Vertical biogenic particle flux during Austral summer in the Antarctic Peninsula area

During both FRUELA cruises, we performed eight circum-diel stations in the Bellingshausen Sea (South of Drake Passage), and the Bransfield and Gerlache straits, where MULTITRAP sediment-traps were deployed for periods of 24 h in order to study carbon (C) and nitrogen (N) export from the photic layer. Two types of regions were visited: High chlorophyll a (Chl a) areas dominated by large-sized diatoms (>10 μm), and low Chl a areas dominated by Cryptomonas sp. or microflagellates. The vertical fluxes of C, N, and Chl a, and the number of fecal pellets (FP) were measured, and the taxonomic composition of the sedimented microplankton was analysed; the results compared to the standing stocks in the water column overlaying the sediment traps. We measured higher carbon export rates in the diatom-dominated regions (Gerlache Strait), than in the stations dominated by small phytoplankton (Bransfield Strait and Bellingshausen Sea). The measured C-fluxes ranged from 115 to 800 mg C m−2 day−1. Typical C : N ratio (by atoms) varied from 5.5 to 16.4. Nitrogen export, however, was not directly related to C export and, thus, we measured changes in the C : N ratio with stations having higher abundance of FP or detritus presenting higher C : N ratios. Variations in the vertical C-flux were related to biological variables such as FP number, algal community composition (quantitative counts and qualitative SEM observations), and photosynthetic and prokaryotic activity of the sedimented material. We calculated photic layer loss rates of C, N, Chl a (0.53–8.0% day−1) and of the different microplankton taxonomic entities (0.27–9.5% day−1). Some exceptionally high loss values were found for Cryptophyceae during Fruela 96. The obtained results are discussed within the conceptual framework set by the relationships between phytoplankton size and food web structure, and concomitant carbon export from the photic layer.