Interannual variation in discharge controls ecosystem metabolism along a grassland river continuum

Longitudinal patterns of ecosystem metabolism were measured in a 310-km grassland river continuum (Taieri River, New Zealand) during two consecutive summer-autumn periods with contrasting discharge (average 1993-1994 = 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-channel 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 natural communities of autotrophs (bankside light intensities greater than or equal to 250 mu mol . m(-2). s(-1)) was observed only in the headwaters 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 regions with oceanic climates, river discharge varies widely within and between years. In such regions autotrophic river reaches should extend 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.