The role of fecal production by the sea urchin Strongylocentrotus droebachiensis in nutrient cycling and energy flow in a rocky subtidal ecosystem

In the marine environment, the biogeochemical roles of fecal production by pelagic zooplankton, reef fish, and suspension feeding bivalves have been extensively investigated. In contrast, the role of fecal production by nearshore grazers has received little attention. The green sea urchin Strongylocentrotus droebachiensis is the dominant grazer in the shallow rocky subtidal zone of the Northwest Atlantic, forming dense feeding aggregations (fronts) and driving the transition from highly productive kelp beds to less productive urchin barrens. To assess the role of fecal production by S. droebachiensis in nutrient cycling and energy flow in the shallow rocky subtidal zone, we examined the production rate, quality, dispersal, and decay of the fecal material. During the transition between community states, 20 % of the kelp ingested is released as fecal pellets, representing a production rate of 106 g feces m-1 front d-1 at a dense urchin grazing front. The fecal material produced is of low quality with a C:N ratio of 30.7. However, the fecal material is rapidly degraded and microbial enrichment during the second week after egestion leads to a decrease in the C:N ratio to 15.9, increasing the quality of the pellets as a food source for detritivores. Decay of the feces also leads to a decrease in pellet density and an increase in dispersal, suggesting that higher quality aged pellets are likely deposited at depths deeper than that of fresh feces where they may be an important food source for suspension- and deposit-feeding invertebrates. We also examined the production rate, quality, and dispersal of feces when urchins were fed kelp encrusted with the invasive bryozoan Membranipora membranacea and the invasive green alga Codium fragile spp. tomentosoides. The urchins produced less fecal material when fed encrusted kelp or C. fragile (82 and 31 g feces m -1 front d-1, respectively) compared to non-encrusted kelp; however, the feces were of much higher quality with C:N ratios of 11.4 and 8.5, respectively. The denser pellets of encrusted kelp are likely deposited at shallower depths (22 to 40 m) than kelp pellets, while pellets of C. fragile are likely transported to the deeper depths (33 to 55 m). Changes in the organic matter pool, food quality, and dispersal of feces in the invaded community states, will likely impact both the local nutrient cycling and energy flow within the detrital food web in the shallow rocky subtidal zone.