H. Kennedy

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...

The remineralization rate of sedimentary organic carbon (Rorg) and the depth-integrated, diffusion-supplied O2 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 O2 profiles yielded an IOC of 0.45±0.07 mmol O2 m−2 day−1 at the deepest site (n=3) and ca. 1–1.5 mmol O2 m−2 day−1 at the shallowest site (n=2). The Rorg was independently estimated at all three sites from ex-situ pore water profiles of the isotopic composition of total dissolved inorganic carbon (δ13CT), assuming that the concentration and isotopic composition of pore water CT with depth in the sediment was controlled only by microbial oxidation of isotopically depleted sedimentary organic carbon. The Rorg was thus estimated to be ca. 0.5–0.6 mmol C m−2 day−1 at the shallowest site and ca. 0.3–0.4 mmol C m−2 day−1 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 Rorg estimates from the ex-situ pore water δ13CT 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.

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.

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 t...

ABSTRACT Seasonal thermal stratification is the dominant hydrodynamic phenomena of tide-dominated shelf seas in the middle and high latitudes. Stratification occurs when summer heating of the sea surface exceeds tidal stirring and the resultant fronts,separating mixed from stratified water, are zones of enhanced primary production and support a coupled pelagic-benthic ecosystem which influences organic sedimentation and the production/preservation of microfossils. The potential of shelf sequences for preserving a long-term record of stratification dynamics was first provided by M2 palaeotidal models of the NW European shelf seas. These model data, which indicated changes in frontal position and the areal extent of summer stratification for selected Holocene timeslices, have been successfully tested using stable isotopic records from a long well-dated Holocene record from a stratified location in the Celtic Sea. Coeval positive and negative trends, in benthic foraminiferal delta 18O and delta 13C respectively, indicate early Holocene cooling of bottom water and organic matter remineralisation, both of which are consistent with a transition from seasonally tidally mixed to stratified water at this core location. Isotopic offsets between species imply a seasonal contribution to the vital effect and a diagenetic effect related to epifaunal/infaunal habitat. Multivariate statistical analyses of dead and live benthic foraminiferal distributions from across the Celtic Sea front define distinct assemblages related to mixed, frontal and stratified watermasses. These same assemblages occur downcore and are stratigraphically consistent with the mixed to frontal gradient indicated by the stable isotopic data. Isotopic evidence from living Celtic Sea foraminifera supports the concept of a "seasonal effect" in which different species calcify their tests at different times of the year and therefore in water of different temperature. Sea surface reconstructions have been based on dinoflagellate cyst stratigraphy calibrated via mulitivariate statistical analyses of modern distribution data. These complement the bottom water interpretations based on the benthic foraminiferal and stable isotopic proxies in providing a record of stratification at the sea surface. This multi-proxy approach, combining benthic foraminiferal and dinoflagellate cyst stratigraphies with stable isotope geochemistry, provides a methodological protocol for long-term integrated bottom water-sea surface reconstructions of seasonal stratification dynamics in shelf seas.