In 1976 streamside nutrient-enrichment experiments were conducted using wooden troughs. Tripling of the PO 4 -P concentration, with or without a similar increase of NO 3 -N, increased algal biomass on the troughs by 8 times after 35 days. Increasing NO 3 -N alone had no appreciable effect on algal growth. A sloughing of algal biomass in August 1976 is believed to have been due to the instability of the heavy algal mat on the troughs and to the very poor light conditions that prevailed throughout August. Visual observation indicated that the relatively heavy algal population in Carnation Creek rapidly declined concurrent with the decline in the troughs. The percentage of diatoms in the algal assemblage remained the same in all troughs, and Fragilaria vaucheriae replaced Achnanthes minutissima as dominant on the phosphorus enriched trough. No shift to green or blue-green algal dominated assemblages occurred despite alteration of the N:P ratio. The dynamics of species succession, distribution, and growth, with and without nutrient addition, are discussed.
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Stockner et al. (1978) studied this question.