Distributions of phytoplankton in a coastal lagoon of Mahin, Ondo, Western Nigeria

Abstract

Distributions of phytoplankton were investigated in Mahin Lagoon within Transgressive mud coast of Ondo, Western Nigeria. Collections and analyses of samples of phytoplankton and surface waters (for some physico-chemical parameters) were done at fifteen stations along the stretch of the lagoon in October 2013 and 2014 as well as March 2014 and 2015. Higher salinity levels were recorded at stations close the mouth of the Atlantic Ocean than those further upstream. Biochemical oxygen demand was high (7.7±2.3-10.6±1.8 mg/L) at stations with high levels of anthropogenic activities. Division Bacillariophyta (diatoms) dominated phytoplankton community in term of species diversity. Most species recorded during rainy season had relatively even spatial distributions. During March 2014 and 2015, high brackish water species such as Chaetoceros affinis, Asterionela japonica and Fragilaria construens were recorded only close to the Atlantic Ocean while stations further upstream had freshwater species such as Aulacoseira granulata, Synedra acus and Eudorina cylindrical. Stations close to high human population and anthropogenic activities had low phytoplankton abundance and diversity. Pollution indicator species, such as Microcystis aeruginosa and Oscillatoria borneti were recorded with relatively high abundance across fifteen stations. Implications of the anthropogenic activities on phytoplankton distributions and sustainable management of the lagoon are discussed.

Keywords: Algae, species diversity, brackish waters, seasonality, anthropogenic activities