Print PageLotic currents regulate the spatial distribution of benthic algal biomass primarily through the opposing effects of drag which removes biomass, and its growth stimulatory effect through favouring nutrient uptake by reducing the thickness of the diffusive boundary layer that surrounds benthic algal cells. Nutrients also regulate the biomass of benthic algae, and in the absence of other limiting factors increased concentrations produce higher biomass, but whether the response of algae to higher nutrient concentrations is uniform across current speeds is unclear. We tested the interaction between current speed and nutrient concentration on algal biomass by conducting a nutrient addition experiment in a tropical Australian river during a long period of base flow. We tested the hypothesis that the response of algal biomass to nutrient addition, compared to controls, would be dependent on current speed. The experiment was conducted at 4 current speeds (0.27, 0.48, 0.78 and 0.98 m/s) on periphyton established on pavers. We also examined grazer macro-invertebrate communities to gain an insight into top-down control of algal biomass that may confound an interaction between the two bottom-up factors, current speed and nutrients.
Results/Conclusions
The benthic algal biomass (measured as chl a) response ratio (test/control chl a biomass) increased linearly with current speed, accounting for 99% of the ratio’s variation. Such pronounced explanatory power of current speed was attributed to the domination of filamentous chlorophytes across current speeds as well as test and control sites, the negligible effect of invertebrate benthic grazers, and oligotrophic conditions. The result of the experiment implies the interaction between current and nutrient concentration contributes to the spatial distribution of benthic algal biomass. It also implies the impact of nutrient pollution on benthic algae may not be uniform but dependent on current speed.
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