The coincidence of environmental and climatic extremes and the response of primary producers in marine coastal habitats and open systems

Understanding how global change affects primary productivity in natural systems is of crucial importance. The majority of ecological studies on climate change focused the attention on the impact of mean changes in environmental conditions, but there is increasing evidence indicating that ecological responses may depend as much upon environmental variation and extremes. Extreme weather events, such as severe droughts, heavy rainfalls, heat waves and hot spells, are increasing in severity and frequency and are likely to cause severe impacts at all levels of biological organization. So far, most manipulative field experiments have examined the effects of individual extreme events, with little attention to the possible synergistic effects of multiple extreme disturbances. My thesis addresses the need to examine the combined effects of compounded extreme events on natural population. I focused on phytoplankton and intertidal epilithic microphytobenthos (EMPB) biofilms as model systems. EMPB has been used to test the hypothesis that the concomitance of distinct environmental extreme events elicits larger effects compared to the expected cumulative effect of individual extreme events. The importance of stochastic and determinist environmental changes in driving extreme events has been evaluated through the environmental bootstrap method applied at the scale of the Mediterranean basin. Research on biofilms started from basic descriptions of spatial organization, as understanding these patterns is necessary to interpret the effects of climate extremes. Results indicated that microalgae develop scale-invariant structures, reflecting the influence of multiple processes operating at different spatial scales and possibly self-organization. A manipulative experiment was set up in order to test the separate and combined effects of warming and runoff following heavy rainfalls. Although a general pattern of reduced EMPB biomass in the clustered than the non-clustered scenario emerged in the first trial of the experiment, the hypothesis that compounded extreme events would elicit larger impacts than extremities of individual disturbances was not supported. EMPB biomass was susceptible to both warming and runoff, but the effects of the combination of these stressors was complex and context-dependent. Thus, repeating this experiment at different times will be necessary before generalities about EMPB responses to environmental extremes can be drawn. The environmental bootstrap method resampled short-term data of sea surface temperature and patterns of geostrophic currents to obtain an ensemble of hypothetical time series that, when combined with a predictive model of chlorophyll a concentration allowed me to make inferences on primary productivity response to environmental extremes. The output of this analysis is a map of the distribution of chlorophyll a concentration values with 100 years return time periods for the Mediterranean basin, which highlights the areas that are likely to harbour exceptional greening events.