A major decrease in air temperature with an increase in wind stress created thermal instability resulting in overturn. Surface oxygen values decreased from 12.6 (170% saturation) prior to overturn to 0.4 mg . 1-l (5.7%) after overturn. Recovery of surface oxygen concentrations to supersaturation took approximately three months. Changes in the nitrogen balance after overturn indicated that extremely high rates of nitrification occurred. Little change in phosphorus concentration was observed. Surface chlorophyll concentrations decreased from 235 to approximately 2 mg . m-3, resulting in very low primary production values (4Yl,X = 12.4 mg C m-3-h-l). Reduced rates of primary production were maintained for several months after overturn, mainly owing to the low ratio of Z,,/Z, (0.17). No significant changes occurred in the total bacterial population other than a redistribution within the water column. Significant changes occurred in the distribution and density of the zooplankton population. No fish kills were observed although fish populations were oxygen-stressed.

The incorporation of [methyl-3H]thymidine into three macromolecular fractions, designated as DNA, RNA, and protein, by bacteria from Hartbeespoort Dam, South Africa, was measured over 1 year by acid-base hydrolysis procedures. Samples were collected at 10 m, which was at least 5 m beneath the euphotic zone. On four occasions, samples were concurrently collected at the surface. Approximately 80% of the label was incorporated into bacterial DNA in surface samples. At 10 m, total incorporation of label into bacterial macromolecules was correlated to bacterial utilization of glucose (r = 0.913, n = 13, P < 0.001). The labeling of DNA, which ranged between 0 and 78% of total macromolecule incorporation, was inversely related to glucose uptake (r =-0.823), total thymidine incorporation (r =-0.737), and euphotic zone algal production (r =-0.732, n = 13, P < 0.005). With decreased DNA labeling, increasing proportions of label were found in the RNA fraction and proteins. Enzymatic digestion followed by chromatographic separation of macromolecule fragments indicated that DNA and proteins were labeled while RNA was not. The RNA fraction may represent labeled lipids or other macromolecules or both. The data demonstrated a close coupling between phytoplankton production and heterotrophic bacterial activity in this hypertrophic lake but also confirmed the need for the routine extraction and purification of DNA during [methyl-3H]thymidine studies of aquatic bacterial production.

No abstract

The number of metabolically active bacteria was measured with nalidixic acid over two annual cycles at three depths in the epilimnion of hypertrophic Hartbeespoort Dam, South Africa. Concurrent measurements were made of water temperature, DOC, phytoplankton production of dissolved (EDOC) and particulate organic carbon, chlorophyll a and the uptake of glucose (Vma,,). The objective was to determine the dominant factors correlated to the number of metabolically active bacteria and the relationship between active bacterial numbers and heterotrophic activity.The number of active bacteria was usually highest at the surface and ranged between 0.70 and 6.82 x 106 cells ml-' . The dominant factors correlated to the number of bacteria at the surface were water temperature (r = 0.65, n = 54, p<0.001), primary production (r = 0.53, n = 51, p < 0.001) and EDOC (r = 0.37, n = 45, p = 0.005). Surface Vm,, for glucose ranged between 0.11 and 4.0 ItgC 1-1 h -1 and was positively correlated to the number of active bacteria (r = 0.61, n = 53, p < 0.001). The specific activity index (10-12 gC cell-' h-l) varied between 80 and 2290 at the surface and was most strongly correlated to EDOC (r = 0.70, n = 48, p < 0.001). Relationships between active bacterial numbers, water temperature, phytoplankton activity and glucose uptake were also found at two additional depths within the epilimnion. These data suggest that bacterial populations in nutrient enriched lakes contain a large number of metabolically active cells with high individual activity as a result of enhanced phytoplankton growth.

The study provides a 2 .5 year record of Rhenosterkop Dam (KwaNdebele, South Africa) plankton population dynamics and production in relation to physical and chemical changes which occurred during the trophic depression and stabilization phases of the reservoir . The mean volume of the reservoir was 4 % of full storage capacity . Water temperatures ranged from 14 ° C to 27 'C . Due to inorganic suspensoids, the euphotic zone averaged 2 .6 m . An anaerobic zone developed each summer . The nitrogen, soluble reactive phosphorus (SRP) and silica concentrations did not display a seasonal pattern, but the latter two nutrients declined over the study . The dominant phytoplankton group was the cryptophytes while the zooplankton population was dominated by crustaceans . Chlorophyll a concentrations ranged from 1 .1 to 27 mg m-3 and were positively correlated to silica and SRP concentrations and inversely with NH 4-N concentrations . Primary production ranged from 22 .6 to 375 mgC m -2 h-' ; changes in A,,, a, were positively correlated to silica and SRP concentrations . Total zooplankton dry weight biomass varied from < 0 .5 to > 4 mg I -' . Annual zooplankton (secondary) production was 8 to -15 gC m -3 a -'; both primary and secondary production were greatest in the first 12 months of study and remained at low levels for the remainder, similar to the trends for silica and SRP . The data indicate that the reservoir shifted from eutrophic to mesotrophic during the study, typical of events in new reservoirs, and that changes in the plankton populations were largely the result of changing nutrient concentrations .Introduction 1990). Groundwater resources are limited and the surface waters are relied upon for urban, indusThe majority of water resources in southern Aftrial and agricultural purposes . Irrigation is the rica are riverine and south of the Zambezi River major use of surface waters in this region, adding there are very few natural lakes (Allanson et al., to the load placed on limited surface resources