Search

The Laurentian Great Lakes have experienced multiple anthropogenic changes in the past century, including cultural eutrophication, phosphorus abatement initiatives, and the introduction of invasive species. Lake Ontario, the most downstream lake in the system, is considered to be among the most impaired. The benthos of Lake Ontario has been studied intensively in the last six decades and can provide insights into the impact of environmental changes over time. We used multivariate community analyses to examine temporal changes in community composition over the last 54 years and to assess the major drivers of long-term changes in benthos. The benthic community of Lake Ontario underwent significant transformations that correspond with three major periods. The first period, termed the pre/early Dreissena period (1964–1990), was characterized by high densities of Diporeia, Sphaeriidae, and Tubificidae. During the next period defined by zebra mussel dominance (the 1990s) the same groups were still prevalent, but at altered densities. In the most recent period (2000s to present), which is characterized by the dominance and proliferation of quagga mussels deeper into the lake, the community has changed dramatically: Diporeia almost completely disappeared, Sphaeriidae have greatly declined, and densities of quagga mussels, Oligochaeta and Chironomidae have increased. The introduction of invasive dreissenids has changed the Lake Ontario benthic community, historically dominated by Diporeia, Oligochaeta and Sphaeriidae, to a community dominated by quagga mussels and Oligochaeta. Dreissenids, especially the quagga mussel, were the major drivers of these changes over the last half century.

In the context-dependent world of biological invasions, biologists understand few general patterns of spread and impact. One possible exception is the zebra mussel (Dreissena polymorpha), an invader that routinely restructures food webs through an ecosystem engineering process termed “benthification.” By efficiently consuming phytoplankton, zebra mussels can increase light penetration and nutrient concentrations in the benthos of a lake, thereby stimulating growth of benthic periphyton (phytobenthos) and macroinvertebrates (zoobenthos). Few studies monitor the response of these benthic communities to invasion. We documented early changes in phytobenthos and zoobenthos as zebra mussels invaded eutrophic Lake Mendota (Wisconsin, USA). From 2015 to 2018, the number of zebra mussel individuals reached densities >30 000 m−2 on hard substrates and 3000 m−2 in macrophyte beds. Community data showed classic signs of benthification, including 300% increases in (non-zebra mussel) zoobenthos and phytobenthos abundance on average across a depth gradient, including significant increases at depths where zebra mussels did colonize. Deep macrophyte biomass increased 900%, but water clarity showed no significant rapid increase. We speculate that nutrient enrichment may be more strongly responsible than increased light penetration for the benthic response of Lake Mendota. Continued integration of benthic production and processes into our study of lake ecosystems will be critical to understanding whole ecosystem function, especially as zebra mussels continue to “benthify” lakes within their invaded range.

The impacts of species invasions can subside over time as ecosystems ‘adapt’ and invaders decline or increase over time as additional species invade. The character and timescales of invasion impacts provide important insights into ecosystem dynamics and management. Yet long-term studies of invasion impacts remain rare and often confound invasive species impacts with coincident environmental change. One way to address this challenge is to ask: what ecological changes over time since invasion are recapitulated in ecosystems that span a range of conditions, are located in different regions, and were invaded in different decades? We synthesize many-decade time series across seven ecosystems to resolve shared changes in seven key ecosystem features following invasion by zebra mussels and subsequent invasion by quagga mussels. These two congeners are among the most widespread invasive species that re-engineer and increasingly co-invade freshwater ecosystems. Seven polymictic shallow lakes with long-term data sets reveal remarkably similar trends, with the strongest ecosystem impacts occurring within 5–10 years of zebra mussel invasion. Surprisingly, plankton communities then exhibited a partial, significant recovery. This recovery was absent, and impacts of initial invasion amplified, in four lakes where quagga mussels outcompeted zebra mussels and more completely depleted phytoplankton. Thus, we show that the ecosystem impacts of invasive species can subside over time but amplify with serial introductions of competing, even closely similar, taxa.

Size spectra analysis (SSA) is used to detect changes in food webs by simplifying complex community structures through abundance-versus-biomass considerations. We applied SSA to 10 years (2006–2015) of data on Great Lakes organisms ranging in size from picoplankton to macrozooplankton. Summer pelagic size spectra slopes were near the theoretical value of −1.0, but spring slopes were steeper, reflecting seasonal differences in abundance of small and large individuals. Pelagic size spectra slopes were relatively stable over the time period we examined. Height (the predicted number of organisms at the spectra midpoint) varied among lakes and was slightly higher in summer than spring in more productive basins. Including benthic data led to shallower slopes when combined with pelagic data, suggesting benthic organisms may increase food web efficiency; height was less affected by benthic data. Benthic data are not routinely included in SSA, but our results suggest they affect slopes and therefore SSA-based predictions of fish abundance. The ability of SSA to track changes in trophic energy transfer makes it a valuable ecosystem monitoring tool.

