Effects of shade from multiple kelp canopies on an understory algal assemblage

Abstract

We examined the effects of shade from multiple kelp canopy layers (surface Macrocystis pyrifera canopy, understory Pterygophora californica canopy), both individually and in combination, on an understory algal assemblage in a central Californian kelp forest. The removal of both kelp canopies resulted in a dense recruitment of the understory brown alga Desmarestia ligulata that formed a third canopy layer, which significantly decreased bottom light and the abundance of understory red algae. We subsequently created an additional canopy treatment by removing this third canopy layer. In general, the understory red algae fluctuated seasonally with winter swell intensity, changes in kelp canopy cover, and grazing; and although species richness increased significantly with increased bottom light, red algal bottom cover did not respond significantly to the canopy clearings until 2 yr after the canopies were initially cleared. Red algal cover within the Pterygophora canopy treatment was similar to that in the control treatment. In the absence of Pterygophora, the Macrocystis canopy treatment and 'no canopy' treatment were found to have greater red algal cover and species richness. Individual understory species were rare, which resulted in small effects sizes and thus low statistical power. However, when grouped post hoc, according to how they responded to the canopy clearings (i.e. response groups), we were able to detect canopy treatment effects as much as 1 yr earlier. This method identified that some understory red algae adapted to areas of canopy removal (light-adapted), and others adapted to a variety of light regimes (shade-tolerant). We were able to classify the light-adapted algae into 2 subgroups: the high-light species and the intermediate-light species. Although this method of grouping was done post hoc, our results indicate that it may provide the clearest assessment of how understory algae respond to shading from kelp canopies.KEY WORDS: Canopy shading · Community structure · Kelp forest · Light-adapted · Macrocystis · Pterygophora · Red algae · Shade-tolerantResale or republication not permitted without written consent of the publisher

Recruitment determines differences between assemblages on shaded or unshaded seawalls

“…Larvae of sessile invertebrates have been shown to respond to light, with many recruiting in greater numbers to shaded surfaces (Maughan 2001). Algae, in contrast, have been shown to recruit in greater numbers to unshaded than shaded patches of shore (Clark et al 2004). A similar pattern has been demonstrated for subtidal artificial structures.…”

“…Algae are generally less abundant in shaded habitats (Dayton 1975, Goldberg & Foster 2002, Clark et al 2004). Glasby (1999) similarly found less algal cover on wooden pilings compared to natural reefs, which was shown experimentally to be due to shading by marinas associated with the pilings.…”

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

“…Shading, in this case by a Macrocystis canopy, can benefit sessile suspension feeders by reducing competition for space with understory algae (Miller and Etter 2008;Arkema et al 2009) and can also suppress the growth and reproduction of mobile predators that forage in understory algae (Holbrook et al 1990). Similar interactions can occur between other species of canopy-forming kelps and the autotrophs living beneath them (Johnson and Mann 1988;Kennelly 1989;Clark et al 2004). The very different ecological roles of giant kelp, understory algae, and phytoplankton means that factors affecting their relative abundance in space or time will likely have complex ecosystem ramifications.…”

“…However, these perspectives ignore the potential for interactions between benthic and pelagic autotrophs on intermediate scales, from hundreds of meters to kilometers, such as among reefs with and without kelp canopies, and much of the variation in coastal production can occur at this scale (Broitman and Kinlan 2006). Giant kelp, Macrocystis spp., a foundation species and ecosystem engineer, creates dense forests that provide food and physical habitat to many animal species (Foster and Schiel 1985;Graham 2004), slow water flow (Jackson and Winant 1983;Gaylord et al 2007), absorb light (Stewart et al 2009), and take up nutrients (Hurd 2000), thus shaping the entire benthic community (Dayton 1985;Clark et al 2004;Arkema et al 2009). Controlled removal experiments have shown that giant kelp suppresses the biomass of understory macroalgae (Dayton et al 1984;Reed and Foster 1984;Clark et al 2004).…”

“…Giant kelp, Macrocystis spp., a foundation species and ecosystem engineer, creates dense forests that provide food and physical habitat to many animal species (Foster and Schiel 1985;Graham 2004), slow water flow (Jackson and Winant 1983;Gaylord et al 2007), absorb light (Stewart et al 2009), and take up nutrients (Hurd 2000), thus shaping the entire benthic community (Dayton 1985;Clark et al 2004;Arkema et al 2009). Controlled removal experiments have shown that giant kelp suppresses the biomass of understory macroalgae (Dayton et al 1984;Reed and Foster 1984;Clark et al 2004). Standing crop biomass of giant kelp varies dramatically both seasonally and interannually, with much of the variation driven by wave disturbance (Graham et al 2007;.…”