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An assessment of upper ocean carbon and nitrogen export fluxes on the boreal continental shelf: A 3-year study in the open Baltic Sea comparing sediment traps, 234Th proxy, nutrient, and oxygen budgets

Ö. Gustafsson, J. Gelting, P. Andersson, U. Larsson, P. Roos

Limnol. Oceanogr. Methods 11:495-510 (2013) | DOI: 10.4319/lom.2013.11.495

ABSTRACT: Six different approaches for estimating the upper ocean export of carbon and nitrogen has been assessed using 2-3 years of observations in the open Baltic Sea. The long-term average export ratios, relative to primary production, were similar from approaches based on upper ocean N budget (0.29 ± 0.02), upper ocean O budget (0.25 ± 0.02), collection-efficiency-corrected sediment traps (0.30 ± 0.03), and 234Th-derived POC export (0.21 ± 0.03; POC/234Th ratio from traps). Estimates of carbon export efficiency based on direct sediment traps (0.14 ± 0.02) likely reflected under trapping whereas filter-based 234Th-derived POC export (0.86 ± 0.03) indicated that POC/234Th ratio from filters were larger than the ratio on truly settling matter; these two approaches and estimates are thus not recommended. This study provides improved constraints on the upper ocean C and N export in offshore temperate-boreal continental shelf regimes such as the open Baltic Proper. The long-term export fluxes derived by averaging the four geochemically consistent yet independent approaches were for POC 44.6 ± 4.0 gC m-2 y-1 and for PON 7.0 ± 0.6 gN m-2 y-1. This multi-technique approach improves understanding of carbon and nitrogen cycling in the upper ocean by constraining the export efficiencies through comprehensive time-series measurements.