Trophic status of the restored South-West and non-restored East Beira Lakes

Abstract

This study was carried out in the restored South West and non-restored East Beira Lakes, with the objectives of determining their current trophic status and investigating the effects of restoration in the South-West Lake with respect to selected water quality parameters. Thirteen (13) stations in the South-West Lake and forty five (45) stations in the East Lake were sampled from 14.02.2005 to 17.03.2005. The mean values of the investigated parameters of South-West Lake and East Lake respectively are as follows: pH 9.3±0.5 and 8.8±0.5, electrical conductivity 0.35±0.03 ms cm1 and 1.8±0.4 ms cm-1, Secchi depth 14.5±1.5 cm and 12.5±1.6 cm, PO43--P (orthophosphate) 0.21±0.04 mg L1 and 0.52±0.2 mg L-1, NO3 -N 1.5±0.07 mg L-1 and 1.3±0.2 mg L-1, NO2-N 0.01±0.02 mg L-1 and 0.14±0.2 mg L-1,chlorophyll-a 0.4±0.1 mg L-1 and 0.59±0.2 mg L-1. All investigated parameters showed significant differences between the restored and non-restored sections of the lake during the period of study. According to the of Organization for Economic Cooperation and Development (OECD) Fixed Boundary System, the trophic status of both the South-West and the East Lake were found to be hypereutrophic. The level of hypereutrophy according to the Carlson Trophic State Index based on chlorophyll-a content and Secchi disc depth was significantly lower in the restored South-West Lake when compared to the non-restored East Lake. However, the restoration objectives with respect to chlorophyll-a and Secchi disc depth were not achieved in South-West Lake.