Dreissenid bivalves (Dreissena polymorpha and D. rostriformis bugensis) are considered the most aggressive freshwater invaders inflicting profound ecological and economic impacts on the waterbodies that they colonize. Severity of these impacts depends on dreissenid population sizes which vary dramatically across space and time. We developed a novel method that analyzes video recorded using a Benthic Imaging System (BIS) in near real-time to assess dreissenid distribution and density across large waterbodies and tested it on Lake Erie. Lake Erie basins differ dramatically in morphometry, turbidity, and productivity, as well as in Dreissena distribution, density, and length-frequency distribution, providing an excellent model to test the applicability of our method across large and dynamic environmental gradients. Results of rapid assessment were subsequently compared with dreissenid density obtained from Ponar grab samples collected at the same sites. In the eastern and central basins, the differences in basin-wide density estimations from BIS and Ponar were 3% and 23%, respectively. In the western basin, this method had limited application due to high turbidity and abundance of small (

Despite significant growth in the information on introduced species (NIS), agreement on their comparative fitness and effects on native (NAT) communities is heterogeneous. Hypotheses aimed at explaining their fitness and impacts are as often supported as challenged. In order to investigate whether origin is associated with fitness or impact, we reviewed 72 meta-analyses of trait differences between NAT and NIS, and impacts of NIS on NAT and the environment, covering terrestrial and aquatic organisms. Interpretation of the trends was based on overall meta-analysis conclusions, and on numbers of point estimates (individual effect sizes) within and across studies. Mixed results were found in 44% of the studies. Higher NIS fitness and significant impacts were found in 31%, and 25% concluded that significant outcomes are largely absent. Point estimates showed that non-significant outcomes (57%) are more common than significant ones. Two thirds yield mixed trait differences and impacts, or do not support the concept that NIS are more fit or have negative effects on NAT. When trait differences or impacts are significant, NAT are usually less fit, or are negatively impacted, but this trend is influenced by the fact that most surveys are based on the most damaging invasive species.

Lake Michigan benthic macrofauna have been studied for almost a century, allowing for a unique analysis of long-term changes in community structure. We examined changes in abundances of three major taxonomic groups of benthic macroinvertebrates (Diporeia, Oligochaeta, and Sphaeriidae) in southern Lake Michigan from 1931 to 2015 and identified the most likely causes for these changes. Abundances of all three groups increased during 1931–1980 with the bulk of these increases occurring in nearshore (≤50 m) waters and coincident with increased loading of phosphorus (P) and subsequent increased primary production. Abundances of all three taxa declined during 1980–2000 again mostly in nearshore waters and coincident with decreased P loading. The quagga mussel (Dreissena rostriformis bugensis) invasion was associated with a further decline in phytoplankton primary production during 2000–2015. Both Diporeia and Sphaeriidae declined in abundance during that time, with Diporeia exhibiting the more pronounced decrease of the two groups. In contrast, Oligochaeta increased in abundance during 2000–2015. The quagga mussel has become, by far, the most abundant benthic macroinvertebrate species in terms of density and biomass. Overall, the primary driver of changes in the abundances of the three major taxa during this 85-year period appeared to be changes in phytoplankton primary production due to changing P loadings and, later in the time series, Dreissena filtering. The dreissenid mussels invasions coincided with a rapid decline of Diporeia abundance, but the mechanism of this negative effect remains unidentified. In contrast, Oligochaeta likely benefited from the quagga mussel invasion, perhaps via quagga-generated food supply.

We examined three decades of changes in dreissenid populations in Lake Ontario and predation by round goby (Neogobius melanostomus). Dreissenids (almost exclusively quagga mussels, Dreissena rostriformis bugensis) peaked in 2003, 13 years after arrival, and then declined at depths 90 m depths. Although the size distributions indicate that round goby is affecting mussel recruitment, we did not find a decline in dreissenid density in the nearshore and mid-depth ranges where goby have been abundant since 2005. The lake-wide densities and biomass of quagga mussels have increased over time, due to both the growth of individual mussels in the shallower depths, and a continuing increase in density at >90 m. Thus, the ecological effects of quagga mussels in Lake Ontario are likely to continue into the foreseeable future.

