The diversity and dynamics of phytoplankton at a tropical tidal creek in Lagos, in relation to water chemistry changes, were investigated for twenty-four months. Water samples were collected with a 75cl plastic bottle whereas phytoplankton samples were collected by a 55 µm mesh size standard plankton net towed horizontally  for 5 mins at <4 Km/h. Water samples were processed to the laboratory for analysis and the phytoplankton samples were preserved by the addition of 4% unbuffered formalin. Water chemistry characteristics were measured using standard methods. The drop count microscopic analysis method was used to study the phytoplankton. Notable parameters that showed marked variations with season include salinity (0.17 – 30.4‰), rainfall (6 – 330mm), transparency (26 – 191cm), total suspended solids (30 - 360 mg/L), total hardness (18 - 3760 mg/L), chloride (60 - 11165 mg/L), conductivity (263 - 32900 µS/cm), total dissolved solids (197 - 21230 mg/L), sulphate (18.9 - 1140 mg/L), nitrate (4.1 - 42 mg/L), calcium (4.9 – 400.2 mg/L), Magnesium (1.4 - 648 mg/L) and iron (0.08 – 0.99 mg/L). A total of sixty-four phytoplankton species (dominated by diatoms) from five (5) algal classes namely Bacillariophyceae (43 species), Cyanophyceae (11 species), Chlorophyceae (7 species), Euglenophyceae (2 species) and Chrysophyceae (1 species) were observed. The green algae, euglenoids and golden brown algae were more frequently occurring during the wet season. The species with the highest biomass (in terms of numbers) was Aulacoseira granulata var. augustissima. The preponderance of very few species primarily Aulocoseira and Microcystis species in the wet season and Coscinodiscus, Actinoptycus, Odontella and Parabelius species in the dry season is a reflection of the changing water conditions within the tidal creek system from season to season and from fresh to high brackish water conditions. Freshwater taxa in the lagoon were primarily recruited from the freshwater areas especially upstream and including the Ologe lagoon from where they existed in higher concentration and usually brought in with floodwater inflow during the rains. Species of Surirella where likely scoured from the creek substratum whereas Parabelius (tube diatoms) were detached from attached submerged hard substrates