Zooplankton distribution in a temperate estuary (Mondego estuary southern arm: Western Portugal)

Abstract

The composition and spatio-temporal distribution of zooplankton abundance and diversity were studied monthly from July 1999 to June 2000 in the southern arm of the Mondego estuarine system (Western Portugal). Two sampling stations with different salinity conditions were selected. Zooplankton samples were obtained using 63 and 125 μm mesh nets. In both taxocenosis densities were higher in autumn and late spring-summer with copepodits and adult copepods among the most abundant zooplankters, representing more than 90.0% of the total densities. Abundance increased with increasing temperature, salinity and chlorophyll a values. The most abundant species were Oithona nana, Acartia tonsa, Acartia clausi, Euterpina acutifrons, Oithona similis, Temora longicornis, Clausocalanus arcuicornis, Paracalanus parvus, and Acartia bifilosa var. inermis. In the 63 μm taxocenosis, diversity was higher closest to the mouth of the estuary, during late spring and summer, and in the inner estuary, during autumn and winter. Cluster analysis showed that spatial distribution dominated over seasonal patterns, i.e. the similarities between the clusters grouping the samples of different months is high, which was also confirmed by ANOVA analysis.