Heat waves and their significance for a temperate benthic community: A near‐natural experimental approach. (16th May 2018) - Pansch, Christian
Abstract: Climate change will not only shift environmental means but will also increase the intensity of extreme events, exerting additional stress on ecosystems. While field observations on the ecological consequences of heat waves are emerging, experimental evidence is rare, and lacking at the community level. Using a novel "near‐natural" outdoor mesocosms approach, this study tested whether marine summer heat waves have detrimental consequences for macrofauna of a temperate coastal community, and whether sequential heat waves provoke an increase or decrease of sensitivity to thermal stress. Three treatments were applied, defined and characterized through a statistical analysis of 15 years of temperature records from the experimental site: (1) no heat wave, (2) two heat waves in June and July followed by a summer heat wave in August and (3) the summer heat wave only. Overall, 50% of the species showed positive, negative or positive/negative responses in either abundance and/or biomass. We highlight four possible ways in which single species responded to either three subsequent heat waves or one summer heat wave: (1) absence of a response (tolerance, 50% of species), (2) negative accumulative effects by three subsequent heat waves (tellinid bivalve), (3) buffering by proceeding heat waves due to acclimation and/or shifts in phenology (spionid polychaete) and (4) an accumulative positive effect by subsequent heat waves (amphipod). The differential responses to single or sequential heat waves at the species level entailed shifts at the community level. Community‐level differences between single and triple heat waves were more pronounced than those between regimes with vs. without heat waves. Detritivory was reduced by the single heat wave while suspension feeding was less common in the triple heat wave regime. Critical extreme events occur already today and will occur more frequently in a changing climate, thus, leading to detrimental impacts on coastal marine systems. Abstract : In the present experimental study, we investigated whether summer heat waves have detrimental consequences on the macrofauna of a temperate coastal community in the Baltic Sea and whether sequential heat waves provoke an increase or decrease in sensitivity to thermal stress, or might even be beneficial. The experimental approach included three treatments: no heat wave (grey), one heat wave (green) and three heat waves (yellow), as well as natural seasonality and stochastic and diurnal temperature variability. We found that heat waves can have detrimental as well as positive effects on single compartments of a temperate benthic community of the western Baltic Sea. We highlight four possible ways in which single species responded to either three subsequent heat waves or one summer heat wave: (1) absence of a response (tolerance, 50% of species), (2) negative accumulative effects by three subsequent heat waves, (3) buffering by proceeding heat waves due to acclimation and/or shifts in phenology and (4) an accumulative positive effect by subsequent heat waves. The responses to single or sequential heat waves at the species level entailed shifts at the community level, with likely consequences for marine temperate ecosystems, such as the Baltic Sea. As critical thresholds such as maximum temperature, amplitude or duration of heat waves will occur more frequently in a changing climate, changes in the structure and functioning of coastal marine ecosystems can be expected.