Fecal production by sea urchins in native and invaded algal beds

To examine the role of fecal production by the sea urchin Strongylocentrotus droebachiensis in native and invaded algal assemblages in Nova Scotia, Canada, we examined the quantity (production rate by urchin feeding aggregations, or 'fronts') and quality (organic carbon and nitrogen content) of feces egested on 3 diets: (1) a native kelp Saccharina longicruris, (2) kelp encrusted with the invasive bryozoan Membranipora membranacea, and (3) an invasive green alga Codium fragile ssp. fragile. Relatively low absorption of clean and encrusted kelp resulted in high fecal output from urchins grazing native and invaded kelp beds (74 and 81 g feces m^-1 front d ^-1, respectively). In contrast, high absorption of C. fragile resulted in low fecal output from urchins grazing meadows of the invasive alga (31 g feces m^-1 front d^-1). Low C:N ratios of feces obtained from grazing C. fragile (8.5) or encrusted kelp (11.4), compared to clean kelp (30.7), suggest that feces from invaded assemblages are a higher quality food source for microbes and detritivores. To compare dispersal characteristics of feces from each diet, we measured pellet shape, size, density, and settling and critical erosion velocities. Using a wave model, we estimated the depth at which feces would be deposited under seasonally varying wave conditions. We found that pellets of C. fragile are likely deposited at the greatest depths (33 to 55 m), pellets of clean kelp at intermediate depths (28 to 47 m), and pellets of encrusted kelp at the shallowest depths (22 to 40 m). Our findings suggest a smaller amount of higher quality feces enters the detrital food web at greater depths from Codium meadows than from native kelp beds.