Additive response to multiple environmental stressors in the seagrass Zostera marina L.

Ongoing climate changes alter a broad range environmental variables in coastal ecosystems, which may lead to changes in species range distributions or collapse of populations. It is often assumed that changes in environmental parameters affect organisms in a nonadditive way and multifactorial experiments are therefore needed to obtain a better prediction of the effects of climate change. We studied experimentally how combinations of high temperature, low light and high nitrogen (HN) availability affect eelgrass (Zostera marina), which is an important foundation species on the northern hemisphere. Plants were cultured under optimal (15°C) and high (25°C) temperature, limiting and saturating light, and low and high ammonium availability in a 3‐factorial design. We found that individual exposure to heat and shade, respectively, had a negative effect on eelgrass while the effect of exposure to HN‐availability alone was less harmful. Exposure to combinations of two or three stressors simultaneously had a stronger (negative) effect on eelgrass than exposure to each stressor individually, but the combined effects of exposure to multiple stressors were largely similar to the expected additive responses. A synergetic effect was only found in case of mortality when eelgrass plants were exposed to all three stressors simultaneously. The effects of exposure to single or multiple stressors could largely be explained from alterations in the C‐budget of the plants: exposure to one or more stressors lowered the acquisition of C, drained the C‐reserves and sometimes allocated energy to defense or repair processes instead of growth resulting in reduced growth and higher mortality.