Seasonal variations in the zooplankton and in the population structure of Acartia tonsa in a very eutrophic area: La Habana Bay (Cuba)

Abstract

Seasonal changes in the abundance of total zooplankton and of main components were studied at a fixed station in La Habana Bay. Special attention was given to the dominant species. Acania tonsa. which constituted 89% of the total number of zooplanktonic organisms. The variations in some of its population parameters (eggs, nauplii. copepodites. adults, sex ratio, number of eggs per female) were analysed in relation to temperature, salinity and chlorophyll. The abundance of eggs was independent of the environmental factors, particularly of chlorophyll which was always present in excess. The number of eggs per female showed several peaks, but was also independent of environmental factors. Nauplii were poorly correlated with egg numbers, but depended on the abundance of chlorophyll in particles of size <55 \im. This suggested that the part of edible food suitable for them in size or quality was not always in excess in the available seston. provoking a temporary increase in naupliar mortality. The temporal evolution of the following successive stages was nearly synchronous: thus, the changes observed in their abundance were not caused by fluctuations in the reproductive potential of females, but appeared to be related to a periodic decrease in the total zooplankton density due to the flushing effect of inland water in the bay.