Impacts on communities (mesocosm):

Testing climate change impact on biodiversity and ecosystem functions using near-natural conditions (that is, including the natural diurnal variability) is a major challenge and requires sophisticated experimental design and methodologies. There are two realistic approaches to tackle this challenge.

(1) “Natural experiments” in which scientist use places where extreme conditions, similar to those predicted in the future, exist already. These can be, for example, as heat plumes near power plant outflows (for warming) and CO2 vents near volcanic islands (for acidification).

(2) “Mesocosms”, which are relatively large confined areas where the tested parameters can be overlaid on the natural (in-situ) fluctuations, either in the field or lab. With mesocosms, you can test the impact of more than one parameter, which represents a more realistic representation of the future, as warming and acidification are unfolding simultaneously, and we are also interested in their potential synergistic or antagonistic impacts on biodiversity and ecosystem functions.

As part of the BALTMED project, we tested the separate and combined impacts of warming and acidification on Levant shallow subtidal rocky communities using a novel benthocosm (benthic mesocosm) system. This work is done with collaboration with the lab of Prof. Dr. Martin Wahl from GEOMAR (Kiel, Germany) as part of a bi-national collboration funded by BMBF-MOST. In this work we compared the sensitivity of the Baltic Sea and the easteren Meditteranean shallow subtidal benthic communities to climate change. The novel aspect of the benthocosm system is its ability to work with “delta-treatments”, i.e. a continuous flow-through of natural seawater that allows the expression of all natural fluctuations within the system (pO2, temperature, pCO2, pH, nutrients, salinity...), while treatments (warming, acidification) may be added “on-top” of these fluctuations without altering, a priori, their natural amplitudes and frequencies. In this system we test the impact of changes caused by climate change on both community structure (and biodiversity) and the ecosystem functions, by measuring the biogeochemistry of the water and metabolic rates. So far, we have conducted two experiments, one in during winter-spring and one in the summer months. In these experiments we found both antagonistic and synergistic effects of warming and acidification on community structure and metabolic functions.