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Capturing the Light Fantastic: Oceanographic and Tidal Influences on Intertidal Algae

Abstract

Seaweeds experience many challenges to their persistence in intertidal zone habitats. Their growth rates must exceed losses associated with a range of ecological and physiological factors including desiccation, herbivory and wave forces. Growth rates depend on an alga's ability to capture and process light to build carbon‐based molecules. We examined local (tidal height) and large (oceanographic) scale influences on algal photosynthetic efficiency and light climate, respectively. At the local scale, we combined periodic measurements of physiological state using PAM fluorometry with traditional demographic monitoring of a Postelsia palmaeformis, population over a tidal height gradient. Parameter estimates derived from rapid fluorescence‐irradiance curves were correlated with longer‐term ecological performance measures including growth rate, morphology, survivorship and reproductive output. At larger scales, we made continuous in situ measurements of chlorophyll fluorescence and light attenuation in the intertidal zone at six sites during 2001 and 2002. Light attenuation to the benthos was sharply reduced at sites when chl‐a fluorescence was high. Long‐term, large‐scale monitoring of intertidal zone chl‐a and macroalgal abundances documents that striking differences among sites are persistent and associated with oceanographic factors. The light saturation parameter and maximum photosynthetic rate calculated for several common intertidal macrophytes, along with published values of the irradiance needed to saturate their growth rates, suggest that underwater light levels may limit intertidal algal growth where phytoplankton blooms are common and persistent. We conclude that physiological stress associated with tidal and oceanographic factors contribute to macroalgal distributions.

Effects of Nutrients, Herbivory, and Depth on the Macroalgal Community in the Rocky Sublittoral

“…Depth gradients may generate variation in the relative importance of top-down and bottom-up control. Due to the zonation of algal and faunal species (e.g., Kautsky et al 1986, Kautsky and Kautsky 1989, Underwood et al 1991, Nielsen et al 2003 and the consequent differences in species identities between the zones, interspecific interactions may differ along the depth gradient. Light levels attenuate and water motion declines with increasing depth, causing reduced photosynthesis, reduced nutrient uptake rates (reviewed in Hurd [2000]), increased sedimentation, and increased herbivore density and grazing efficiency (e.g., Lubchenco and Menge 1978, Nielsen 2001, Robles et al 2001.…”

“…We further expected certain interactions of nutrients, herbivory, and depth: (H 3 ) Mesoherbivores may strongly reduce algal colonization at natural levels of nutrients, thus decreasing species richness; with increasing nutrient levels the effect of grazers on species richness will turn positive; at the highest nutrient level herbivores may be unable to control algal densities leading to a drop in species richness (Proulx and Mazumber 1998, Kondoh 2001. Grazing may be more efficient in deep water, where the environmental stress for grazers, in terms of wave action, is less severe and algae, due to reduced photosynthesis, can utilize nutrients less efficiently than close to the surface (Lubchenco and Menge 1978, Robles et al 2001, Nielsen et al 2003. Thus, (H 4 ) we expected the grazing effects on algal colonization to be more pronounced in deep than in shallow environment.…”