Primary production rates, chlorophyll and phytoplankton biovolume were measured monthly from April 2003 to November 2004 in Lake Tana, a large tropical lake in the highlands of Ethiopia. The lake is characterised by low nutrient concentrations, and a low water transparency due to high silt load of the inflowing rivers during the rainy seasons (May-November) and daily resuspension of sediments in the inshore zone. The mean chlorophyll-a concentrations varied seasonally and ranged from 2.6 mg m -3 to 8.5 mg m -3 (mean: 4.5 mg m -3 ) in the offshore zone. Primary production was measured using the light-dark bottles technique. We incubated only at three depths, i.e. 0.6, 1.2 and 1.8 m. Therefore, we may have missed a substantial part of the depth production profile and probably also frequently missed P max . Gross primary production in the openwater averaged 2.43 g O 2 m -2 d -1 and ranged between 0.03 g O 2 m -2 d -1 and 10.2 g O 2 m -2 d -1 ; production was significantly higher in the inshore zone. The highest production rates were observed in the post-rainy season (Oct-Nov), which coincided with a bloom of Microcystis and higher chlorophyll levels. This seasonal high production is probably caused by a relatively high nutrient availability in combination with favourable light conditions. The gross primary production rates of L. Tana are among the lowest compared with other tropical lakes. This will be partly the result of our underestimation of gross primary production by often missing P max . Another cause is the oligotrophic nature of the lake in combination with its relatively low water transparency. The gross primary production per unit chlorophyll in the openwater zone was in the same range as in 30 other tropical lakes and reservoirs. The higher primary production in the inshore zone is probably the result of the daily water column mixing (Z mix ‡ Z t ) in this area, enhancing nutrient recycling. A large proportion of the annual primary production is realised in one of the four seasons only. This productive post-rainy season is relatively short (2 months) and therefore efficiency of transfer of matter between the first and second trophic level of the Lake ecosystem will be poor.

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Urbanization and human settlement in close proximity to the Ethiopian lakes are among the potential causes of changes in water quality and quantity. The drastic changes occurred into one of the Bishoftu crater lakes (Kilole) best exemplify this phenomenon. The purpose of this study was ecological assessment of Lake Hora using benthic and weed-bed fauna. Samples of benthic and weed-bed were collected monthly from September 2009 to March 2010 at 3 sampling stations (A, B, C), with a standard Ekman grab. Station A is in front of Ras Hotel, Station B is place of Irecha and station C was to the south crater of the lake. The benthic and weed-bed fauna of Lake Hora included a total of 6958 individuals within 27 taxa belonging principally to Copepod (2812) and Chironomidae (1460) and Ecdyonuridae (735). A high number of organisms were observed mainly at stations B and A (3198 and 2342respectively). The correlation result indicates that oxygen showed strong relation to benthic and weed bed fauna distribution and abundance. There were high number of individuals, taxa diversity, evenness and grate number of rare taxa of benthic and weed-bed fauna at stations A and B, but these stations were affected by the community around the lake area for different reasons (for example washing clothes, boat parking and others).However low density and abundance of macroinvertabrates at station C could be due to: low organic matter load at station C which was free of human interactions; steeply slope geographical setting of the profoundly and its catchment and low vegetation cover. The Family Biotic Index result for all the sampling stations was 7.55, according to Hilsenhoff Family Biotic Index this value is indicating likely severe organic pollution and very poor water quality in all sampling sites. As this research finding indicates Lake Hora needs protection management strategies to maintain its sustainable use.

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