Nutrient release and ecological stoichiometry of freshwater mussels (Mollusca:Unionidae) in 2 small, regionally distinct streams

Ecological stoichiometry is the study of the balance of multiple elements in ecological interactions and processes. We investigated the ecological stoichiometry of freshwater mussels across 3 seasons at 2 sites each in an agricultural watershed (Little Darby Creek [LD], Ohio) and a forested watershed (Upper Ouachita River [OR], Arkansas) (2 species/stream). We used nutrient-release experiments to determine C, N, and P content and elemental ratios in seston and in consumer-driven nutrient recycling (CNR) components (mussel soft tissues, shells, biodeposited material, and excreted nutrients). We focused on seasonal patterns of: 1) biodeposition and excretion rates; 2) seston and CNR component % C, % N, % P, C:N, C:P, and N:P; and 3) degree of homeostasis in mussels. Differences in mass-specific biodeposition and excretion rates were driven largely by seasonal factors. Percent P, C:N, C:P, and N:P of seston were seasonally variable in LD, and % C, % N, % P, C:N, C:P, and N:P of seston were seasonally variable in OR. Mussel tissues and biodeposited materials were variable for 3 to 5 of the 6 nutrient metrics in both LD and OR. Mussel shell and soft-tissue nutrient stoichiometry were relatively homeostatic and fell within stoichiometric ranges of other macroinvertebrates, except that mussel tissue had higher % P and lower C:P and N:P than are usually observed in other macroinvertebrates. Mussels can repackage nutrients in stream seston and fine benthic organic matter in the form of biodeposited material and excreted nutrients, thereby providing a significant source of C, N, and P for other benthic organisms.