Showing papers by "Hilary Kennedy published in 2001"

Individual variability in diel vertical migration of a marine copepod: Why some individuals remain at depth when others migrate

Abstract: The diel vertical migration (DVM) of the copepod Metridia pacifica was examined in Dabob Bay (47°45.05′N, 122°49.71′W), a fjord in Washington state. Although the population showed deep daytime residence (75–175 m), a proportion of the population was found in the surface waters at night. For individuals that migrated to the surface, the mean size of the oil sac was much smaller than those that remained at depth (mean lengths of oil sac 0.25 mm for individuals collected between 0 and 25 m at night, compared with 0.43 mm for individuals from between 125 and 175 m). Similarly, the C:N ratio was lower for animals collected from near the surface, indicative of their lower lipid reserves. These results suggest that individual variability in DVM was influenced by body condition, with those animals with larger lipid stores not needing to risk coming to the surface to feed at night.

Dissolved organic matter in Antarctic sea ice

Abstract: Sea ice supports highly dynamic, and at times highly productive assemblages of auto- and heterotrophic organisms. Central to the biogeochemical cycling of carbon and nitrogen within sea ice is the production and lability of dissolved organic matter (DOM). DOM is produced through excretion, mortality, cell lysis, and amplified by inefficient feeding by grazers. It has been hypothesised that there are significant 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. The dissolved organic carbon and nitrogen (DOC, DON) of sea ice from the Amundsen, Bellingshausen and Weddell Seas were investigated in 1992, 1994 and 1997. Measurements were made on melted sea ice sections in 1994 and 1997 and in sea ice brines in 1992. These studies confirm that DOC and DON can reach up to 30 and 8 times higher concentrations compared to those in surface waters, respectively. This DOM tends to be carbon enriched with a mean DOC:DON ratio of 14. Measurements made on a limited data set showed that carbohydrates constitute on average 35% of the DOC pool with highly variable contributions.

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 seawater 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 seawater 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 an indoor basin at -15*C. Three 1 m3 tanks, to which synthetic seawater, 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 the dilution line. No difference in bacteria numbers was observed between water, ice and brine. No bacteria growth was measured, and therefore had no influence on the 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.

Oxygen and carbon stable isotopic profiles of the fan mussel, Pinna nobilis , and reconstruction of sea surface temperatures in the Mediterranean

Abstract: Stable oxygen and carbon isotope profiles (δ18Oskeletal and δ13Cskeletal), taken along the direction of growth from the umbo to the shell margin in shells of the pinnid Pinna nobilis, were used to reconstruct sea surface temperatures (SST) in the south-east Mediterranean and ontogenetic records of metabolic CO2 incorporation. Comparison of the seasonal cycle of SST, predicted from the δ18Oskeletal record of a small (young) rapidly growing pinnid and temperature measured with a continuous in situ recorder showed that P. nobilis calcifies under isotopic equilibrium with surrounding seawater, thus indicating that P. nobilis shells can be used as a reliable predictor of SST. A 10-year SST record for the south-east Mediterranean was reconstructed from the shell profiles of four pinnid shells of different sizes and ages collected in 1995 and 1996. Reliable resolution of the seasonal SST could only be achieved during the first 4 years of shell growth. As the pinnids grew older, the temperature record was poorly resolved because the shell growth had diminished with age, resulting in time-averaging of the record. The amplitude of the generated seasonal temperature cycle compared favourably (±2°C) with a long-term temperature record from northern Mediterranean waters. Clear seasonal cycles in δ13Cskeletal were observed with an amplitude of ~1.0‰, similar to the calculated seasonal changes in δ13C of seawater (0.6‰) overlying seagrass meadows. An ontogenetic trend towards less positive δ13Cskeletal values was too large to be attributed to any decrease in δ13C in seawater resulting from the invasion of anthropogenic CO2. It is suggested that the temporal changes of δ13Cskeletal are due to incorporation of respiratory CO2 into the extrapallial fluid and reflect changes in the metabolic activity of the pinnid rather than changes in the isotopic composition of dissolved inorganic carbon within the surrounding seawater.

