Response of plankton assemblage to nutrient and environmental parameters in a tropical coastal water

Abstract

Seasonal variations of nutrients and water quality parameters in relation to plankton abundance were investigated in Ogun coastal water, Ogun State, Nigeria from February to July, 2015 at three different stations of 5 km apart along the 15 km Ogun coastal stretch line. The study revealed that nutrient concentrations were influenced by seasonal changes. Mean nutrient values and important variables determining plankton assemblages were recorded as follows: nitrate (0.73mg/l), salinity (30.95mg/l), dissolved oxygen (6.74mg/l), phosphorus (0.12 mg/l) and temperature (28.43 ^oC). A total population of 1571 planktons was identified which consists of 29 and 45 species of phytoplankton and zooplankton respectively. Bacillariophyceae (46.4%) dominated the phytoplankton community followed by Dinophyceae (31.0%) while the Cyanobacteria (0.95%) occurred least. Two important taxa of zooplankton i.e. Cladocera (36.2%) and Rotifera (21.4%) were encountered. An increased number of planktons was recorded during the wet season (61.3%) with a corresponding reduced diversity. Analysis of the ecological indices by the routines in Primer (v6) showed a high level of evenness (1.744 and 1.211) with a Margalef of (3.093 and 1.4412) for the phyto- and zoo-planktons respectively. A total species richness of 29 phto- and 45 zoopankton identified in the study area competes favourably when compared to other tropical aquatic ecosystems and temperate inland waters. The study revealed that water nutrient concentrations were influenced by seasonal changes; increased levels of PO₄-P and salinity were recorded during the dry season while higher NO₃–N content coupled with increased number of planktons was experienced during the wet season. This study provides considerable advances in understanding the seasonality of nutrient distribution in the study area and its effect on plankton assemblage.

Keywords: Plankton, Seasonal, Nutrient, Dynamics, Correlation, Assemblage