PHYSIOLOGICAL INDICATORS OF NUTRIENT DEFICIENCY IN CLADOPHORA (CHLOROPHYTA) IN THE CLARK FORK OF THE COLUMBIA RIVER, MONTANA1

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Abstract
Cellular nutrient concentrations and nutrient uptake rates of Cladophora glomerata (L.) Kuetzing were determined during summer and fall in 1989–1990 at a site on the upper Clark Fork of the Columbia River, Montana. Both physiological tests indicated that Cladophora growth is likely to be limited by nitrogen during late summer‐early fall. Maximum uptake rates of ammonia‐N and nitrate‐N were 5935–6991 and 507–984 μg · g DW−1· h−1, respectively, during July–October when dissolved inorganic nitrogen (DIN) concentrations in the river were less than 10 μg · L−1. During November‐December, when DIN was 72–376 μg · L−1, maximum ammonia‐N uptake was 1137–1633 μg · g DW−1· h−1 and maximum nitrate‐N uptake was 0–196 μg · g DW−1· h−1. Cellular nitrogen during summer–early fall was 0.78–1.80% of Cladophora dry weight, frequently at or below 1.1%, a level suggested as a critical minimum N concentration for maximum growth. In contrast, cellular P was 0.18–0.36% of dry weight, 3–6 times the suggested critical P concentration of 0.06%. Molar ratios of cellular N:P (< 16:1) and DIN: SRP (< 4:1) during late summer‐early fall also indicated potential N limitation. Cellular N and P from Cladophora collected from a second site influenced by a municipal wastewater discharge in 1990 displayed similar seasonal trends. At both sites, seasonal fluctuations in DIN were closely tracked by changes in cellular N, Cellular P, however, increased through the growing season despite declining levels of SRP in the river.

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