Plankton experiment: Climate change in a tankResearchers have experimentally investigated the impacts of climate change on the spring bloom of phytoplankton. Tanks measuring 1.4 cubic metres were filled with planktonic organisms corresponding to the developmental stage of the phytoplankton in late winter. The tanks were then exposed to different amounts of light and different patterns of spring temperature development in climate chambers. The experiments simulated the present-day average spring temperature sequence, as well as patterns reflecting warming of 2, 4, and 6 degrees Celsius. The results were impressive. The spring bloom occurred 1 to 1.5 days earlier per degree of temperature increase. An increased light supply amplified this effect. The zooplankton reacted even more strongly to warming: the copepod larvae, called nauplii, hatched up to 9 days earlier per degree of temperature increase. The consequences of this were disastrous because most of the nauplii hatched before the spring bloom of phytoplankton. There was no food available for them so they starved, and an entire generation was lost. Not only did the warming cause a shift in the timing of the spring bloom. The total biomass of phytoplankton as well as its composition was also altered – to the detriment of the zooplankton. Under normal conditions large-celled diatoms dominated, which are a good food base for copepods. Under warmer conditions the smaller flagellates dominated. These, however, are not an optimal food source for copepods. The implications are obvious: the animals grow more slowly; they produce fewer eggs and therefore fewer offspring. But the warming of seawater can have further negative consequences beyond those for the food chain “phytoplankton – zooplankton – fish”. It also impacts on the storage capacity of the greenhouse gas CO2 in the ocean, the “biological CO2 pump” (see text box on the following page). Under warmer conditions the respiration of zooplankton and bacteria is enhanced, which produces CO2. This means that the CO2 initially taken up by phytoplankton is released back into the surface water. The proportion of CO2 that remains fixed in the biomass and sinks to the sea floor as organic material, to finally be incorporated as carbon in the sediments, is therefore reduced. This is a serious problem because it represents a fatal feedback mechanism for climate change: Due to climate change a natural process is weakened that has until now been able to extract a portion of the anthropogenic CO2 that is so harmful to the climate.
5.5 > Phytoplankton reproduction (green line) normally begins with the increase in light availability around the end of winter, before the hatching of the zooplankton larvae (nauplii, red line). This ensures that there is enough food available for the zooplankton. But if less light is available and the water is 6 degrees warmer, the zooplankton hatch before the phytoplankton bloom, and the larvae starve. This is an especially disturbing scenario because it is exactly what researchers are predicting for the Baltic Sea: less light due to increased cloud cover with an accompanying rise in water temperatures due to climate change. © maribus (after Sommer, Lengfellner et al. (in prep.))