Phytoplankton Community Response to Seasonal Changes in Chlorophyll a and Nitrate-Phosphate Concentrations in a Tidal Blackwater River in Niger Delta

Abstract

The study evaluated the seasonal abundance of standing stock of phytoplankton groups in a tidal blackwater system of the New Calabar River in Nigeria. The focus was on observable patterns in phytoplankton community structure as it relates to the variations in the supplies of production such as Chlorophyll a and growth-limiting nutrients such as Nitrate and Phosphate. The results showed there were significant seasonal variation in the cell counts of the dominant family groups such as Bacillariophyceae (70%:90%) and Chlorophyceae (3%:30%) to total phytoplankton between wet and dry season respectively. The results also produced a community structure composed of four groups with a consistent seasonal dominance order of Bacillariophyceae> Chlorophyceae> Cyanophyceae> Dinophyceae. The observation contrasts with studies of blackwater systems which has at varying levels other groups such Euglenophyceae, Charophyceae, Chrysophyceae, Cryptophyceae, Haptophyceae and Rhodophyceae. The three parameters of Nitrate, Phosphate and Chlorophyll a evaluated to explain the abundance in cell counts provide contrasting differences in their relationship to each of the phytoplankton groups between seasons. The results showed that seasonal changes in cell numbers were inversely related with changes in chlorophyll a concentration which suggests that it is not a reliable indicator of increase in cell numbers of the Phytoplankton groups within the study area. In comparison the Phosphate and Nitrate components had a moderate positive correlation with phytoplankton cell standing stock. This observation was interpreted as a probable effect of uptake and drawdown ratios of bioavailable nitrates and phosphate by phytoplankton in proportions that allow only marginal increase in Chlorophyll a production.