The impacts of species invasions can subside or amplify over time as ecosystems “adapt” or additional invaders arrive. These long-term changes provide important insights into ecosystem dynamics. Yet studies of long-term dynamics are rare and often confound species impacts with coincident environmental change. We synthesize many-decade time-series across ecosystems to resolve shared changes in seven key features following invasion by quagga and zebra mussels, two widespread congeners that re-engineer and increasingly co-invade freshwaters. Six polymictic shallow lakes with long-term data sets reveal remarkably similar trends, with the strongest ecosystem impacts occurring within 5-10 years of zebra mussel invasion. Surprisingly, plankton communities then exhibited a partial, significant recovery. This recovery was absent, and impacts of initial invasion amplified, in lakes where quagga mussels outcompeted zebra mussels and more completely depleted phytoplankton. Thus, invasion impacts subside over time but can amplify with serial introductions of competing, even closely similar, taxa.

Pelagic-oriented alewife (Alosa pseudoharengus) and benthic-oriented round goby (Neogobius melanostomus) are two important prey fishes in the Laurentian Great Lakes. In 2015, we evaluated their seasonal total energy (TE) across nine Lake Michigan transects. Round goby contained at least 48% more kilojoules of TE than alewife of equal length during spring and summer. TE varied spatially for both species, but only large alewife exhibited a consistent pattern, with higher values along the eastern shoreline. Variation in TE was not explained by site-specific prey densities for either species. Round goby energy density (ED) was higher in Lake Michigan than in central Lake Erie, but comparable to other regions of the Great Lakes. Alewife ED in 2015 was similar to that in 2002–2004 in Lake Michigan, with the exception of November (small alewife ED was 21% higher) and April (large alewife ED was 30% lower). Despite oligotrophication, our study suggests that starvation of juvenile and adults has not been directly contributing to overall declining prey fish abundance, although future research should evaluate the potential for overwinter starvation.

The frequency and intensity of droughts are predicted to increase over the next few decades and understanding the impacts of drought on mussels is imperative for species conservation. Our objective was to examine the impacts of an exceptional drought in 2011 on mussel communities in four tributaries of the Colorado River basin in central Texas by testing hypotheses that post-drought community composition depends on (1) relative abundances pre-drought, (2) life history strategy of the mussels, (3) ability of sites to retain water, (4) changes in temperature and discharge. Surveys were conducted pre- (2005–2011) and post-drought (2017) and environmental conditions during the drought were examined using discharge data, satellite imagery, and water temperature estimations. No mussels were found at 9 out of 30 sites where mussel populations were present pre-drought. The most abundant species pre-drought tended to be the most abundant species post-drought and no significant difference between life history strategies was found. No clear effect of the ability of a site to retain water was detected, but the largest declines in species richness occurred in the tributaries with the lowest mean discharge and highest water temperatures, suggesting that mussels may be especially at risk in rivers with lower discharge.

Benthic invertebrates are important trophic links in aquatic food webs and serve as useful bioindicators of environmental conditions because their responses integrate the effects of both water and sediment qualities. However, long-term data sets for benthic invertebrate assemblages across broad geographic areas are rare and, even if collected, historic data sets are often not readily accessible. This data set provides densities of benthic macroinvertebrates for all taxa collected during lake-wide surveys in Lake Ontario, a Laurentian Great Lake, from 1964 to 2018. This information resulted from surveys funded by the governments of the United States and Canada to investigate the status and changes of Lake Ontario benthic community. Of the 13 lake-wide benthic surveys conducted in Lake Ontario over the course of 54 yr, we were able to acquire taxonomic data to the species level for 11 of the surveys and data to the group level for the other two surveys. Density data are provided for taxa representing the Annelida, Arthropoda, Mollusca, Cnidaria, Nemertea, and Platyhelminthes phyla. Univariate and multivariate analyses revealed that the compositional structure of Lake Ontario invertebrate assemblages differed markedly by depth and were also significantly altered by the Dreissena spp. invasion in early 1990s. The introduction of invasive dreissenids has changed the community historically dominated by Diporeia, Oligochaeta, and Sphaeriidae, to a community dominated by quagga mussels and Oligochaeta. Considering the rarity of long-term benthic data of high taxonomic resolution in lake ecosystems, this data set could be useful to explore broader aspects of ecological theory, including effects of different environmental factors and invasive species on community organization, functional and phylogenetic diversity, and spatial scale of variation in community structure. The data set could also be useful for studies on individual species including abundance and distribution, species co-occurrence, and how the patterns of dominance and rarity change over space and time. Use of this data set for academic or educational purposes is encouraged as long as the data source is properly cited using the title of this Data Paper, the names of the authors, the year of publication, the journal name, and the article number.

