HYDROGRAPHIC CONDITIONS, COMPOSITION AND DISTRIBUTION OF PLANKTON IN RELATION TO POTENTIAL RESOURCES OF PARAÍBA RIVER ESTUARY.

Abstract

The present work briefly reports the findings on hydrological conditions, composition and abundance of plankton in relation to the potential resources of the Paraíba river estuary. A vertical salinity stratification was observed at the river mouth and the harbour areas and a horizontal salinity gradient extends up the estuarine waters landward which is very much influenced by weather conditions. The surface temperature distribution also shows a horizontal gradient but this tends to decrease seaward. The visibility and pH values generally increase seaward. The estuarine segments, particularly stations 2 and 3, are extremely rich in organic debris and nutrients which have profound effect upon the productivity of the plankton, especially the diatoms. The Atlantic water penetrates into the estuary and lifts the river water to the surface which extends seaward beyond the river mouth. A total of 87 planktonic organisms belonging to the two major groups of phyto- and zooplankton are recorded and their seasonal fluctuations and total numbers are compared, in relation to variations of salinity and temperature. The average total phytoplankton ranged between 270,000 to 1,050,000/100 ml. Among the phytoplankton, the diatoms constituted the most commonest and dominant component. Cosinodiscus centralis, Cosinodiscus sp., Streptotheca tamesis and Skeletonena costatum were the most abundant and they occured frequently. The others in numerical abundance were Ditylum brightwellii, Biddulphia sinensis, Gyrosigrne sp., Melosfra sulcata, and Chaetoceros. The Cosinodiscus centralis and Gyrosigma sp. generally showed increasing concentration up the estuarine waters with decreasing salinity and this tendency was more so with Gyrosigma sp. The second abundant group of phytoplankton was Dinophyceae with Ceratium furca being the common and dominant species. The other groups encountered were Sillcoflageilates and Cyanophycea but these were found only in smaller numbers. The seasonality of the zooplankton was relatively uniform than the phytoplankton. However, two distinct peaks of zooplankton were noticed, the major peak in October and the second during December - January. The mean total zooplankton generally ranged between 100,000 to 5 10,000/ 100 ml. But these were dominated by copepods (40 to 66%). The calanoid copepods were more numerous and represcnted by Labidocera fluviatilis, Eucalanus elongatus, Acartia longiremis, Temora stylifera and Centropages furcata; others were the harpacticoid Euterpina acutifrons and the cyclopoid Oithona and Copilia in apparent order of their numerical abundance. The calonoid copepods generally increased in numbers seaward, while the harpacticoid Euterpina acutifron was widely distributed in most segments of estuary proximal to the sea and appeared to prefer an optimally decreased salinity. The crustacean larvae dominated, particularly at station 2, where the waters of different origins mix and these conditions appear to favour the spawning and developmental processes of the crustaceans. The other commonest zooplankton found in abundance were coelenterates, including ctenophores, but these always dominated the statjons 3 and 4 perhaps, they prefer the reduced pH associated with the water. The cirripede nauplll were, though common, but found in such abundance and they exceeded 80% of the zooplankton population especially at station 5. Station 6 was always dominated by ciliates (60%) tintinnids, and cladocerans is numerical abundance. The findings, so far, show no direct correlation between diatom production and zooplankton abundance in the estuarine system, but in the coastal and tiver mouth waters an inverse relationship does occur. The enormous quantity of diatom production in the estuary, influenced by rich nutrients, might be consumed by herbivorous pelagic fishes of Mugilidae family, crustaceans such as prawns and shrimps and the rich benthic filter-feeders, particularly, shellflsh. These form some of lhe important economic resources of lhe estuary with great commerciai potential. The Chaetognatha was represented by Sagitta enflata. These findings are of considerable importance for further investigations.