Photosynthetic rates in four tropical African fresh waters

SUMMARY. Seasonal changes in photosynthetic rates of phytoplankton and underwater light in four Kenyan fresh waters (Lake Naivasha, Crescent Island Crater, Oloidien Lake and Winam Gulf) were measured in 1973 and 1974. Daily variations in water temperature were only slightly less than seasonal changes, and complete vertical mixing usually occurred each day. Well oxygenated water from top to bottom provided evidence for daily mixing.The vertical profiles of photosynthetic rates in the four lakes had the familiar feature of subsurface maxima. Of the photosynthetically available radiation (PAR), 1% coincided with the limit of detection of photosynthesis in Naivasha and Crescent Island Crater. The PAR at the depth of the maximal photosynthetic rate was higher for Naivasha (mean= 1.71 einsteins (E) m−2) than for the Oloidien and Crescent Island Crater (means = 1.23 and 1.20 E m−2 h−1, respectively).The maximal rates of photosynthesis (mg O2 m−3 h−1) were usually higher in Oloidien (260–750) and Winam Gulf (230–640) than in Naivasha (150–240) and were least in Crescent Island Crater (50–180). Chlorophyll‐a concentrations were directly related to photosynthetic rates. Because of the inverse relation between euphotic zone depth and maximal photosynthetic rates, the areal rates varied less among the lakes than the volumetric rates (i.e. 3.0–12.1 g O2 m−2 day−1).The efficiency of utilization of PAR expressed on a molar basis ranged from 6.1 to 1.9 mmol O2 E−1, but was usually 2–4 mmol O2 E−1.The seasonal changes over a 12‐month period in the areal photosynthetic rates ranged from a 3‐fold variation in Oloidien and Crescent Island Crater to a 1.5‐fold variation in Naivasha. In Winam Gulf over a 5‐month period a 2.3‐fold variation occurred.