MEPS 569:145-161 (2017)  -  DOI: https://doi.org/10.3354/meps12086

ABSTRACT: In coastal sediments, macrofauna can trap macrophyte detritus in their burrows, modifying organic matter retention/export. However, the resulting effects on ecosystem function are unclear. We conducted a field experiment to assess how detrital processing by a common bioturbating crab Austrohelice crassa influences benthic metabolism, primary production, and ammonium regeneration at a sand and muddy-sand site. Since the functional role of crabs and detrital decay rates vary with mud content, we hypothesised their interactive effects on ecosystem function would also vary. Sixteen cages (0.36 m²) were established at each intertidal site, and cages were allocated a crab (0 or 35 ind. cage^-1) and seagrass detritus (0 or 120 g dry weight [DW] cage^-1) treatment. Ten days later, sediment-water solute fluxes were measured. Treatment effects on ecosystem function were site-specific: detritus stimulated benthic metabolism in muddy-sand (by 12 to 29%, p = 0.01), but in sand, effects were dominated by crabs. Metabolism increased by 12 to 21% (p = 0.04), of which 24 to 52% could be attributed to crab respiration. Crabs enhanced ammonium regeneration in sand (p < 0.003), of which <26% could be attributed to crab excretion, but in muddy-sand, the presence of detritus masked this positive effect (dark ammonium flux, crab × detritus interaction: p = 0.03). Crabs and detritus had negative effects on primary production in sand, where crabs reduced primary production by 57% and detritus by 73%. In muddy-sand, variable light conditions made primary production estimates unreliable. Our results emphasise that context is paramount when understanding the effects of reductions in the densities of key species or changes in detrital inputs on soft-sediment ecosystem function.

KEY WORDS: Bioturbation · Austrohelice crassa · Detrital decay · Primary production · Nutrient flux · Grapsid · Seagrass · Benthic community metabolism