Degradation of sea urchin feces in a rocky subtidal ecosystem: implications for nutrient cycling and energy flow

Abstract

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.

Detrital subsidy from subtidal kelp beds is altered by the invasive green alga Codium fragile ssp. fragile

“…Consumption by sea urchins will greatly accelerate the degradation process, as large fragments of algae are reduced to small (~2.4 mm diameter) fecal particles (Sauchyn & Scheibling 2009a). These feces are of higher nutritional quality (lower C/N ratio) than fresh algal material (Sauchyn & Scheibling 2009b), and degrade more rapidly than fresh kelp (Sauchyn & Scheibling 2009b). Our cages excluded other large detritivores and predators of macrofauna, such as lobsters and crabs, which also may contribute to the degradation of algal thalli and influence the structure of associated macrofaunal assemblages.…”

Production and fate of kelp detritus

“…This translates to an estimated fecal production rate of 74 to 81 g yr −1 per linear meter of front, or 20720 kg d −1 across an estimated 280 km of coastline spanned by these grazing fronts in the mid to late 1990s (Sauchyn & Scheibling 2009a, Sauchyn et al 2011). The small (~2 mm diameter) fecal pellets are a highly nutritious food source relative to live kelp, and nitrogen, lipid, and available energy content increase rapidly during degradation (Sauchyn & Scheibling 2009b). …”

Sea urchin fecal production and accumulation in a rocky subtidal ecosystem

“…In turn, microbes increase the nutritional value of the feces by contributing to the labile organic matter fraction (González & Biddanda 1990, Fabiano et al 1994 through remineralization of dissolved organic matter (Urban-Rich 1999, Povero et al 2003) and uptake of dissolved inorganic nutrients from the water column , Povero et al 2003. Microbial degradation of sea urchin feces leads to an increase in the available energy content and a decrease in the fecal C:N ratio (Sauchyn & Scheibling 2009b), suggesting that the degraded feces are an important food source for suspension and deposit feeding invertebrates in the rocky subtidal zone and in adjacent sedimentary habitats in deeper water (Newell 1965, Frankenberg & Smith 1967, Lopez & Levinton 1987. While fecal production and biodeposition have been well studied in suspension feeding invertebrates such as bivalves (Haven & MoralesAlamo 1966, Kautsky & Evans 1987, Fabiano et al 1994, Norling & Kautsky 2007, the role of sea urchins and other benthic grazers in these processes remains poorly understood (Mamelona & Pelletier 2005, Sauchyn & Scheibling 2009a.…”

“…Degradation of feces through time was modelled by Sauchyn & Scheibling (2009b) using an exponential decay function:…”