The Gulf of Trieste: a changing ecosystem

Understanding the impact of climate change on zooplankton populations is of major importance, as they represent the basis for higher trophic levels in the marine food web. In this study we analyze the 36-year copepod abundance time series in the Gulf of Trieste, northern Adriatic, to investigate its interannual variability, with particular attention to species trends and phenology. Following the analysis of the local winter sea surface temperature, two periods are identified: 1970-1987 and 1988-2005. These periods are characterized by ecosystem-wide changes: a circa doubling in total copepod abundance, the arrival of a new species (Diaixis pygmoea), the rise (Paracalanus parvus, Oncaea spp., Oithona spp., and Euterpina acutifrons) or decline (Pseudocalanus elongatus, Clausocalanus spp.) of several taxa, and changes in the phenology in several species, with predominantly forward shifts in the timing of the maximum peak. While Acartia clausi remains the dominant species, there is a general trend toward smaller species in the second period. Our results also indicate the large, possibly critical, reduction in the abundance of the species Pseudocalanus elongatus. We hypothesize that the changes in copepod abundances and community composition in the Gulf of Trieste are related to the general warming in the sea surface temperature, and associated northerly displacement of the ecosystem, and to the changes in the Mediterranean circulation that began at the end of the 1980s and affected the whole basin in the following years as part of the phenomenon called the Eastern Mediterranean Transient.