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

Point quadrat (PQ) and video transect (VT) techniques were compared to determine the feasibility of using the latter to sample subtidal red algal assemblages. More taxa were found by PQs than by VTs, both overall and on a per quadrat basis. VTs tended to overestimate the percent cover of rock and articulated coralline algae. VTs were incapable of resolving many of the less common red algae. Video sampling required less time in the field than point quadrats but laboratory analysis was very time consunling. Lack of resolution and extensive laboratory analysis make this technique insufficient for sampling benthic red algal populations. However, VTs may be useful in documenting large-scale differences between regions, such as the percent cover of understory kelps or the density of large mobile invertebrates in central California kelp forests. Video sampling has been successfully used in the intertidal but traditional, random point quadrats s t d appear to be the most efficient and accurate way to sample benthlc communities in kelp forests.