Seagrass ecosystems provide numerous ecosystem services that support coastal communities around t... more Seagrass ecosystems provide numerous ecosystem services that support coastal communities around the world. They sustain abundant marine life as well as commercial and artisanal fisheries, and help protect shorelines from coastal erosion. Additionally, seagrass meadows are a globally significant sink for carbon and represent a key ecosystem for combating climate change. However, seagrass habitats are suffering rapid global decline. Despite recognition of the importance of ‘Blue Carbon’, no functioning seagrass restoration or conservation projects supported by carbon finance currently operate, and the policies and frameworks to achieve this have not been developed. Yet, seagrass ecosystems could play a central role in addessing important international research questions regarding the natural mechanisms through which the ocean and the seabed can mitigate climate change, and how ecosystem structure links to service provision. The relative inattention that seagrass ecosystems have receiv...
An amendment to this paper has been published and can be accessed via a link at the top of the pa... more An amendment to this paper has been published and can be accessed via a link at the top of the paper.
Liquid oceans and ice caps, along with ice crusts, have long been considered defining features of... more Liquid oceans and ice caps, along with ice crusts, have long been considered defining features of the Earth, but space missions and observations have shown that they are in fact common features among many of the solar system's outer planets and their satellites. Interactions with rock-forming materials have produced saline oceans not dissimilar in many respects to those on Earth, where mineral precipitation within frozen seawater plays a significant role in both determining global properties and regulating the environment in which a complex ecosystem of extremophiles exists. Since water is considered an essential ingredient for life, the presence of oceans and ice on other solar system bodies is of great astrobiological interest. However, the details surrounding mineral precipitation in freezing environments are still poorly constrained, owing to the difficulties of sampling and preservation for laboratory analysis, meaning that predictive models have limited empirical underpinn...
Antarctic pack ice is inhabited by a diverse and active microbial community reliant on nutrients ... more Antarctic pack ice is inhabited by a diverse and active microbial community reliant on nutrients for growth. Seeking patterns and overlooked processes, we performed a large-scale compilation of macro-nutrient data (hereafter termed nutrients) in Antarctic pack ice (306 ice-cores collected from 19 research cruises). Dissolved inorganic nitrogen and silicic acid concentrations change with time, as expected from a seasonally productive ecosystem. In winter, salinity-normalized nitrate and silicic acid concentrations (C*) in sea ice are close to seawater concentrations (Cw), indicating little or no biological activity. In spring, nitrate and silicic acid concentrations become partially depleted with respect to seawater (C* < Cw), commensurate with the seasonal build-up of ice microalgae promoted by increased insolation. Stronger and earlier nitrate than silicic acid consumption suggests that a significant fraction of the primary productivity in sea ice is sustained by flagellates. By...
The sea ice cover of high latitude oceans contains concentrated brines which are the site of in-s... more The sea ice cover of high latitude oceans contains concentrated brines which are the site of in-situ chemical and biological reactions. The brines become supersaturated with respect to mirabilite (Na 2 SO 4 • 10H 2 O) below −6.4 • C, and the associated removal of Na + and SO 2− 4 from the brine results in considerable non-conservative changes to its composition. The changes are reflected in the brine salinity, which is a fundamental physico-chemical parameter in the sea ice brine system. Here, measurements of electrical conductivity and brine composition in synthetic sea ice brines between −1.8 and −20.6 • C, obtained during a comprehensive investigation of the brine-mirabilite equilibrium at below-zero temperatures reported elsewhere, are combined with modelled estimates to assess the behaviour of the absolute (S A) and practical (S P) salinities of sea ice brines. Results display substantial divergence of S P from S A below −6.4 • C, reaching a 7.2 % difference at −22.8 • C. This
The protection of organic carbon stored in forests is considered as an important method for mitig... more The protection of organic carbon stored in forests is considered as an important method for mitigating climate change. Like terrestrial ecosystems, coastal ecosystems store large amounts of carbon, and there are initiatives to protect these &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;blue carbon&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;stores. Organic ...
Seagrass meadows provide numerous ecosystem services and their rapid global loss may reduce human... more Seagrass meadows provide numerous ecosystem services and their rapid global loss may reduce human welfare as well as ecological integrity. In common with the other 'blue carbon' habitats (mangroves and tidal marshes) seagrasses are thought to provide coastal defence and encourage sediment stabilisation and surface elevation. A sophisticated understanding of sediment elevation dynamics in mangroves and tidal marshes has been gained by monitoring a wide range of different sites, located in varying hydrogeomorphological conditions over long periods. In contrast, similar evidence for seagrasses is sparse; the present study is a contribution towards filling this gap. Surface elevation change pins were deployed in four locations, Scotland, Kenya, Tanzania and Saudi Arabia, in both seagrass and unvegetated control plots in the low intertidal and shallow subtidal zone. The presence of seagrass had a highly significant, positive impact on surface elevation at all sites. Combined data from the current work and the literature show an average difference of 31 mm per year in elevation rates between vegetated and unvegetated areas, which emphasizes the important contribution of seagrass in facilitating sediment surface elevation and reducing erosion. This paper presents the first multi-site study for sediment surface elevation in seagrasses in different settings and species.
