Experimental Assessment of the Effects of Shade on an Intertidal Kelp: Do Phytoplankton Blooms Inhibit Growth of Open-coast Macroalgae?

Benthic primary producer abundance and satellite-derived chlorophyll concentrations are strongly negatively correlated along the Oregon and Northern California coast, suggesting an antagonistic interaction. Direct field observations of interannual and among-site changes in abundance of low intertidal macrophytes also suggest a negative interaction. Several years (2001–2006) of quantification of surf-zone chlorophyll concentrations and light reaching the intertidal benthos suggested that severe light attenuation from dense phytoplankton blooms is an important mechanism underlying this inverse phytoplankton–macrophyte relationship. From early June to September of 2004 we quantified the response of the intertidal kelp Saccharina sessile to experimentally manipulated light regimes that mimicked the attenuation during blooms. Shading frames were installed in the low intertidal zone to manipulate the light levels available to the benthos at two sites of contrasting long-term differences in average phytoplankton abundance. Treatments included shaded and unshaded plots that were 0.25 m2 in area. Although the magnitude of the effect was context-dependent, shading strongly decreased growth rates and abundance of S. sessile. These results are comparable to results in estuarine studies that have demonstrated adverse effects on benthic macrophytes as a result of eutrophication and subsequent light limitation and are the first demonstration of phytoplankton-induced light limitation for energetic open coasts. In these systems where annual benthic production can exceed the pelagic production and where perennial macrophytes such as kelp and surf grasses are important habitat modifiers, large-scale reduction of macrophytes via phytoplankton shading could lead to profound modifications of coastal ecosystem dynamics.