Predicting the ecosystem effects of invasive species and the best control strategies requires understanding population dynamics and population regulation. Invasive bivalves zebra and quagga mussels (Dreissena spp.) are considered the most aggressive invaders in freshwaters and have become major drivers of ecosystem processes in the Laurentian Great Lakes. Combining all lake-wide studies of Dreissena spp. conducted in the Great Lakes, we found that invasion dynamics are largely governed by lake morphometry. Where both species are present, quagga mussels generally become dominant in 8–13 years. Thereafter, zebra mussels remain common in shallow lakes and embayments and lake-wide Dreissena density may remain similar, while in deep lakes quagga led to a near-complete displacement of zebra mussels and an ensuing dramatic increase in overall dreissenid density. In deep lakes, overall Dreissena biomass peaked later and achieved ~ threefold higher levels than in shallow lakes. Comparison with 21 waterbodies in North America and Europe colonized by both dreissenids confirmed that patterns of invasion dynamics found in the Great Lakes are very consistent with other waterbodies, and thus can be generalized to other lakes. Our biophysical model predicted that the long-term reduction in primary producers by mussel grazing may be fourfold less in deep compared to shallow lakes due to thermal stratification and a smaller proportion of the epilimnion in contact with the bottom. While this impact remains greatest in shallow areas, we show that when lakes are vertically well-mixed, dreissenid grazing impact may be greatest offshore, revealing a potentially strong offshore carbon and phosphorus sink.

Long time series data can provide insights into dynamics of large lakes. We use the USEPA-GLNPO biological monitoring dataset (phytoplankton, benthos, zooplankton and water quality), collected from 1996 through 2016, to identify whether there is evidence of concordant linear or non-linear trends in community composition, density/biomass/biovolume and major environmental parameters. We show changes in biotic assemblages and water quality variables, particularly in Lakes Michigan and Huron. These include changes in phytoplankton biovolume and zooplankton biomass, increasing invasive Dreissena abundance and decreasing densities of other benthos. Biotic changes are accompanied by pronounced changes in water quality and nutrient ratios. Species change-points, identified using threshold indicator taxon analysis, are often less abrupt, but there are clear shifts in a large proportion of species in each assemblage. The concordance of breakpoints among assemblages or lack thereof provides valuable insight into potential drivers of ecosystem change.

We used the results of seventeen years of Great Lakes benthic monitoring conducted by the U.S. EPA’s Great Lakes National Program Office to describe the spatial and temporal patterns of benthic communities, assess their status, trends, and main drivers, and to infer the potential impact of these community changes on ecosystem functioning. Benthic abundance and diversity were higher at shallow (

The U.S. EPA Great Lakes National Program Office (GLNPO) implements long-term monitoring programs to assess Great Lakes ecosystem status and trends for many interrelated ecosystem components, including offshore water quality as well as offshore phytoplankton, zooplankton and benthos; chemical contaminants in air, sediments, and predator fish; hypoxia in Lake Erie's central basin; and coastal wetland health. These programs are conducted in fulfillment of Clean Water Act mandates and Great Lakes Water Quality Agreement commitments. This special issue presents findings from GLNPO's Great Lakes Biology Monitoring Program, Great Lakes Water Quality Monitoring Program, Lake Erie Dissolved Oxygen Monitoring Program, Integrated Atmospheric Deposition Network, Great Lakes Fish Monitoring and Surveillance Program, and Great Lakes Sediment Surveillance Program. These GLNPO programs have generated temporal and spatial datasets for all five Great Lakes that form the basis for assessment of the state of these lakes, including trends in nutrients, key biological indicators, and contaminants in air, sediments and fish. These datasets are used by researchers and managers across the Great Lakes basin for investigating physical, chemical and biological drivers of ongoing ecosystem changes; some of these analyses are presented in this special issue, along with discussion of new methods and approaches for monitoring.

The lower food webs of Lake Huron and Lake Michigan have experienced similar reductions in the spring phytoplankton bloom and summer populations of Diporeia and cladocerans since the early 2000s. At the same time phosphorus concentrations have decreased and water clarity and silica concentrations have increased. Key periods of change, identified by using a method based on sequential t-tests, were 2003–2005 (Huron) and 2004–2006 (Michigan). Estimated filtration capacity suggests that dreissenid grazing would have been insufficient to directly impact phytoplankton in the deeper waters of either lake by this time (mid 2000s). Despite some evidence of decreased chlorophyll:TP ratios, consistent with grazing limitation of phytoplankton, the main impact of dreissenids on the offshore waters was probably remote, e.g., through interception of nutrients by nearshore populations. A mass balance model indicates that decreased phosphorus loading could not account for observed in-lake phosphorus declines. However, model-inferred internal phosphorus dynamics were strongly correlated between the lakes, with periods of increased internal loading in the 1990s, and increased phosphorus loss starting in 2000 in Lake Michigan and 2003 in Lake Huron, prior to dreissenid expansion into deep water of both lakes. This suggests a limited role for deep populations of dreissenids in the initial phosphorus declines in the lakes, and also suggests a role for meteorological influence on phosphorus dynamics. The high synchrony in lower trophic level changes between Lake Michigan and Lake Huron suggests that both lakes should be considered when investigating underlying causal factors of these changes.