[1] Seagrass meadows are highly productive habitats found along many of the world's 6 coastline, ... more [1] Seagrass meadows are highly productive habitats found along many of the world's 6 coastline, providing important services that support the overall functioning of the coastal 7 zone. The organic carbon that accumulates in seagrass meadows is derived not only from 8 seagrass production but from the trapping of other particles, as the seagrass canopies 9 facilitate sedimentation and reduce resuspension. Here we provide a comprehensive 10 synthesis of the available data to obtain a better understanding of the relative contribution 11 of seagrass and other possible sources of organic matter that accumulate in the sediments 12 of seagrass meadows. The data set includes 219 paired analyses of the carbon isotopic 13 composition of seagrass leaves and sediments from 207 seagrass sites at 88 locations 14 worldwide. Using a three source mixing model and literature values for putative sources, 15 we calculate that the average proportional contribution of seagrass to the surface 16 sediment organic carbon pool is ∼50%. When using the best available estimates of 17 carbon burial rates in seagrass meadows, our data indicate that between 41 and 18 66 gC m −2 yr −1 originates from seagrass production. Using our global average for 19 allochthonous carbon trapped in seagrass sediments together with a recent estimate of 20 global average net community production, we estimate that carbon burial in seagrass 21 meadows is between 48 and 112 Tg yr −1 , showing that seagrass meadows are natural hot 22 spots for carbon sequestration. 27 [2] Although seagrass meadows account for only a rela-28 tively small area of the coastal ocean (∼0.1%), they play an 29 important role in the coastal zone and provide ecosystem 30 goods and services that have been estimated to be of high 31 value compared with other marine and terrestrial habitats 32 [Costanza et al., 1997]. The annual rate of carbon accu-33 mulation in seagrass meadows of ∼83 gC m −2 yr −1 [Duarte 34 et al., 2005a] is larger than that in most terrestrial ecosys-35 tems [Pidgeon, 2009] and their global carbon burial rates of 36 27-44 Tg C yr −1 are an important component (10-18%) of 37 the total carbon burial in the ocean. A proportion of the 38 carbon that accumulates in seagrass meadows derives from 39 excess photosynthetic carbon fixation within the meadows, 40 some of which is placed directly into the sediments as roots 41 and rhizomes [Duarte and Cebrián, 1996]. Alongside this 42 direct source of carbon from seagrass tissues, organic matter 43 from other sources accumulates in the sediments due to the 44 seagrass canopies acting as efficient filters, stripping parti-45 cles from the water column and adding them to the sediment 46 load [Hendriks et al., 2007]. As a result of these processes 47 there is a net transfer of allochthonous carbon to the sedi-48 ments of seagrass meadows that enhances their capacity for 49 long-term carbon sequestration.
