Climate Change and Copepod Size Distribution: Comparison of Two Coastal Long-Term Series in the Western Mediterranean Sea

Abstract

This thesis was aimed at investigating long-term dynamics of zooplankton communities in relation to climate change. It represents a pioneer study in comparing Mediterranean time series with a standardised methodology, i.e., the ZooScan, a digital imaging system for counting and sizing mesozooplankton from preserved samples. This study has proven that copepod size spectra (i.e., histogram of organisms arranged by size classes) obtained with the ZooScan is a powerful synthetic index to monitor changes in the pelagic system. Copepod abundance and size spectra were analyzed in the zooplankton time series conducted at stn MC (Gulf of Naples, Tyrrhenian Sea) and Point B (Villefranche Bay, Ligurian Sea) for the years 1986-2005 and 1974-2003, respectively. In both time series, the proportion of large individuals in the copepod community increased over the years, with a shift in the early 1990s at stn MC and in 1987 at Point B. In both cases, the 1990s copepod reproduction might have decreased due to earlier and stronger stratification driven by rising temperature. At stn MC, the shift to dominance of small phytoplankton cells in the 1990s seemed to be the direct cause. At Point B, in addition to the detrimental effect of earlier stratification in the 1990s, the 1980s seemed to be very productive due to strong winter convection driven by cool salty winters. High salinity was related to low winter atmospheric pressure linked to the North Atlantic Oscillation (+ winter NAO). In both locations, the frequency of occurrence of typical offshore species increased over the years, suggesting changes in coastal-offshore interactions. Mechanisms controlling the copepod communities in both sites seem to be different. Stn MC has a more coastal character than Point B and thus the former is more affected by local conditions as terrestrial nutrient inputs. At Point B, stratification-destratification dynamics seem to control production.