Hilary Kennedy

Seagrass sediments as a global carbon sink: Isotopic constraints

Seagrass meadows are highly productive habitats found along many of the world’scoastline, providing important services that support the overall functioning of the coastalzone. The organic carbon that accumulates in seagrass meadows is derived not only fromseagrass production but from the trapping of other particles, as the seagrass canopiesfacilitate sedimentation and reduce resuspension. Here we provide a comprehensivesynthesis of the available data to obtain a better understanding of the relative contributionof seagrass and other possible sources of organic matter that accumulate in the sedimentsof seagrass meadows. The data set includes 219 paired analyses of the carbon isotopiccomposition of seagrass leaves and sediments from 207 seagrass sites at 88 locationsworldwide. Using a three source mixing model and literature values for putative sources,we calculate that the average proportional contribution of seagrass to the surfacesediment organic carbon pool is ∼50%. When using the best available estimates ofcarbon burial rates in seagrass meadows, our data indicate that between 41 and66 gC m

The Mediterranean climate as a template for Mediterranean marine ecosystems: the example of the northeast Spanish littoral

Abstract 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.

Dissolved organic matter in Antarctic sea ice.

Abstract It has been hypothesized that there are significant dissolved organic matter (DOM) pools in sea-ice systems, although measurements of DOM in sea ice have only rarely been made. The significance of DOM for ice-based productivity and carbon turnover therefore remains highly speculative. DOM within sea ice from the Amundsen and Bellingshausen Seas, Antarctica, in 1994 and the Weddell Sea, Antarctica, in 1992 and 1997 was investigated. Measurements were made on melted sea-ice sections in 1994 and 1997 and in sea-ice brines in 1992. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations in melted ice cores were up to 1.8 and 0.78 mM, respectively, or 30 and 8 times higher than those in surface water concentrations, respectively. However, when concentrations within the brine channel/pore space were calculated from estimated brine volumes, actual concentrations of DOC in brines were up to 23.3 mM and DON up to 2.2 mM, although mean values were 1.8 and 0.15 mM, respectively. There were higher concentrations of DOM in warm, porous summer second-year sea ice compared with colder autumn first-year ice, consistent with the different biological activity supported within the various ice types. However, in general there was poor correlation between DOC and DON with algal biomass and numbers of bacteria within the ice. The mean DOC/DON ratio was 11, although again values were highly variable, ranging from 3 to highly carbon-enriched samples of 95. Measurements made on a limited dataset showed that carbohydrates constitute on average 35% of the DOC pool, with highly variable contributions of 1−99%.

Analysis of total and organic carbon and total nitrogen in settling oceanic particles and a marine sediment : an interlaboratory comparison

Abstract 10 laboratories, using their routine methods, collaborated in a comparison of analyses of total carbon, nitrogen and organic carbon in a single sample of marine sediment and a pooled sample of settling particulate material collected by sediment trap. The standard error of the means from all individual laboratories was ±3% of the mean for all total carbon results, and for total nitrogen in the marine sediment, but was ±7% of the mean for total nitrogen in the sediment trap material. Larger errors (±8% of the mean) were found for organic carbon results from both the marine sediment and the sediment trap material, with a range of results (expressed as a percentage of the untreated dry weight) for the sediment trap sample analyzed here of 5.60–8.03%. When this range is compared with that of particulate organic carbon concentration in sediment trap material obtained by different workers at various deep ocean sites (2.4–10.2%) [Wefer, G., 1989. Particle flux in the ocean: Effects of episodic production, in: W.H. Berger, V.S. Smetacek, G. Wefer (Eds.), Productivity of the Ocean: Present and Past, John Wiley & Sons, pp. 139–154.] it is apparent that a large proportion of the observed oceanic variability could be accounted for by difference in analytical technique. We suggest that the problem stems from the difficulty of accurate separation of the organic and inorganic phases and discuss the inaccuracies involved in the separation. We present evidence that the range of results is partly due to either volatilisation of organic carbon during acid treatment, or incomplete removal of organic matter during thermal treatment. If results from different laboratories are to be compared, as is presently required for basin scale and global studies of ocean carbon cycling and budgets, there is clearly a need for suitable reference materials, rigorous intercalibration and technique development.

Sea ice contribution to the air-sea CO2 exchange in the Arctic and Southern oceans

Although salt rejection from sea ice is a key process in deep-water formation in ice-covered seas, the concurrent rejection of CO2 and the subsequent effect on air–sea CO2 exchange have received little attention. We review the mechanisms by which sea ice directly and indirectly controls the air–sea CO2 exchange and use recent measurements of inorganic carbon compounds in bulk sea ice to estimate that oceanic CO2 uptake during the seasonal cycle of sea-ice growth and decay in ice-covered oceanic regions equals almost half of the net atmospheric CO2 uptake in ice-free polar seas. This sea-ice driven CO2 uptake has not been considered so far in estimates of global oceanic CO2 uptake. Net CO2 uptake in sea-ice–covered oceans can be driven by; (1) rejection during sea–ice formation and sinking of CO2-rich brine into intermediate and abyssal oceanic water masses, (2) blocking of air–sea CO2 exchange during winter, and (3) release of CO2-depleted melt water with excess total alkalinity during sea-ice decay and (4) biological CO2 drawdown during primary production in sea ice and surface oceanic waters.

