AME prepress abstract  -  doi: 10.3354/ame01998

ABSTRACT: The response of the phytoplankton and bacterial spring succession to the predicted warming of sea surface temperature in temperate climate zones during winter was studied using an indoor-mesocosm approach. The mesocosms were filled with winter water from the Kiel Fjord, Baltic Sea, containing sufficient amounts of nutrients. Two of the mesocosms were started at 2°C and temperature was subsequently increased according to the decadal temperature profile of the Fjord (ΔΤ = 0°C, baseline treatment). Additional mesocosms were run at 3 elevated temperatures with differences of ΔΤ +2, +4 and +6°C compared to the baseline treatment. All mesocosms were exposed to the same light conditions. Peak timing of phytoplankton primary production (PP) during the experimental spring bloom was not significantly influenced by increasing incubation temperatures, whereas the peak of bacterial secondary production (BSP) was accelerated by about 2 d per °C. This suggests that, in case of warming, the spring peak of bacterial degradation of organic matter (in terms of BSP) would occur earlier in the year. Furthermore, the lag time between the peaks of PP and BSP (about 16 d in the ΔΤ = 0°C treatments) would diminish progressively at elevated temperatures. The average ratio between BSP and PP increased significantly from 0.37 in the coldest mesocosms (ΔΤ = 0°C) to 0.63 in the warmest ones (ΔΤ = +6°C). Community respiration and the contribution of picoplankton (<3 µm fraction) to which variable also increased at elevated temperatures. These observations suggest that, at elevated temperatures, bacterial degradation of organic matter produced by phytoplankton occurs earlier and is stronger than at lower temperatures. Our results lead to the prediction that climate warming during the winter/early spring in temperate climate zones will tighten the coupling between phytoplankton and bacteria. However, if PP is reduced by warming, as in our experiments, this will not necessarily lead to increased recycling of organic matter (and CO₂).