Seasomal Studies of Phytoplankton Primary Production In Relation To Light and Nutrients in Lake Awasa
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Date
1985-10
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Addis Ababa University
Abstract
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.
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Biology