Silica, nitrate, total and dissolved phosphorus, and conductivity were measured during spring and summer in Lake Powell, Utah‐Arizona. Phytoplankton productivity was also determined. Conductivity is used as a tracer for delineating the advective influence of inflows from the Colorado and San Juan Rivers on nutrient delivery and distribution in the reservoir. High spring runoff (1,000–2,000 m3·s−1) enters the lake essentially as an overflow and dominates the nutrient cycle in the epilimnion. The interaction of advective nutrient delivery and high turbidity controls the distribution of phytoplankton productivity and nutrient depletion.

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A newly formed reservoir in the southwestern part of the United States was analyzed for man's impact on the eutrophication of the impoundment. The analysis of the 14C net productivity (mg '2C/m2 per day) indicated that the area studied was naturally eutrophic. Significant differences in net production were observed among the sites, as the area where man's recreational activities are highly concentrated had a significantly higher production rate than the other sites investigated.Mean monthly estimate of production for all the sites, and monthly and yearly estimates for the area studied are also included.

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