Effects of Ocean Alkalinity Enhancement on mesozooplankton communities during seasonal mesocosm studies in the western Baltic Sea (Kiel Fjord, Germany)

durch Silke Lischka (GEOMAR)

Mittwoch 18.06.2025, 13:00 → 14:30 Europe/Berlin

8A-002 - Hörsaal Ostufer / Lecture Hall East (GEOMAR - Standort Ostufer / GEOMAR - East Shore)

Ocean Alkalinity Enhancement (OAE) is a technology for marine Carbon Dioxide Removal (mCDR) that accelerates the natural process of rock weathering. While its impacts on the environment are largely unexplored, the associated changes in carbonate chemistry and pH could potentially affect marine food webs. Using in situ mesocosms, we investigated the effects of OAE on natural plankton communities in the western Baltic Sea (Kiel Fjord, Germany) across three different seasons (spring, summer and autumn) to examine seasonal variations in their response to OAE. We tested calcium hydroxide and magnesium hydroxide, two real-world candidates for mineral-based OAE, and analyzed zooplankton abundance and diversity to evaluate food web productivity and stability. A total alkalinity gradient (ΔTA) was created across twelve mesocosms (six for each mineral type) ranging from ambient levels to a ΔTA of 750 μmol kg⁻¹, with steps of 150 μmol kg⁻¹. Preliminary results suggest potential effects of OAE on the zooplankton community, revealing both seasonal differences and similarities depending on the zooplankton taxon. Total zooplankton abundance decreased consistently with increasing ΔTA in each season, with reductions ranging up to 77%. All three studies showed changes in the composition of the copepod community, with for example the proportion of Acartia spp. in the total copepod abundance increasing significantly with increasing ΔTA (roughly 13–35% depending on the season). It still remains to be seen whether these are direct responses to OAE or if they could have been indirectly mediated, for instance through impacts on food availability and quality.