Food sources, behaviour, and distribution of hydrothermal vent shrimps at the Mid-Atlantic Ridge

Five species of bresilioid shrimp were investigated at seven hydrothermal sites on the Mid-Atlantic Ridge: Menez Gwen, Lucky Strike, Rainbow, Broken Spur, TAG, Snake Pit and Logatchev. Samples were prepared for analysis of stable isotopes, elemental composition and lipids. Shrimp behaviour was observed from the submersible ‘Alvin’ and in the laboratory aboard RV ‘Atlantis’. The distribution and zonation of the shrimp species was recorded. Juvenile shrimp of all species arrive at the vents carrying reserves of photosynthetic origin, built-up in the pelagic larval stages. These reserves are used while the shrimp metamorphose to the adult form and, in Rimicaris exoculata and Chorocaris chacei , while they develop epibiotic bacteria supporting structures, the modified mouthparts and the inside of the carapace. The main food of adult R. exoculata is filamentous bacteria that grow on these structures. The intermediate sizes of C. chacei also feed on such bacteria, but the final stage gets some food by scavenging or predation. Mirocaris species scavenge diverse sources; they are not trophically dependent on either R. exoculata or mussels. Adults of Alvinocaris markensis are predators of other vent animals, including R. exoculata . The dense swarms of R. exoculata , with their exosymbionts, can be compared to endosymbiont-containing animals such as Bathymodiolus and the vestimentiferan tube-worms of the Pacific vents. Such associations, whether endo- or ectosymbiotic, may be necessary for the development of flourishing communities at hydrothermal vents.

Isotopic partitioning between scallop shell calcite and seawater: Effect of shell growth rate

Abstract The relationship between molluscan shell growth rate and skeletal δ18O and δ13C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ18O and δ13C. At low shell growth rates, shell δ18O were more positive (of the order +0.4‰) and δ13C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ18O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ13C were consistent with incorporation of isotopically depleted respiratory 13C (i.e., a metabolic effect). A trend toward more depleted shell δ18O and δ13C values occurred at higher shell growth rates, with negative δ18O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ18O and δ13C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study.

Experimental investigation into partitioning of stable isotopes between scallop (Pecten maximus) shell calcite and sea water

Stable isotopic compositions of bivalve shells have often been used for the reconstruction of high-resolution records of palaeotemperature and palaeoproductivity cycles. A major assumption in such studies is that isotopic equilibrium between shell carbonate and sea water is maintained at the time of precipitation. This assumption was tested in the laboratory for scallops, Pecten maximus, cultured over the temperature range 10–17°C. At the low shell growth rates exhibited (<0.1 mm day−1), deviations of shell δ18O from equilibrium were +0.6‰ over the experimental temperature range, a temperature equivalency of approximately −3°C. This is hypothesised as reflecting possible differences in the solution carbonate chemistry at the site of mineralisation in the extrapallial fluid (EPF) as compared to that of the external sea water medium, from which the EPF is isolated. Measured depletions of shell δ13C (of the order of −2.0‰) are interpreted as resulting from introduction of 13C-depleted respiratory CO2 into the EPF and subsequent incorporation into the shell.

Behaviour of dissolved organic matter and inorganic nutrients during experimental sea ice formation.

Abstract It is well established that during sea-ice formation, crystals aggregate into a solid matrix, and dissolved sea-water constituents, including inorganic nutrients, are rejected from the ice matrix. However, the behaviour of dissolved organic matter (DOM) during ice formation and growth has not been studied to date. DOM is the primary energetic substrate for microbial heterotrophic activity in sea water and sea ice, and therefore it is at the base of the trophic fluxes within the microbial food web. The aim of our study was to compare the behaviour of DOM and inorganic nutrients during formation and growth of sea ice. Experiments were conducted in a large indoor ice-tank facility (Hamburg Ship Model Basin, Germany) at −15°C. Three 1 m3 tanks, to which synthetic sea water, nutrients and dissolved organic compounds (diatom-extracted DOM) had been added, were sampled over a period of 5 days during sea-ice formation. Samples were collected throughout the experiment from water underlying the ice, and at the end from the ice as well. Brine was obtained from the ice by centrifuging ice cores. Inorganic nutrients (nitrate and phosphate) were substantially enriched in brine in comparison to water and ice phases, consistent with the processes of ice formation and brine rejection. Dissolved organic carbon (DOC) was also enriched in brine but was more variable and enriched in comparison to a dilution line. No difference in bacteria numbers was observed between water, ice and brine. No bacteria growth was measured, and this therefore had no influence on the measurable DOC levels. We conclude that the incorporation of dissolved organic compounds in newly forming ice is conservative. However, since the proportions of DOC in the brine were partially higher than those of the inorganic nutrients, concentrating effects of DOC in brine might be different compared to salts.

Dissolved carbohydrates in Antarctic sea ice

Concentrations of dissolved monocarbohydrates (MCHO) and polycarbohydrates (PCHO) were analysed in a variety of ice habitats from summer Weddell Sea sea ice (surface ponds, ice cores, gap layers and platelet ice). The dissolved organic carbon (DOC) pool in these habitats was also measured and the contribution of carbohydrate to this pool was assessed. The DOC concentrations within all sea ice habitats were high compared to surface seawater concentrations with values up to 958μMC being measured. Total carbohydrates (TCHO) were highest in the ice cores and platelet ice samples, up to 31% of the DOC pool, a reflection of the high algal biomass in these two habitat classes. TCHO in the other habitats ranged between 10% and 29% of DOC. The ratios of MCHO to PCHO varied considerably between the ice habitats: in surface ponds and ice cores MCHO was 70% of the TCHO pool, whereas in gap layers and platelet ice there were lower PCHO concentrations resulting in MCHO being 88% of TCHO.