Native freshwater mussel effects on nitrogen cycling: Impacts of nutrient limitation and biomass dependency

Like many aggregating animals, unionid mussels occurring at high densities can create patches or 'hotspots' of biogeochemical activity directly through nutrient regeneration and material flux and indirectly by movement of sediments through physical activities of the organism. We examined the effect of unionid mussels and nutrient limitation on denitrification and nitrification rates across a gradient of ambient mussel diversity and density in the Kiamichi River, Oklahoma, USA. We incubated pooled sediment samples and measured denitrification rates (with amendments of N as KNO₃, C as dextrose, N and C together, and no amendment), nitrification rates, and pore-water chemistry at each site. Mussel biomass and alleviation from N limitation influenced denitrification rates. In unamended sediments, denitrification rates decreased with increasing mussel densities, and were not limited by N or C. However, sediments with mussels had higher denitrification rates with the addition of N. Nitrification rates were not correlated with mussel biomass, but rates did increase marginally with the proportion of Actinonaias ligamentina (a relatively more mobile species) in each mussel bed. Pore-water concentrations of NO₃ ^- and NH₄⁺ were highest at high-mussel-biomass sites. Our study builds on existing evidence of the importance of unionid mussels in nutrient cycling in streams, by relating their effects to organism biomass and alleviation from nutrient limitation. The N limitation of denitrification rates found in our study may mean that mussel beds become denitrification hotspots given anticipated increases in nutrient availability. However, the continued decline of these organisms probably will reduce the denitrification capacity of mussel beds.