It is well documented that the introduction of dreissenid bivalves in eutrophic lakes is usually associated with decreases in turbidity and total phosphorus concentrations in the water column, concomitant increases in water clarity, as well as other physical changes to habitat that may have cascading effects on other species in the invaded waterbody. In contrast, there is a paucity of data on the ecological ramifications of the elimination or decline of dreissenids due to pollution, bottom hypoxia, or other mechanisms. Using data collected by the U.S. Environmental Protection Agency Great Lakes National Program Office's Long-Term Biology and Water Quality Monitoring Programs, we analyzed the impacts of the hypoxia-induced declines in Dreissena densities in the central basin of Lake Erie on major water chemistry and physical parameters. Our analysis revealed that the decline in Dreissena density in the central basin was concomitant with a decrease in spring dissolved silica concentrations and an increase in total phosphorus and near bottom turbidity not seen in the western or eastern basins. In contrast, opposite patterns in water quality were observed in the eastern basin, which was characterized by a high and relatively stable Dreissena population. We are the first to report that dreissenid-related shifts in water quality of invaded waterbodies are reversible by documenting that the sharp decline of Dreissena in the central basin of Lake Erie was concomitant with a shift from clear to turbid water.

Finding remnant populations of species that are of conservation concern can be difficult, particularly in aquatic habitats. Models of ecological niches can aid in the discovery of refuges. ...

Species distribution models have become widespread in benthic ecology; however, they have mostly been applied to marine and lentic systems. In this study, we applied a maximum entropy model by using remote sensing-derived environmental variables to predict the distribution of 4 major benthic communities dominated by Tubificidae, Naididae, Echinogammarus ischnus, and Dreissena spp, respectively, in the lower Niagara River, NY, USA. The model showed very good accuracy for benthic communities with a narrow distribution range (Tubificidae and Naididae) indicated by the area under the curve test values of 0.906 and 0.987, respectively. In contrast, the model showed poor performance for E. ischnus and Dreissena indicated by the low area under the curve values of 0.615 and 0.618, respectively. Both communities are known to cope with a wide variety of environmental factors and habitats, making their accurate predictions difficult using presence-only data. Our results can further be used to locate important feeding grounds for higher trophic levels, to assess the potential spread of exotic species, and to identify areas for restoration.

Unionid freshwater mussels are a threatened fauna, and understanding their distribution is essential to aid and promote conservation efforts. Therefore, we (1) compared patterns of species richness and endemism of Texas mussel and fish species, as mussels depend on fish for their reproduction and dispersal; (2) examined how distribution and community composition of mussels and fishes varies across river basins; and (3) how much variation in mussel community composition could be explained by the distribution of potential host fish, river basin (as a spatial component), ecoregion (as proxy for large-scale environmental differences), and flow variability. Mussel and fish community compositions in rivers differed significantly between river basins with an east-to-west gradient of decreasing species richness following the transition from sub-humid to arid climate. River basin explained 25% of the variation in mussel community composition, and potential host fish presence explained 20%. The total variation explained by both variables was 34%, as part of the variation in host fish presence (11.5%) was spatially structured by differences in river basins. Flow variability explained an additional 14% of the variation in mussel community composition, and ecoregion an additional 9% compared with river basin alone. Locations of significantly higher mussel species richness and/or endemism were present in rivers from all regions of Texas. These locations should be protected, especially as human population continues to expand and urbanize in these regions. A better understanding of mussel–host fish relationships and the impact of flow variation on the distribution of mussels will be needed to inform conservation efforts.

