Using functional traits to assess the influence of burrowing bivalves on nitrogen-removal in streams

Functional traits define an organism in terms of how the organism interacts with its environment and determine the influence the organism has on dynamic ecosystem processes, such as biogeochemical nutrient cycling. Freshwater mussels (Bivalvia: Unionidae) form hotspots of biogeochemical activity in benthic environments by alleviating nutrient limitation at the sediment–water interface; however, little is known about the influence of mussel functional traits on sediment biogeochemical cycles, particularly nitrogen (N)-removal via denitrification and anammox. Our aim was to model the influence of two mussel functional traits: ammonium (NH₄⁺) excretion and organic matter (OM) biodeposition, on N-removal in stream sediments. We quantified mussel excretion and biodeposition and incubated mussels in microcosms containing river sediment using flow-through methods. We measured nutrient and gas fluxes to quantify real-time ambient N-removal (denitrification + anammox), and conducted isotope pairing techniques to determine the maximum N-removal potential for denitrification and anammox pathways. NH₄⁺ excretion was shown to be a significant predictor of ambient N-removal, whereas OM biodeposition significantly increased the maximum N-removal potential in the sediment. Our study is the first of its kind to link mussel functional traits specifically to N-removal and contributes to the growing knowledge of the role these highly imperiled organisms play by directly and indirectly influencing ecosystem-scale processes in lotic systems.