Theory holds that changes to lake ecosystem structure and function are intertwined with nutrient enrichment, while changes to stream ecosystem responses to nutrient enrichment are generally more reliant upon geophysical and hydraulic conditions independent of nutrient enrichment. Stream macrophytes may connect nutrient fluxes with the geophysical and hydraulic characteristics of the stream by altering streamflow variability and stabilizing sediments. Macrophytes may thus be capable of providing a link between nutrients and geophysical changes, and we thus hypothesized that a macrophyte-dominated stream experiencing nutrient enrichment, such as the Henry’s Fork River (ID), should express longitudinal patterns in physicochemical and biotic characteristics analogous to those of lakes across a gradient of nutrient enrichment alone. As autotrophic structure reacts to nutrient enrichment, the Henry’s Fork should display a longitudinal gradient of increasing then decreasing macrophyte coverage. Structural changes to autotrophs should precipitate parallel increase-decrease patterns in stream ecosystem complexity, including habitat complexity (proxy: variance in velocity), invertebrate diversity, food web complexity (proxy: proportion of predatory invertebrate taxa), and the proportion of nutrient flux as organic material. Understanding macrophyte-dominated river response to nutrient enrichment is important given anthropogenic nutrient loading and the increasing prevalence of macrophytes in streams due to increased river regulation.
Results/Conclusions
We found macrophyte coverage increased logarithmically downstream, with associated measures of hydraulic complexity—including velocity variance and stage shift—increasing with macrophyte coverage. This differs from expectations of parabolic increase-then-decrease patterns in macrophyte coverage with nutrients, as described for lakes. This could be because the Henry’s Fork is too shallow to experience algal turbidity-driven shading that can occur in lakes, or nutrient concentrations were too low to precipitate shading by epiphyton. Invertebrate metrics also did not follow expectations; the proportion of predators (a proxy for food web complexity) showed no significant trend, while invertebrate diversity logarithmically increased, and population size decreased longitudinally (i.e. along a gradient of increasing nutrient and macrophyte coverage). We conclude that the hydraulic effects of macrophyte growth interact with nutrient increases, but not in ways that are completely analogous to lake ecosystem regime shifts. Further study of macrophyte-dominated streams may provide insight into the relative importance of physical structure and nutrient enrichment in stream ecosystems.