Human activity has dramatically accelerated both species extinctions and introductions, and the balance of these two processes is generally expected to reduce biodiversity and increase taxonomic homogenization. However, few tests of this hypothesis have been made. We tested whether new macroinvertebrate invaders in North American freshwaters can replace the recent loss of biodiversity, particularly focusing on molluscs. We found that both crustaceans and molluscs are overrepresented among endangered and recently extinct species, as well as among invaders. For molluscs, the number of recently extinct species (79 species) was more than twice that for exotic species (38 species). In addition, molluscan invaders are from different taxonomic families than recently extinct or endangered species. While most extinct and endangered molluscs are from streams and rivers, invaders preferentially colonize lakes and reservoirs. The impact of humans on species introductions and extinctions increases with spatial scale (from local to continental scales), resulting in the increased phylogenetic dissimilarity between introduced species and native communities. Construction of dams and alteration of the flow regimes of lotic systems will continue to lead to the extinction of native species, and promote the spread of invaders, resulting in a loss of biodiversity and taxonomic homogenization.

Lampsilis bracteata (Gould), the Texas Fatmucket, is a regional endemic species in the central Texas biogeographic province which is a candidate to be listed as threatened or endangered under the Endangered Species Act of 1973. Lampsilis bracteata is morphologically similar to the common species L. hydiana (Lea). Here, we examine the molecular taxonomic identification of L. bracteata, and compare its historical range with its current geographic distribution. Tests of genetic affinities based on two mitochondrial genes typically used for DNA barcoding (cytochrome oxidase subunit 1, COI and NADH dehydrogenase subunit 1, ND1) support recognition of L. bracteata as a full species. An unexpected spin-off result was that ND1 sequences of L. satura (Lea), a threatened species in Texas, formed a highly supported cluster within putative L. cardium Rafinesque. As an endemic species, the distribution of L. bracteata has been historically restricted; however, poor land and water management practices have further reduced its distribution from eighteen to just eight streams in the Colorado River drainage and to one stream in the Guadalupe River drainage. For L. bracteata, as for many other imperiled freshwater mussel species, effective conservation measures rely on correct species identification, definition of its geographic range and assessment of its changes in the recent past.

Modification of flow regimes and habitat degradation are the strongest, most common, and often co-occurring human activities affecting riverine populations. Ongoing efforts to restore peak flow events found under pristine flow regimes could increase advection-driven dispersal for many species. In rivers with extensive habitat loss, increased advection could transport individuals from remnant populations into degraded downstream areas, causing restored flow regimes to decrease persistence of threatened species. To resolve the capacity for such ‘washout’ effects across imperiled taxa, we evaluate population growth in spatial matrix models of insect, fish, and mollusc taxa experiencing advective dispersal and either long-term habitat loss or temporary disturbances. As a case study to quantify advective dispersal in threatened species, we use intensive mark-recapture methods in a Rio Grande population of the federally endangered unionid mussel Texas horhshell (Popenaias popeii). Among unionids, the most threatened freshwater taxa of North America, we find high levels of annual downstream emigration (16-51%) of adultP. popeii, concomitant with strong immigration from upstream habitats. For different taxa experiencing such advective dispersal during specific life stages, our population model shows that washout effects strongly reduce population recovery under high levels of habitat loss. Averting this negative consequence of restoring hydrology requires simultaneously restoring or protecting long, contiguous stretches of suitable habitats. Across taxa in heavily impacted systems, we suggest integrating hydrodynamic studies and field surveys to detect the presence of advective dispersal and prioritize areas for habitat restoration to enhance population persistence.

Modification of flow regimes and habitat degradation are the strongest, most common, and often co-occurring human activities affecting riverine populations. Ongoing efforts to restore peak flow events found under pristine flow regimes could increase advection-driven dispersal for many species. In rivers with extensive habitat loss, increased advection could transport individuals from remnant populations into degraded downstream areas, causing restored flow regimes to decrease persistence of threatened species. To demonstrate such possible 'washout' effects across imperiled taxa, we evaluate population growth in spatial models of insect, fish, and mollusc taxa that experience advective dispersal and either long-term habitat loss or temporary drought disturbances. As a case study to quantify advective dispersal in threatened species, we use intensive mark-recapture methods in a Rio Grande population of the endangered mussel Popenaias popeii belonging to the Unionida order, the most threatened faunal taxa worldwide. Our mark-recapture models estimate high levels of annual downstream emigration (16-51%) and immigration from upstream habitats (32-48%) of adult P. popeii, a result consistent with hydrodynamic experiments. Across taxa where such advective dispersal occurs in specific life stages, our population model suggests that washout effects might strongly reduce population recovery under high levels of habitat loss, especially for sessile or shorter lived species. Averting this potential negative consequence of restoring hydrology requires simultaneously restoring or protecting long, contiguous stretches of suitable habitats. In heavily impacted systems, we suggest integrating hydrodynamic studies and field surveys to detect the presence of advective dispersal and prioritize areas for habitat restoration to enhance population persistence.

