Linking traits to species level response to a pulse stress event in transitional waters: the dystrophic crisis of Lesina lagoon

Benthic macro-invertebrates represent a key component on Mediterranean lagoon ecosystems being the group of species whose life cycle and behavioural patterns fits are more exposed to the abiotic variability characteristic of these ecosystems. Here we describe the results of a field experiment carried out during the dystrophic crisis affecting Lesina lagoon in summer 2008. Benthic macro-invertebrate guilds were studied at weekly interval in two stations differently exposed to the crisis. Seasonal data on the same stations were also collected. The aim of the study was to evaluate the resistance and resilience components of structural and functional metrics of macro-invertebrate guilds, where body size structure and trophic groups were considered as functional metrics. Dystrophic crisis determined significant changes in all metrics considered: taxonomic richness decreased, species composition changed, functional richness decreased and size distribution widths decreased in the area affected by the crisis with respect to the control, unaffected, area. In the station inside the crisis area, only more opportunistic species (e.g., Chironomus salinarius, Hydrobia ventrosa) were able to outstand the anoxic conditions characterizing the dystrophic crisis. However, the time lag of the response, its amplitude and the recovery times vary greatly among taxonomic, functional and size structure descriptors of benthic macro-invertebrate guilds. The trajectories of change for everyone of these descriptor along the spatial and temporal gradients set by the dystrophic crisis are analysed and discussed. The overall data set seems to indicate that functional responses, including the metabolic based ones, are faster and more consistent than those shown by the taxonomic components, suggesting that a niche filtering process on functional traits is a driving force in the organization of macro-invertebrate guilds in Mediterranean lagoon ecosystems.