Okpoka Creek of the Upper Bonny Estuary in the Niger Delta is a tidal creek receiving organic anthropogenic effluents from its environs. The study investigated the influence of tides (low and high) on the species composition, diversity, abundance, and distribution of phytoplankton. The surface water and phytoplankton samples were collected monthly from May 2004 to April 2006 at both tides from ten stations according to standard methods. Phytoplankton was identified microscopically. Species diversity was calculated using standard indices. Data analyses were done using analysis of variance, Duncan multiple range, and descriptive statistics. Phosphate and ammonia exceeded international acceptable levels of 0.10？mg/L for natural water bodies indicating high nutrient status, organic matter, and potential pollutants. A total of 158 species of phytoplankton were identified. Diatoms dominated the phytoplankton (62.9%). Diversity indices of diatoms were (Margalef) and (Shannon). Pollution-indicator species such as Navicula microcephala, Nitzschia sigma, Synedra ulna (diatoms), Cladophora glomerata (green alga), Euglena acus (euglenoid), Anabeana spiroides (blue-green alga), and Ceratium furca (dinoflagellate) were recorded at either only low, high or both tides. Concerted environmental surveillance on Upper Bonny Estuary is advocated to reduce the inflow of pollutants from the Bonny Estuary into this Creek caused by tidal influence. 1. Introduction Estuaries are unique aquatic environments that have an additional source of buoyancy input derived from riverine freshwater inflow and an additional source of mechanical energy input from the tides (tidal stirring). Estuarine ecosystems are very favourable for algae and animal life. They play an important ecological role because they are natural habitants of water. They are widely used as an indicator of water pollution [1]. The Bonny Estuary is one of the several estuaries in the Niger Delta swamps through which the Lower Niger and Benue Rivers flow into the ocean. The estuary is richly endowed with abundant aquatic resources (creeks, distributaries, flood plains and mangrove swamps with fin/shell fish resources, etc.). The estuary, its creeks, and tributaries consist of rich collection of flora and fauna constituting a unique tropical biodiversity. The vegetation of Bonny Estuary is dominated by the red mangrove Rhizophora racemosa and R. mangle [2]. The mangrove provides nurseries and feeding grounds for commercially important species of finfish and shellfish. Despite its good environment for aquatic life forms,
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