Biological Ramifications of Spatial Discontinuities Created by Tributary Junctions

A variety of aquatic species require access to multiple linked habitats to grow, survive and reproduce. In river systems, there is also the simple concept that processes occurring upstream can influence downstream habitat via the transport of water, sediment, organic matter, and nutrients. Thus, maintaining habitat linkages is essential for maintaining ecosystem structure and function through a variety of mechanisms.  We examined the importance of physical linkages in three river systems over three years by conducting intensive surveys around 13 tributary junctions. We hypothesized that tributary streams would increase habitat heterogeneity and productivity in the main stem rivers they entered thereby affecting insect and fish populations.  There was considerable among- and within basin variation in physical, chemical and biological characteristics. Some of the within basin variation was explained by tributary junctions: we observed distinct discontinuities in main stem water temperature and limiting nutrients (nitrogen and phosphorus) at tributary junctions. Furthermore, we found that in some cases river biota responded to these discontinuities.  We also found that the effect of tributaries on main stem habitat was correlated to tributary size, with larger tributaries having a greater effect than smaller tributaries.  This study showed the importance of maintaining the physical linkages in river ecosystems and points out that maintaining these linkages may have important restoration and conservation implications.