Surface mesozooplankton in open waters of the Western Mediterranean

The seasonal abundance and distribution of mesozooplankton were studied in relation to the presence of floating objects, physical and biological varia bles from 1995 to 1997 in the epipelagic waters off Majorca. Zooplankton we re sampled fortnightly using a vertically hauled WP-2 net (330 mum). Compar isons between zooplankton abundance at four fixed stations with Fish Aggreg ating Devices (FADs) and four open water control stations were conducted to 1) describe the seasonal succession of mesozooplankton organisms in relati on to physical parameters and 2) to test for any attraction phenomena of zo oplankton towards floating objects. Mean zooplankton densities were low and ranged from 4 ind.m(-3) to 659 ind.m(-3). Species composition showed no si gnificant inter-annual variations and a dominance of copepods. Average dry weight biomass was 3 mg.m(-3) Environmental factors did not directly determ ine the total zooplankton community structure but influenced the seasonal p atterns of the individual taxa. Although, in the past, floating structures have been shown to increase spatial heterogeneity in the pelagic environmen t and enhance the aggregation of several fish species, the present study fo und no evidence of mesozooplankton aggregation below FADs. No significant d ifferences were found in the abundance or biomass of zooplankton at sites w ith FADs, suggesting that the intrinsic variability in zooplankton override s any possible FAD effect on plankton aggregation. We hypothesise that at t he mesoscale level FADs do not provide a microhabitat with sufficiently dif ferent physical conditions, although they could locally induce patchiness i n resource availability and predation linked with the presence of shoals of juvenile fishes aggregated underneath the floating device.