Journal of Geophysical Research Biogeosciences, Feb 1, 2009
1] We tested the role of solar irradiance and ammonium inputs on phytoplankton bloom formation in... more 1] We tested the role of solar irradiance and ammonium inputs on phytoplankton bloom formation in Antarctic coastal waters (62°39.576 0 S; 60°22.408 0 W, Livingston Island, South Sethlands) through the combination of a large-scale, in situ mesocosm experiment and a small-scale experiment. Phytoplankton growth, nutrient use, and biomass development remained low at ambient irradiances and increased greatly (greater than thirtyfold) to yield large (up to 93 mg chlorophyll a l À1 ) phytoplankton blooms in response to moderate shading. The phytoplankton communities tested were light limited when irradiance was reduced below 30% of the incident irradiance and stressed by high irradiance at the full ambient irradiance. Ammonium additions greatly stimulated phytoplankton growth, biomass, and stimulated the use of the large nitrate pool present in the Antarctic waters and lead to a decline in the specific UV absorption by mycosporinelike amminoacids. The small-scale experiment confirmed the role of UV irradiance in inhibiting phytoplankton growth and the capacity of ammonium inputs to overcome this inhibition. The alleviation of the high-irradiance stress by ammonium additions provided evidence of a key role of ammonium inputs in allowing phytoplankton to resume growth and nutrient use. The results demonstrate that there is a narrow window of irradiance where phytoplankton growth is adequate and that coastal Antarctic phytoplankton communities, examined here, are either light limited or stressed by high irradiance at irradiances outside this range. More research is needed to analyze the interplay between light climate, ammonium, and bloom initiation in Antarctic coastal waters to test the generality of the results obtained. Citation: Agustí, S., C. M. Duarte, M. Llabrés, N. S. R. Agawin, and H. Kennedy (2009), Response of coastal Antarctic phytoplankton to solar radiation and ammonium manipulation: An in situ mesocosm experiment,
The Mediterranean climate exerts a major influence on the basic properties of the Mediterranean S... more The Mediterranean climate exerts a major influence on the basic properties of the Mediterranean Sea, which constrains the structure and dynamics of the ecosystem. Seasonal variations in the marine climate follow the expected unimodal seasonality only for temperature, while most other forcing factors show a complex variance structure, with dominant time scales of 50-100 days (e.g. wave action), and with some of the factors acting as random factors (‘white noise’) at the annual scale (e.g. rainfall), thereby limiting the predictability of the system. The resulting ecosystem seasonality is unconventional and poorly linked to temperature. The prolonged period of high atmospheric pressure and associated high irradiance and calm waters in late winter is the main seasonal trigger in the NW Mediterranean Sea, setting the development of a phytoplankton bloom, as well as the recruitment of the benthos. Decadal changes in the Mediterranean marine climate are characterized by the dominance of oscillations with a 22-year period, suggesting an important solar forcing on the climate. This forcing masks the monotonous trends, such as the warming and increased sea level in the Mediterranean, expected from anthropogenic forcing. Records of decadal changes in the ecosystem often display a monotonous trend in the deterioration of water quality, indicative of human effects as the main forcing agent, while climatic forcing, which displays oscillatory variation, is of secondary importance. The paucity of long-term records precludes a robust analysis of ecosystem response to decadal climatic forcing. This absence can be partially remediated by the ability to interrogate the long-lived organisms that represent an important, albeit endangered component of Mediterranean biodiversity, to extract records (e.g. growth, temperature, changes in the nature of the dissolved inorganic carbon pool) of the changes they have witnessed.
The remineralization rate of sedimentary organic carbon (R org ) and the depth-integrated, diffus... more The remineralization rate of sedimentary organic carbon (R org ) and the depth-integrated, diffusion-supplied O 2 consumption rate (IOC) during microbial metabolism in sediments was investigated in three deep-sea sites at 1100, 2000 and 3500 m water depth in the eastern north Atlantic during the spring and summer 1998. In-situ pore water O 2 profiles yielded an IOC of 0.45F0.07 mmol O 2 m À2 day À1 at the deepest site (n=3) and ca. 1-1.5 mmol O 2 m À2 day À1 at the shallowest site (n=2). The R org was independently estimated at all three sites from ex-situ pore water profiles of the isotopic composition of total dissolved inorganic carbon (d 13 C T ), assuming that the concentration and isotopic composition of pore water C T with depth in the sediment was controlled only by microbial oxidation of isotopically depleted sedimentary organic carbon. The R org was thus estimated to be ca. 0.5-0.6 mmol C m À2 day À1 at the shallowest site and ca. 0.3-0.4 mmol C m À2 day À1 at the two deeper sites. Stoichiometric and isotopic constraints indicated that oxic remineralization of sedimentary organic matter was the dominant metabolic pathway in the sediments at 3500 m water depth. Similarly, stoichiometric and isotopic constraints suggested that the R org estimates from the ex-situ pore water d 13 C T profiles from 1100 and 2000 m water depth were likely to be minimum values and provided evidence for the occurrence of post-oxic remineralization processes. Post-oxic metabolism in the sediments of these sites could be linked to, or even augmented by, the non-diffusive mode of supply of organic matter mediated by infaunal organisms below the oxic sediment layer. D
The integrated iodate and total-iodine concentrations accompanying accelerated growth of natural ... more The integrated iodate and total-iodine concentrations accompanying accelerated growth of natural phytoplankton in eight 14-m deep mesocosm experiments did not vary significantly. Growth was induced by the addition of nutrients, while light irradiance was controlled by neutral density screens. These measures resulted in a range of particulate organic carbon concentrations of between 13 and 220 mol L Ϫ1 , that is, covering some that are well in excess of blooms generally found in the natural environment. This, together with most earlier results obtained from phytoplankton culturing and much hydrographic survey, is used as an opportunity to question whether phytoplankton growth can be the cause of iodate reduction in seawater.
