The impact of climate warming on plankton spring succession: a mesocosm study

Abstract

Our mesocosm studies focused on marine plankton spring succession in Kiel Bight under climate change conditions. Kiel Bight serves as a model for moderately deep water bodies of temperate regions where the plankton bloom can start before the onset of thermal stratification. Plankton spring blooms are critical periods in the seasonal cycle because they form the first food impulse in the year and thus can be linked to reproductive success of many species. The conducted experiments stand out in their complex nature, monitoring simultaneously plankton food web processes from nutrients up to the copepod level under predicted global warming conditions. In the present thesis, results from phytoplankton, ciliates and copepods were combined and comprehensively examined. In three subsequent years (Spring seasons 2005 - 2007), indoor mesocosms were stocked with plankton spring communities from Kiel Bight and subjected to temperature regimes warmed by 0°C, 2°C, 4°C and 6°C above the decadal mean in situ temperatures (1992 - 2003) of Kiel Bight. Between the years, we varied mean intensities of the daily light dose for the mixed water column. Several effects emerged from our studies: At higher temperatures, we observed higher metabolic rates in the key copepod species Pseudocalanus sp. as well as increased rates of feeding. At the same time, the overall net growth efficiency in Pseudocalanus sp. decreased. Together with increased grazing in ciliates this resulted in stronger top-down control of the phytoplankton at warmer temperatures. Phytoplankton biomass at the bloom peak was diminished at higher temperatures, probably the effect of both, high grazing pressure and reduced growth efficiency in phytoplankton. Further, depending on the level of available food, elevated temperatures decreased reproductive success in Pseudocalanus sp. and seemingly prevented younger individuals to develop successfully, which in turn lead to a stronger decline in populations at higher temperatures. With respect to trophic levels, we demonstrated different temperature sensitivity in autotrophs and heterotrophs: whereas the timing of phytoplankton biomass maxima was rather insensitive towards temperature change and seemingly fixed to a critical mean daily light dose, ciliate and copepod peaks advanced strongly under global warming conditions. The interplay of light intensity and temperature ultimatively determined whether this differential temperture sensitivity translated into trophic mismatch situations and detrimental effects on copepod offspring and copepod population development. In case of ciliates, temperature increase resulted in a stronger coupling with the phytoplankton bloom and thus likely enhanced energy transfer towards the microbial loop. Further, higher temperatures partly promoted faster dynamics in species diversity and overall changed population and community size structure in copepods and phytoplankton: during the bloom peak, phytoplankton mean size was shifted towards smaller species; adult copepods showed reductions in prosome length at the end of the experiments. This favouring of small size might emerge as a new rule for how global change affects the biosphere