Microscale measurements reveal contrasting effects of photosynthesis and epiphytes on frictional drag on the surfaces of filamentous algae

Filamentous algae are widespread in freshwater ecosystems worldwide with a significant presence in streams, rivers and lakes with sufficient light and nutrients. Although typically not a preferred food source for grazers, dense filamentous mats provide surfaces for epiphytic microorganisms that are more palatable, thus adding to stream productivity. We tested the hypothesis that epiphytes change velocity gradients, and consequently shear stress and skin friction drag, near the surface of algal filaments. Using both digital holography and particle image velocimetry to measure micrometre-scale velocity fields, we found that the surface shear stress on filamentous algae was much greater when the algae were actively photosynthesising. The presence of attached diatoms significantly reduced surface shear stress, while those filaments were photosynthesising, compared with bare filaments. A nutrient flux model, based on boundary layer thickness and surface shear stress, predicts that nutrient flux to the surface of a photosynthesising filament under measured flow conditions will be 1.5 times greater than for a preserved (i.e. dead) filament. Modelled nutrient flux to filaments with epiphytic assemblages dominated by diatoms is 75% of the flux to bare filaments under similar flow conditions. The proposed positive feedback between photosynthesis, surface shear stress and nutrient flux could be an important biophysical mechanism that overcomes diffusion limited nutrient supply within dense algal mats, enhancing algal survival through increased nutrient flux to actively photosynthesising filaments and decreased sloughing risk for filaments with lower rates of photosynthesis (due to epiphyte coverage or other light-limiting factors).