Abstract
SUMMARY: The annual cycle of plankton was studied over 14 years from 1984 to 2000 at a coastal station in the Gulf of Naples, with the aim of assessing seasonal patterns and interannual trends. Phytoplankton biomass started increasing over the water column in February-early March, and generally achieved peak values in the upper layers in late spring. Another peak was often recorded in autumn. Diatoms and phytoflagellates dominated for the largest part of the year. Ciliates showed their main peaks in phase with phytoplankton and were mainly represented by small (< 30 µm) naked choreotrichs. Mesozooplankton increased in March-April, reaching maximum concentrations in summer. Copepods were always the most abundant group, followed by cladocerans in summer. At the interannual scale, a high variability and a decreasing trend were recorded over the sampling period for autotrophic biomass. Mesozooplankton biomass showed a less marked interannual variability. From 1995 onwards, phytoplankton populations increased in cell number but decreased in cell size, with intense blooms of small diatoms and undetermined coccoid species frequently observed in recent years. In spite of those interannual variations, the different phases of the annual cycle and the occurrence of several plankton species were remarkably regular.
Highlights
Current views about the functioning of marine pelagic ecosystems are based on the conceptualization and generalization of patterns observed in different areas (Sverdrup, 1953; Mann and Lazier, 1996; Longhurst, 1998 and references therein)
The existing paradigm for temperate latitudes is a bimodal distribution of autotrophic biomass, with a spring bloom at the start of thermal stratification of the water column and a second bloom during early fall, when the inversion of the buoyancy flux causes the deepening of the seasonal thermocline (Longhurst, 1998; Cebrián and Valiela, 1999)
Salinity values for the same layer (Fig. 2b) showed frequent spikes of lower and more rarely higher values, superimposed to a sinusoidal pattern which is similar to that observed at other Mediterranean sites (Dyfamed dataset, available at http://www.obsvlfr.fr/jgofs2/sodyf/home.htm) and temperate regions
Summary
Current views about the functioning of marine pelagic ecosystems are based on the conceptualization and generalization of patterns observed in different areas (Sverdrup, 1953; Mann and Lazier, 1996; Longhurst, 1998 and references therein). Zooplankton peaks driven by food availability would follow and control phytoplankton biomass. This broad simplification is contradicted by several cases of inconsistencies. In coastal and shelf waters, plankton abundance and species composition are characterised by a very high degree of spatial and temporal variability. This reflects the variety of terrestrial and offshore as well as atmospheric forcing and internal biological processes to which these boundary areas are subject. The overall complexity explains why, despite the establishment of a small set of paradigms, no simple and wide-ranging rules have been agreed upon for the annual cycle of plankton and for the functioning of the pelagic ecosystem in coastal waters
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