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Nutrient cycling is a key process linking organisms in ecosystems. This is especially apparent in stream environments in which nutrients are taken up readily and cycled through the system in a downstream trajectory. Ecological stoichiometry predicts that biogeochemical cycles of different elements are interdependent because the organisms that drive these cycles require fixed ratios of nutrients. There is growing recognition that animals play an important role in biogeochemical cycling across ecosystems. In particular, dense aggregations of consumers can create biogeochemical hotspots in aquatic ecosystems via nutrient translocation. We predicted that filter-feeding freshwater mussels, which occur as speciose, high-biomass aggregates, would create biogeochemical hotspots in streams by altering nutrient limitation and algal dynamics. In a field study, we manipulated nitrogen and phosphorus using nutrient-diffusing substrates in areas with high and low mussel abundance, recorded algal growth and community composition, and determined in situ mussel excretion stoichiometry at 18 sites in three rivers (Kiamichi, Little, and Mountain Fork Rivers, south-central United States). Our results indicate that mussels greatly influence ecosystem processes by modifying the nutrients that limit primary productivity. Sites without mussels were N-limited with ∼26% higher relative abundances of N-fixing blue-green algae, while sites with high mussel densities were co-limited (N and P) and dominated by diatoms. These results corroborated the results of our excretion experiments; our path analysis indicated that mussel excretion has a strong influence on stream water column N:P. Due to the high N:P of mussel excretion, strict N-limitation was alleviated, and the system switched to being co-limited by both N and P. This shows that translocation of nutrients by mussel aggregations is important to nutrient dynamics and algal species composition in these rivers. Our study highlights the importance of consumers and this imperiled faunal group on nutrient cycling and community dynamics in aquatic ecosystems.

This work was supported by: i) POSEUR-03-2215-FC-000096 and ICNF funds under project CP01-MARG-QUERCUS/2018; ii) Project Reviving Douro Basin funded by MAVA, Fondation pour la Nature and iii) European Investment Funds by FEDER/COMPETE/POCI – Operational Competitiveness and Internationalization Program, under Project POCI-01-0145-FEDER-006958 and National Funds by FCT - Portuguese Foundation for Science and Technology, under the projects UID/AGR/04033/2013 and UID/AGR/00690/2019. FCT also support MLL with a doctoral grant (SFRH/BD/115728/2016).

Freshwater mussels are one of the most endangered animal groups globally, making them a high conservation priority. Conservationists increasingly employ translocation or captive breeding procedures to support ailing populations, and the ecosystem engineering capabilities of mussels are being increasingly harnessed in bioremediation projects. However, there is little consideration of the risk of pathogen transmission when moving mussels from hatcheries or wild donor populations into new habitats. This is of significant concern as recent developments suggest parasites and diseases are highly prevalent and have contributed to several mass population‐level die‐offs. Here, we explicitly highlight the risks of pathogen spread in mussel translocations, explore how these risks are mediated, and provide recommendations for both research and action to avoid the inadvertent spread of virulent pathogens when conserving vulnerable mussel populations. While targeted at freshwater conservationists, this perspective has relevance for considering translocation‐mediated disease and parasite spread in any study system.

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.

Differences in animal distributions and metabolic demands can influence energy and nutrient flow in an ecosystem. Through taxa-specific nutrient consumption, storage, and remineralization, animals may influence energy and nutrient pathways in an ecosystem. Here we show these taxa-specific traits can drive biogeochemical cycles of nutrients and alter ecosystem primary production and metabolism, using riverine systems that support heterogeneous freshwater mussel aggregations. Freshwater unionid mussels occur as distinct, spatially heterogeneous, dense aggregations in rivers. They may influence rates of production and respiration because their activities are spatially concentrated within given stream reaches. Previous work indicates that mussels influence nutrient limitation patterns, algal species composition, and producer and primary consumer biomass. Here, we integrate measures of organismal rates, stoichiometry, community-scaled rates, and ecosystem rates, to determine the relative source–sink nutrient dynamics of mussel aggregations and their influence on net ecosystem processes. We studied areas with and without mussel aggregations in three nitrogen-limited rivers in southeastern Oklahoma, USA. We measured respiration and excretion rates of mussels and collected a subset of samples for tissue chemistry and for thin sectioning of the shell to determine growth rates at each site. This allowed us to assess nutrient remineralization and nutrient sequestration by mussels. These rates were scaled to the community. We also measured stream metabolism at three sites with and without mussels. We demonstrated that mussel species have distinct stoichiometric traits, vary in their respiration rates, and that mussel aggregations influence nutrient cycling and productivity. Across all mussel aggregations, we found that mussels excreted more nitrogen than they sequestered into tissue and excreted more phosphorus than they sequestered except at one site. Furthermore, gross primary productivity was significantly greater at reaches with mussels. Collectively, our results indicate that mussels have ecosystem-level impacts on nutrient availability and production in nutrient-limited rivers. Within these streams, mussels are affecting the movement of nutrients and altering nutrient spiralling.

