The effect of single versus multiple heat waves on seagrass communities growing under naturally fluctuating temperature conditions

Temperature fluctuations are much stronger in coastal regions than in the open ocean and in the deep sea. The intensity, duration and frequency of temperature fluctuations depends on, for example, weather, tides, seasons, currents and upwelling/mixing events. Organisms living in coastal habitats are adjusted to these conditions and are able to cope with, at least transient, temperature extremes. Climate Change, however, has the potential to increase the severity and frequency of extreme events, for example, of heat waves. In an outdoor mesocosm experiment (2015), we investigated the effect of heat waves on Baltic Sea seagrass communities. We tested whether organisms and communities are more likely to cope with extreme water temperatures in late summer, when they have experienced “heat waves” in spring and early summer, already. Experiments were conducted under close to natural conditions, using Baltic Sea water flow through and simulating natural diurnal and day-to-day temperature variations of a representative average year of the last 20 years. Heat waves were simulated by adding a “delta”-treatment of 4-5 °C. This is one of the rare studies on Climate Change effects by taking natural fluctuations into consideration, providing a more realistic outlook on how coastal ecosystems may respond to Climate Change.