@ 1980, by the American Society of Limnology and Oceanography, Inc. Advective control of nutrient dynamics in the epilimnion of a large reservoir1

Ah true t Silica, nitrate, total and dissolved phosphorus, and conductivity were measured during spring and summer in Lake Powell, Utah-Arizona. Phytoplankton productivity was also de-tcrmined. Conductivity is used as;L tracer for delineating the advective influence of inflows from the Colorado and San Jtlan Rivers on nutrient d&very and distribution in the reservoir. High spring runoff (l,OOO-2,000 m3*s-l) enters the lake essentially as an overflow and dom-inates the nutrient cycle in the epilimnion. The interaction of advective nutrient d&very and high turbidity controls the distribution of phytoplankton productivity and nutrient depletion. The cycling of nutrients in most lakes is controlled primarily by internal pro-cesses (cf. Wetzel 1975; Likens 1975). Regeneration of nutrient levels to the trophogenic zone of lakes depends on wind-induced mixing or diffusion through the water column during periods of little or no density stratification. Reservoirs, however, are often very different from natural lakes in their hydrological cycles (Wright 1967; Wunderlich 1971). The hy-drology o.f a reservoir can influence its nutrient dynamics through reservoir mor-phometry, inflow-outflow regimes, depth of discharge, and density stratification characteristics. Wright’s (1967) hypothe-sis that deep discharge reservoirs will have nutrient cycles opposite to those of natural lakes (i.e. a progressive loss of nutrients) has been supported by recent work. Martin and Arneson (1978) com-pared nutrient export from a deep dis-charge reservoir and surface discharge lake in Montana where the reservoir dis