Mussels, drought and ecosystem services

Background/Question/Methods

Freshwater mussels are large, long-lived suspension feeders that provide important ecosystem services in rivers such as biofiltration, nutrient recycling and nutrient storage. Mussels occur in speciose aggregations (mussel beds) at high biomass, and there are differences in biofiltration and nutrient excretion rates across species.  An exceptional drought affected many rivers in the southern plains in 2011.  In the Kiamichi River, Oklahoma, drought conditions were exacerbated by management of a tributary reservoir and by upstream pumping for irrigation.  This study quantified the mussel-provided biophysical processes influencing water quality and ecosystem function in the Kiamichi River before and after the drought. From previous studies, we knew the location, size, species composition, and biomass of most mussel beds in the river, and the filtration rates, nutrient excretion rates, and tissue nutrient concentration of dominant mussel species. We used these data to calculate mussel aggregate biofiltration and nutrient stores and fluxes in the river (1) during a typical summer and (2) following the summer 2011 drought. We then compared the estimated dollar values of mussel-provided services with the costs of providing similar services through human engineered and best management practices.

Results/Conclusions

Mussel losses in the Kiamichi River were high.  For example, at one large mussel bed we have monitored for > 20 years we observed 19 species of freshly dead mussels and estimated 66 kg (dry mass) of mussel soft tissue was lost.  For the river overall, we estimated that mussel biomass and species reductions from the drought resulted in losses of 2.6 kg/day in N recycling, 720 kg of N storage, and 19,783,840 L/day of biofiltration.  Many of these losses could have been prevented had water been released from an upstream tributary to alleviate high water temperatures which exceeded 40°C in some locations.  Drought in the southern plains is cyclical and mussels evolved under these conditions.  However, human water management is a new constraint. Drought in this region is predicted to become both more frequent and more severe with climate change, and the human population is growing and using more water.  While we can’t control the frequency and severity of droughts, we can control how we manage water resources to maintain populations of freshwater mussels and the ecosystem services they provide.