Includes bibliographies.Blackwater lakelets in the south-western Cape are amongst the most darkly coloured humic waters in the world. In addition the aquatic invertebrate fauna of this region represents a relict and highly endemic group of the South Temperate Gondwanian fauna. The major environmental and biological variables were investigated over a period of fifteen months in six south-western Cape vleis (Suurdam, Gillidam, Sirkelsvlei, Grootrondevlei, Grootwitvlei and Rondevlei), which range in colour from a very dark brown to only slightly stained and vary in pH from 3.7 to 10.1. Using absorbance and fluorescence measurements and the Folin-Ciocallteu reagent, relative measures of the quantity and quality of humic compounds were obtained. Suurdam, Gillidam and Sirkelsvlei contained waters of greatest humic content (Suurdam mean A₂₉₀ = 6.561), with a common mid-winter to spring minimum; levels were lower in Grootrondevlei and Grootwitvlei (Grootrondevlei mean A₂₉₀ = 0.996), with a common mid-winter to spring maximum; and lowest in Rondevlei (mean A₂₉₀ = 0.284), with a slight peak in winter. Maxima were related to increased inflow of water with winter rain. Humic compounds in Suurdam, Gillidam and Grootrondevlei were of relatively high molecular weight and phenolic content, indicating an allochthonous origin. In Sirkelsvlei humic compounds were of lower molecular weight and phenolic content, possibly as a result of precipitation of the higher molecular weight fraction due to the high total salinity. Grootwitvlei and Rondevlei had lower molecular weight fractions probably as a result of autochthonous humic production, precipitation with calcium and greater rates of humic degradation; a higher molecular weight allochthonous fraction was present in winter in these two vleis. Buffering at low pH and the complexation of both iron and soluble reactive phosphorous were evident in Suurdam and Gillidam. Catchment geology, atmospheric precipitation, evaporation and the input of vertebrate excreta explain the inorganic chemical environments of the vleis. Acid, well-leached soils and calcareous sands resulted in mean pH values of 3.8 and 4.2 in Suurdam and Gillidam and 8.0 and 8.6 in Grootwitvlei and Rondevlei. The cation composition of the vleis on well-leached acid soils was primarily determined by the atmospheric precipitation of marine salt; evaporation and calcareous sands increased the salinity or relative calcium concentrations in some vleis. Animal excreta in Grootwitvlei and Rondevlei also alter the cation composition slightly, and increase the load of major nutrients. Nitrogen and phosphorous are largely of biological origin. Nutrient levels in the vleis are variously affected by marginal macrophytes, the sediments, primary production and the levels of humics. Chlorophyll a levels indicated low phytoplankton biomass in Suurdam, Gillidam and Grootrondevlei (Suurdam, mean chlorophyll a = 0.9ug1-¹ ) and no distinct seasonality; intermediate levels in Sirkelsvlei (mean chlorophyll a = 11.6ug1-¹ ) and high levels in Grootwitvlei and Rondevlei (Rondevlei, mean chlorophyll a = 60.4ug1-¹ ) displayed a common mid- to late-summer peak. Summer stratification was present only in Suurdam and Gillidam. Maximum phytoplankton biomass of the different vleis related to pH, the quantity and quality of the humic substances, and the nutrient loading. Chlorophyll b: chlorophyll a ratios and chlorophyll c: chlorophyll a ratios indicated a dominance of b- and c-containing species in Suurdam, Gillidam and Grootrondevlei and a dominance of species containing only chlorophyll a in Grootwitvlei and Rondevlei. Multi-dimensional scaling showed four distinct zooplankton community groupings. In Suurdam, the community was dominated by Microcyclops crassipes and was characterised by low zooplankton abundance (mean no.m-³ = 1783), a low species richness, an absence of limnetic cladocerans, and high species diversity (H') and evenness (J') indices. The community appeared to be limited by the low pH and the pH-dependent humic toxicity of the water. The communities in Gillidam, Grootrondevlei and Grootwitvlei were dominated by Metadiaptomus purcelli; otherwise that of Gillidam showed similar characteristics to that of Suurdam and was probably limited by the same factors. Both communities contained individuals of small mean size, more likely a result of limitation by the chemical environment rather than of predation pressure. A number of large-bodied limnetic cladoceran species and Lovenula simplex were present in Grootrondevlei, but absent from Grootwitvlei, possibly as a result of a visual predation pressure. In Sirkelsvlei the community was dominated by Metadiaptomus capensis and Lovenula simplex was present in lower numbers. Species richness, species diversity (H') and evenness (J') were low, with few limnetic cladoceran species present. High total salinity probably determines the community composition and seasonal variation. The eutrophic Rondevlei contains a community dominated by cosmopolitan and common Pan-Ethiopian species, in contrast to the endemic south temperate Gondwanian species of the other vleis. The community was dominated by Thermocyclops oblongatus, Brachionus calyciflorus and Brachionus rubens, and was characterized by greater species richness, and high species diversity (H') and evenness (J'). Total zooplankton abundances were much greater than in the other vleis (mean no.m-³ = 618944). Size-selective predation and the quantity and quality of the phytoplankton probably determine community composition. It is concluded that zooplankton diversity and abundance are influenced, both directly and indirectly, by the concentration and character of the humic substances and the pH, particularly at high concentration and low pH
