Nutrient regeneration from feces and pseudofeces of mussel Mytilus edulis spat

Suspension-feeding mussels exert top-down grazing control on primary producers, and provide bottom-up feedback of regenerated nutrients. Besides direct excretion, an important pathway of nutrient regeneration is through the decomposition of feces and pseudofeces, of which mussels can produce large quantities. Information on mussel feces and pseudofeces quality and nutrient regeneration rates is scarce. Feces and pseudofeces, produced in varying proportions, are commonly treated as one pool. We determined nutrient regeneration rates of feces and pseudo feces decomposition in incubations using natural seawater and juvenile Mytilus edulis from spat collectors. Apart from one 1993 trial, our results are the first to present nutrient regeneration dynamics of feces and pseudofeces separately. Dissolved inorganic nitrogen (DIN) and phosphate regeneration continued at stable rates for approximately 3 wk, after which 13.1 and 12.4% of the available N and 8.7 and 7.9% of the available P was regenerated from feces and pseudofeces, respectively. Rates of silicate regeneration declined continuously, which we attribute to its accumulation in the experimental setup. Coinciding environmental levels of DIN and silicate at growth-limiting levels indicate the potential ecological relevance of biodeposit decomposition. Overall DIN regeneration rates were similar between feces and pseudofeces, but depletion of ammonia was initially more rapid for pseudofeces due to stronger nitrification. Phosphate and silicate regeneration rates were 1.1 and 1.4 times greater from feces than pseudofeces, respectively. Future research should clarify the role of bivalve suspension feeders in controlling Si and P availability in coastal ecosystems in relationship to the proportion of pseudofeces generated, which depends on food concentration.