PLANKTONIC HERBIVOROUS FOOD WEBS IN THE CATALAN SEA (NW MEDITERRANEAN) - TEMPORAL VARIABILITY AND COMPARISON OF INDEXES 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-Novembe r 1992) and ME93 (June 1993), at two fixed stations placed in a transe ct crossing the Catalan density front (NW Mediterranean), one located in the vicinity of the density front and another offshore. Both statio ns were sampled at variable intervals during each cruise in order to d etermine possible tendencies in the temporal changes in the phyto-zoop lankton coupling. This coupling (i.e. the matter and energy transfer t hrough 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 consumer s; (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 offsho re stations coincided for the two cruises and were similar to the aver age values for the whole area observed in previous cruises At stations near the front, while structural indices suggested a more intense phy to-zooplankton coupling during FR92 (although differences between crui ses were not statistically significant), functional indices indicated an opposite trend, being significantly higher during ME93. The differe nt functional indices allowed coincident tendencies on the phyto-zoopl ankton coupling and seemed to respond faster to any change in the rela tive importance of planktonic food webs than structural ones. This sug gests that the relative importance of planktonic herbivorous food webs can be better estimated through the relationships between rate proces ses of producers and consumers, than through the relationships between more conservative state variables, like biomass or community structur e.