LITTORAL-PELAGIC PHOSPHORUS DYNAMICS DURING NIGHTTIME CONVECTIVE CIRCULATION

Littoral phosphorus dynamics and P exchanges between littoral and pelagic zones, driven by nighttime convective circulation, were examined in Eau Galle Reservoir, Wisconsin, during summer 1989. Rates of P release from littoral sediments, measured in laboratory incubation systems, increased linearly with pH under both aerobic and anaerobic conditions. Estimated rates of P release from littoral sediments, based on field measurements of pH and oxygen, averaged 3.6 mg m-2 d-1 under aerobic conditions and were in close agreement with independently determined diffusional fluxes across the littoral-sediment interface. Marked vertical gradients in P developed during summer in bottom waters of the littoral zone. The littoral zone cooled more rapidly at night than the pelagic zone on 72% of midsummer days (June-August), promoting nighttime convective circulation. On the basis of patterns of dye dispersion during these periods of circulation, cooler littoral bottom water moved into the pelagic zone as an interflow confined to the base of the epilimnion, while warmer pelagic water moved into the littoral zone as a surface flow. Hourly volumetric flow rates were linearly related to mean negative horizontal temperature gradients that developed at night. Net total P flux from the littoral to the pelagic zone ranged from 0. 12 to 0.43 Mg m-2 h-1 (avg. 0.22) for the summer. Nighttime convective circulation seems to be an important mechanism for movement of littoral P to the pelagic zone in this reservoir.