http://astonjournals.com/faj 1 Fisheries and Aquaculture Journal, Volume 2011: FAJ-13 Influence of Seasons on Water Quality, Abundance of Fish and Plankton Species of Ikwori Lake, South-Eastern Nigeria * BO Offem, EO Ayotunde, GU Ikpi, SN Ochang, FB Ada Department of Fisheries and Aquatic Sciences, Cross River University of Technology, PMB 102 Obubra Campus, Cross River State, Nigeria. * Correspondence to: Benedict O Offem, benbeff06@yahoo.com Accepted: December 6, 2010; Published: January 5, 2011 Abstract This study is aimed at assessing the effect of seasons on quality of lake water in relation to changes in fish, plankton abundance and environmental variables. Standard methods were used to analyze monthly fish, plankton and water samples collected from Ikwori Lake during wet and dry seasons for 24 months (January 2008 to December 2009). All measured water parameters except dissolved oxygen (3.3 ± 0.10mgl -1 ), total dissolved solids (103.2 ± 2.22mgl -1 ), acidity (4.44 ± 3.91mgl -1 ), color (60.33 ± 8.38pt. co) and pH (5.8 ± 0.119) showed significant seasonal variation. The highest values of the parameters occur between February and March (dry season) and lowest between M ay and August (wet season). In addition, total phytoplankton (cells/litre), were significantly more during the dry season than the wet. Out of 16 fish species sampled, 7 species were restricted to dry season, only one i.e. Polypterus senegalus was recorded in the wet while 8 species occurred in both seasons. Seasonal differentiation in the richness and diversity indices of all species sampled revealed higher values for the dry season than the wet. Therefore, the influence of season is important in the management of water quality, plankton and fish production of the aquatic system. Keywords: Phytoplankton; Zooplankton; Fish species; Macrophytes; Diversity indices; Lake Ikwori; Nigeria. 1. Introduction The Ikwori Lake is used for fishing activities and as a source of domestic water supply to the host community and therefore vulnerable to anthropogenic impact. However, if the lake is properly managed, estimated potential yield of over 6000 metric tonnes per annum is possible. The effects of inputs on water quality and aquatic life particularly in the Niger-Delta region of Nigeria have been investigated. Akpan and Offem [1] observed that decrease in productivity of most inland wetlands of the region has been attributed to poor water quality caused by anthropogenic inputs from neighboring communities and oil industries. Inadequate management of the resources [2,3] is another reason. Impacted changes in water quality as reflected in the biodiversity, has also been investigated and observation made that both the development period and the hatching period for aquatic organisms are generally shorter at higher temperature than at lower temperature [4]. Species differ in their temperature tolerance during development but generally, there are temperatures which are too low and too high for physiological processes [5]. Other studies on Nigerian aquatic ecosystem impairment have been reported and most of the researchers established that a knowledge of hydrological conditions of any body of water is not only useful in assessing its productivity but will also permit a better understanding of the population and life cycle of the fish community [6]. Major industries responsible for water pollution in Nigeria have been listed to include petroleum, mining, wood, pulp, pharmaceuticals, textiles, plastics, iron and steel, brewing, distillery, fermentation, paints, beverages, food and agriculture [7]. Fertilizer effluents from industrial city of Kano polluted Jakara reservoir [8]. High levels of toxic heavy metals including copper, zinc, chromium, iron and manganese in fish from Warwade Reservoir in Kano was detected [9]. Increased pollution levels of the coastal areas of Cross River State by petroleum product spillage, that is a problem to the fishery industry, had been reported [10]. However, no work had been carried out on seasonality of water quality and biodiversity of lakes in the southeast region of Nigeria. The objective of the study is to provide a quantitative record of the seasonal changes of some macro and micro fauna and flora and to examine their distribution and diversity in relation to environment gradients in a tropical lake.