Abstract
AbstractWe determined the gross and net primary production (GPP and NPP) for the total community and the < 10 μm 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 μm 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 μmol 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.
Highlights
IntroductionThe ecosystem will typically be somewhere in between these extremes, with a temporal development toward a balanced state after a perturbation (e.g., a pulse of inorganic nutrients)
There was some evidence suggesting that the GPPB was slightly higher in the Archipelago Sea (ArS) compared with Gulf of Finland (GoF) and Baltic Proper (BP) and it was lower in the Bay of Bothnia (BoB) compared with the rest of the subbasins
In terms of bloom phase, the GPPB decreased from the growth to the peak phase, but increased again in the post-bloom phase, and there was no difference in the GPPB between the growth and the postbloom phases
Summary
The ecosystem will typically be somewhere in between these extremes, with a temporal development toward a balanced state after a perturbation (e.g., a pulse of inorganic nutrients). In areas close to or at balanced state, it is relatively easy to measure and model carbon fluxes when carbon fixation matches carbon respiration. It is in the areas with high productivity, governed by perturbations, where the carbon fluxes are more complex and difficult to model
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