Biogeochemistry of platelet ice: its influence on particle flux under fast ice in the Weddell Sea, Antarctica

Abstract: An array of four sediment traps and one current meter was deployed under a well-developed platelet layer for 15 days in the Drescher Inlet in the Riiser Larsen ice shelf, in February 1998. Traps were deployed at 10 m (just under the platelet layer), 112 m (above the thermocline), 230 m (below thermocline) and 360 m (close to sea floor). There was a substantial flux of particulate organic material out of the platelet layer, although higher amounts were collected in the traps either side of the thermocline. Material collected was predominantly composed of faecal pellets containing diatom species growing within the platelet layer. The size classes of these pellets suggest they derive from protists grazing rather than from larger metazoans. Sediment trap material was analysed for particulate organic carbon/nitrogen/phosphorus (POC/PON/POP) and δ 13CPOC (carbon isotopic composition of POC). These were compared with organic matter in the overlying platelet layer and the water column. In turn, the biogeochemistry of the platelet layer and water column was investigated and the organic matter characteristics related to inorganic nutrients (nitrate, nitrite, ammonium, silicate, phosphate), dissolved organic carbon/nitrogen (DOC/DON), pH, dissolved inorganic carbon (DIC), oxygen and δ 13 CDIC (carbon isotopic composition dissolved inorganic carbon).

Dissolved carbohydrates in Antarctic sea ice

Abstract: 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, platelet ice there were lower PCHO concentrations resulting in MCHO being 88% of TCHO.

Diet and association of Pontonia pinnophylax occurring in Pinna nobilis: insights from stable isotope analysis

Abstract: Stable carbon isotope measurements (δ 13 C) were used to assess the sources of carbon assimilated by the fan mussel Pinna nobilis, in sea grass Posidonia oceanica meadows, and an associated shrimp Pontonia pinnophylax which occurs within this bivalve's mantle cavity. The primary carbon sources available to both animals displayed a wide range of δ 13 C values, from - 12.3 to -22.3‰. The δ 13 C and δ 15 N of Pinna nobilis and Pontonia pinnophylax suggest that they assimilate carbon from similar sources, occupy comparable trophic levels and that their association is commensal.

An autonomous benthic lander: preliminary observations from the UK BENBO thematic programme.

Abstract: A new, multi-purpose autonomous benthic lander is described, and preliminary experimental data are presented relating to deployments in the Atlantic Frontier (eastern north Atlantic) during the recent UK Thematic Programme ‘BENBO’. The autonomous lander was deployed at two contrasting sites — Site A (mouth of Rockall Trough; 3570 m) and Site B (Hatton-Rockall Bank; 1100 m) — before and following the spring-time surface ocean phytoplankton bloom (May & July, 1998, respectively). Diffusive oxygen uptake and nutrient flux data were obtained using two interchangeable modules — a profiling oxygen micro-electrode unit and a benthic chamber unit. Diffusive O 2 uptake across the sediment–water interface and the O 2 penetration depths within the sediment were determined from the oxygen micro-profiles. The shallower site, which had previously received phyto-detrital input, had a comparatively large diffusive oxygen uptake within the sediment (1.2 mmol m −2 d −1 ) and a maximum penetration depth of only 21 mm. The deeper site had greater oxygen penetration depths (∼80 mm) but a lower diffusive oxygen uptake of 0.6 mmol m −2 d −1 , indicative possibly of little or no phyto-detrital input. Visual observations of retrieved sediment cores support this conclusion, however Site A has also displays generally lower organic content and lower macrobenthic biomass which may contribute to this observation. Nutrient pore water profile data indicated fluxes of nitrate of 0.161 mmol m −2 d −1 and phosphate 0.0008 mmol m −2 d −1 into the overlying water. However, the benthic chamber studies showed virtually no change in nutrient concentrations, due probably to the relatively short deployment time used.

Different energy sources for three symbiont-dependent bivalve molluscs at the Logatchev hydrothermal site (Mid-Atlantic Ridge)

Abstract: The vent mussel Bathymodiolus puteoserpentis, a large vesicomyid clam and a smaller thyasirid were collected from an area of sediment subject to diffuse hydrothermal flow. The mussels live on the surface, the vesicomyids are partly buried and the thyasirids burrow in the sediment. The fine structure of the gills differs in the three bivalves. Bathymodiolus puteoserpentis hosts two types of bacterial symbiont, one methanotrophic, and another probably thiotrophic. The other two bivalves have single types of symbiont of different shapes. Stable isotope ratios of carbon and nitrogen indicate thiotrophy in the vesicomyid and thyasirid, but a predominance of methanotrophy in the mussel. This is the first time that such an assemblage has been found at a hydrothermal site on the Mid-Atlantic Ridge (MAR), with the different faunistic elements exploiting different energy resources.