Global warming may be linked to increasing mass deaths of marine life in the Mediterranean Sea.

By analysing past temperature changes and simulating climate change effects on a type of coral, a new Spanish study indicates that climate change may indeed be to blame. There are concerns that climate change is already reducing marine biodiversity and this may worsen in the future.

Recently, the coral community, one of the most diverse communities in the Mediterranean Sea, has suffered several mass mortality events (where large numbers of individuals die) and many species have been badly affected. The causes remain unknown but it is thought they are related to distinct changes in climate in late summer and early autumn. For example, two of the largest mass mortality events coincided with temperatures reaching around 3 to 4�C above average.

The study focused on a specific group of marine animals called benthic suspension feeders. These include sponges and gorgonians (a variety of coral) which live on the seabed and feed by filtering microscopic particles. They have been particularly vulnerable in recent mass mortality events and the reasons for this are thought to be twofold. Firstly, the increase in temperature causes high respiratory demand, and secondly, it decreases the amount of food and nutrients available due to 'thermal stratification', or layering of the water according to temperature. These are typical summer conditions in the Mediterranean and place energetic constraints on benthic suspension feeders, which go into dormancy, or decreased activity, during this time. However, summer conditions have become much more prolonged in recent decades, which the authors suggest are difficult for the benthic suspension feeders. The study looked at temperature variations in the Mediterranean Sea between 1974 and 2006 and demonstrated that global warming had enhanced thermal stratification and produced about a 40 per cent increase (i.e. 40 days) in the time period that benthic suspension feeders are exposed to summer conditions.

To investigate this proposed causal link the study examined the consequences of warming and lengthening summer conditions on a well-studied gorgonian - Paramuricea clavata. The branches of this coral were exposed to different combinations of temperature and food concentration. Partial mortality was observed in a high-temperature/lower-food condition and about 49 per cent of biomass was lost under these conditions. These conditions reflect those found during mass mortality events. In comparison, branches in the high-temperature/high-food treatment lost about 31 per cent of biomass. Losses of more than 35 per cent of biomass result in mortality similar to that recorded in field observations during mass mortality events. This trend in environmental conditions and the identification of a cause-and-effect relationship between temperature and mortality supports the theory that mass mortality events in the Mediterranean are linked to climate change. It appears that temperature changes are the underlying cause of events and limits on food and energy are the main trigger. The physiological stress this places on benthic suspension feeders may also make them more vulnerable to disease, leading to further deaths.

Source: Coma, R., Ribes, M., Serrano, E. et al. (2009). Global warming-enhanced stratification and mass mortality events in the Mediterranean. Proceedings of the National Academy of Sciences. 106 (15): 6176-6181. Contact: coma@ceab.csic.es