Effect of decreasing inorganic N:P ratio on the plankton community - INN:PP

Our mesocosm experiment is aimed at understanding the effect of the decreasing inorganic N:P ratio on the plankton community of the Baltic Sea. It was held for three weeks, from 5th until 23rd of June 2021, at the facility of Tvärminne Zoological Station of the University of Helsinki (Finland).

With increasing light and nutrients in the water after winter, there is an algal bloom during spring, and this ends when dissolved inorganic nitrogen (DIN) becomes depleted. Until the mid-1990s, phytoplankton consumed all DIN and dissolved inorganic phosphorus (DIP), but the Baltic Sea has been affected by eutrophication for several decades, which increased inorganic P-pools. This resulted in a regime-like shift in the mid-1990s when the DIP pools started to be at elevated levels following the DIN depletion. Low N:P ratio can cause shifts in phytoplankton communities and could favour N-fixing cyanobacteria. However, the dominant cyanobacterial species in the Baltic Sea, Nodularia spumigena and Aphanizomenon flos aquae, grow relatively slowly, especially in the relatively cold water in May and June, and typically the DIP is gone by the time cyanobacteria start growing. At present, it is unclear what organisms take up the remaining DIP, something we wanted to find out in this AQUACOSM TA supported project INN:PP.

Our international team, including TA users and national collaborators, consisted of three young researchers (PhD students): Cristian Villena-Alemany (Institute of Microbiology, Centre ALGATECH, Czech Academy of Sciences), Mariano Santoro (Leibniz-Institute for Baltic Sea Research, Germany) and Mari Vanharanta (SYKE: Finnish Environment Institute), and two PIs: Kasia Piwosz (National Marine Fisheries Research Institute, Poland, kpiwosz@mir.gdynia.pl) and Kristian Spilling (SYKE, Finland, kristian.spilling@syke.fi). We were supported by Jonna Piiparinen (SYKE) who measured bacterial production, and four interns (Emma Forss, Neea Hanström, Anni Leinonen, Marlena Grönqvist).