Stream size and human influences on ecosystem production in river networks

The quality and amount of productivity plays a central role in determining community structure and biogeochemical dynamics in ecosystems. Humans have altered river ecosystems in multiple ways that are likely to influence annual rates of ecosystem productivity and metabolism, but the net affects of such modifications on these processes are poorly known, especially at annual scales. An analysis of annual, whole ecosystem productivity for over 200 streams and rivers shows that human activities shift the amount, quality, and spatial distribution of production in river networks via enrichment of resources that limit autotrophic production. Human actions increased primary production to a greater degree than heterotrophic respiration, shifting streams (i.e., sites <100 km(2) watershed area) toward greater autochthony. Whereas human activities change multiple aspects of environmental conditions in streams, and often result in habitat degradation, reduction in biodiversity, and elevated contaminants, these analyses show that primary and secondary productivity of strongly impacted streams increased by over 600% relative to reference (pristine/lightly modified) sites due to increased resource availability and quality. A human-driven increase in autotrophic-based production is thus likely a major factor in ongoing global shifts in biodiversity, trophic dynamics and element cycling in streams.