Climate Change killing Mediterranean Sea Life
Today is the inaugural UN-designated ‘World Oceans Day’, intended to highlight the value of our oceans and the parlous state many of them have fallen into. The theme this year is ‘One Ocean, One Climate, One Future’, and we therefore thought it appropriate to highlight the findings of a recent Spanish study on the effects of climate change in the Mediterranean.
Recently, the coral community, one of the most diverse communities in the Med, has suffered several mass mortality events, and many species have been badly affected. The causes remain unknown but it is thought that they are related to distinct changes to the climate in late summer/early autumn (two of the largest events coincided with temperatures reaching around 3-4 degrees Celsius above average at this time of year).
The study focused on benthic suspension feeders, a group which includes sponges and gorgonians (a variety of coral) which live on the seabed and feed by filtering microscopic particles. This group has been particularly affected by the mass mortality events, which is put down to the increase in temperature causing high respiratory demand and decreasing the amount of food and nutrients available due to thermal stratification (layering of the water according to temperature).
The proposed causal link between climate change and the mortality events was evaluated 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. Losses of more than 35 per cent of biomass result in mortality similar to that recorded in field observations during mass mortality events.
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.
For further details of the study, please see: 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.
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