Primary production and grazing in freshwater and intertidal reaches of a coastal stream, Southeast Alaska

Abstract

During summer 1981, primary production of periphyton and grazing by macroinvertebrates were measured in Porcupine Creek, a small stream in Southeast Alaska. The intertidal reach was in a meadow primarily of sedge (Carex sp.); the freshwater reach was in a mature forest of western hemlock (Tsuga heterophylla) and Sitka spruce (Picea sitchensis). Daily net primary production was 50% greater in the intertidal reach (mean = 486 mg C·m‒2) than in the freshwater reach (mean = 324 mg C·m‒2). In the freshwater reach, periphyton had high biomass (mean = 7.8 g C·m‒2; 95% C.I. = 4.9–10.7 g C·m‒2) and low production: biomass ratio (0.04), and shade of the forest canopy limited primary production. In the intertidal reach, periphyton had low biomass (mean = 2.0 g C· m‒2; 95% C.I. = 1.3–2.7 g C·m‒2) and high production: biomass ratio (0.24), and grazing macroinvertebrates limited primary production by reducing the biomass of periphyton. Mean daily net primary production in the intertidal reach could supply about 3.8 kcal·m‒2·d‒1 and satisfy 200% of the daily energy requirement for maintenance and activity of the population of macroinvertebrates in the reach, primarily the isopod Gnorimosphaeroma oregonensis and amphipods Paramoera columbiana and Eogammarus confervicolus. Autochthonous primary production by periphyton is probably the dominant source of energy in summer for secondary producers in the intertidal reach of Porcupine Creek.