Planktonic herbivorous food webs in the catalan Sea (NW Mediterranean): Temporal variability and comparison of indices of phyto-zooplankton coupling based on state variables and rate processes

The structural properties and functional relationships between phyto- and zooplankton were studied during two cruises, FR92 (October-November 1992) and ME93 (June 1993), at two fixed stations placed in a transect crossing the Catalan density front (NW Mediterranean), one located in the vicinity of the density front and another offshore. Both stations were sampled at variable intervals during each cruise in order to determine possible tendencies in the temporal changes in the phyto-zooplankton coupling. This coupling (i.e. the matter and energy transfer through planktonic herbivorous food webs) was estimated by means of two categories of quantitative indicators: (i) structural indices, based on the relationships between state variables of producers and consumers; (ii) functional indices, based on their rate processes. Structural indices showed lower temporal variability, both at a short time scale (within cruises) and interannually (amongst cruises), than those based on rate processes. The values of both categories of indices at offshore stations coincided for the two cruises and were similar to the average values for the whole area observed in previous cruises At stations near the front, while structural indices suggested a more intense phyto-zooplankton coupling during FR92 (although differences between cruises were not statistically significant), functional indices indicated an opposite trend, being significantly higher during ME93. The different functional indices allowed coincident tendencies on the phyto-zooplankton coupling and seemed to respond faster to any change in the relative importance of planktonic food webs than structural ones. This suggests that the relative importance of planktonic herbivorous food webs can be better estimated through the relationships between rate processes of producers and consumers, than through the relationships between more conservative state variables, like biomass or community structure.