ECOSYSTEM METABOLISM, DISTURBANCE, AND STABILITY IN A PREALPINE GRAVEL-BED RIVER

The extent of temporal variation in ecosystem metabolism was determine d in a 6th-order, gravel-bottomed, prealpine river (mean discharge 4.6 m(3)/s), which is subject to disturbance by bed-moving spates. Daily integrals of gross primary production and ecosystem respiration were m easured for 447 d between May 1992 and November 1994 based on single s tation diel oxygen curves. In 1993 gross primary production averaged 2 .5 g O-2 m(-2)d(-1) and ecosystem respiration averaged 3.5 g O-2 m(-2) d(-1) (maximum daily rates observed in summer were 12.6 and 11.7 g O-2 m(-1)d(-1)). During the investigation, 32 bed-moving spates (peak dis charge >28 m(3)/s) occurred. Bed-moving spates distinctly depressed pr imary production and ecosystem respiration. Spates shifted ecosystem m etabolism toward heterotrophy (decreased P/R) because gross primary pr oduction was more affected than ecosystem respiration. Recovery rates of gross primary production were high in summer and low in winter. Rec overy of ecosystem respiration showed less distinct seasonal variation . Energy flow through the system was calculated as the sum of gross pr imary 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 flo w, which ranged from 6.1 to 9.6 indicated that resistance to spates wa s small, at least after extended periods free of spates (5-8 wk).