Daniel E. Spooner

Community and foodweb ecology of freshwater mussels

Abstract Freshwater mussel (Superfamily Unionoidea) communities are important components of food webs, and they link and influence multiple trophic levels. Mussels filter food from both the water column and sediment with ciliated gills. Differences in cilia structure and arrangement might allow mussel species to partition food resources. Mussels are omnivores that feed across trophic levels on bacteria, algae, detritus, zooplankton, and perhaps, dissolved organic matter. Living mussels and their spent shells provide or improve habitat for other organisms by providing physical structure, stabilizing and bioturbating sediments, and influencing food availability directly and indirectly through biodeposition of organic matter and nutrient excretion. Effects of mussel communities on nutrient translocation and cycling depend on mussel abundance, species composition, and environmental conditions. Nutrient-related mussel effects influence multiple trophic levels. Healthy mussel communities occur as multispecies assemblages in which species interactions are probably very important. Food limitation and competition among species have been documented, but so have positive species interactions, and rare species have been shown to benefit energetically from living in species-rich communities. Effects of mussel species on ecosystem services and food webs vary across spatial and temporal scales, and the relative importance of competition and facilitation might change at different scales.

Ecosystem Processes Performed by Unionid Mussels in Stream Mesocosms: Species Roles and Effects of Abundance

Unionid mussels are a guild of freshwater, sedentary filter-feeders experiencing a global decline in both species richness and abundance. To predict how these losses may impact stream ecosystems we need to quantify the effects of both overall mussel biomass and individual species on ecosystem processes. In this study we begin addressing these fundamental questions by comparing rates of ecosystem processes for two common mussel species, Amblema plicata and Actinonaias ligamentina, across a range of abundance levels and at two trophic states (low and high productivity) in stream mesocosms. At both low and high productivity, community respiration, water column ammonia, nitrate, and phosphorus concentrations, and algal clearance rates were all linearly related to overall mussel biomass. After removing the effects of biomass with ANCOVA, we found few differences between species. In a separate series of experiments, nutrient excretion (phosphorus, ammonia, and molar N:P) and biodeposition rates were only marginally different between species. For the species studied here, functional effects of unionids in streams were similar between species and linearly related to biomass, indicating the potential for strong effects when overall mussel biomass is high and hydrologic residence times are long.

A trait-based approach to species' roles in stream ecosystems: climate change, community structure, and material cycling

The sustained decline in habitat quality and community integrity highlights the importance of understanding how communities and environmental variation interactively contribute to ecosystem services. We performed a laboratory experiment manipulating effects of acclimation temperature (5, 15, 25, and 35°C) on resource acquisition, assimilation and subsequent ecosystem services provided by eight freshwater mussel species. Our results suggest that although freshwater mussels are broadly categorized as filter feeders, there are distinct nested functional guilds (thermally tolerant and sensitive) associated with their thermal performance. At 35°C, thermally tolerant species have increased resource assimilation and higher rates of contributed ecosystem services (nutrient excretion, benthic–pelagic coupling). Conversely, thermally sensitive species have decreased assimilation rates and display an array of functional responses including increased/decreased benthic–pelagic coupling and nutrient excretion. Although thermally sensitive species may be in poorer physiological condition at warmer temperatures, their physiological responses can have positive effects on ecosystem services. We extrapolated these results to real mussel beds varying in species composition to address how shifts in community composition coupled with climate change may shift their contributed ecological services. Comparative field data indicate that two co-existing, abundant species with opposing thermal performance (Actinonaias ligamentina, Amblema plicata) differentially dominate community biomass. Additionally, communities varying in the relative proportion of these species differentially influence the magnitude (benthic–pelagic coupling) and quality (N:P excretion) of ecosystem services. As species are increasingly threatened by climate change, greater emphasis should be placed on understanding the contribution of physiological stress to the integrity and functioning of ecosystems.

Unionid mussels influence macroinvertebrate assemblage structure in streams

Abstract Unionid mussels often occur as multispecies aggregates called mussel beds and in dense patches within the mussel beds themselves. Thus, their distributions are patchy at 2 spatial scales. We examined the association between mussel assemblage structure and macroinvertebrate assemblage structure at these 2 spatial scales in rivers of the Ouachita Highlands, Arkansas and Oklahoma, USA. We used multivariate variation partitioning techniques to relate variation in benthic macroinvertebrate distribution and abundance to variation in mussel assemblages, environmental variables, spatial variables, and overlapping or shared variation between these components. At the patch scale, total densities of macroinvertebrates and dominant groups (Oligochaeta, Chironomidae, Ephemeroptera, and Trichoptera) were significantly higher in patches containing mussels than where mussels were absent, and densities of macroinvertebrates were positively correlated with unionid density. In variation partitioning analyses, mussel assemblages explained almost ½ of the variation in macroinvertebrate assemblages at both spatial scales, even after removing effects of similar habitat (environmental variables) and biogeographic history (spatial variables).

CONTEXT‐DEPENDENT SPECIES IDENTITY EFFECTS WITHIN A FUNCTIONAL GROUP OF FILTER‐FEEDING BIVALVES

We asked whether species richness or species identity contributed more to ecosystem function in a trait-based functional group, burrowing, filter-feeding bivalves (freshwater mussels: Unionidae), and whether their importance changed with environmental context and species composition. We conducted a manipulative experiment in a small river examining the effects of mussel assemblages varying from one to eight species on benthic algal standing crop across two sets of environmental conditions: extremely low discharge and high water temperature (summer); and moderate discharge and water temperature (fall). We found strong species identity effects within this guild, with one species (Actinonaias ligamentina) influencing accrual of benthic algae more than other species, but only under summer conditions. We suspect that this effect is due to a combination of the greater biomass of this species and its higher metabolic and excretion rates at warm summer temperatures, resulting in increased nitrogen subsidies to benthic algae. We also found that Actinonaias influenced the condition of other mussel species, likely through higher consumption, interference, or both. This study demonstrates that species within trait-based functional groups do not necessarily have the same effects on ecosystem properties, particularly under different environmental conditions.

