INTERANNUAL VARIATION IN DISCHARGE CONTROLS ECOSYSTEM METABOLISM ALONG A GRASSLAND RIVER CONTINUUM

Longitudinal patterns of ecosystem metabolism were measured in a 310-k m grassland river continuum (Taieri River, New Zealand) during two con secutive summer-autumn periods with contrasting discharge (average 199 3-1994 = 76.8 m(3) . s(-1); 1994-1995 = 6.6 m(3) . s(-1)). Metabolism was estimated from diel oxygen curves using the single-station open-ch annel regression method. Ratios of gross primary production (<0.3-9.6 g O-2 . m(-2). day(-1)) to community respiration (0.7-9.8 g O-2 . m(-2 ). day(-1)) ranged to 6.5, while net daily metabolism ranged from -8.2 to 6.2 g O-2 . m(-2). day(-1). Only the upper 70 lan of the river was autotrophic during 1993-1994, while most of the river was autotrophic during 1994-1995. Light saturation of gross primary production by nat ural communities of autotrophs (bankside light intensities greater tha n or equal to 250 mu mol . m(-2). s(-1)) was observed only in the head waters during 1993-1994, but throughout the continuum during 1994-1995 . Patterns of gross primary production were controlled by fluctuations in water depth and turbidity that determined light availability. In r egions with oceanic climates, river discharge varies widely within and between years. In such regions autotrophic river reaches should exten d down continua during low-discharge periods, and retreat during high discharge. This conceptual model explains how hydrological variability may control longitudinal patterns of river metabolism.