Carbon flow through inshore marine environments of the Vestfold Hills, East Antarctica

The first integrated study in the Antarctic region of the marine carbon cycle over an entire annual period, including organic carbon formation, remineralisation, sedimentation and burial, was undertaken during two summers and the intervening winter from December 1993 to February 1995 at two sites in the Vestfold Hills region, East Antarctica: offshore at O'Gorman Rocks and in semi-enclosed Ellis Fjord. A single peak in uptake of dissolved inorganic carbon (DIC) occurred at the O'Gorman Rocks site in the first summer, while three peaks, resulting from different phytoplankton communities, were observed in the second. Net organic carbon (OC) production calculated from the uptake of DIC reached 2.1 mg kg-1. The sum of particulate and dissolved OC was typically only 30 % of the calculated total, suggesting rapid sedimentation of OC or transfer to higher trophic levels. After the end of OC production in autumn the concentrations of DIC and nutrients increased slowly and steadily to late winter maxima, primarily as the result of remineralisation rather than nutrient supply by deep-water upwelling. Sediment trap flux of OC at O'Gorman Rocks totalled 10 g m -2 year-1, with peak sedimentation coinciding with maximum OC production, but with a significant proportion (30 %) occurring outside summer. The development of a bottom ice algal community in October contributed a pulse of sedimentation which was characterised by OC with high 813C (-13 % compared to the annual average -19 %0). Comparison of the fluxes with the OC content of a sediment core showed that 2 gm-2 year-1 OC was buried, with the majority (80 %) remineralised by the benthic community. In Ellis Fjord DIC and nutrient uptake began earlier and ended later than at O'Gorman Rocks, though total DIC uptake and annual OC sedimentation were similar to those offshore. Early in summer the C:N uptake ratio calculated from concentrations in the water column reached 40, considerably higher than the expected Redfield Ratio of 16:1. Sedimentation of bottom ice algae again resulted in the transfer of particulate matter with high 613C to the sediment. This study shows that OC production in the nearshore Antarctic environment does not necessarily occur in a single pulse after the loss of the ice cover, but rather can begin under the ice and continue after refreezing. Furthermore, OC production can be of similar magnitude in ice-covered systems to areas which are ice-free for part of the summer. Bottom ice algae play an important role in total OC production, and are a source of OC for sedimentation before and after the period of peak water column production. This community is also important in the formation of OC with elemental ratios and 613C different to material produced in open water. Interannual variability in OC production and the algal and phytoplankton communities, however, can be large. These factors must be taken into account when developing biological carbon uptake estimates from models of nutrient utilisation, in interpreting sedimentary isotope records, and considering the effects of climate change on Antarctic ecosystems.