In terrestrial and aquatic ecosystems, animals frequently occur in dense aggregations. Examples include herds of deer, schools of fish, flocks of birds, and beds of mussels. Research into the effects of such aggregations on ecosystems has typically focused on a single species or animal group. In stream ecosystems, fish and mussels have the strongest animal-mediated biogeochemical and ecosystem effects. Fish schools or mussel beds can increase nutrient concentrations, alter their ratios, increase algal growth, and affect the growth of aquatic insects. These effects have been investigated separately for each group, however, in most streams, dense aggregations of fish and mussels periodically overlap. Our research, funded by NSF DEB-1457542, explored the ecosystem and biogeochemical responses when fish and mussel aggregations overlapped.

We investigated these responses with a field survey, field experiment, and several artificial stream experiments with the help of undergraduate students, graduate students, and post-doctoral researchers. In the field, we surveyed the abundances of fish and mussel species along the Kiamichi, Little, and Glover Rivers in Oklahoma and then scaled up their abundances to estimate their impacts on nutrient concentration and stoichiometry. We found that the abundance of fish on mussel beds was driven by hydrology, and that fish mass on mussel beds was highest when the river flows were low. During these periods we found that the overlap had the greatest impact on the modeled ratios of nutrients on those beds. Further field experimentation manipulating the density of mussels revealed that fish were attracted to the structure of mussel shells. Because mussel beds typically occupy areas of rivers where water is always available (even during drought), mussels may provide an important underwater landmark for fish seeking refugia that won’t dry out during drought. In an experimental stream experiment simulating seasonal low flows, we showed that fish survived longer in mesocosms with mussels than those without and at extreme levels of low flow – when streams and mussel beds dry out – we found that mussel bed die-offs can have both short- and long-term influences on the ecosystem services of rivers.

Along with this work, we manipulated fish presence on mussel beds. This work extended our understanding of the food web effects of overlapping fish schools and mussel beds. Importantly our work highlights interconnections between animal and microbial ecosystems. Our research provided fundamental insight into how animals influence the organic energy available to microbial community function. Such insight will be important for modeling and anticipating the microbial ecosystem and biogeochemical responses to ongoing losses in animal biomass and biodiversity.

Some key publications from this work:

Lopez, Jonathan W., Thomas B. Parr, Daniel C. Allen and Caryn C. Vaughn. 2020. Animal aggregations increase emergent plant growth at the aquatic-terrestrial interface. Ecology, e02126. Lopezetal.Ecology.2020

Hopper, Garrett W., Traci G. DuBose, Keith B. Gido and Caryn C. Vaughn. 2019. Freshwater mussels alter fish distributions at fine spatial scale through habitat subsidies. Freshwater Science 38:702-712, DOI: 10.1086/705666. Hopperetal.FWS.2019

DuBose, Traci P., Carla L. Atkinson, Caryn C. Vaughn and Steve Golladay. 2019. Drought-induced, punctuated loss of freshwater mussels alters ecosystem function across temporal scales. Frontiers in Ecology and Evolution DOI: 10.3389/fevo.2-19.00274

Parr, Thomas B., Caryn C. Vaughn and Keith B. Gido. 2019. Animal effects on dissolved organic carbon lability in an algal controlled ecosystem. Freshwater Biology 65:1298-1310. Parr_et_al-2019-Freshwater_Biology

Hopper, Garrett W., Keith B. Gido, Caryn C. Vaughn, Thomas B. Parr, Traci G. Popejoy, Kiza K. Gates, and Carla L. Atkinson. 2018. Biomass distribution of fishes and mussels mediates spatial and temporal heterogeneity in nutrient cycling in streams. 2018. Oecologia 188:1133-1144, Hopperetal.Oecologia.2018 

mesocosm experiment at the University of Oklahoma

Our lab uses an integrative approach combining comparative field studies with experiments at multiple scales (field, mesocosm, and laboratory) to understand the functional role of freshwater mussels in ecosystems. Over the past 30 years, our work has demonstrated that mussels are important “biofilters” that transfer energy and nutrients in aquatic food webs and even subsidize terrestrial ecosystems. The biophysical processes performed by mussels vary with species and environmental conditions, particularly flow and temperature, and are highest in the most diverse communities. The processes performed by mussels result in ecosystem services to humans, such as water purification, nutrient recycling and storage, and habitat for other species.

Current work includes investigating how mussel excreta influences terrestrial vegetation and herbivores (Jonathan Lopez), the role of the mussel microbiome in nitrogen cycling (Ed Higgins), and synthesizing long-term data on mussel communities and traits to predict future ecosystem function with climate change.

Some of this work is summarized in these two recent review articles:

Vaughn, Caryn C.  and Timothy J. Hoellein. 2018. Bivalve impacts in freshwater and marine ecosystems. Annual Review of Ecology, Evolution, and Systematics 49:183-208. Vaughn&Hoellein.ARES.2018

Vaughn, Caryn C. 2018. Ecosystem services provided by freshwater mussels. Hydrobiologia 810:15-27. Vaughn.Hydrobiologia.2018


Freshwater is vital for both humans and fish and wildlife, but humans are using freshwater more rapidly than it can be replenished. The Kiamichi River watershed in southeastern Oklahoma is at the center of intense conflict over water ownership and use. Missing from these disputes are the needs of the watershed’s rich animal and plant life, including three federally endangered freshwater mussels. Our lab has used an ecosystem services framework to examine how different water management/environmental flow scenarios in the Kiamichi River watershed affect the delivery of ecosystem services, and thus contribute to the wellbeing of people living both in and outside the watershed. Our approach involves mapping the spatial delivery of a selection of watershed ecosystem services, and then exploring the tradeoffs between their biophysical, socio-cultural and economic values.

Some key publications from this work:

Castro, Antonio J., Caryn C. Vaughn, Jason P. Julian and Maria Garcia Llorente. 2016. Social demand of ecosystem services for watershed management. Journal of the American Water Resources Association. 52: 209-221. Castroetal.JAWRA.2016

Gates, Kiza K., Caryn C. Vaughn and Jason P. Julian. 2015. Incorporating species traits in a guild approach to develop environmental flow recommendations for freshwater mussels. Freshwater Biology 60:620-635. Gatesetal.FWB.2015

Vaughn, Caryn C., Carla L. Atkinson and Jason P. Julian. 2015. Drought-induced changes in flow regimes lead to long-term losses in mussel-provided ecosystem services. Ecology and Evolution 5:1291-1305. Vaughnetal.EcolEvol.2015