Experimental drought changes ecosystem structure and function in a macrophyte-rich stream

Abstract

Water abstraction continues to increase worldwide, causing periods with extreme low-flow in many streams, which will likely intensify in the future due to climate change. Extreme low-flow may have major effects on in-stream habitats, organisms, and consequently ecosystem functions. We investigated the effects of a 2 months experimentally induced extreme low-flow scenario on the physical, biological, and functional characteristics in a macrophyte-rich lowland stream using a before-after, control-impact (BACI) approach. We quantified nutrient dynamics, including inorganic nitrogen and phosphorus concentrations, ammonium uptake, and whole-stream metabolism. We found a significant decline in the stream wetted habitat area, an increase in water temperature, and an increase in the accumulation of fine organic matter with reduced flow, but no significant changes in dissolved oxygen or benthic chlorophyll a concentrations. Furthermore, the relative demand and overall uptake of ammonium was lower in the low-flow reach relative to the control reach, whereas the relative demand and uptake of phosphate were higher at low-flow. Our results demonstrate that low-flow conditions cause resource limitation in stream biota most likely due to increased thickness of the diffusive boundary layers and an enhanced heterotrophic activity in the accumulated fine organic matter. Our results imply that the basal resources for productivity shift from autotrophic towards more heterotrophic resources causing a shift at higher trophic levels towards more detritivore based and less herbivore based food webs with implications for the invertebrate community composition and the distribution of functional feeding groups. Based on the strong links observed between low-flow and nutrient uptake, we suggest that functional metrics are suitable to assess the effects of low-flow conditions in small streams.