Global warming-enhanced stratification and mass mortality events in the Mediterranean

Abstract

Summer conditions in the Mediterranean Sea are characterized by high temperatures and low food availability. This leads to ''summer dormancy'' in many benthic suspension feeders due to energetic constraints. Analysis of the most recent 33-year temperature time series demonstrated enhanced stratification due to global warming, which produced a Ϸ40% lengthening of summer conditions. Many biological processes are expected to be affected by this trend, culminating in such events as mass mortality of invertebrates. Climatic anomalies concomitant with the occurrence of these events represent prolonged exposure to warmer summer conditions coupled with reduced food resources. Simulation of the effects of these conditions on a model organism demonstrated a biomass loss of >35%. Losses of this magnitude result in mortality similar to that noted in field observations during mass mortality events. These results indicate that temperature anomalies are the underlying cause of the events, with energetic constraints serving as the main triggering mechanism.climatic anomalies ͉ summer lengthening ͉ energetic constraints ͉ benthic suspension feeders ͉ extreme events I n a rapidly warming world, marine and terrestrial biodiversity and ecosystems are responding through shifts in distribution and changes in abundance, phenology, structure, and functioning (1-3). Furthermore, global environmental change is expected to lead to a higher frequency and shorter return time of extreme events (4). An example of this is the occurrence of mass mortality events, which are affecting an increasing number of marine species and ecosystems worldwide (5, 6). These events have been well documented in coral reef ecosystems, where they have produced dramatic phase shifts in community structure (7-9). In recent years, the coralligenous community, one of the most diverse in the Mediterranean Sea (Ϸ1,666 species; ref. 10), where suspension feeders are dominant, has been strongly affected by several mass mortality events [supporting information (SI) Table S1]. Engineer species, including gorgonians and sponges, have been the most affected taxa down to depths of 45 m (11-13). The impact of these events on populations is similar to that observed in the 1980s and 1990s in the Caribbean Sea (14, 15). But despite the wide extent of these effects on Caribbean populations, identification of the causes has not always been possible; however, some were associated with temperature increases (15), and others were associated with diseases (14). Understanding mass mortality events is becoming increasingly important, given the escalating presence of anthropogenic stressors that affect marine ecosystems (5, 6, 16).The causes of mass mortality events in the Mediterranean remain unknown, and our ability to predict the effects of these events depends on characterizing them and elucidating trends exhibited by potentially causative factors. Current hypotheses about the causes of these events in the Mediterranean focus on their relationship with the occurrence of distinctive cl...

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