Stream metabolism consists of two fundamental processes, primary production and ecosystem respiration which, collectively, produce and transform energy and define the basal food web structure in lotic ecosystems. Standard assessments of stream metabolism measure changes in dissolved oxygen in rivers to infer metabolic rates at a whole-system scale. This method provides insights on overall river health, but a focus on large scales cannot elucidate factors that drive variations in production and transformation of energy among rivers or locales. These factors may operate at smaller scales but are ignored by a scale-blindness that limits our understanding of fundamental processes driving metabolism in ecosystems.
We present a review and synthesis of theoretical frameworks (River Continuum Concept, Flood Pulse Concept, etc.) used to conceptualise metabolic processes in rivers. Specifically, three knowledge gaps are assessed: (1) Lack of a unifying framework – competing stream metabolism theories oversimplify the metabolic behaviour of lotic systems, with spatially focused frameworks that impose longitudinal gradients on energy dynamics that may not exist, and temporally focused frameworks that assume steady-state systems; (2) Spatial partitioning of energy dynamics within sites – partitioning between benthic and water column metabolism is neglected even though this can deliver valuable information on small-scale metabolic patterns and processes; (3) Sources of energy production and transformation – the role of biofilms in energy dynamics remains a black box, despite the potential to contribute fundamentally to energetic processes that underpin local food webs.
Our synthesis reveals an abundance of competing theoretical concepts, suggesting that they impede a unified understanding of the mechanisms driving stream metabolism at various scales. By combining spatial and temporal complexity, partitioning of benthic and water column metabolism, and introducing biofilm dynamics into theoretical frameworks, we may enable more holistic assessments of stream metabolism, its underlying processes, and the energetic outcomes for ecosystems.