Deep Sea Research Part Ii Topical Studies in Oceanography, 1995
The concentration and isotopic composition of carbon was measured in suspended particulate organi... more The concentration and isotopic composition of carbon was measured in suspended particulate organic matter in surface waters of the Southern Pacific Ocean along an 88°°W transect from 67-54°S during the Austral summer. Concurrent measurements of total dissolved inorganic carbon (ΣCO 2), pCO 2, particulate organic carbon (POC), salinity, chlorophyll and temperature provide an opportunity to study the covariance of these parameters with changes in the isotopic composition of particulate organic carbon (δ 13C-POC). The south-north transect did not show any significant changes in POC (10-19 mmol m -3) or chlorophyll (0.5-1 mg m -3), while δ 13C-POC variations of approximately 4%o were recorded. Although surface water CO 2(aq) is substantially out of equilibrium with the local atmosphere, the isotopic changes are negatively correlated with the concentration of dissolved carbon dioxide along the transect, and follow the general trends predicted from previously determined relationships. Estimates of ɛp ( ɛ#p) are less well constrained and show a weaker correlation with [CO 2 (aq)]. Where ɛ#p varies independently of [C0 2(aq)] small changes in the concentration gradient between extra- and intra-cellular CO 2 can be predicted. The difficulty in obtaining independent measures of growth rate and matching these measurements to the timescales over which the 613C of the phytoplankton is integrated is examined.
Rapid Communications in Mass Spectrometry, Jun 15, 2003
The conversion of ammonium (NH(4) (+)) to 1-sulfonato-iso-indole has been examined as a method fo... more The conversion of ammonium (NH(4) (+)) to 1-sulfonato-iso-indole has been examined as a method for natural abundance measurement of delta(15)N of NH(4) (+). The reaction is complete within 2 h and is based on the derivatisation of NH(4) (+) by o-phthaldialdehyde and sodium sulfite at a high pH, 11.2. The product is readily concentrated from dilute solutions by reverse-phase solid-phase extraction (SPE). The method is compound-specific despite partial derivatisation of potentially interfering amino acids, as their derivatives are not extracted by SPE. delta(15)N values of NH(4) (+) in KCL soil extracts can be measured within 48 h by automated continuous-flow IRMS with a precision of 0.23 per thousand (1 SD). Parallel measurements of NH(4) (+) standards of known delta(15)N are made to allow correction for the isotopic dilution by non-sample NH(4) (+). The practicality of this method is demonstrated by measuring the changes in NH(4) (+) concentration and delta(15)N following the addition of urea as a nitrogen source to inorganic N-depleted soil.
Temporal and spatial variation of d 34 S, total sulfur (TS) concentration, and elemental sulfur c... more Temporal and spatial variation of d 34 S, total sulfur (TS) concentration, and elemental sulfur concentration (S 0 ) in leaves, roots, and rhizomes of Zostera marina was followed between June 2002 and May 2003 at four locations in Roskilde Fjord and Øresund, Denmark. These were related to temporal changes in sediment sulfide concentrations, sulfur pool size, and sulfur pool d 34 S. The d 34 S of Z. marina was most negative in the roots, followed by rhizomes and leaves, indicating that roots were mostly affected by sulfide. A significant relationship between decreasing d 34 S and increasing TS in the plant tissues indicated that sulfide accumulated in the plant and, furthermore, a positive relation between TS and S 0 in the plant suggests that part of the sulfide is reoxidized to S 0 . There were marked temporal changes in all variables at all sites, but the pattern of change varied between sites. The temporal and spatial heterogeneity in plant d 34 S, TS, and S 0 depended on a variety of factors, such as sediment sulfide concentrations and the below : aboveground biomass ratio of the plants. This suggests that mechanisms of sulfide invasion are complex, and several factors (plant morphology, environmental variables) acting in concert or against each other need to be considered to successfully predict sulfide invasion in seagrasses.
Seagrass ecosystems provide numerous ecosystem services that support coastal communities around t... more Seagrass ecosystems provide numerous ecosystem services that support coastal communities around the world. They sustain abundant marine life as well as commercial and artisanal fisheries, and help protect shorelines from coastal erosion. Additionally, seagrass meadows are a globally significant sink for carbon and represent a key ecosystem for combating climate change. However, seagrass habitats are suffering rapid global decline. Despite recognition of the importance of ‘Blue Carbon’, no functioning seagrass restoration or conservation projects supported by carbon finance currently operate, and the policies and frameworks to achieve this have not been developed. Yet, seagrass ecosystems could play a central role in addessing important international research questions regarding the natural mechanisms through which the ocean and the seabed can mitigate climate change, and how ecosystem structure links to service provision. The relative inattention that seagrass ecosystems have receiv...
