Ecosystem Metabolism, Disturbance, and Stability in a Prealpine Gravel Bed River

Abstract

The extent of temporal variation in ecosystem metabolism was determined in a 6th-order, gravel-bottomed, prealpine river (mean discharge 4.6 m³/s), which is subject to disturbance by bed-moving spates. Daily integrals of gross primary production and ecosystem respiration were measured for 447 d between May 1992 and November 1994 based on single station diel oxygen curves. In 1993 gross primary production averaged 2.5 g O₂ m^-2d^-1 and ecosystem respiration averaged 3.5 g O₂ m^-2d^-1 (maximum daily rates observed in summer were 12.6 and 11.7 g O₂ m^-2d^-1). During the investigation, 32 bed-moving spates (peak discharge >28 m³/s) occurred. Bed-moving spates distinctly depressed primary production and ecosystem respiration. Spates shifted ecosystem metabolism toward heterotrophy (decreased P/R) because gross primary production was more affected than ecosystem respiration. Recovery rates of gross primary production were high in summer and low in winter. Recovery of ecosystem respiration showed less distinct seasonal variation. Energy flow through the system was calculated as the sum of gross primary production and ecosystem respiration. Upper (maximum energy flow) and lower (resistant energy flow) boundaries were estimated, within which energy flow varied. The ratio of maximum to resistant energy flow, which ranged from 6.1 to 9.6, indicated that resistance to spates was small, at least after extended periods free of spates (5-8 wk).