Dreissena spp. are aggressive invaders of many waterbodies worldwide. However, the accurate assessment of their spatial distribution in large rivers is difficult using traditional sampling techniques such as Ponar grabs or SCUBA diving. The aim of this study was to use sonar technologies and underwater imagery (videos, still images) in tandem with traditional Ponar sampling to predict Dreissena presence, and produce a habitat suitability map to enhance our understanding of its spatial distribution in the lower Niagara River, New York, USA. Geo-referenced maps of environmental variables were generated using three sonar technologies: side scan sonar, multibeam sonar, and an Acoustic Doppler Current Profiler. Dreissena presence/absence was determined at 102 sites along a 10 km stretch using Ponar grabs supplemented by an underwater imagery. Substrate and near-bottom flow were the most important variables affecting Dreissena distribution. Habitats with coarse substrate and near-bottom flow of 0.6–0.80 m/s were predicted to be most often occupied. The habitat suitability model indicates that almost 90% of the stream bed in the river can be considered highly- or moderately suitable habitat. Our results demonstrate that supplementing traditional sampling with sonar technologies and underwater imagery can greatly improve Dreissena distribution assessment at the ecosystem scale.

It is well documented that the introduction of dreissenid bivalves in eutrophic lakes is usually associated with decreases in turbidity and total phosphorus concentrations in the water column, concomitant increases in water clarity, as well as other physical changes to habitat that may have cascading effects on other species in the invaded waterbody. In contrast, there is a paucity of data on the ecological ramifications of the elimination or decline of dreissenids due to pollution, bottom hypoxia, or other mechanisms. Using data collected by the U.S. Environmental Protection Agency Great Lakes National Program Office’s Long-Term Biology and Water Quality Monitoring Programs, we analyzed the impacts of the hypoxia-induced declines in Dreissena densities in the central basin of Lake Erie on major water chemistry and physical parameters. Our analysis revealed that the decline in Dreissena density in the central basin was concomitant with a decrease in spring dissolved silica concentrations and an increase in total phosphorus and near bottom turbidity not seen in the western or eastern basins. In contrast, opposite patterns in water quality were observed in the eastern basin, which was characterized by a high and relatively stable Dreissena population. We are the first to report that dreissenid-related shifts in water quality of invaded waterbodies are reversible by documenting that the sharp decline of Dreissena in the central basin of Lake Erie was concomitant with a shift from clear to turbid water.

Due to cultural eutrophication and global climate change, an exponential increase in the number and extent of hypoxic zones in marine and freshwater ecosystems has been observed in the last few decades. Hypoxia, or low dissolved oxygen (DO) concentrations, can produce strong negative ecological impacts and, therefore, is a management concern. We measured biomass and densities of Dreissena in Lake Erie, as well as bottom DO in 2014 using 19 high frequency data loggers distributed throughout the central basin to validate a three-dimensional hydrodynamic-ecological lake model. We found that a deep, offshore hypoxic zone was formed by early August, restricting the Dreissena population to shallow areas of the central basin. Deeper than 20 m, where bottom hypoxia routinely develops, only young of the year mussels were found in small numbers, indicating restricted recruitment and survival of young Dreissena. We suggest that monitoring Dreissena distribution can be an effective tool for mapping the extent and frequency of hypoxia in freshwater. In addition, our results suggest that an anticipated decrease in the spatial extent of hypoxia resulting from nutrient management has the potential to increase the spatial extent of profundal habitat in the central basin available for Dreissena expansion.

Freshwater mussels of the Order Unionida provide important ecosystem functions and services, yet many of their populations are in decline. We comprehensively review the status of the 16 currently recognized species in Europe, collating for the first time their life-history traits, distribution, conservation status, habitat preferences, and main threats in order to suggest future management actions. In northern, central, and eastern Europe, a relatively homogeneous species composition is found in most basins. In southern Europe, despite the lower species richness, spatially restricted species make these basins a high conservation priority. Information on freshwater mussels in Europe is unevenly distributed with considerable differences in data quality and quantity among countries and species. To make conservation more effective in the future, we suggest greater international cooperation using standardized protocols and methods to monitor and manage European freshwater mussel diversity. Such an approach will not only help conserve this vulnerable group but also, through the protection of these important organisms, will offer wider benefits to freshwater ecosystems.

