Punctuated, mass mortality events are increasing for many animal taxa and are often related to climatic extremes such as drought. Freshwater mussels are experiencing increased mass mortality events linked to hydrologic drought. Because mussels play important functional roles in rivers it is important to understand the ecosystem effects of these die-offs. Here, we address how mass mortality events of mussels caused by drought may impact stream ecosystem function. We first present a conceptual model, based on the literature, of how mussel mass mortality should affect different ecosystem functions across various ecological time scales, from hours to decades. Next, we highlight two case studies of drought-linked, mussel-mass mortality events from rivers in the southern U.S. We then present the results of an experiment we performed quantifying the ecosystem effects of a punctuated mussel die-off. Finally, we combine our experimental results with field data from a recent mussel die-off to predict how mussel losses will influence ecosystem function. Based on the presented case studies, our mesocosm experiment, and our extrapolated nutrient pulse due to a mussel die-off, we conclude that stream ecosystems are extensively altered following mussel mass mortality events. Mussel loss is governed by drought severity, location within the river network, and species-specific drought tolerances. In the short term, decomposing carrion from mussel die-offs releases a large pulse of nutrients into the water which stimulates food web productivity. In the long term, the overall loss of mussel biomass, and the loss of functional traits as more sensitive species decline, leads to decreases in ecosystem function which may take decades to recover. Drought and human demand for water will make mussel die-offs more likely in the future and it is unlikely that drought sensitive species will recover without changes in water management and restoration of populations through mussel propagation. Our research provides an example of how the loss of an abundant, long-lived organism has cascading, and long-term impacts on ecosystems.

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1. Freshwater mussels (Bivalvia: Unionoida) are globally imperilled and are the subjects of wide-ranging conservation initiatives. This study combined traditional species-monitoring surveys with a novel functional trait classification scheme and publicly available environmental data to assess potential environmental drivers of declining mussel abundance and species richness. 2. Surveys to document mussel abundance and assemblage composition in south-east Oklahoma, United States were conducted on the Glover, Mountain Fork and upper Little rivers. Present day survey results (2015-2021) were compared with those from previous studies (1993-1999, 2010) to document long-term changes in the species and functional composition of mussel assemblages and concurrent changes in climate and land use. 3. Mussel catch per unit effort declined by 71.5% between historical and present day surveys. Species richness declined by 44.4% over this same period. Using a novel classification of mussel drought sensitivity, it was found that the declines were associated with a disproportionate loss of drought-sensitive taxa (67.0% decline)those classified as drought-tolerant did not decline in abundance. Mussel declines coincided with the loss of open surface waters (such as streams, ponds and lakes) and riparian wetlands, increased local air temperatures and longer and more intense hydrological drought. 4. These findings indicate that for a complete understanding of the causes and consequences of mussel declines, conservation biologists must not only monitor the species composition and abundance of threatened organisms, but also consider functional traits. The results further underscore the importance of long-term monitoring for long-lived organisms owing to the decadal time scales over which climate and land use change occur.

Between global climate change and anthropogenic water needs, freshwater systems are becoming more intermittent, stressing organisms adapted to perennial waters. Drought‐induced intermittency concentrates aquatic organisms into drying pools. These pools represent refugia from desiccation but apply other stressors, such as extreme temperatures and increased competition for dwindling resources. In the Southern United States, fishes and freshwater mussels are often concentrated together in drying pools during seasonal, summer droughts. This can result in increased competition for food among invertivorous fish, but also increased food abundance for these fish because mussels increase macroinvertebrate abundance. Further, since mussels require fish as hosts for their ectoparasitic larval phase, glochidia, competition with their host during this biologically active time is detrimental to mussels. We conducted an experiment to examine the effects of freshwater mussels on trophic resources and fish survival in drying pools. We stocked mesocosms with juvenile largemouth bass that were infected or uninfected with glochidia and tracked abiotic conditions, trophic resources and fish survival for 10 days. We found that fish survived longer in the presence of adult mussels, regardless of their infection status. We suspect that prey items supplemented by adult freshwater mussels increased the survival of fish. Thus, the presence of adult mussels and the resources that increase in their presence potentially mitigate stress to fish in “ecological crunch times.” By conserving mussels, fish populations might withstand droughts more easily.

