The photosynthesis-irradiance

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.

The effects of biomass accumulation and community age on the photosynthesis-irradiance response of periphyton were examined in developmental sequences from three stream locations that differed in canopy cover. Photosynthesis-irradiance (P-Z) curves for four developmental stages from each location were constructed from 14C uptake at six irradiances (22-1,100 pmol quanta m-2 s-l) in the laboratory. Photoinhibition in the periphyton from the two shaded sites declined with development, indicating that self-shading significantly influenced photosynthesis in older communities. Photoinhibition was absent, however, in all of the developmental stages from the high light site. Maximum photosynthesis (P,,,,,) and slope ((x) of the light-limited portion of the chlorophyll-specific light response decreased > 40% in developing periphyton at all sites. Significant negative correlations between biomass and biomass-specific P,,,,, suggested that light or metabolic substrates became progressively more limiting during development. Although there was little community-level evidence of shade adaptation during development (photosynthesis did not saturate at lower light intensities as development proceeded), periphyton understories appear to retain considerable photosynthetic potential despite significant shading by upper layers.

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No abstract