The light environment in a giant kelp forest: influence of Macrocystis pyrifera on spatial and temporal variability
- Cite this article as:
- Gerard, V.A. Mar. Biol. (1984) 84: 189. doi:10.1007/BF00393004
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Quantum irradiance was measured at several depths in forests of the giant kelp Macrocystis pyrifera (L.) C. Agardh in southern California, USA, during the summer of 1983. The greatest reduction in irradiance occurred in the uppermost 1 m of the water column, where the floating portions of kelp fronds formed surface canopies with high blade densities. Average irradiances low enough to limit macroalgal photosynthesis (<200 μE m-2 s-1) were recorded at 1 m depth below dense kelp canopies under sunny surface conditions. Light penetration was exponentially related to canopy density, but was higher than predicted from transmission through individual kelp blades dae to the heterogeneous distribution of canopy tissue. Light penetration was considerably lower immediately adjacent to an individual kelp plant than in the interplant spaces, indicating significant self-shading. Spatial and temporal variability in both irradiance and light penetration were highest just below the canopy and decreased with increasing depth. Sunflecks (>200 μE m-2 s-1), ranging from <1 to 60s or more in duration, composed a considerable proportion of total irradiance at 1 m depth, even during recordings with low average irradiances. These fluctuations probably affect photosynthetic efficiency of kelp tissues within and just below the canopy. Predictions of kelp productivity based on irradiances averaged over hours or days and on photosynthetic parameters determined under laboratory conditions may be inaccurate, due to the spatial and temporal variability characteristic of the kelp forest light-environment.