A baseline limnological study of Wagendrift Dam (Thukela basin, KwaZulu-Natal)

Abstract

Basic features of the physical and biological limnology of Wagendrift Dam, a moderately large impoundment on the Boesmans River, were studied between July 1989 and May 1990. This man-made like showed a typical monomictic pattern of summer stratification (November through April) and holomictic winter circulation (May to October). Water quality was chemically good (Department of Water Affairs and Forestry records), with little evidence of nutrient enrichment. Water clarity during the study was moderately low (mean SD and KdPAR values of 0.45 m and 2.55 m-1), with little evidence of persistent mineral turbidity suggesting reasonably satisfactory catchment conditions. In keeping with the low nutrient status, surface chlorophyll content remained below 5 μu l-1 throughout the study. in terms of ecological groupings, the phytoplankton was numerically dominated mostly by 'Competitive' green algae for most of the year. 'Stress-tolerant' cyanophytes were continuously sparse, but were marginally more abundant during summer stratification. Diatoms and other disturbance-tolerant ruderal' algae persisted throughout the year. No spring peak in ruderal algae was evident but it may have been masked by high zooplankton grazing pressure. An autumnal peak in ruderal algae coincided broadly with flood-related inflows and annual de-stratification. Zooplankton was dominated (numerically and/or gravimetrically) by typical clear-water taxa. The copepod Tropodiaptomus spectabilis and cladoceran Daphnia pulex along with various unidentified cyclopoids and rotifers were seasonally persistent while other cladoceran taxa (D. laevis and Diaphanosoma excisum) showed considerable seasonal periodicity. Zooplankton standing stock was collectively substantial, annually averaging around 0.65 g m-2 DM, and approaching 1 g m-2 DM in spring/early summer, implying potentially significant natural grazer-control of phytoplankton, aided perhaps by large numbers of benthic bivalve molluscs (Unio caffer), observed stranded in littoral margins following reservoir draw-down.