Freshwater mussels (Unionida) are among the world’s most imperiled taxa, but the relationship between freshwater mussel mortality events and infectious disease is largely unstudied. We surveyed viromes of a widespread and abundant species (mucket, Actinonaias ligamentina; syn: Ortmanniana ligamentina) experiencing a mortality event of unknown etiology in the Huron River, Michigan, in 2019–2020 and compared them to viromes from mucket in a healthy population in the St. Croix River, Wisconsin and a population from the Clinch River, Virginia and Tennessee, where a mortality event was affecting the congeneric pheasantshell (Actinonaias pectorosa; syn: Ortmanniana pectorosa) population. We identified 38 viruses, most of which were associated with mussels collected during the Huron River mortality event. Viral richness and cumulative viral read depths were significantly higher in moribund mussels from the Huron River than in healthy controls from each of the three populations. Our results demonstrate significant increases in the number and intensity of viral infections for freshwater mussels experiencing mortality events, whereas individuals from healthy populations have a substantially reduced virome comprising a limited number of species at low viral read depths.

Designing effective surveys for freshwater mussels (Unionidae) is a challenge, because they are spatially clustered and often found in low densities. The objective of this study was to examine how the performance (in respect to species richness, the total number of mussels per search effort, species composition and size distribution) of three different survey methods (timed searches, transect method, and adaptive cluster method) varied between different habitats at six sites in rivers of Central Texas. Habitat conditions affected the performance of search methods by either facilitating or hindering the detection of mussels, e.g., sandy substrate facilitating the detection of mussels seemed to enhance the performance of the adaptive cluster method in respect to the number of mussels found per unit search effort. Differences in detectability of mussels was also associated with the size of mussels, their behavior and morphology. Timed searches detected a larger proportion of larger mussels that tended to be less burrowed and that had shells with more sculpturing compared to quantitative methods. Our results suggest that to design effective surveys variation in detectability of mussels must be considered which depends on local habitat conditions, experience of surveyor, behavior, size and morphology of mussels.

Native freshwater mussels form a critical component of benthic foodwebs, but are endangered worldwide, making their study an important conservation issue. Many unionids live in shallow environments where they are potentially vulnerable to natural disturbances, but the impact of physical forces on their growth and the role of sediments as a refuge is poorly understood. Here, we validate the use of two types of shell internal lines (nacreous, prismatic) as indicators of physical disturbance and shell coloration as an indicator of sedimentary habitat. We use these indicators to test (1) whether the sediments provide an effective refuge for juvenile and young adult mussels from natural disturbances and (2) whether disturbance events affect their growth. Elliptio complanata (Eastern Elliptio) emerge from the sediments when they are 20–50 mm in size and 2.5–7 years old. Juvenile and young adults lay down more disturbance lines at more exposed nearshore sites, but also in small lake basins with dense mussel populations. Disturbance lines are produced during both endo- and epibenthic growth periods, but in contrast to adults, they are not associated with growth anomalies. Sediments accumulating in shallow nearshore areas of lakes provide an imperfect but effective refuge for native mussels that warrant protection.

This work was supported by: i) POSEUR-03-2215-FC-000096 and ICNF funds under project CP01-MARG-QUERCUS/2018; ii) Project Reviving Douro Basin funded by MAVA, Fondation pour la Nature and iii) European Investment Funds by FEDER/COMPETE/POCI – Operational Competitiveness and Internationalization Program, under Project POCI-01-0145-FEDER-006958 and National Funds by FCT - Portuguese Foundation for Science and Technology, under the projects UID/AGR/04033/2013 and UID/AGR/00690/2019. FCT also support MLL with a doctoral grant (SFRH/BD/115728/2016).

Animals can play important roles in cycling nutrients [hereafter consumer-driven nutrient dynamics (CND)], but researchers typically simplify animal communities inhabiting dynamic environments into single groups that are tested under relatively static conditions. We propose a conceptual framework and present empirical evidence for CND that considers the potential effects of spatially overlapping animal groups within dynamic ecosystems. Because streams can maintain high biomass of mussels and fish, we were able to evaluate this framework by testing if biogeochemical hotspots generated by stable aggregations of mussels attract fishes. We predicted that spatial overlap between these groups may increase the flux of mineralized nutrients. We quantified how different fish assemblage biomass was between mussel bed reaches and reaches without mussels. We compared fish and mussel biomass at mussel beds to test whether differences in animal biomass mediate their contributions to nutrient cycling through nitrogen and phosphorous excretion. We estimated areal excretion rates for each group by combining biomass estimates with measured excretion rates. Fish biomass was homogeneously distributed, except following a period of low flow when fish were more concentrated at mussel beds. Mussel biomass was consistently an order of magnitude greater than fish biomass and mussel areal excretion rates exceeded fish excretion rates. However, the magnitude of those differences varied spatially and temporally. Mussel excretion stoichiometry varied with changes in assemblage composition, while fish excretion stoichiometry varied little. Biogeochemical hotspots associated with mussels did not generally overlap with fish aggregations, thus, under these conditions, animal processes appear to exert additive ecosystem effects.