Phytoplankton diversity and Stoichiometric nutrient limitation in the Lagos Harbour and adjacent sea, Southwestern Nigeria

Abstract

Harbours and their adjacent seas are increasingly stressed by maritime activities and development of coastal cities worldwide. This is especially true for the Lagos Harbour. The aim of this study was to investigate the trophic status, nutrient stoichiometry and phytoplankton assemblage of Lagos Harbour and the adjacent sea. Water and plankton samples were collected for a period of eighteen months from January 2015 to June 2016 between 06.00 and 11.00 hrs each time. Phytoplankton samples were investigated using a Leica DMLB microscope with 100x full oil immersion optics and 1.35 numerical aperture equipped with a Nikon Coolpix 995 CCD digital camera of 3.3 megapixel resolution. Relevant statistical tests such as Principal Components Analysis (PCA) were employed to determine major controlling water quality indices and Canonical Correspondence Analysis (CCA) for determination of the water quality indices that influenced the monthly and spatial phytoplankton occurrences. A total of 108 phytoplankton taxa belonging to five classes were recorded in this study. Bacillariophyceae comprised eighty-four (84) taxa (78%), Chlorophyceae had two (2) taxa (2%), Cyanophyceae were represented by 11 taxa (10%), Dictyochophyceae had one (1) taxon (1%) and Dinophyceae had 10 taxa (9%). CCA revealed a clear seasonal pattern; it also showed that phosphate was the major controlling factor in the Lower Lagos Harbour and the creeks in the months of July, August and September 2015. Trophic state index (TSI) using Carlson's indices revealed eutrophic to hypereutrophic conditions. The comparison of ambient nutrient ratios with Redfield ratio (N:P = 16:1) revealed clear spatial and temporal variations in the Lagos Harbour and adjacent sea. The significant difference shown by phosphate in the wet season could be linked to allochthonous and autochthonous inputs from rainfall.