Short-term variability of the planktonic size structure in a Mediterranean coastal lagoon

The seasonal variability of the planktonic size structure of a Mediterranean hypersaline coastal lagoon was analysed by applying the size-spectra theory (Platt and Denman, 1977, 1978). The dataset used comprised a one-year time series of weekly samples analysed by non-destructive image analysis techniques from the pico- to mesoplankton size range. The resulting model suggested that, for the overall size range, the biovolume increased with individual body size by a factor of approximately 2 if the classical size ranges from pico-, nano-, micro- and mesoplankton were considered. Two antagonistic types of spectra were used to illustrate different configurations of the food web, the first with a volumetric autotroph–heterotroph ratio slightly higher than 1 (A:H = 1.02), the second largely dominated by heterotrophs (A:H = 0.02). The density spectra slope from the nano- tomesoplankton size range followed a seasonal trend, showing more negative values in late summer (September) and less negative values in winter. Both the variability range of the slope and the seasonal trend were lessened when considering the picoplankton size range, suggesting that the microbial compartment plays a relevant role in the configuration of the food web in the lagoon.