability of the oceanographic processes in the region. These
Antarctic Nutrient Cycles and Food Webs, 1985
ABSTRACT
Polar Biology, 2004
The US Antarctic Marine Living Resources (AMLR) program has, since 1990, conducted annual surveys... more The US Antarctic Marine Living Resources (AMLR) program has, since 1990, conducted annual surveys from early January through mid-March in a large sampling grid around Elephant Island, Antarctica. Approximately 100 hydrographic stations were occupied twice during each field season, with physical, chemical, optical, and biological data acquired from the surface to 750 m depth or to within 10 m of the bottom at shallower stations. During these 14 years, most of the stations in pelagic waters to the northwest of Elephant Island had very low chlorophyll-a (Chl-a) concentrations (<0.2 mg m À3) in the upper mixed layer (UML) of $45 m, but a deep chlorophyll-a maximum (DCM) existed between 50 and 100 m, with a peak at approximately 75 m. This was in contrast to adjacent stations which had higher Chl-a concentrations (approximately 1.0 mg m À3) in the UML and no DCM. We provide evidence that the higher Chl-a concentrations that occur at depths of 50-100 m result from increased photosynthetic activity and not from a passive sinking of cells from the UML or by the intrusion of Chl-a rich coastal waters. Data to support this conclusion include (1) elevated dissolved oxygen concentrations between 50 and 100 m, (2) evidence of active photosynthesis at the depth of the DCM as indicated by increased natural upwelling radiation at 683 nm, and (3) water samples obtained from the DCM at 75 m and incubated under simulated conditions of temperature and light had assimilation numbers of approximately 1.0-1.5 mg carbon fixed per milligram Chl-a per hour. DCMs occur in the same depth range as the temperature minimum layer (TML) (the winter remnant of Antarctic surface water, AASW), which is known to have elevated concentrations of inorganic nutrients essential for growth of phytoplankton. Our data indicate that a DCM develops as a result of (1) depletion of iron (Fe) in the UML with the onset of the summer season, and (2) growth of phytoplankton in the TML where both Fe concentrations and solar irradiance levels are high enough to permit an increase of phytoplankton biomass.
An ocean color chlorophyll-a (chl-a) algorithm (SPGANT) has been developed by combining new in si... more An ocean color chlorophyll-a (chl-a) algorithm (SPGANT) has been developed by combining new in situ spectral reflectance and chl-a data obtained during the US JGOFS AESOPS (Antarctic Environment and Southern Ocean Process Study) and the AMLR (Antarctic Marine Living Resources) programs. The results are compared to previous ocean color algorithms recommended for the Southern Ocean (Mitchell and Holm-Hansen, Deep Sea
Limnology and Oceanography, 2007
Processes influencing phytoplankton bloom development in the southern Drake Passage were studied ... more Processes influencing phytoplankton bloom development in the southern Drake Passage were studied using shipboard iron-enrichment incubations conducted across a surface chlorophyll gradient near the Antarctic Peninsula, in a region of water mass mixing. Iron incubation assays showed that Antarctic Circumpolar Current (ACC) waters were severely iron limited, while shelf waters with high ambient iron concentrations (1-2 nmol L 21) were iron replete, demonstrating that mixing of the two water masses is a plausible mechanism for generation of the high phytoplankton biomass observed downstream of the Antarctic Peninsula. In downstream highchlorophyll mixed waters, phytoplankton growth rates were also iron limited, although responses to iron addition were generally more moderate as compared to ACC waters. Synthesizing results from all experiments, significant correlations were found between the initial measurements of Photosystem II (PSII) parameters (F v : F m , s PSII , and p) and the subsequent responses of these waters to iron addition. These correlations indicate that PSII parameters can be used to assess the degree of iron stress experienced in these waters and likely in other regions where photoinhibition and nitrogen stress are not confounding factors.
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Papers by Christopher Hewes