Interactions between the mat-forming alga Didymosphenia geminata and its hydrodynamic environment

Algae require dissolved nutrients to grow, but nutrient concentrations are often very low in unpolluted streams and rivers. The supply of dissolved nutrients to benthic (bottom-dwelling) algae increases with increasing river flow, but higher flows also produce drag forces that can damage the algae. To persist under high-flow, low-nutrient conditions, benthic algae must balance the conflicting requirements of minimal drag (to minimize detachment risks) and maximal exposure to turbulent flow (to maximize nutrient supplies). We explored flow-algae interactions using the freshwater alga Didymosphenia geminata. D. geminata forms dense mats in swift, nutrient-poor rivers, and is of particular interest because it is a highly invasive species in North American and New Zealand. To identify the properties that help D. geminata grow rapidly and resist detachment, we transplanted cobbles with attached D. geminata mats from a river to a laboratory flow chamber and measured flow properties above and within the mats. We then removed the mats from the cobbles and repeated the measurements. We found that water flow near the streambed is strongly altered in the presence of D. geminata mats. The dense, interwoven D. geminata stalks produce high friction and reduce flow fluctuations around and within the mats. The reduction in flow fluctuations around mats may reduce the risk of detachment and help retain dissolved nutrients within mats. We also found that D. geminata mats increase the roughness of riverbeds relative to bare cobbles, which may increase the rate of nutrient supply from the water to mat surfaces. Benthic autotrophs in oligotrophic rivers must adapt to and modify their hydrodynamic environment to balance the conflicting requirements of minimal drag (to minimize detachment risks) and maximal exposure to turbulent flow (to maximize nutrient acquisition). We explored flow–organism interactions using the benthic, freshwater alga Didymosphenia geminata. D. geminata forms large mats in swift, oligotrophic alluvial rivers. The physical properties that allow D. geminata to resist detachment and proliferate under these harsh conditions are unknown. We transplanted cobbles with attached D. geminata mats from a riverbed to a flume and used velocimetry and microelectrode profiling to measure hydrodynamic and transport conditions above and within the mats over a wide range of flows. We then removed the mats from the cobbles and repeated the velocimetry measurements. Experiment results indicated that D. geminata mats reduce form-induced stresses and near-bed turbulent velocity fluctuations, which may reduce the risk of detachment. D. geminata mats also increase turbulent shear stress just above mat surfaces, which may enhance water column–mat solute exchange. High friction associated with flow at mat surfaces leads to very low velocities and predominantly diffusive transport within mats, which may in turn favor the retention of solutes derived from organic matter within and below mats. Enhanced mass transfer at mat surfaces and effective solute retention in mat matrices suggest a mechanism by which D. geminata cells acquire nutrients from different sources: advection-dominated transport of water-column nutrients to cells at mat surfaces, and diffusion-dominated transport from decomposing organic matter within mats, with minimal advective losses