An amendment to this paper has been published and can be accessed via a link at the top of the pa... more An amendment to this paper has been published and can be accessed via a link at the top of the paper.
Liquid oceans and ice caps, along with ice crusts, have long been considered defining features of... more Liquid oceans and ice caps, along with ice crusts, have long been considered defining features of the Earth, but space missions and observations have shown that they are in fact common features among many of the solar system's outer planets and their satellites. Interactions with rock-forming materials have produced saline oceans not dissimilar in many respects to those on Earth, where mineral precipitation within frozen seawater plays a significant role in both determining global properties and regulating the environment in which a complex ecosystem of extremophiles exists. Since water is considered an essential ingredient for life, the presence of oceans and ice on other solar system bodies is of great astrobiological interest. However, the details surrounding mineral precipitation in freezing environments are still poorly constrained, owing to the difficulties of sampling and preservation for laboratory analysis, meaning that predictive models have limited empirical underpinn...
Antarctic pack ice is inhabited by a diverse and active microbial community reliant on nutrients ... more Antarctic pack ice is inhabited by a diverse and active microbial community reliant on nutrients for growth. Seeking patterns and overlooked processes, we performed a large-scale compilation of macro-nutrient data (hereafter termed nutrients) in Antarctic pack ice (306 ice-cores collected from 19 research cruises). Dissolved inorganic nitrogen and silicic acid concentrations change with time, as expected from a seasonally productive ecosystem. In winter, salinity-normalized nitrate and silicic acid concentrations (C*) in sea ice are close to seawater concentrations (Cw), indicating little or no biological activity. In spring, nitrate and silicic acid concentrations become partially depleted with respect to seawater (C* < Cw), commensurate with the seasonal build-up of ice microalgae promoted by increased insolation. Stronger and earlier nitrate than silicic acid consumption suggests that a significant fraction of the primary productivity in sea ice is sustained by flagellates. By...
The sea ice cover of high latitude oceans contains concentrated brines which are the site of in-s... more The sea ice cover of high latitude oceans contains concentrated brines which are the site of in-situ chemical and biological reactions. The brines become supersaturated with respect to mirabilite (Na 2 SO 4 • 10H 2 O) below −6.4 • C, and the associated removal of Na + and SO 2− 4 from the brine results in considerable non-conservative changes to its composition. The changes are reflected in the brine salinity, which is a fundamental physico-chemical parameter in the sea ice brine system. Here, measurements of electrical conductivity and brine composition in synthetic sea ice brines between −1.8 and −20.6 • C, obtained during a comprehensive investigation of the brine-mirabilite equilibrium at below-zero temperatures reported elsewhere, are combined with modelled estimates to assess the behaviour of the absolute (S A) and practical (S P) salinities of sea ice brines. Results display substantial divergence of S P from S A below −6.4 • C, reaching a 7.2 % difference at −22.8 • C. This
The protection of organic carbon stored in forests is considered as an important method for mitig... more The protection of organic carbon stored in forests is considered as an important method for mitigating climate change. Like terrestrial ecosystems, coastal ecosystems store large amounts of carbon, and there are initiatives to protect these &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;blue carbon&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;stores. Organic ...
Seagrass meadows provide numerous ecosystem services and their rapid global loss may reduce human... more Seagrass meadows provide numerous ecosystem services and their rapid global loss may reduce human welfare as well as ecological integrity. In common with the other 'blue carbon' habitats (mangroves and tidal marshes) seagrasses are thought to provide coastal defence and encourage sediment stabilisation and surface elevation. A sophisticated understanding of sediment elevation dynamics in mangroves and tidal marshes has been gained by monitoring a wide range of different sites, located in varying hydrogeomorphological conditions over long periods. In contrast, similar evidence for seagrasses is sparse; the present study is a contribution towards filling this gap. Surface elevation change pins were deployed in four locations, Scotland, Kenya, Tanzania and Saudi Arabia, in both seagrass and unvegetated control plots in the low intertidal and shallow subtidal zone. The presence of seagrass had a highly significant, positive impact on surface elevation at all sites. Combined data from the current work and the literature show an average difference of 31 mm per year in elevation rates between vegetated and unvegetated areas, which emphasizes the important contribution of seagrass in facilitating sediment surface elevation and reducing erosion. This paper presents the first multi-site study for sediment surface elevation in seagrasses in different settings and species.
