No abstract

No abstract

Invertebrate zooplankton predators are generally less diverse in average species numbers in tropical than in temperate lakes and reservoirs . Predatory Copepoda which comprise the majority of limnetic predators are particularly low in species numbers in the tropics . Predatory Cladocera are confined to the North Temperate zone. Chaoborus appears to be cosmopolitan . Among Rotifera, only the cosmopolitan predator Asplanchna occurs in tropical waters while the other common limnetic carnivorous genus Ploesoma is restricted to higher latitudes . Hydracarina, and insects besides Chaoborus, are generally restricted to the littoral and appear to be more diverse in the tropics . Lakes Awasa and Zwai, Ethiopia, are almost devoid of predators in the limnetic, which is invaded by a littoral chydorid Alona diaphana . Low diversity of lake types and low production of tropical zooplankton could restrict predator diversity too . Very low diversity of invertebrate predators in the limnetic and higher diversity in the littoral may characterize tropical lakes in contrast to temperate lakes, which have more invertebrate predators in the limnetic and perhaps relatively less in the littoral . Tropical zooplankton in freshwaters, appears to be a very immature community . Hence opportunistic species can readily invade the limnetic and even dominate in isolated situations as has been shown for Alona davidi, Hydracarina and some other unconventional forms .

Lake Abijata lies in a shallow depression (maximum depth 8-9.5 m); the water is green with phytoplankton and it supports large fish and bird communities. Lake Shala lies in a deep caldera (maximum depth reputedly 260 m); phytoplankton is sparse and fish and bird communities scanty.Lakes Abijata and Shala, sampled in January, 1985, had conductivities of 14000 and 21000 microSiemens cml at 25 °C respectively, mainly due to high sodium, carbonate and chloride ions. Calcium concentrations are very low.The benthic fauna was studied with an Ekman grab to a depth of 8.5 m in Abijata and 15.5 m in Shala and was found to be dense in both lakes but varying greatly in composition at different depths. In Abijata the benthos consisted mainly of Ostracoda and Chironomidae, and in Shala mainly of Tubificidae, Ostracoda and Chironomidae. There were very few Nematoda. No true halophilic species were found but the community consisted of euryhaline forms found also in non-saline waters. Predatory invertebrates were absent and many of the dominant species, notably of the Chironomidae, were different from those of non-saline lakes nearby.

Koka Reservoir in the Ethiopian Rift Valley (altitude about 1600 m) appears to be similar to natural lakes in the region. Its water is turbid because of suspended inorganic material. At the time of measurement it was supersaturated with oxygen to a depth of about 8 m, and displayed a pronounced conductivity stratification, due probably to the incomplete mixing of two inflows. The phytoplankton was dominated by Microcystis. The zooplankton was of low diversity but abundant. There was a large population of benthic invertebrates.Finchaa Reservoir in west central Ethiopia (altitude about 2200 m) is very different. It covers the former Ch'omen Swamp and so contains many floating islands and large quantities of decomposing vegetation, causing undersaturation with oxygen even at the surface. The water is somewhat turbid owing to the presence of organic debris, of low conductivity, slightly acidic, and apparently calcium-dominated. A species of Microcystis was the most abundant phytoplankter. The zooplankton was sparse and of very low diversity. There were almost no benthic invertebrates.

No abstract

1. Quantitative benthic samples were collected monthly between June 1984 and May 1985 from seven stations at various depths in Lake Zwai, one of the Ethiopian Rift Valley lakes, situated in the northern part of the Rift Valley. The most common species of ostracods were Limnocylhere thomasi thoma.si Martens (a subspecies endemic to Lake Zwai), Gomphocylhere angulata Lowndes (common in, but restricted to. East Africa) and Darwinula stevensoni (Brady & Robertson) (cosmopolitan).2. Absolute densities of all species varied considerably over the year. In all stations, there were population peaks during the two dry seasons, followed by as yet unexplained population crashes during the short and the long rains.3. The three common species had different habitat preferences. Darwinula stevensoni avoids parts of the lake where temperatures rise high and both G. angulata and L. thomasi had a preference for sheltered habitats close to vegetation and/or for a specific grain size of the sediment (300-350

Consensus-building univariate and multivariate data analyses were used to identify patterns in space and time over seven years among 12 sampling stations in a 720 km2 hardwater Canadian lake (Lake Simcoe, Ontario, Canada). There were 15 copepods and 26 cladoceran species identified in samples collected throughout the May-October periods of 1986-1992 from Lake Simcoe. Eleven crustacean zooplankters accounted for 88% of the total average density of all recorded species in the lake. Most of these (the main exceptions being Eubosmina coregoni and Daphnia pulicaria) are ubiquitous taxa with wide environmental tolerances. Multivariate analyses of these data identified spatial differences and a temporal trend in community composition. The use of Cao et al.’s (1997a) "CY-dissimilarity" measure combined with Ward's Linkage clustering algorithm and non-metric multidimensional scaling ordination resulted in several clearly defined groups of sampling units (SUs), which apparently were separated predominantly on the basis of variables related to space (sampling station) and time (year). The 7-year record suggested several lines of evidence for trends in community structure. Averaged over all stations, the yearly intra-station dissimilarities steadily increased 78% by 1992. The greatest rates of increase in yearly intra-station dissimilarity with 1986 baselines were found for the deeper sampling stations. These whole community-level trends were the net results of a many different patterns of change identified in species-level analyses. Such patterns included increasing densities over the 7-year period of Daphnia longiremis and Diaphanosoma birgei and declining densities of Daphnia pulicaria. Many other taxa showed highest values in the middle years of the study. Community-level temporal trends were least evident at sampling locations in the shallower and warmer Cook’s Bay, despite clear trends in several individual species. A significantly greater representation by large-bodied zooplankton species in the latter years of the study was consistent with declining stocks of rainbow smelt (Osmerus mordax) and lake herring (Coregonus artedii), suggesting the associated relaxation of predation pressure as a possible cause. Several species indicative of eutrophic states (Acanthocyclops vernalis, Chydorus sphaericus and Bosmina longirostris) were common in the samples. The scarcity of the calanoid Senecella calanoides and the absence of another classic oligotrophic indicator calanoid, Limnocalanus macrurus in the Lake Simcoe samples suggests impairment of their deep, cold-water habitat and is consistent with the modelled historical loss of hypolimnetic dissolved oxygen in Lake Simcoe