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 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 AE 0.03 (SE) to 0.15 d −1 AE 0.02 (SE), the contribution of the < 10 μm fraction increased from 14% AE 2.5 (SE) to 47% AE 4.0 (SE) and the percent extracellular release (PER) increased from 3.8% AE 0.7 (SE) to 8.9% AE 1.5 (SE). The assimilation number, was on average 0.13 mol C (g Chl a) −1 h −1 AE 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.

Bacterioplankton dynamics driven by interannual and spatial variation in diatom and dinoflagellate spring bloom communities in the Baltic Sea

“…The environmental conditions varied largely within and between the sampling years due to the different sub‐basins and phytoplankton bloom phases covered (Table 1; Fig. 1A, Spilling et al 2019). Salinity ranged from ~ 8 in the southernmost station of the BP to ~ 2 in the northernmost part of the BoB sub‐basin (Table 1).…”

“…The PP was calculated from the incorporated 14 C after incubation knowing the total amount of added isotope and the DIC concentration according to Gargas (1975). A more detailed description of the method can be found in Spilling et al (2019).…”

“…Different phases of the spring bloom, named as Growth (Gr), Peak (Pe), Decline (De), and Postbloom (PB) were defined for each station and sampling year, as described in detail in Spilling et al (2019). In brief, the phases were defined based on the concentration of inorganic nutrients (PO 4 and NO 3 + NO 2 ) and Chl a (Supporting Information Table S1).…”

Viability of pico- and nanophytoplankton in the Baltic Sea during spring

“…Franklin et al (2009) demonstrate, high F v /F m value might not be a clear sign of absence of dead cells, although cells with reduced viability likely have lower photosynthetic efficiency (Veldhuis et al 2001). A drop in F v /F m might also be a transient response to stress, as phytoplankton continuously acclimates to their surroundings (Halsey and Jones 2015), and during the spring bloom in the Baltic Sea, the primary production output per Chl a unit (the assimilation number) is not affected by the inorganic nutrient concentration (Spilling et al 2019). Therefore, it might be better to consider measurements of F v /F m and viability as complementary assays for the physiological state of phytoplankton communities.…”

Baltic Earth Assessment Report on the biogeochemistry of the Baltic Sea

“…Most commonly measured by isotope 14 C incubations, data reflect a continuum between gross and net primary production, i.e. between the carbon uptake rate into phytoplankton cells and the net effect of carbon uptake and release due to cellular respiration and exudation (Milligan et al, 2015;Sakshaug et al, 1997;Spilling et al, 2019). Primary production is rarely measured in-situ but mostly by incubating samples at selected light levels and temperatures, which introduces methodological problems (ICES Working Group on Primary Production, 1987; Platt and Sathyendranath, 1993;Sakshaug et al, 1997).…”