Surface mesozooplankton in open waters of the Western Mediterranean

Abstract

The seasonal abundance and distribution of mesozooplankton were studied in relation to the presence of floating objects, physical and biological variables from 1995 to 1997 in the epipelagic waters off Majorca. Zooplankton were sampled fortnightly using a vertically hauled WP-2 net (330 μm). Comparisons between zooplankton abundance at four fixed stations with Fish Aggregating Devices (FADs) and four open water control stations were conducted to 1) describe the seasonal succession of mesozooplankton organisms in relation to physical parameters and 2) to test for any attraction phenomena of zooplankton towards floating objects. Mean zooplankton densities were low and ranged from 4 ind·m-3 to 659 ind·m-3. Species composition showed no significant inter-annual variations and a dominance of copepods. Average dry weight biomass was 3 mg·m-3. Environmental factors did not directly determine the total zooplankton community structure but influenced the seasonal patterns of the individual taxa. Although, in the past, floating structures have been shown to increase spatial heterogeneity in the pelagic environment and enhance the aggregation of several fish species, the present study found no evidence of mesozooplankton aggregation below FADs. No significant differences were found in the abundance or biomass of zooplankton at sites with FADs, suggesting that the intrinsic variability in zooplankton overrides any possible FAD effect on plankton aggregation. We hypothesise that at the mesoscale level FADs do not provide a microhabitat with sufficiently different physical conditions, although they could locally induce patchiness in resource availability and predation linked with the presence of shoals of juvenile fishes aggregated underneath the floating device.