Experimental assessment of the effects of shade on an intertidal kelp: Do phytoplankton blooms inhibit growth of open coast macroalgae?

Abstract

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)(2002)(2003)(2004)(2005)(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 m 2 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.

Partitioning of primary production among giant kelp (Macrocystis pyrifera), understory macroalgae, and phytoplankton on a temperate reef

“…The autotrophs in these systems can be broadly divided into pelagic phytoplankton and benthic macrophytes. Both groups can contribute substantially to ecosystem production (Gattuso et al 1998;Cebrian 1999) and can potentially affect each other through competition for light and nutrients (Smith and Horne 1988;Kavanaugh et al 2009). Canopy-forming kelps, particularly, can reduce the biomass of lower lying autotrophs through shading (Reed and Foster 1984;Santelices and Ojeda 1984).…”

Population consequences of biomass loss due to commercial collection of the wild seaweed Postelsia palmaeformis

“…The results have important management implications for regulating commercial take from Postelsia populations, but also shed light on how non-lethal disturbances that remove substantial biomass or limit productivity of seaweeds, such as herbivory (Graham 2003), changes in oceanographic climate (Dayton et al 1999, Edwards & Estes 2006 or light limitation (Kavanaugh et al 2009), can affect algal population dynamics. The frond trimming method voluntarily adopted by many commercial sea palm collectors in northern California is not lethal and allows fronds to regrow and produce zoospores.…”

“…The abundance of kelps and other seaweeds can vary dramatically among years and locations, and is typically associated with disturbances that cause large removal of biomass or limit productivity including storms (Dayton & Tegner 1984), herbivory (Graham 2002), light limitation (Kavanaugh et al 2009), bleaching (Harley 2003) and nutrient limitation or oceanographic climate (Dayton et al 1999, Edwards & Estes 2006. The rate of recovery from such disturbances depends on propagule (zoospore) supply and mortality of early life history stages.…”

Congeneric variation in surfgrasses and ocean conditions influence macroinvertebrate community structure

“…Research by Menge and colleagues (Menge et al 1997a,b, 2004, Connolly et al 2001, Leslie et al 2005, Barth et al 2007, Freidenburg et al 2007, Broitman et al 2008, Kavanaugh et al 2009 shows that seasonal oceanic upwelling processes are critical in structuring sessile invertebrate and macrophyte communities along the Pacific coast. Blooms of phytoplankton caused by upwelling of nutrients from depth can have several effects on intertidal biota, including direct bottom-up effects on growth of filter-feeding invertebrates, as well as effects on larval development and recruitment (Menge et al 1997b, Barth et al 2007, Broitman et al 2008.…”

“…Blooms of phytoplankton caused by upwelling of nutrients from depth can have several effects on intertidal biota, including direct bottom-up effects on growth of filter-feeding invertebrates, as well as effects on larval development and recruitment (Menge et al 1997b, Barth et al 2007, Broitman et al 2008. Phytoplankton abundance can negatively influence macrophyte communities if fast-growing invertebrates are able to preempt space or if phytoplanktonic blooms attenuate light availability (Menge et al 1997a, Freidenburg et al 2007, Kavanaugh et al 2009). …”