The Effects of Morphology and Water Flow on Photosynthesis of Marine Macroalgae

We present observations and experimental results supporting the view that macroalgal morphology can represent a balance between maximizing photosynthetic ability and limiting susceptibility of the thallus to mechanical damage. The relationships among morphology, water flow, and photosynthesis were examined for five algal species representing three functional forms of marine macroalgae at Santa Catalina Island, California, USA. The relationship between surface area/volume ratio (SA/V) and rates of biomass-specific net photosynthesis (NP) across functional-form groups was examined to test the hypothesis that NP is related positively to SA/V. The rate of NP increased with increasing SA/V across functional groups. This relationship then was examined within a functional group for two species, Dictyopteris undulata and Zonaria farlowii, which are morphologically variable among sites. Flow measurements and morphometric analysis of thalli of D. undulata and Z. farlowii at two wave-exposed and two wave-protected sites indicated that intraspecific morphological variation corresponded to variation in wave exposure. A field experiment that manipulated rates of flow in situ resulted in morphological changes in D. undulata and Z. farlowii, indicating that water motion was responsible for the observed morphological plasticity. To examine the interaction between morphology and water flow on algal physiology, NP was estimated for morphotypes of D. undulata and Z. farlowii from wave-exposed and wave-protected sites. At saturating photon flux densities, NP was affected significantly by flow speed, particularly for morphotypes from the wave-protected site. For these morphotypes, increased flow speeds resulted in sustained, then decreased NP as flow speeds exceeded 17 cm/s, suggesting that water flow may affect NP at low flow speeds by mitigating mass transfer limitation and at high flow speeds by changing light interception and flow within the compacted thallus. The plasticity of morphologies and the interaction of these morphotypes with flow may represent a trade-off between maximizing NP and reducing the susceptibility of the thallus to mechanical damage and/or dislodgement by hydrodynamic forces.