Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awasa, Ethiopia

Abstract

The biomass and primary production of phytoplankton in Lake Awasa, Ethiopia was measured over a 14 month period, November 1983 to March 1985. The lake had a mean phytoplankton biomass of 34 mg chl a m−3 (n = 14). The seasonal variation in phytoplankton biomass of the euphotic zone (mg chl a m−2 h−1) was muted with a CV (standard deviation/mean) of 31%. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light inhibition on all but overcast days. The maximum specific rates of photosynthesis or photosynthetic capacity (Omax) for the lake approached 19 mg O2 (mg chl a)−1 h−1, with high values during periods of low phytoplankton biomass. Areal rates of photosynthesis ranged between 0.30 to 0.73 g O2 m−2 h−1 and 3.3 to 7.8 g O2 m−2 d−1. The efficiency of utilisation of PhAR incident on the lake surface varied from 2.4 to 4.1 mmol E−1 with the highest efficiency observed corresponding to the lowest surface radiation. Calculated on a caloric basis, the efficiency ranged between 1.7 and 2.9%. The temporal pattern of primary production by phytoplankton showed limited variability (CV = 21 %).