1 Aquatic Ecology and Water Quality Management Group, Agricultural University, P.O. Box 8080 Wageningen, The Netherlands. E-mail: victor.langenberg@aqec.wkao.wau.nl
2 Department of Biology, University of Turku, FIN-20014, Turku, Finland

The effect of wind induced water movements on nutrients, chlorophyll a and primary production on Lake Tanganyika

We studied the spatial and temporal distribution of major plant nutrients, dissolved oxygen, chlorophyll a and primary production at three deep-water locations in Lake Tanganyika. Our purpose was to determine patterns of water displacements and to interpret the impact of wind-generated changes in nutrients on the timing and magnitude of phytoplankton production.
The onset of cooler, windier conditions during May-September caused stratification weakening, tilting of isotherms and upwelling of dissolved inorganic nitrogen and soluble reactive phosphorus from nutrient-rich deeper waters in the south. These factors coincided with an increase in chlorophyll a concentrations, but there was no definitive evidence of increased primary production. When a seiche occurred, it seemed associated with similar but less pronounced events at the other stations after cessation of the winds.

We suggest that seiching can spread kinetic energy throughout the lake. This will trigger an upward nutrient flux through mixing elsewhere but that the response of water quality to water displacements (in terms of nutrient supply to the epilimnion), seems to be governed by differences in both the strength in vertical temperature, nutrient gradients and dissolved oxygen and local bottom morphology. At relatively shallow north stations these gradients were strongest and closest to each other and throughout the year primary production rates, nutrients and chlorophyll a concentrations in the euphotic zone appeared to be in general higher than at the other stations.

We emphasise that the intra- and inter-annual differences between the strength in lake stratification and yearly wind regime found are very important in determining seiching strength and duration that will consequently affect the extent of internal nutrient loading and will play an important role in the lake trophic status.