Plankton- Based Assessment of the Trophic State of Three Tropical Lakes

Benedict Obeten Offem,Gabriel Ujong Ikpi,S N Ochang,F B Ada,Ezekiel Olatunji Ayotunde

doi:10.4236/jep.2011.23034

Abstract

In developing countries, lakes being important sources of water supply and fishing are vulnerable to anthropogenic impact, yet knowledge of their trophic state in relation to changes in species composition, and environmental variables, are limited. This study is aimed at assessing the trophic status of lakes by monthly sampling of three lakes located along the floodplain of Cross River, Nigeria between January 2008 and December 2009. Samples were analyzed for water quality parameters, zooplankton and phytoplankton composition and distribution. Results were subjected to community structure analysis using trophic state index, species richness and diversity indexes. Essential primary productivity nutrients, nitrates, sulphates and phosphates were highest in Ejagham Lake, and lowest in Ikot Okpora Lake. Dominant phytoplankton species Oscillatoria lacustria (Cyanophyceae), Cyclotella operculata (Bacilliarophyceae) and zooplankton Keratella tropica, Keratella quadrata, Filinia longiseta, Branchionus anguillaris and Trichocerca pusilla (rotifers) all typical of eutrophic communities were recorded in high densities in Ejagham Lake in both dry and wet seasons while Cladocerans, Bosmina longirostris and Moina micrura and copepods considered indicators of oligotrophy and mesotrophy were recorded in large numbers in Ikot Okpora and Obubra Lakes respectively. Higher values of species richness, Evenness and Shannon-Wiener diversity index for both phytoplankton and zooplankton, were recorded in Ejagham Lake during the dry season than wet. Also values of the Trophic state index were generally highest at the Ejagham Lake in the savanna region of the floodplain and lowest at Ikot Okpora in the forest region of the floodplain. Forest region is therefore a limiting factor in the productivity of lakes in the tropics.

Highlights

Biological approaches to evaluating water quality involve assessing communities of organisms
Nitrates, sulphates and phosphates were highest in Ejagham Lake, and lowest in Ikot Okpora Lake
Dominant phytoplankton species Oscillatoria lacustria (Cyanophyceae), Cyclotella operculata (Bacilliarophyceae) and zooplankton Keratella tropica, Keratella quadrata, Filinia longiseta, Branchionus anguillaris and Trichocerca pusilla all typical of eutrophic communities were recorded in high densities in Ejagham Lake in both dry and wet seasons while Cladocerans, Bosmina longirostris and Moina micrura and copepods considered indicators of oligotrophy and mesotrophy were recorded in large numbers in Ikot Okpora and Obubra Lakes respectively

Summary

Introduction

Biological approaches to evaluating water quality involve assessing communities of organisms. “The basis for this approach is that different species have varying tolerances to environmental stressors” [1]. Fish productivity of water bodies is connected to primary production by many intermediate trophic links. The four groups of organisms that appear in The European Water Framework Directives WFD (Phytoplankton, Zooplankton, Fish and macrophytes), represent water ecological structure over a range of temporal and spatial scales and functional roles. It was recommended that the above biological indicators and, in addition to a range of supporting hydro-morphological and physico-chemical elements should form the core of any monitoring program on lakes [2]. The qualitative and quantitative estimates of the plankton provide good indices of quality and productive capacity of water

Objectives

Methods

Conclusion