HOW BENTHIC MACROALGAE COPE WITH FLOWING FRESHWATER: RESOURCE ACQUISITION AND RETENTION
Authors: Raven John A. 
Published: October 2004

The unidirectional water flow to which stream macro‐algae are exposed imposes mechanical strseses that vary with the life‐form of the algae. The stress, and thus the possibility of breaking or detaching with loss of previously accumulated resources, is least in the boundary‐layer forms (crusts, turfs) and greatest in the semierect forms (mucilaginous and nonmucilaginous filaments, and tissuelike thalli). Present evidence on the resource costs of providing semierect forms with the observed safety factor in minimizing the chance of breakage under normal flow conditions does not permit useful comparisons to be made with resource acquisition benefits of exposure to rapid water flow. These benefits relate to minimizing boundary‐layer thickness, with a consequent increase in the potential flux of nutrient solutes to the algal surface. Such enhanced fluxes are significant in habitats in which (as is often the case) algal growth is nutrient‐limited. By contrast, crusts and turfs have lower potential nutrient fluxes due to the thick boundary layers around the substratum to which they are attached. A possible advantage for resource acquisition of the thick boundary layers associated with crusts and turfs relates to the use of extracellular catalysis by the alga of the conversion of the supplied form of nutrient (e.g. HCO3‐, organic P, Fe3+) into the form taken up (CO2, inorganic P, Fe2+, respectively). A thick boundary layer restricts the loss of the transformed nutrient species to the bulk, rapidly flowing medium thus favoring uptake of CO2, inorganic P, or Fe2+. The influence of the stream on photosynthesis and growth via light supply involves shading by riparian vegetation and shading of low‐growing algae (crusts and turfs) by semierect algae as well as by semierect submerged bryophytes and tracheophytes. The shading of boundary‐layer forms by semierect plants further constrains the photo synthetic rate achievedin situby crusts and turfs. The negative effect of shading on productivity is mitigated by the generally low (less than 25% of full sunlight) photon flux density needed for photosyn‐thetic (and net productivity) saturation of stream macro‐algae, especially when nutirents are limiting. There is relatively little acclimation to changed total photon flux density and its spectral distribution within the natural range. Problems of upstream migration could be partially resolved by herbivorous motile animals as vectors. Despite potential problems for grazers on semierect algae due to rapid water movement, it is not clear that such algae are less grazed than crusts or turfs. Further work in certain areas could yield important results using currently available techniques. Examples are 1) the role of water flow per se (independent of boundary layer effects) on growth, 2) the allocation of resources to structural elements as a function of water flow, 3) the effect of flow on bidirectional transmembrane fluxes (influx and efflux measured with tracers) of nutrients (corrected for boundary‐layer effects), and 4) the role of animal vectors in upstream dispersal. Positive results in 1–3, that is, an effect of increased flow on growth, resource allocation, and transplasmalemma fluxes, could indicate a role for mechanosensitive (= stretch‐sensitive) ion channels and other membrane transport systems.