[1] Seagrass meadows are highly productive habitats found along many of the world's 6 coastline, ... more [1] Seagrass meadows are highly productive habitats found along many of the world's 6 coastline, providing important services that support the overall functioning of the coastal 7 zone. The organic carbon that accumulates in seagrass meadows is derived not only from 8 seagrass production but from the trapping of other particles, as the seagrass canopies 9 facilitate sedimentation and reduce resuspension. Here we provide a comprehensive 10 synthesis of the available data to obtain a better understanding of the relative contribution 11 of seagrass and other possible sources of organic matter that accumulate in the sediments 12 of seagrass meadows. The data set includes 219 paired analyses of the carbon isotopic 13 composition of seagrass leaves and sediments from 207 seagrass sites at 88 locations 14 worldwide. Using a three source mixing model and literature values for putative sources, 15 we calculate that the average proportional contribution of seagrass to the surface 16 sediment organic carbon pool is ∼50%. When using the best available estimates of 17 carbon burial rates in seagrass meadows, our data indicate that between 41 and 18 66 gC m −2 yr −1 originates from seagrass production. Using our global average for 19 allochthonous carbon trapped in seagrass sediments together with a recent estimate of 20 global average net community production, we estimate that carbon burial in seagrass 21 meadows is between 48 and 112 Tg yr −1 , showing that seagrass meadows are natural hot 22 spots for carbon sequestration. 27 [2] Although seagrass meadows account for only a rela-28 tively small area of the coastal ocean (∼0.1%), they play an 29 important role in the coastal zone and provide ecosystem 30 goods and services that have been estimated to be of high 31 value compared with other marine and terrestrial habitats 32 [Costanza et al., 1997]. The annual rate of carbon accu-33 mulation in seagrass meadows of ∼83 gC m −2 yr −1 [Duarte 34 et al., 2005a] is larger than that in most terrestrial ecosys-35 tems [Pidgeon, 2009] and their global carbon burial rates of 36 27-44 Tg C yr −1 are an important component (10-18%) of 37 the total carbon burial in the ocean. A proportion of the 38 carbon that accumulates in seagrass meadows derives from 39 excess photosynthetic carbon fixation within the meadows, 40 some of which is placed directly into the sediments as roots 41 and rhizomes [Duarte and Cebrián, 1996]. Alongside this 42 direct source of carbon from seagrass tissues, organic matter 43 from other sources accumulates in the sediments due to the 44 seagrass canopies acting as efficient filters, stripping parti-45 cles from the water column and adding them to the sediment 46 load [Hendriks et al., 2007]. As a result of these processes 47 there is a net transfer of allochthonous carbon to the sedi-48 ments of seagrass meadows that enhances their capacity for 49 long-term carbon sequestration.
Journal of Geophysical Research Biogeosciences, Feb 1, 2009
1] We tested the role of solar irradiance and ammonium inputs on phytoplankton bloom formation in... more 1] We tested the role of solar irradiance and ammonium inputs on phytoplankton bloom formation in Antarctic coastal waters (62°39.576 0 S; 60°22.408 0 W, Livingston Island, South Sethlands) through the combination of a large-scale, in situ mesocosm experiment and a small-scale experiment. Phytoplankton growth, nutrient use, and biomass development remained low at ambient irradiances and increased greatly (greater than thirtyfold) to yield large (up to 93 mg chlorophyll a l À1 ) phytoplankton blooms in response to moderate shading. The phytoplankton communities tested were light limited when irradiance was reduced below 30% of the incident irradiance and stressed by high irradiance at the full ambient irradiance. Ammonium additions greatly stimulated phytoplankton growth, biomass, and stimulated the use of the large nitrate pool present in the Antarctic waters and lead to a decline in the specific UV absorption by mycosporinelike amminoacids. The small-scale experiment confirmed the role of UV irradiance in inhibiting phytoplankton growth and the capacity of ammonium inputs to overcome this inhibition. The alleviation of the high-irradiance stress by ammonium additions provided evidence of a key role of ammonium inputs in allowing phytoplankton to resume growth and nutrient use. The results demonstrate that there is a narrow window of irradiance where phytoplankton growth is adequate and that coastal Antarctic phytoplankton communities, examined here, are either light limited or stressed by high irradiance at irradiances outside this range. More research is needed to analyze the interplay between light climate, ammonium, and bloom initiation in Antarctic coastal waters to test the generality of the results obtained. Citation: Agustí, S., C. M. Duarte, M. Llabrés, N. S. R. Agawin, and H. Kennedy (2009), Response of coastal Antarctic phytoplankton to solar radiation and ammonium manipulation: An in situ mesocosm experiment,
