Robert C. Carpenter, Department of Biology, California State University, Northridge, CA 91330-8303, USA.
Algal morphology can be affected by a variety of physical and biological factors including light, salinity, temperature, and herbivory. Water motion also has been demonstrated to alter morphology of large kelps. We have been investigating the role of water motion in altering the morphology of some smaller, subtidal macroalgae that live in kelp forest environments and how morphology mediates the interaction of the thallus with the physical environment to influence rates of metabolism. For at least two species of phaeophytes, morphology varies consistently between wave-exposed and wave-protected sites with a stunted, narrow-bladed morphology most common in the former and a more foliose morphology occuring in the latter. Rates of biomass-specific photosynthesis are higher for the foliose morphology suggesting a trade-off between morphology and metabolism that appears to be mediated by water motion. In tropical environments, coral reef algal turfs are comprised of several species of morphologically similar algal species that primarily are filamentous. We have demonstrated that the metabolism of algal turfs is related significantly and positively to water flow speed, consistent with the hypothesis that metabolism is limited by mass transfer of a compound across the boundary layer. Boundary layer dynamics are determined in part by the physical architecture of the algal turf that results from variable density, height, and diameter of algal filaments, which are in turn determined primarily by physical and biological disturbance. These results suggest that patchiness in the architecture of algal turfs will alter the local flow environment and have a significant effect on rates of algal metabolism and growth. The form and function of algae in both temperate and tropical environments are affected significantly by water motion. Water motion may be the cause of morphological variation and/or the flow environment may be altered by the physical structure of the algae. Models designed to explain and predict algal growth and morphology should incorporate water motion as an important factor influencing both form and function.