Primary production, carbon release, and respiration during spring bloom in the Baltic Sea

Abstract

We determined the gross and net primary production (GPP and NPP) for the total community and the < 10 μ m size fraction, the net release of dissolved organic carbon (DOC), and the microbial respiration in the Baltic Sea during the spring bloom. Samples ( n = 126) were taken from the surface (3 m depth) covering most subbasins and different phases of the bloom, defined by the inorganic nutrient and Chlorophyll a (Chl a ) concentrations. During the course of the bloom, the NPP rate (i.e., growth rate) decreased from 0.34 d −1 ± 0.03 (SE) to 0.15 d −1 ± 0.02 (SE), the contribution of the < 10 μ m fraction increased from 14% ± 2.5 (SE) to 47% ± 4.0 (SE) and the percent extracellular release (PER) increased from 3.8% ± 0.7 (SE) to 8.9% ± 1.5 (SE). The assimilation number, was on average 0.13 mol C (g Chl a ) −1 h −1 ± 0.01 (SE), and the average GPP:NPP rate was 1.25. The respiration increased with growth rate and was 21% of the GPP rate. The net release of DOC was relatively constant over the bloom phases, with increasing PER compensating for the reduction in biomass, and estimated to 24–36 μ mol DOC L −1 during the whole spring bloom period in all subbasins except in the Bay of Bothnia where it was 75% lower. The assimilation number was surprisingly stable, suggesting it is uncoupled from the inorganic nutrient concentration, likely a reflection of physiological acclimation and changing phytoplankton community.