This study was embarked upon with the aim of addressing spatial and temporal variations in phytoplankton and zooplankton communities in the Gulf of Guinea large marine ecosystem, and how the distribution patterns were influenced by hydrological and climatic factors. Availability of long term-data from the coastal waters of Ghana also allowed trend analyses of zooplankton biomass and the role of local and/or remote biological and physical factors on the observed trends. Such information is relevant in understanding the ecosystem dynamics and productivity of the Gulf of Guinea.

The hydrographic changes in the region are in the form of minor and major upwelling alternating with hydrographic stratification. The major upwelling phenomenon differs from that of other eastern boundary systems and is strongest off Cote d'Ivoire and Ghana. Temperature is directly related to seasonal changes in the hydrography and also plays a key role in the distribution pattern of plankton during the major upwelling.

The use of continuous plankton recorder in this study is novel in the Gulf of Guinea and despite limitations such as large mesh size, small entrance aperture and limited sampling depth, it afforded an excellent means of sampling an extensive area.

Spatial variation in the distribution pattern of zooplankton was driven mostly by phytoplankton abundance in all seasons except the major upwelling, during which period water temperature governed most of the distribution. As much as 60% of the variation in zooplankton distribution was accounted for by sea surface temperature during this season.

Phytoplankton communities exhibited variable temporal patterns with respect to the hydrographic regimes. The following taxa were identified as contributing significantly to the distribution patterns: Hyalochaete spp., Phaeceros spp. Thalassionema nitzschioides, Trichodesmium spp., Rhizosolenia hebetata semispina, Rhizosolenia calcar avis, Ceratium extensum and Ceratium minutum. With regard to the zooplankton, their community structure during the major upwelling was significantly different from the other hydrographic seasons. Foraminifera, Chaetognath, Oithona spp., Oncaea spp, Echinoderm larvae, Para-pseudocalanus, Temora stylifera, Eucalanus spp., and Clausocalanus spp. contributed significantly to observed variations. Multivariate analysis also suggested that Corycaeus spp. and Oncaea spp. (Copepoda: Cyclopoida) shared similar distribution patterns within nearshore and shelf waters.

Long-term temporal investigation in the coastal waters of Ghana indicated significant decline in zooplankton biomass of 6.33 ml/1000m^3/year from 1969 to 1992. The mechanisms proposed to explain the observed trend were warming of sea surface temperature as a result of global "green house" effect, and predation pressure by Sardinella larvae, regulated by hydro-climatic factors. Using a 24-year average, the best model (p <0.001) which accounted for the greatest proportion (83.3%) of the variability in seasonal biomass of zooplankton incorporated sea surface temperature and Sardinella larvae abundance as predictor variables.