Sea urchin fecal production and accumulation in a rocky subtidal ecosystem

We used a spatial model of destructive grazing of kelp beds by dense aggregations (fronts) of sea urchins Strongylocentrotus droebachiensis, together with measures of sea urchin absorption efficiency and fecal degradation rate, to predict rates of fecal production and accumulation during the transition from a kelp bed to a barrens state in a rocky subtidal ecosystem of Nova Scotia, Canada. The rate of fecal production as dry mass reached 9.17 g m ^-2 d ^-1 at 10 yr after the formation of a grazing front, with associated rates of carbon and nitrogen production of 2.12 g C m ^-2 d ^-1 and 0.21 g N m ^-2 d ^-1. Fecal production rate exceeded the rates of physical, chemical, and microbial degradation, such that up to 193.6 g of feces, 70.2 g of C, and 4.0 g of N accumulated per linear m of front after 10 yr. The extent of sea urchin grazing fronts along the Atlantic coast of Nova Scotia was estimated at 280 km in 2000, which translates to a total fecal production rate of 6423 kg of feces d ^-1 and an accumulation of 54 200 kg of feces after 10 yr of front formation, based on our model. This temporally variable flux of organic matter likely has a profound effect on adjacent deeper-water macrobenthic communities utilizing the feces as a food source.