Interactions between the mat‐forming algaDidymosphenia geminataand its hydrodynamic environment

Abstract

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: advectiondominated transport of water-column nutrients to cells at mat surfaces, and diffusion-dominated transport from decomposing organic matter within mats, with minimal advective losses.

Hydrodynamic control of benthic mats of Didymosphenia geminata at the reach scale

“…This relationship provides further support for the observation that D. geminata is well adapted to living in high turbulent, fast flowing mountain streams and that this might be due to the influence of the mats themselves on controlling the near-bed hydrodynamics (Larned et al 2011). This relationship provides further support for the observation that D. geminata is well adapted to living in high turbulent, fast flowing mountain streams and that this might be due to the influence of the mats themselves on controlling the near-bed hydrodynamics (Larned et al 2011).…”

Responses of lotic periphyton to pulses of phosphorus: P‐flux controlled growth rate

“…, 1981). However, the multitude of factors affecting physical mixing processes within and between periphytic communities and the adjacent boundary layer (Larned et al. , 2011), would make generalisations about its significance problematic.…”

Ecological eustress? Nutrient supply, bloom stimulation and competition determine dominance of the diatom Didymosphenia geminata

“…This template must be modified for D. geminata in which increased biomass results not from cell division but from the prolific basal stalk production stimulated by nutrient limitation (Cullis et al., ; Kilroy & Bothwell, ; Whitton, Ellwood, & Kawecka, ). Stalk production due to nutrient stress may be unique to D. geminata where it may be related to phosphorus acquisition due to phosphatase production associated with stalk material, or mass transfer processes through placing the terminal cell higher in the water column (Bray, O'brien, & Harding, ; Ellwood & Whitton, ; Larned, Packman, Plew, & Vopel, ). Other species similarly create extracellular polymeric substances (EPS) such as mucilage and may also produce associated phosphatases, or accumulate storage granules under nutrient limitation (Bray et al., ; de Brouwer, Wolfstein, Ruddy, Jones, & Stal, ; Ellwood & Whitton, ).…”