Distribution of mesozooplankton resting eggs in seabottom sediments of Thermaikos gulf (NW Aegean Sea, Greece) and possible effects of sediment resuspension

Abstract

The distribution of mesozooplankton resting eggs was studied in the bottom sediments of Thermaikos Gulf (Aegean Sea, Greece), within the framework of the EU project INTERPOL; possible effects of sediment resuspension, due to physical forcing (storm events) and/or to anthropogenic activity (trawling) on the egg assemblages were investigated. Sediment core slices of 2 cm height were collected down to 10 cm depth at 6 stations (from water depth ranging from 25 to 60 m); mesozooplankton samples from the water column were obtained simultaneously. Sampling was performed in September 2001 (before trawling activities and without storm events), in late October 2001 (after the start of the trawling activities) and in February 2002 (after 120 days of intensive trawling activities and the stormy season). The total abundance of eggs was higher at stations located close to the rivers’ mouths, with the muddy sediments of the bed and the water column rich in zooplankters, than at stations located at greater depths and with muddy-sand bed sediments. At the former stations, the vertical distribution of resting eggs has revealed a trend of homogenisation within the sediment column, from September to February; this is related probably to sediment resuspension. At the latter stations, no clear temporal variability in the vertical distribution was detected. The eggs found in the bottom sediments of Thermaikos Gulf were assigned to 16 morpho-types. Amongst these, eggs assigned to Paracartia latisetosa and Labidocera wollastoni (as well as one type called as “Calanoida 3”), were found to be dominant at all the stations and during all the sampling periods, both as full and empty eggs.