Biogeochemical measurements at the Antarctic Peninsula

Size-fractionated chlorophyll a and photosynthetic carbon incorporation, microbial oxygen production and respiration and particulate vertical flux were measured in January 1996 at three regions, characterized by distinct hydrographic fields and planktonic communities, of the Antarctic Peninsula: (1) a diatom-Phaeocystis sp., dominated community associated with the relatively stratified waters of the Gerlache Strait, (2) a nanoplankton-Cryptomonas sp. dominated assemblage at the Gerlache-Bransfield confluence; and (3) a nano- and picoplankton community in mixed waters of the Bransfield Strait. Despite the marked differences in both community structure and total phytoplankton biomass and primary production, and against predictions from models about trophic control of C export, the lowest respiration rates were measured at Bransfield (pico- and nanoplankton), and no difference was observed between the Gerlache (large diatoms) and Bransfield stations in relative vertical particle flux (6.4 vs. 5.1 % of suspended C; 14.9 vs. 10.4 % of net community production, respectively). Growth and loss rates of the phytoplankton population studied for each community indicate that microbial populations can be explained by in situ growth, but spatial (diatom-Phaeocystis sp., bloom) and temporal (diatom-Phaeocystis sp. bloom and nanoplankton communities) scales of study were shown to be insufficient for addressing the coupling between primary production and biogenic carbon export, especially after the appreciation of the accumulation of dissolved organic carbon in the water column. This would explain the unexpected results and highlights the necessity of including the mechanisms controlling accumulation and consumption of dissolved organic matter into conceptual models about the trophic control of C export.