William Ross Hunter, Neil Ogle and Nessa O’Connor
Current rates of extinction in the oceans are comparable with past mass extinctions. Unlike past mass extinctions, however, current species loss disproportionately affects larger predatory animals living close to the sea floor. Alongside marine species loss the Intergovernmental Panel for Climate Change predict that oceans will warm by at least 2ºC over the course of this century. Ocean warming and the loss of larger animals therefore represent major threats to seabed ecosystem functioning. Yet we know little about their combined effects upon the marine carbon cycle. In this study, we experimentally tested how faunal species loss affected the absorption and storage of carbon in non-vegetated coastal sediments by manipulating the presence / absence of the shore crab Carcinus maenas under both the current climate and predicted future warming conditions (ambient + 2°C). We traced the uptake and retention of a stable-isotope labelled carbon source within the sediment.
Our results show that crab presence increased the absorption and retention of carbon within the sediment under the predicted future warming conditions. Yet the retention of this carbon within the microbial community decreased under future-warming conditions. Crabs also played an important role in controlling the sediment microbial community. Under the present climatic conditions, biomarkers for marine fungi increased when crabs were absent, whilst interactions between crab presence and temperature altered the activity levels of bacteria, single-celled algae and fungi. Given that temperature is a key control upon the activity of marine organisms (from bacteria to animals), we propose that interactions between marine species loss and ocean warming will have a pronounced effect upon marine carbon budgets.