In streams, unionoid mussels and fish form aggregations that exert bottom‐up and top‐down effects on food webs, but the magnitude and spatial extent of their effects are controlled by species traits. Sedentary mussels live burrowed in the sediment in patchily distributed dense aggregations (mussel beds) where they filter seston and provide a local, relatively constant nutrient subsidy. In contrast, fish move on and off mussel beds, and thus comprise a transient nutrient subsidy. We asked how overlap between fish and mussels influences nutrient recycling and resource distribution in streams. We conducted an 8‐week study in experimental streams where we created mussel beds (comprised of two species, Actinonaias ligamentina and Amblema plicata), manipulated the occurrence of a grazing minnow (Campostoma anomalum), and tracked nutrient (nitrogen and phosphorus) and resource (algae, detritus, and chironomids) abundance up and downstream of the mussel beds. In general, neither consumer had strong effects on the concentration or spatial distribution of nutrients. Water turnover time in our experimental streams may have diluted fish and mussel nutrient excretion effects, making it difficult to detect spatial patterns during a given sampling period. Fish controlled the abundance and productivity of algae. In treatments without fish, large mats of filamentous algae formed early in the experiment. These algae senesced, decomposed, and were not replaced. When fish were present, algae consisted of attached biofilms with consistent biomass and spatial distribution over time. Although previous work has shown that mussels can have strong, seasonal bottom‐up effects on both primary and secondary production, our results suggested that adding grazing mobile fishes, led to a more consistent and homogenous supply of algal resources. Because mussels rarely occur in the absence of fish, considering their combined influence on ecosystem dynamics is likely to be important.

As body size often predicts energetic requirements and fecundity, understanding the drivers behind size variation is important. Neo‐Bergmann's rule states that larger individuals are found at higher latitudes and this size variation is attributable to temperature gradients. In ectotherms, this macroecological pattern has mixed support within the literature—both the direction and mechanism of size correlation with latitude varies. We asked if two species of freshwater mussels with different thermal niche preferences, Amblema plicata and Lampsilis cardium, follow neo‐Bergmann's rule and what mechanisms might drive that latitudinal variation. Lampsilis cardium is a thermally sensitive species intolerant of high temperatures, and A. plicata is more tolerant of a wide range of temperatures. We predicted that the thermally sensitive species at southern latitudes would have stunted growth in the summer and that this stunting would produce a steeper relationship with latitude than in the thermally tolerant species. We collected and thin‐sectioned 113 A. plicata shells from 23 sites and 85 Lampsilis cardium shells from 12 sites across a latitudinal gradient in the eastern U.S.A. We used back‐calculated size‐at‐age data to determine logistic growth parameters for each taxon across this gradient. We used Bayesian model selection to evaluate how environmental information correlated with potential maximum size. We found that both mussel species reached a larger potential maximum size at higher latitudes, with the thermally sensitive species having the larger increase in potential maximum size across the gradient when compared to the thermally tolerant species. Average annual water temperature explained mussel potential maximum size better than annual catchment precipitation, annual minimum flow ratio, and species identity. Both increased average water temperature and annual catchment precipitation, two predicted outcomes of climate change, were associated with smaller potential maximum size. Reductions in mussel maximum size due to climate change induced shifts in temperature regimes and precipitation patterns could lead to reduced reproductive output of this threatened guild and subsequent changes in ecosystem function and services, such as decreased biofiltration. As climate change alters precipitation patterns and stream productivity, and increases water temperature, understanding size variation and its cause in aquatic organisms will be important for managing these vulnerable populations.

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The status of species in freshwater systems shift over time due to natural and anthropogenic causes. Determining the magnitude and cause of these shifts requires a long‐term perspective. This process is complicated when there are also questions about the taxonomic validity of a species. Addressing these issues is important because both can undermine conservation and management efforts if incorrect. Pleurobema riddellii, Louisiana Pigtoe, is under review for protection under the U.S. Endangered Species Act, but its status in the Trinity River basin, where the taxon was described, remains in doubt due to questions about its taxonomy and occurrence within this basin. To address these questions, we compared shell morphometrics of P. riddellii dating to the late Holocene with modern P. riddellii, late Holocene Fusconaia sp., and modern Fusconaia sp. using multivariate analyses to test associations between the putative morphotypes. Based on these analyses, we demonstrate that P. riddellii was likely present in the Trinity during the late Holocene, which indicates questions about its taxonomic validity or presence in this basin are unfounded. Our study further highlights the role zooarchaeological studies can play in status assessments and their utility in better understanding biogeographic patterns for rare species.