Tributary streams create spatial discontinuities in habitat, biological productivity, and diversity in mainstem rivers

Abstract

Lotic ecosystems are made up of numerous tributary streams forming a complex branching network. The point where smaller tributaries flow into larger rivers, or tributary junctions, may be sites in the network where spatial discontinuities or “hot spots” are created and maintained, because small streams funnel important materials captured from the surrounding landscape and carry them by gravity downstream. We hypothesized that habitat complexity, environmental productivity, and abundance of primary consumers and predators peak in mainstem rivers at or downstream of tributary junctions. We conducted surveys in three river basins and 13 reaches to examine interdependence between tributary streams and the larger rivers they enter. Wood abundance and volume, variability in median substrate size (i.e., substrate heterogeneity), concentrations of nitrogen and phosphorus in water, algal biomass, and abundance of consumers and predators peaked with a higher frequency at or downstream of tributary junctions. For several variables, the size of the tributary relative to the main stem contributed to the strength of tributary affect. These findings suggest that some tributary streams have fundamental effects on the larger rivers they enter. We argue that maintaining the integrity of connections among and between ecosystems is essential for promoting habitat complexity and community structure within river networks.