A Seasonal Study on Phytoplankton Primary Production in Relation to Light and Nutrients in Lake Ziway (Ethiopia)
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Date
1988-06
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Addis Ababa University
Abstract
Primary production by phytoplankton of Lake Ziway, Ethiopia, was st.udied
from February. 1987 to Fehruary.1988 by the oxygen light-dark bottle
technique. Supporting information included data on thermal characteristics,
water chemistry, light penetration and meteorological conditions.
Isothermal or near isothermal conditions ~Iere fr~quently observed.
These were ensured by ~Iindy condition prevail ing on the large and
unsheltered lake surface. I'lell oxygenated water from top to bottom
provided evidence for frequent mixing.
In all the sampling periods sodium clearly dominated the cations and
carbooate-bic~rbD~Nte rlnmi~tQd the anions. The cationic proportion of the
lake followed the order Na » Ca > f1g > K \'Ihile its aniont.c proportion
fnl..l..owed the order HC03- + C03= » CD S04. The lake \'las found to have a
htgh concentratIon of siHca and low concentrations of phosphate ond
nitrate.
The pattern of underwater light penetration was similar in all the
sampling periods. The highest penetration occurred in the red, and the
10~lest in the blue spectral region. Solar irradiance incident on the lake
surface di d not va ry markedly except for the 10\'1 va lues record(:d on cloudy
days.
The maximum photosynthetic rates of bottled phytoplankton varied between
1640 and 4070 mg 02.m-3.h-1 and areal rates ranged from 288 to 1625 mg
02m-2h-l. A principal reason for high rates of photosynthesis WIIS the
combined effect of high chlorophyll .i!. content of the euphotic zone (81.1-
191.6 mg ChI lI.m- 2 ) and high photosynthetic capacity 1I1.0 - 22.5 mg 02
(mgChla)-1 h-I ) Efficiency of utilization of photosynthetically available radiation
(Ph.A.R), Q, ranged from 2.4 to 9.6 mmol 02 per Einstien Ph.A.R. incident
on the lake surface. An inverse relation bet\'leen efficiency and irradiance
~Ias observed.
The temporal variation in al gal biomass was more pronounced when
measured on a per unit area basis (cv R 23.9%), than when measured on a per
unit volume basis (cv = 10.1%). This \'las due to the marked variation in
the underwater light climate of the lake. High algal. biomass and sustained
product ion seem8d to be maintained more by nutrient recycl ing than by
nutrient input as the lake \'las of 10\'1 nutrient status through out the year.
Hydrographic (wat2r column structure) factors seemed to play an over
riding role in determining the rate of nutrient recycling, the lIndOrl'later
light climate, the nxtent of algal gro\'lth in the lake, and subsequently the
spatial (vertical) and temporal pattern of phytoplanktonic photosynthesis.
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Biology