A trophic cascade triggered by ocean acidification: implications for pelagic ecosystem structure, trophic transfer, and biogeochemical cycling

Chair: Ana Queiros

 

U. Riebesell (1)*, N. Aberle-Malzahn (2), L.T. Bach1, J.-M. Bouquet (3), T. Boxhammer (1), I. Dörner (1), M. Haunost (1), S. Ismar (1), V. Kalter (1), A. Lechtenbörger (1), S. Lischka (1), K. Lohbeck (1,4), A. Ludwig (1), R. Meichßner (1), J. Meyer (1), D. Michaelis (1), F. Minutolo (1), A. Nauendorf (1), C. Spisla (1), M. Sswat (1), P. Stange (1), J. Taucher (1)

1 GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, D-24105 Kiel, Germany

2 Alfred Wegener Institute, Kurpromenade, D-27498 Helgoland, Germany

3 University of Bergen, Thormøhlensgate 55, N-5020 Bergen, Norway

4 University of Gothenburg, Carl Skottsbergs Gata 22B, S-41319 Göteborg, Sweden

Background
Individual responses to ocean acidification (OA) vary greatly between different taxonomic groups or even between different species of the same taxon, suggesting that there will be winners and losers of ocean acidification (as well as species not directly affected by it). How OA sensitivities play out in the natural environment, where trophic interactions act in concert with physiological responses, cannot be easily extrapolated from the species’ individual reactions. For example, we know that some coccolithophore species are negatively affected by ocean acidification in terms of their rate of calcification and the frequency of coccolith malformations. Does that make them losers of ocean acidification in a natural environment when exposed to competition, predation and viral infection? And if so, is there sufficient redundancy and biological buffering in the system to ensure that ecosystem function and biogeochemical cycling remain unchanged? To address questions along these lines we have performed a series of sea-going mesocosm experiments in a range of climatic zones and under a variety of nutritional conditions.

Methods
To study the effects of ocean acidification on marine plankton communities and potential impacts on ecosystem functioning and biogeochemical cycling, large-scale mesocosm experiments were conducted between 2010 and 2015 using the Kiel Off-shore Mesocosms for Future Ocean Simulations (KOSMOS). Nine mesocosms, each enclosing 55 m3 of seawater, were deployed at near-shore locations in different climate zones ranging from the high Arctic to the subtropical North Atlantic. After CO2 manipulation, the enclosed plankton communities were closely monitored for 5 to 15 weeks. This presentation will focus on the most recent KOSMOS study conducted in the spring/summer of 2015 in the Raunefjord, Norway.

Findings
The KOSMOS 2015 study revealed direct positive and negative effects as well as indirect effects of ocean acidification on the enclosed plankton community. Among the groups with direct positive effects were picoeukaryotic phytoplankton and the appendicularium Oikopleura dioica. Groups with direct negative impacts included the coccolithophore Emiliania huxleyi and the pteropod Limacina helicina. Indirect effects, due to trophic cascades, were observed in cyanobacteria and microzooplankton. The combination of direct and indirect effects of ocean acidification significantly altered the trophic transfer and modified elemental cycling in the enclosed systems.

Conclusions
Through a combination of direct effects via physiological responses and indirect effects via trophic interactions, ocean acidification can greatly alter ecosystem structure, trophic transfer, and biogeochemical cycles.