Ocean acidification and desalination: climate-driven change in a Baltic Sea summer microplanktonic community

Abstract

Helcom scenario modelling suggests that the Baltic Sea, one of the largest brackish-water bodies in the world, could expect increased precipitation (decreased salinity) and increased concentration of atmospheric CO₂ over the next 100 years. These changes are expected to affect the microplanktonic food web, and thereby nutrient and carbon cycling, in a complex and possibly synergistic manner. In the Baltic Proper, the extensive summer blooms dominated by the filamentous cyanobacteria Aphanizomenon sp., Dolichospermum spp. and the toxic Nodularia spumigena contribute up to 30% of the yearly new nitrogen and carbon exported to the sediment. In a 12 days outdoor microcosm experiment, we tested the combined effects of decreased salinity (from 6 to 3) and elevated CO₂ concentrations (380 and 960 µatm) on a natural summer microplanktonic community, focusing on diazotrophic filamentous cyanobacteria. Elevated pCO₂ had no significant effects on the natural microplanktonic community except for higher biovolume of Dolichospermum spp. and lower biomass of heterotrophic bacteria. At the end of the experimental period, heterotrophic bacterial abundance was correlated to the biovolume of N. spumigena. Lower salinity significantly affected cyanobacteria together with biovolumes of dinoflagellates, diatoms, ciliates and heterotrophic bacteria, with higher biovolume of Dolichospermum spp. and lower biovolume of N. spumigena, dinoflagellates, diatoms, ciliates and heterotrophic bacteria in reduced salinity. Although the salinity effects on diatoms were apparent, they could not clearly be separated from the influence of inorganic nutrients. We found a clear diurnal cycle in photosynthetic activity and pH, but without significant treatment effects. The same diurnal pattern was also observed in situ (pCO₂, pH). Thus, considering the Baltic Proper, we do not expect any dramatic effects of increased pCO₂ in combination with decreased salinity on the microplanktonic food web. However, long-term effects of the experimental treatments need to be further studied, and indirect effects of the lower salinity treatments could not be ruled out. Our study adds one piece to the complicated puzzle to reveal the combined effects of increased pCO₂ and reduced salinity levels on the Baltic microplanktonic community.

Fig. 1

Non-metric multidimensional scaling of relative species abundance on Day 12, displayed with sample (a) and species (b) scores. The dissimilarity matrix was calculated with Bray–Curtis dissimilarity. PERMANOVA revealed that salinity treatment had a significant effect on the species composition (pseudo-F_(1,12) = 14.2, p < 0.001), which is illustrated with the grey ellipses in (a)

Fig. 2

Biovolumes of diazotrophic cyanobacteria (a) and (b) biovolumes of flagellates, green algae, diatoms and dinoflagellates. The different treatments are four combinations of salinity (S6, S3) and carbon dioxide concentrations (Amb, High). Vertical lines show standard deviation (n = 4)

Fig. 3

Bacterial cell numbers (a) and (b) cell-specific bacterial productivity (CSP, estimated by thymidine uptake rates). The different treatments are four combinations of salinity (S6, S3) and carbon dioxide concentrations (Amb, High). Vertical lines show standard deviation (n = 4). NB: Data points are positively nudged on the x axis to properly display the error bars

Fig. 4

The filamentous cyanobacteria Nodularia spumigena with associated heterotrophic bacteria. Effects of climate change on the autotrophic community may have indirect effect on closely associated heterotrophic bacteria and their biogeochemical interactions

Fig. 5

Hourly measurements of effective quantum yield (ΔF/F_m′) during 30 h under ambient PAR (400–700 nm) conditions. Active down-regulation of photosynthesis is observed during midday trough ΔF/F_m′ depression in all treatments. The different treatments are four combinations of salinity (S6, S3) and carbon dioxide concentrations (Amb, High)

Fig. 6

Variation in pH_T over 24 h. The continuous bubbling of CO₂-enriched synthetic air provides a fluctuating pH during the diurnal cycle of primary production. The different treatments are four combinations of salinity (S6, S3) and carbon dioxide concentrations in µatm, (Amb, High) plus in situ sea surface values. Vertical lines show standard deviation (n = 4)

Fig. 7

Daily variations of photosynthetic active radiation (PAR, 400–700 nm) and ultraviolet-A radiation (UV-A, 320–400 nm) during the experimental period. The aquaria and light sensors were placed under a mesh to simulate light intensities experienced in the surface water of the Baltic Sea (see “Experimental setup”)