The Mediterranean climate exerts a major influence on the basic properties of the Mediterranean S... more The Mediterranean climate exerts a major influence on the basic properties of the Mediterranean Sea, which constrains the structure and dynamics of the ecosystem. Seasonal variations in the marine climate follow the expected unimodal seasonality only for temperature, while most other forcing factors show a complex variance structure, with dominant time scales of 50-100 days (e.g. wave action), and with some of the factors acting as random factors (‘white noise’) at the annual scale (e.g. rainfall), thereby limiting the predictability of the system. The resulting ecosystem seasonality is unconventional and poorly linked to temperature. The prolonged period of high atmospheric pressure and associated high irradiance and calm waters in late winter is the main seasonal trigger in the NW Mediterranean Sea, setting the development of a phytoplankton bloom, as well as the recruitment of the benthos. Decadal changes in the Mediterranean marine climate are characterized by the dominance of oscillations with a 22-year period, suggesting an important solar forcing on the climate. This forcing masks the monotonous trends, such as the warming and increased sea level in the Mediterranean, expected from anthropogenic forcing. Records of decadal changes in the ecosystem often display a monotonous trend in the deterioration of water quality, indicative of human effects as the main forcing agent, while climatic forcing, which displays oscillatory variation, is of secondary importance. The paucity of long-term records precludes a robust analysis of ecosystem response to decadal climatic forcing. This absence can be partially remediated by the ability to interrogate the long-lived organisms that represent an important, albeit endangered component of Mediterranean biodiversity, to extract records (e.g. growth, temperature, changes in the nature of the dissolved inorganic carbon pool) of the changes they have witnessed.
The remineralization rate of sedimentary organic carbon (R org ) and the depth-integrated, diffus... more The remineralization rate of sedimentary organic carbon (R org ) and the depth-integrated, diffusion-supplied O 2 consumption rate (IOC) during microbial metabolism in sediments was investigated in three deep-sea sites at 1100, 2000 and 3500 m water depth in the eastern north Atlantic during the spring and summer 1998. In-situ pore water O 2 profiles yielded an IOC of 0.45F0.07 mmol O 2 m À2 day À1 at the deepest site (n=3) and ca. 1-1.5 mmol O 2 m À2 day À1 at the shallowest site (n=2). The R org was independently estimated at all three sites from ex-situ pore water profiles of the isotopic composition of total dissolved inorganic carbon (d 13 C T ), assuming that the concentration and isotopic composition of pore water C T with depth in the sediment was controlled only by microbial oxidation of isotopically depleted sedimentary organic carbon. The R org was thus estimated to be ca. 0.5-0.6 mmol C m À2 day À1 at the shallowest site and ca. 0.3-0.4 mmol C m À2 day À1 at the two deeper sites. Stoichiometric and isotopic constraints indicated that oxic remineralization of sedimentary organic matter was the dominant metabolic pathway in the sediments at 3500 m water depth. Similarly, stoichiometric and isotopic constraints suggested that the R org estimates from the ex-situ pore water d 13 C T profiles from 1100 and 2000 m water depth were likely to be minimum values and provided evidence for the occurrence of post-oxic remineralization processes. Post-oxic metabolism in the sediments of these sites could be linked to, or even augmented by, the non-diffusive mode of supply of organic matter mediated by infaunal organisms below the oxic sediment layer. D
The integrated iodate and total-iodine concentrations accompanying accelerated growth of natural ... more The integrated iodate and total-iodine concentrations accompanying accelerated growth of natural phytoplankton in eight 14-m deep mesocosm experiments did not vary significantly. Growth was induced by the addition of nutrients, while light irradiance was controlled by neutral density screens. These measures resulted in a range of particulate organic carbon concentrations of between 13 and 220 mol L Ϫ1 , that is, covering some that are well in excess of blooms generally found in the natural environment. This, together with most earlier results obtained from phytoplankton culturing and much hydrographic survey, is used as an opportunity to question whether phytoplankton growth can be the cause of iodate reduction in seawater.
Deep Sea Research Part Ii Topical Studies in Oceanography, 1995
The concentration and isotopic composition of carbon was measured in suspended particulate organi... more The concentration and isotopic composition of carbon was measured in suspended particulate organic matter in surface waters of the Southern Pacific Ocean along an 88°°W transect from 67-54°S during the Austral summer. Concurrent measurements of total dissolved inorganic carbon (ΣCO 2), pCO 2, particulate organic carbon (POC), salinity, chlorophyll and temperature provide an opportunity to study the covariance of these parameters with changes in the isotopic composition of particulate organic carbon (δ 13C-POC). The south-north transect did not show any significant changes in POC (10-19 mmol m -3) or chlorophyll (0.5-1 mg m -3), while δ 13C-POC variations of approximately 4%o were recorded. Although surface water CO 2(aq) is substantially out of equilibrium with the local atmosphere, the isotopic changes are negatively correlated with the concentration of dissolved carbon dioxide along the transect, and follow the general trends predicted from previously determined relationships. Estimates of ɛp ( ɛ#p) are less well constrained and show a weaker correlation with [CO 2 (aq)]. Where ɛ#p varies independently of [C0 2(aq)] small changes in the concentration gradient between extra- and intra-cellular CO 2 can be predicted. The difficulty in obtaining independent measures of growth rate and matching these measurements to the timescales over which the 613C of the phytoplankton is integrated is examined.
