Phytoplankton Diversity and Chlorophyll-a Concentration in the Coastal Waters of Cross River State

Phytoplankton are primary producers fundamental to the functioning of coastal marine ecosystems, and their diversity reflects the trophic status and productivity of aquatic environments. This study examined phytoplankton diversity and chlorophyll-a concentration in coastal waters of Cross River State, South South Nigeria. Samples were collected monthly over six months from five stations along the Calabar River estuary using a 25-micrometer mesh phytoplankton net. Identification was conducted under a compound microscope using standard taxonomic references, and chlorophyll-a was measured spectrophotometrically following methanol extraction. Physicochemical parameters including temperature, salinity, turbidity, pH, and inorganic nutrients were analysed concurrently. A total of 48 phytoplankton taxa distributed across three major divisions, Bacillariophyta, Chlorophyta, and Cyanophyta, were identified. Bacillariophyta dominated the assemblage, accounting for 55 percent of total taxa. Chlorophyll-a values ranged from 1.8 to 12.4 micrograms per litre, indicating mesotrophic to eutrophic conditions at certain stations. Elevated nutrient concentrations near urban discharge points correlated positively with phytoplankton density. The Simpson diversity index revealed moderate diversity at the majority of stations, with lower values near the Calabar port. Seasonal patterns showed increased abundance during the rainy season. The findings contribute baseline ecological data vital for environmental impact assessments and fisheries management in the Cross River estuary. Keywords: phytoplankton diversity, chlorophyll-a, Cross River, estuary, eutrophication.