Taucher J, Schulz KG, Dittmar T, Sommer U, Oschlies A, Riebesell U
acidification, temperature increase, DOC, DIC, POC, mesocosm, Kiel indoor, Kiel, Germany, 1.4 m3
Increasing concentrations of atmospheric carbondioxide are projected to lead to an increase in sea surfacetemperatures, potentially impacting marine ecosystems andbiogeochemical cycling. Here we conducted an indoor mesocosmexperiment with a natural plankton community takenfrom the Baltic Sea in summer.We induced a plankton bloomvia nutrient addition and followed the dynamics of the differentcarbon and nitrogen pools for a period of one month attemperatures ranging from 9.5 C to 17.5 C, representing arange of ±4 C relative to ambient temperature. The uptakeof dissolved inorganic carbon (DIC) and the net build-up ofboth particulate (POC) and dissolved organic carbon (DOC)were all enhanced at higher temperatures and almost doubledover a temperature gradient of 8 C. Furthermore, elementalratios of carbon and nitrogen (C : N) in both particulateand dissolved organic matter increased in response tohigher temperatures, both reaching very high C :N ratios of>30 at +4 C. Altogether, these observations suggest a pronouncedincrease in excess carbon fixation in response to elevatedtemperatures. Most of these findings are contrary to resultsfrom similar experiments conducted with plankton populationssampled in spring, revealing large uncertainties inour knowledge of temperature sensitivities of key processesin marine carbon cycling. Since a major difference to previousmesocosm experiments was the dominant phytoplanktonspecies, we hypothesize that species composition might playan important role in the response of biogeochemical cyclingto increasing temperatures.