National Agricultural Research Organisation (NARO), Fisheries Resources Research Institute (FIRRI), P.O. Box 343, Jinja, Uganda. E-mail:firi@infocom.co.ug

Heterogeneity of planktonic nutrient status and algal biomass in Lake Victoria (E.Africa)

Phytoplankton in Lake Victoria are nitrogen (N) and phosphorus (P)-deficient according to the sestonic indicators of nutrient status (particulate N: P, C:P and C:N ratios). Nitrogen and phosphorus deficiency are more common in inshore than in offshore surface waters of the lake. Overall, nitrogen and phosphorus deficiency are present when the lake is thermally stable(September-April) and are absent when the lake is deeply mixing (June-July). In inshore, phosphorus rather than nitrogen deficiency has a stronger control of phytoplankton biomass development, as N-fixers can draw on the atmospheric nitrogen source when the light climate is good. In offshore, phytoplankton is less constrained by nitrogen and phosphorus availability, as N and P-deficiency are absent most of the year. Despite the absence of nutrient deficiency, phytoplankton biomass (as chloropyll-a) and particulate nutrient concentrations were consistently lower offshore. This is a result of light limitation, caused by the deeply mixed water column most of the year. Consequently, total phosphorus (TP) and total nitrogen (TN) are only weakly correlated with chlorophyll-a in Lake Victoria. These weak chl-a-TP and chl-a-TN correlation suggests that further phosphorus and nitrogen input into Lake Victoria may not result in further increases in algal biomass.