The study on 10 lakes within the Ethiopian Rift Valley during March-May 1991 covered a range of conductivity (K 25 ) between 286 and 49100 /S cm '. HCO 3 -CO32-and Na + were the dominant ions in all the lakes. Concentrations of K', Cl-and SO4-increased with increasing salinity and alkalinity, whereas Ca 2+ and Mg 2 + decreased. Comparison of these data with previous records showed that a ten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and about three-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were generally high (12-222 mg SiO 21 -1) and increased with increasing salinity, except for Lake Chamo which showed SiO 2 depletion (to < 1 mg SiO21 -1) over the past three decades.The relationship between ionic concentration and phosphorus was irregular although high phosphorus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler (1974) chlorophyll a -total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo are phosphorus-limited, whereas others have surplus phosphorus.

Bacterioplankton abundance, [3H]thymidine incorporation, 14CO2 uptake in the dark, and fractionated primary production were measured on several occasions between June and August 1982 in eutrophic Lake Norrviken, Sweden. Bacterioplankton abundance and carbon biomass ranged from 0.5 x 109 to 2.4 x 109 cells liter-' and 7 to 47 ,ug of C liter-', respectively. The average bacterial cell volume was 0.185 ,um3. [3H]thymidine incorporation into cold-trichloroacetic acid-insoluble material ranged from 12 x 1012 to 200 x 10-12 mol liter-1 h-1. Bacterial carbon production rates were estimated to be 0.2 to 7.1 p.g of C liter-' h-1. Bacterial production estimates from [3H]thymidine incorporation and 14C02 uptake in the dark agreed when activity was high but diverged when activity was low and when blue-green algae (cyanobacteria) dominated the phytoplankton. Size fractionation indicated negligible uptake of [3H]thymidine in the >3-pum fraction during a chrysophycean bloom in early June. We found that >50% of the 3H activity was in the >3-p.m fraction in late August; this phenomenon was most likely due to Microcystis spp., their associated bacteria, or both. Over 60o of the 14CO2 uptake in the dark was attributed to algae on each sampling occasion. Algal exudate was an important carbon source for planktonic bacteria. Bacterial production was roughly 50% of primary production.

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Models describing the dependence oflake trophic state on external phosphorus loading are of two general types: empirical, steady state models derived from statistical treatment of data from large numbers of lakes, and theoretical, dynamic models based on more or less detailed mathematical descriptions of kinetics of nutrient and population dynamics.In the former type of models, inlake P concentrations have been found to depend mainly on inflow P concentrations and on water residence time. Efforts to explain the remaining variance by other factors such as iron, calcium, alkalinity, etc. have generally not been successful. Relations between lake P concentrations and chlorophyll may be useful for the prediction of maximum chlorophyll levels with some confidence but large variations in the actual chlorophyll concentrations are caused by, e.g. zooplankton grazing, varying chlorophyll content of the algae and other limiting nutrients.Dynamic, theoretical models are more suited for the prediction of rates of response to perturbations than the empirical steady state models. Due to the much larger demand for input data and labour investment, these models are generally more suited for the scientist as tools for formalizing and testing hypotheses, than for the water manager. Simple models with flexible structure generally have advantages over complex models.Both empirical and theoretical models are potentially powerful tools in eutrophication research and management. It is important for both scientists and water managers to select the optimum complexity with regard to the problem to be solved and to the resources available in each case.

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