Seasomal Studies of Phytoplankton Primary Production In Relation To Light and Nutrients in Lake Awasa

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Addis Ababa University


Phytoplankton primary production of Lake Avmsa was studied from Novembelj 1983 to Mnrch, 1985 by the oxygen light and dark bottle technique. Supporting data include water chemistry, light penetration, thermal characteristics and standing crop of phytoplankton. The solar radiation falling on the lake surface did not show pronounced variation apart from one low value recorded on a cloudy day. The lake is of fairly high transpOlrency with red and green the most penetrating components. There has been no indication of deep-seated thermal stratification in Lake Awasa apart from the superficial type of stratification which is generated by solar heating and destroyed by nocturnal cooling or wind-induced vertical mixing. The chemistry of the lake water is basically similar to other East African lake waters with sodium as the '. prec}(lntinant cntion [md bioarbonate+ a nrbonll1Jtt, .. " the predominant anion. The lake \'las found to have high concentration of silica ~.nd 101'1 concentration of phosphate and nitrate. Almost all nutrients were found to show some seasonal ''f ariations •. The algal crop is fairly high and \'las found to change seasonally. The vertic".l distribution of photosynthetic activity per unit water volume was of a typical pattern for viiphytoplankton with light inhibition on nll but the most overcast days, The threshold of inhibition at the lake -2 . surface was around 1500 f Em • Haximum net photosynthesis averaged 211r.6 mg0 ) -3-1 2 (66.95mgG m h and integral photosynthesis ran~;ed from 0.3 to 0.73 g 02 (0.094 to 0.226 gG) -2 -1 m h • The estimated velues of daily integral gross photosynthesis were in the range of 3.3 to 7.8 g 02 (1.03 to 2.4 g G) m-2d-1 • The correlation between gross photosynthesis and standing crop was poor though there was a general tendency for the maximum photosynthesis to increase with the standing crop. Though not pronounced, spatial (vertical) and temporal variations of phytoplankton standing crop and primary production were observed. The vertical va.riations appear to be the result of variation in the underwater radiation which is a function of the lake's transparency and amount of surface radiation. The observed terolporal variations in phytoplankton standing crop and primary production seem to be controlled more by nutrient supply than energy supply. The seasonality of rainfall seems, therefore, to play the pradominant role in determing the nutrient status and extent of algal growth and primary production.