Species traits and environmental conditions govern the relationship between biodiversity effects across trophic levels.

Changing environments can have divergent effects on biodiversity–ecosystem function relationships at alternating trophic levels. Freshwater mussels fertilize stream foodwebs through nutrient excretion, and mussel species-specific excretion rates depend on environmental conditions. We asked how differences in mussel diversity in varying environments influence the dynamics between primary producers and consumers. We conducted field experiments manipulating mussel richness under summer (low flow, high temperature) and fall (moderate flow and temperature) conditions, measured nutrient limitation, algal biomass and grazing chironomid abundance, and analyzed the data with non-transgressive overyielding and tripartite biodiversity partitioning analyses. Algal biomass and chironomid abundance were best explained by trait-independent complementarity among mussel species, but the relationship between biodiversity effects across trophic levels (algae and grazers) depended on seasonal differences in mussel species’ trait expression (nutrient excretion and activity level). Both species identity and overall diversity effects were related to the magnitude of nutrient limitation. Our results demonstrate that biodiversity of a resource-provisioning (nutrients and habitat) group of species influences foodweb dynamics and that understanding species traits and environmental context are important for interpreting biodiversity experiments.

Species richness and temperature influence mussel biomass: a partitioning approach applied to natural communities

To increase the generality of biodiversity-ecosystem function theory, studies must be expanded to include real communities in a variety of systems. We modified J. W. Foxs approach to partition the influence of species richness on standing crop biomass (net biodiversity effect) of 21 freshwater mussel communities into trait-independent complementarity, trait-dependent complementarity (species with particular traits dominate without impacting other species), and dominance effects (species with particular traits dominate at the expense of others). Overall, species-rich mussel communities have greater biomass than predicted based on average biomass across the region. This effect is largely due to trait-independent complementarity with less abundant species having higher body condition and reduced metabolic rates in species-rich communities. These measures are positively correlated with spatial and temporal thermal variation, suggesting that use of thermal niches as habitat may be important to species coexistence and performance, and emphasizing that knowledge of species traits and environmental context are important to understanding biodiversity-ecosystem function dynamics.

Profiles of Biochemical Tracers in Unionid Mussels Across a Broad Geographical Range

ABSTRACT As large, long-lived filter feeders with high clearance rates, unionid mussels are capable of altering nutrient cycling in freshwater food webs. Because the effects of mussel communities on ecosystem processes result largely from their feeding behavior, we need to understand how they sample and process particulate matter. Our objective was to determine whether measuring a suite of biochemical markers in unionid mussels, from different populations of 2 species (Amblema plicata, Actinonaias ligamentina) across a broad geographical range (4 North American rivers), would help identify their primary dietary sources. Stable isotope data were able to differentiate mussels among rivers, and sometimes species. Data on &dgr;15N revealed that mussels fed across trophic levels but that diet varied with habitat and perhaps species. Although mussels contained the same types of fatty acids regardless of river or species, we nonetheless observed considerable variation in essential fatty acid content across rivers. Essential fatty acid profiles were dominated consistently by arachidonic acid. Source-specific fatty acid biomarkers suggested that bacterial and detrital resources might be as important as algae in many habitats. In summary, biochemical metrics differed more among rivers than between species. This suggests that mussels are either considerably adaptable in their dietary requirements or that they feed nonselectively. Understanding nutritional profiles in mussels will help restore unionid communities, which may lead to the reestablishment of their critical role as nutrient recyclers in freshwater food webs.

Status of the Mussel Fauna of the Poteau River and Implications for Commercial Harvest

Abstract The Poteau River, a major tributary of the Arkansas River, flows through the Ouachita Uplands of eastern Oklahoma and western Arkansas. The river has been harvested for mussels, historically by the Caddo Indians and recently for the pearl industry. We documented the current distribution and abundance of mussels in the river, compared this with historical distributions and examined whether the river can sustain current levels of commercial harvest. The Poteau River retains a rich mussel fauna of 35 species. However, mussel abundance is much lower than in surrounding rivers. Mean total mussel densities in the largest beds in the river are 3.8 individuals/m2. Densities of the two commercially harvested species, Megalonaias nervosa (washboard) and Amblema plicata (threeridge), are only 0.15 and 1.25 individuals/m2, respectively. Mussel size distributions indicate very low recent recruitment of both M. nervosa and A. plicata. The majority of M. nervosa exceed the shell height limit of 4 inches established by the state of Oklahoma and, thus, are not protected by this regulation. Our data indicate that the Poteau River cannot sustain commercial mussel harvest. A recently established mussel sanctuary should be retained and further commercial harvest of mussels should be avoided.

A Tale of Two Rivers: Implications of Water Management Practices for Mussel Biodiversity Outcomes During Droughts

Droughts often pose situations where stream water levels are lowest while human demand for water is highest. Here we present results of an observational study documenting changes in freshwater mussel communities in two southern US rivers during a multi-year drought. During a 13-year period water releases into the Kiamichi River from an impoundment were halted during droughts, while minimum releases from an impoundment were maintained in the Little River. The Kiamichi observed nearly twice as many low-flow events known to cause mussel mortality than the Little, and regression tree analyses suggest that this difference was influenced by reduced releases. During this period mussel communities in the Kiamichi declined in species richness and abundance, changes that were not observed in the Little. These results suggest that reduced releases during droughts likely led to mussel declines in one river, while maintaining reservoir releases may have sustained mussel populations in another.