The response of benthic algal biomass to nutrient addition over a range of current speeds in an oligotrophic river

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Current speed could affect algal responses to river eutrophication and flow regulation via its influence on nutrient availability to benthic algae. In laboratory experiments, the thickness of the diffusive boundary layer decreases with increasing current speed, enhancing rates of nutrient mass transfer by molecular diffusion across the layer to the algal cell wall. In rivers and streams, this phenomenon may be masked by grazing and physical losses. We used a 10-wk field experiment to test the hypothesis that the response of benthic algae to nutrient addition was a function of current speed. We deployed clay pavers at the beginning of a 3-mo period of baseflow in the Daly River (tropical Australia) at 5 sites (current speeds <2–98 cm/s). After a 5-wk colonization period, we added nutrients to treatment sites via slow-release fertilizer pellets. After 5 wk, we measured chlorophyll a, algal composition, and macroinvertebrate grazer abundance and composition, nutrient concentrations, and current speed. We measured algal biomass response (BRR) as the ratio of biomass in the nutrient-addition treatment to biomass in the control treatment. BRR increased linearly with current speed between 27 to 98 cm/s and accounted for 99% of biomass variation. At <2 cm/s BRR did not conform to this relationship. The dominance of filamentous chlorophytes, oligotrophic conditions, and weak grazer effects probably contributed to the strong relationship between current speed and BRR. The effect of nutrient pollution on riverine benthic algal biomass could be greatest where current speeds are highest, and flow regulation that reduces current speed could reduce nutrient availability. Algal composition and physiognomy, grazing, and physical losses could reduce this effect. Current-mediated nutrient availability and patchiness of nutrient-driven bottom-up control of benthic algal biomass may influence algal responses to eutrophication and flow regulation.