From seasonal to decadal inter-annual variability of mesozooplankton biomass in the Northern Adriatic Sea (Gulf of Trieste)

Abstract

This paper reports the rate of change of mesozooplankton biomass on seasonal, inter-annual and decadal time scale in the Gulf of Trieste (NE Mediterranean, Northern Adriatic). We measured variability in mesozooplankton dry weight (mg DW m − 3 ), organic carbon and nitrogen contents of the DW in relation to mesozooplankton taxonomic structure and some environmental parameters. The study is based on data obtained from mesozooplankton samples collected monthly by a vertical WP2 net (200 µm) from January 1986 to September 2005 at one monitoring station, a coastal site in the Gulf of Trieste. We considered mesozooplankton DW in relation to copepods, phytoplankton taxonomic structure, water temperature and North Atlantic Oscillation. For further analyses we counted also on data for DW for the period 1972–1980, monthly data for organic carbon (mg C m − 3 ) and nitrogen contents (mg N m − 3 ) of the DW for the period 1991–2005, determined by a CHN Elemental Analyzer. We explored statistically our high temporal resolution time series data picking out the main features: seasonal components and trends. Mesozooplankton DW ranged from only 1 mg m − 3 (January 1977) to 95 mg m − 3 (March 1990) in the coastal ecosystem of the Northern Adriatic during the period 1972–2005. The annual cycle of the DW was found to be bimodal with maximum in late winter–spring and a weaker one in late summer. Maximum DW were regularly recorded when Copepods prevailed the mesozooplankton community structure. Similarly, high organic carbon and nitrogen contents were detected when copepods dominated, although wide-ranging on a seasonal scale. Irregular intra- and inter-annual fluctuations were typical mostly during the 1990s. Mesozooplankton DW sharply shifted since 2001–2002 to the level exceeding the observed one during the regime of the 1980s. Our results indicate substantial changes in the seasonal timing of mesozooplankton DW, which together with decadal inter-annual fluctuations match significantly with shifts in copepods, phytoplankton taxonomic structure, temperature and NAO index. The discussed zooplankton descriptors (DW, C and N contents and C:N ratio) being to a certain degree predictive of phyto- and mesozooplankton composition, in essence might foretell the phenological response of plankton communities to environmental changes in the coastal ecosystem, including climate change.