The extent and type of vegetation within watersheds are critical factors influencing stream water chemistry, specifically nitrogen and phosphorus concentrations. As stream productivity can be limited by nutrient supply, nutrient subsidies from red alder, a nitrogen fixing species prevalent in the Pacific Northwest of North America and other temperate climates, may influence the degree of nutrient limitation in streams. Our study was designed to determine the extent that algae are nutrient limited in three red alder and three coniferous forested second and third-order streams on the Olympic Peninsula, Washington, U.S.A. Nutrient diffusing substrates were used in each stream to test whether nitrogen and phosphorus additions at three concentrations (0.05 M, 0.1 M and 0.5 M) increased chlorophyll a concentrations compared to non-nutrient infused substrates. With the addition of nitrogen or phosphorus, three of six streams (two conifer and one alder stream) had significantly higher chlorophyll a concentrations relative to controls. Within each stream, we saw no difference in chlorophyll a concentrations between nutrient type (nitrogen or phosphorus) or concentration (0.05 M, 0.10 M or 0.50 M). Treatments increased chlorophyll a concentrations in alder streams to twice that of conifer streams. We speculate that other factors, such as algae community composition, contributed to differing algae production in alder and conifer streams in response to nutrient supplements.
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