Grazers and Nutrients Simultaneously Limit Lotic Primary Productivity

Abstract

Nitrate and phosphate were added to two of four flow through channels in a second-order stream in eastern Tennessee to raise nutrient concentrations to 3–4 times background, while two of four sets of colonized ceramic tiles in each channel were raised above the substratum to exclude grazing snails (Elimia clavaeformis). Snail grazing maintained a thin layer of periphyton dominated by Stigeoclonium basal cells, regardless of nutrient regime. Although nutrient effects on periphyton ash-free dry mass were statistically insignificant, nutrient additions significantly increased chlorophyll a, especially where snails were excluded. Snail densities were 89% higher in nutrient-enriched channels. Photosynthesis–irradiance data suggested that nutrient enrichment increased self-shading in the periphyton. Areal-specific productivity was simultaneously limited by grazing and low nutrient concentrations: snail exclusion and nutrient enrichment both increased productivity > 2 times. The negative effect of snails on areal-specific productivity was due to (1) reduction in biomass by cropping and (2) depression of chlorophyll-specific productivity. The means by which Elimia depresses chlorophyll-specific productivity is unclear, but the depression is clearly disadvantageous to food-limited grazers. Because Elimia was the dominant invertebrate, our results indicate that low nutrient concentrations limit secondary as well as primary production in autumn.

Trophic interactions in open systems: Effects of predators and nutrients on stream food chains

“…Their results matched the predictions of classical food-chain theory because increasing productivity caused higher biomass only at the top trophic level and at alternative trophic levels below it. Nutrient manipulations in simpler one-or two-level food chains in streams have, though, given rise to higher biomass at both trophic levels (e.g., Hill et al 1992;Rosemond et al 1993). Our results also differ from the pattern predicted by classical theory because increasing primary production by nutrient addition caused marked increases in both primary producer (algae) and primary consumer (baetid) biomass.…”

Positive effects of mesograzers on epiphytes in an eelgrass system

“…The consumers had created a community dominated mainly by grazing-resistant species, which as a trade-off had relatively low growth rates (Rosemond et al 1993, Hill et al 1992. We found a similar trend for Idotea grazing in the experiment with higher nutrient supply.…”

Stream periphyton response to grazing and changes in phosphorus concentration

“…Grazing by aquatic insects has been shown to have an overriding influence on periphyton in many situations, as evidenced by a decrease in biomass, an altered rate of primary productivity, and/or a change in taxonomic composition and community structure (Hunter, 1980;Kesler, 1981;Lamberti & Resh, 1983;McAuliffe, 1984;Cattaneo & Kalff, 1986;Jacoby, 1987;Lamberti et al, 1987;Steinman et al, 1987a;Steinman et al, 1987b;Hill & Knight, 1988;Feminella et al, 1989;McCormick & Stevenson, 1989;Steinman et al, 1991;Hill et al, 1992). Lack of suitable habitat for grazers was indicated to be an important factor allowing the development of dense mats of filamentous green algae, such as Cladophora, in some environments (Welch et al, 1992).…”