Environmental conditions and primary production in a Sahelian shallow lake (Lake Guiers, North Senegal)

Abstract

This is a study of the environmental conditions and primary phytoplankton production in a Sahelian shallow lake of Senegal, West Africa. Environmental descriptors (nutrient, water transparency, temperature and hydrochemistry) and their effects on primary production were studied. Samples were collected monthly from March 2002 to February 2003 in 3 stations of the central part of water body. Climatic variables show a marked annual pattern following two dominant wind directions: NNE (North-North-East) and NNW (North- North-West). This cyclic evolution affects hydrobiological variables. In this central part of the lake, water body was not stratified with same measures between surface and bottom. Lake water was turbid with lower values during the maritime trade winds period characterized by high water temperatures and conductivity. Water pH was relatively higher than seven (07) associated with high contents of oxygen (approximately 90%) in all stations. The station Typha is marked by a low pH and high fluctuations in the rates of oxygen (20 to 120%). Nutrient concentrations were low, with high variability (from 0 to 30 μg.l^-1 for DIN and from 0 to 18 μg.l^-1 for PO₄). The primary production in Guiers Lake was high; it showed a vertical gradient with production of 2400 mgC.m^-3.days^-1 in surface and a low of 200 mgC.m^-3 days^-1 at the bottom. In this shallow system, environmental conditions regulate the primary production.

Keywords: Environmental descriptors, primary production, shallow lake, Senegal.