During destructive grazing events in the Northwest Atlantic, sea urchins Strongylocentrotus droebachiensis consume large amounts of kelp biomass, transforming this material into feces. To determine the role of urchin fecal material in nutrient cycling and energy flow in the shallow rocky subtidal zone, we monitored the physical, chemical, and microbial degradation of urchin feces at 6, 9, 12, and 16 m depth over 19 d at a wave-exposed site on the Atlantic coast of Nova Scotia by quantifying changes in fecal biochemical composition, pellet size, and settling velocity. We observed an exponential loss of fecal material and rapid degradation of total and labile organic matter fractions; this suggests that urchin fecal production plays an important role in local nutrient cycling and energy flow via the microbial food web. As the feces were further degraded there was a relative increase in organic carbon, nitrogen, lipid, and available energy content, and a decrease in the C:N ratio, suggesting that degraded urchin feces are an important food source for suspension-and deposit-feeding invertebrates. The settling velocity of the feces also decreased over time, likely due to a decrease in fecal pellet density. Older, less dense feces with a relatively high energy content are more likely to be suspended and transported horizontally, providing a mechanism for the export of kelp primary production to deeper, less productive waters.

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-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.