Effects of experimental stream diversions on algal-grazer dynamics

Background/Question/Methods Alteration of flow regimes is among the most serious threats to the ecological sustainability of streams and their associated communities. Water is being extracted from natural watercourses in growing quantities worldwide to meet basic human needs even as it becomes increasingly clear that water removal changes the way streams look, act and function. Decreased benthic current velocity resulting from reduced flows can alter the ability of stream grazer to remove benthic algae, and this has the potential to alter algal accrual, an important ecosystem process. To examine the effect of water abstraction on algal-grazer dynamics, we experimentally diverted 4, first order streams located at the Rocky Mountain Biological Laboratory in Gothic, CO. Benthic samples of macroinvertebrates and algae were collected from streams prior to diversion, and areas upstream of the diversions served as reference sites. During 4 weeks in July and August 2008, we diverted approximately 85% of the water from 300 m sections in each stream. Benthic current and flow were measured at the beginning and end of the experiment, benthic macroinvertebrate and algal samples were collected weekly, and temperature was logged hourly.

Results/Conclusions After 4 weeks of diversion, macroinvertebrate richness declined relative to pre-diversion values (repeated measures ANOVA, P < 0.05), and also in comparison the upstream reference sites (t-test, p< 0.05). Macroinvertebrate abundance, of which grazers comprised > 90%, was not different and actually increased in some diverted stream sections (t-test, p< 0.05). Despite the higher number of grazers, algal abundance (measured as chlorophyll-a) showed a 2-fold increase in diverted sections. Peak temperatures in diversions were greater by 5 degrees C, sedimentation increased 4-fold, and benthic current was slower by half. We speculate that two factors contributed to greater algal abundance in diverted reaches: higher temperatures and decreased grazer efficiency. Previous work has shown that grazers may actually promote algal growth at slower benthic current, and removal may have been hampered by increased sedimentation.