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.

Limited response of a spring bloom community inoculated with filamentous cyanobacteria to elevated temperature and pCO2

“…Nevertheless, no effects of elevated pCO 2 were found on the filamentous cyanobacteria. These results are consistent with some previous studies (Paul et al 2016, Wulff et al 2018, while others show increased growth (Wannicke et al 2012, Eichner et al 2014. Also, filamentous cyanobacteria are known to cope with daily fluctuations in pH measured both in aggregates (Ploug 2008) and in situ conditions (Wulff et al 2018).…”

“…The biovolume of Aphanizomenon sp. decreased at the initiation of the experiment, as also observed by Wulff et al (2018), thus, no temperature effects could be observed.…”

“…Baltic Sea phytoplankton communities experience large daily variation in pH, e.g. ranging from early morning values of 7.8-8.4 in the afternoon during a summer bloom (Wulff et al 2018), similar to pH ranges measured in natural aggregates of filamentous cyanobacteria (Ploug 2008). Hence, natural phytoplankton thrive despite large fluctuations in pH and pCO 2 .…”

“…Manipulations of the carbonate system using air bubbling to mimic future climate change scenarios have previously been used successfully (Gattuso et al 2010, Karlberg and Wulff 2013, Torstensson et al 2013, Wulff et al 2018. A pH difference was established from day 3 onwards, even though specific pCO 2 values are difficult to control because pCO 2 also varies with photosynthesis and respiration rates (Wulff et al 2018). Nevertheless, no effects of elevated pCO 2 were found on the filamentous cyanobacteria.…”

Ecological and functional consequences of coastal ocean acidification: Perspectives from the Baltic-Skagerrak System

“…The likely combined biological effects of this freshening with OA in the Baltic-Skagerrak System have received relatively little attention to date. Available data indicate increased sensitivity to OA in low-salinity populations (crustaceans, Gulf of Bothnia, Wood et al 2016 ), potential for freshening to counteract the positive effects of OA on reproductive success ( Fucus , Baltic, Serraõ et al Serrao et al 1999 ; Graiff et al 2017 ), and no substantive impacts of OA + freshening on microplankton community structure (Baltic Proper, Wulff et al 2018 ; Table 1 ). This important research area requires further experimental investigation before patterns in responses to this interaction can be identified.…”