BOOTSMA, H.A., B. MWICHANDE, J. MWITA, E. ANDRÉ, S. HIGGINS, R. HECKY, M. VOLLMER & R. WEISS

Great Lakes WATER Institute, University of Wisconsin-Milwaukee, 600 E. Greenfield Ave., Milwaukee, WI 53204, U.S.A.

Fish, physics and farming: What controls nitrogen dynamics in Lake Malawi?

Unlike large temperate lakes, the African Great Lakes have low concentrations of dissolved nitrogen in their surface waters. As a result, nitrogen dynamics may play an important role in regulating plankton production and, ultimately, fish production. Using data collected by a number of research programs conducted over the past decade, we have determined the relative importance of various external nitrogen sources to Lake Malawi, and the significance of physical and biological processes as factors controlling the lake's internal nitrogen cycle. New inputs to the lake are in the form of river discharge and atmospheric deposition, which are almost equal in magnitude. A comparison of rivers indicates that catchments with intensive agriculture and sparse forests release much more nitrogen than forested catchments.
Nitrogen inputs to the epilimnion are greater than nitrogen sinking rates measured in sediment traps at the base of the epilimnion. This imbalance is likely the result of high denitrification rates in epilimnetic sediments. Denitrification in the sediments and in the anoxic hypolimnion represents a major nitrogen loss mechanism for the lake. Conversely, in nearshore waters periphyton nitrogen fixation rates are exceptionally high, and may make a significant contribution to the lake's nitrogen budget. Measurements of N and P recycling by fish suggest that the nearshore fish community may play an important role in promoting cyanobacteria dominance and benthic nitrogen fixation.