Rapid Communications in Mass Spectrometry, Jun 15, 2003
The conversion of ammonium (NH(4) (+)) to 1-sulfonato-iso-indole has been examined as a method fo... more The conversion of ammonium (NH(4) (+)) to 1-sulfonato-iso-indole has been examined as a method for natural abundance measurement of delta(15)N of NH(4) (+). The reaction is complete within 2 h and is based on the derivatisation of NH(4) (+) by o-phthaldialdehyde and sodium sulfite at a high pH, 11.2. The product is readily concentrated from dilute solutions by reverse-phase solid-phase extraction (SPE). The method is compound-specific despite partial derivatisation of potentially interfering amino acids, as their derivatives are not extracted by SPE. delta(15)N values of NH(4) (+) in KCL soil extracts can be measured within 48 h by automated continuous-flow IRMS with a precision of 0.23 per thousand (1 SD). Parallel measurements of NH(4) (+) standards of known delta(15)N are made to allow correction for the isotopic dilution by non-sample NH(4) (+). The practicality of this method is demonstrated by measuring the changes in NH(4) (+) concentration and delta(15)N following the addition of urea as a nitrogen source to inorganic N-depleted soil.
Temporal and spatial variation of d 34 S, total sulfur (TS) concentration, and elemental sulfur c... more Temporal and spatial variation of d 34 S, total sulfur (TS) concentration, and elemental sulfur concentration (S 0 ) in leaves, roots, and rhizomes of Zostera marina was followed between June 2002 and May 2003 at four locations in Roskilde Fjord and Øresund, Denmark. These were related to temporal changes in sediment sulfide concentrations, sulfur pool size, and sulfur pool d 34 S. The d 34 S of Z. marina was most negative in the roots, followed by rhizomes and leaves, indicating that roots were mostly affected by sulfide. A significant relationship between decreasing d 34 S and increasing TS in the plant tissues indicated that sulfide accumulated in the plant and, furthermore, a positive relation between TS and S 0 in the plant suggests that part of the sulfide is reoxidized to S 0 . There were marked temporal changes in all variables at all sites, but the pattern of change varied between sites. The temporal and spatial heterogeneity in plant d 34 S, TS, and S 0 depended on a variety of factors, such as sediment sulfide concentrations and the below : aboveground biomass ratio of the plants. This suggests that mechanisms of sulfide invasion are complex, and several factors (plant morphology, environmental variables) acting in concert or against each other need to be considered to successfully predict sulfide invasion in seagrasses.
Seagrass ecosystems provide numerous ecosystem services that support coastal communities around t... more Seagrass ecosystems provide numerous ecosystem services that support coastal communities around the world. They sustain abundant marine life as well as commercial and artisanal fisheries, and help protect shorelines from coastal erosion. Additionally, seagrass meadows are a globally significant sink for carbon and represent a key ecosystem for combating climate change. However, seagrass habitats are suffering rapid global decline. Despite recognition of the importance of “Blue Carbon,” no functioning seagrass restoration or conservation projects supported by carbon finance currently operate, and the policies and frameworks to achieve this have not been developed. Yet, seagrass ecosystems could play a central role in addressing important international research questions regarding the natural mechanisms through which the ocean and the seabed can mitigate climate change, and how ecosystem structure links to service provision. The relative inattention that seagrass ecosystems have received represents both a serious oversight and a major missed opportunity. In this paper we review the prospects of further inclusion of seagrass ecosystems in climate policy frameworks, with a particular focus on carbon storage and sequestration, as well as the potential for developing payment for ecosystem service (PES) schemes that are complementary to carbon management. Prospects for the inclusion of seagrass Blue Carbon in regulatory compliance markets are currently limited; yet despite the risks the voluntary carbon sector offers the most immediately attractive avenue for the development of carbon credits. Given the array of ecosystem services seagrass ecosystems provide the most viable route to combat climate change, ensure seagrass conservation and improve livelihoods may be to complement any carbon payments with seagrass PES schemes based on the provision of additional ecosystem services.
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Papers by Hilary Kennedy