Periphyton production in an Appalachian mountain trout stream

Abstract

Temporal production patterns for periphyton have been poorly quantified in small woodland streams. In this study periphyton production on natural substrates was measured in a second order, western Virginia trout stream and two of its tributaries. Measurements were taken about three times monthly from April 1977 through April 1978 excluding 74 days of inaccessibility due to snow. Primary production rates were estimated in situ using ¹⁴C assimilation in light and dark chambers equipped with circulating systems. Allochthonous input and physicochemical parameters were measured concurrently.

Annual mean photosynthetic rates at midday were 2.27 ± 0.37 SE, 1.65 ± 0.25 SE, and 1.37 .± 0.19 SE (n = 46) for Guys Run (mainstream), Piney Branch, and Glade Brook, respectively. No relationship between the irradiance and photosynthetic rates was observed. Highest photosynthetic rates occurred in early July following the full emergence of the shading forest canopy. Tests comparing primary production rates between variously shaded and unshaded stream sites suggested periphyton light saturation to be about 20-30% of full sunlight. Mean daily irradiance for each season was calculated from light monitored at stream sites and used to estimate seasonal periphyton production in each stream. Annual autochthonous production, on a flat m² basis, was 6.54, 4.10, and 3.71 g C/y for Guys Run, Piney Branch, and Glade Brook, respectively. Compared to allochthonous input the periphyton contributed 3% of the total energy budget of the main stream and about 2% for the tributaries. Of the total autochthonous production, 90 ± 1% occurred during late spring and summer for all streams. In summer when terrestrially derived detritus was poorest in quantity and quality, periphyton was producing a majority of its annual input. Current concepts, which consider contributions by periphyton to consumer production to be extremely small in low order woodland streams, are based upon annual energy budgets. Because of the seasonal timing and high nutritional quality of periphyton, its true importance in low order streams may be greater than typically indicated in gross annual budgets. The roles of periphyton production and allochthonous input need to be reevaluated and better resolved on a seasonal basis.