An Investigation of Wrack at Cattle Point: Algal Composition, Spatial Distribution, and Sampling Method Comparison

Abstract

Deposits of drift macroalgae in the intertidal zone serve as an important source of nutrients for herbivores in beach systems that are otherwise sparsely vegetated. We investigated the algal composition and spatial distribution of the high tide wrack line on a gravel beach at Cattle Point, San Juan Island. We employed two different sampling methods to measure algal composition, spatial distribution, and size distribution, hypothesizing that the two methods would return different results. We expected both size of detritus and algal type to vary spatially along the beach. For the first sampling method, we laid a transect through the high tide wrack line and took point samples of single algal pieces at half meter marks, combining the samples into four groups corresponding to four equal sections of transect. For the second sampling method, we cut 10-cm diameter cores from the wrack line every 5 meters, again grouped by section. We then sorted the samples by algal type and took size measurements from the point samples. Our results for overall algal composition were equivalent between the two sampling methods, showing that both methods were effective for determining composition of the wrack. Kelps were most prevalent in our sampling, followed by other brown algae, filamentous red algae, and red blades. Seagrasses and green algae were least prevalent. We did not find a detectable pattern of spatial distribution by size or algal type between the four sections of the transect, which suggests that size and algal type are roughly evenly distributed along the wrack. These results can potentially provide context to investigations of food web dynamics in marine-terrestrial transition zone ecosystems on San Juan Island.