Although freshwater molluscs in the order Unionoida are considered one of the most endangered groups of animals in the world, sufficient data on their status are lacking for most species. As a result, a species may become rare, endangered, and even extinct before the first population assessment is conducted. This is especially true for endemic species, particularly those limited to remote regions with difficult access. We studied the current distribution and population densities of Popenaias popeii endemic to the Rio Grande drainage in Texas, and developed a method to evaluate changes in the population’s size and distributional range over the last 100 years. Sampling over 250 sites in four rivers that constitute the entire historical range of P. popeii in Texas, we found that this species has likely been extirpated from two rivers. The total length of the rivers populated by this mussel has declined by 75%, and the total P. popeii population size has declined by 72%. The remaining population of this species in the Rio Grande is fragmented, with only one 190-km stretch still supporting high densities. The developed approach could be used for other rare freshwater molluscs to reconstruct their historical range and population size.

Freshwater mussels are a critically imperiled group of mollusks that play key ecological roles and provide important services to humans. The Ambleminae is the only subfamily of these mussels, endemic to North America. Complete mitogenomes have only been sequenced for two of five tribes of the subfamily. Pleurobema oviforme, Amblema plicata, and Popenaias popeii each belong to tribes Pleurobemini, Amblemini, and Popenaidini, respectively, and have not had published mitogenomes. Thus, this study aims to present the complete mitogenomes for these species, to provide a phylogeny of the Ambleminae and confirm the gene arrangements with representation from each of its tribes. The newly sequenced mitogenomes range from 15,852 to 15,993 nucleotides, are composed of 13 PCGs, 22 tRNAs, and two rRNAs and all share the same (UF1) gene order. This work was supported by Portuguese Foundation for Science and Technology (FCT) [grant number ConBioMics/BI-Lic/2019-037 (JTT), grant number SFRH/BD/137935/2018 (AGS)]; COMPETE 2020, Portugal 2020 and the European Union through the ERDF, and by Portuguese Foundation for Science and Technology (FCT) through national funds [UID/Multi/04423/2019] under project ConBiomics: the missing approach for the Conservation of Bivalves Project, and [project number NORTE-01- 0145-FEDER-030286]. Fieldwork in Texas was funded by the U.S. Fish and Wildlife Service, and Texas Parks and Wildlife Department (TPWD) as a Joint Traditional Section 6 Project 407348. info:eu-repo/semantics/publishedVersion

Bivalves are ubiquitous members of freshwater ecosystems and responsible for important functions and services. The present paper revises freshwater bivalve diversity, conservation status and threats at the global scale and discusses future research needs and management actions. The diversity patterns are uneven across the globe with hotspots in the interior basin in the United States of America (USA), Central America, Indian subcontinent and Southeast Asia. Freshwater bivalves are affected by multiple threats that vary across the globe; however, pollution and natural system (habitat) modifications being consistently found as the most impacting. Freshwater bivalves are among the most threatened groups in the world with 40% of the species being near threatened, threatened or extinct, and among them the order Unionida is the most endangered. We suggest that global cooperation between scientists, managers, politicians and general public, and application of new technologies (new generation sequencing and remote sensing, among others) will strengthen the quality of studies on the natural history and conservation of freshwater bivalves. Finally, we introduce the articles published in this special issue of Hydrobiologia under the scope of the Second International Meeting on Biology and Conservation of Freshwater Bivalves held in 2015 in Buffalo, New York, USA., This work was supported by FCT—Foundation for Science and Technology, Project 3599—Promote the Scientific Production and Technological Development and Thematic 3599-PPCDT by FEDER as part of the project FRESHCO: multiple implications of invasive species on Freshwater Mussel co-extinction processes (Contract: PTDC/AGRFOR/1627/2014). FCT also supported MLL under Grant (SFRH/BD/115728/2016).

During the last 50 years the ecosystem of Lake Erie has experienced major environmental changes, from anthropogenic eutrophication in 1930–1960s, to nutrient and pollution abatement in the 1970s, and then the introduction of exotic dreissenids in the 1980s. We used multivariate statistical techniques to examine long-term changes in the zoobenthic community, comparing contemporary collections (2009, 2011–2012) and historical data (1963–1965, 1978–1979, 1993, and 1998). The Lake Erie benthic community underwent significant changes during each decade examined, showing signs of recovery following ecosystem restoration in the 1970s, but then experiencing major structural and functional changes after dreissenid ( Dreissena polymorpha and D. r. bugensis ) introductions. There was a significant temporal trend in community composition changes from 1963 to 2012, and the largest difference was found between pre- and post-dreissenid invasion communities. Currently the lake-wide benthic community is dominated by dreissenids both in density (41%) and total wet biomass (97%), followed by oligochaetes and chironomids. The largest benthic density was found in the central basin, and the greatest biomass in the eastern basin. The number of exotic species found in benthic surveys increased every decade, from 1 in 1963 to 10 in 2009–2012, and the majority of the invaders were molluscs. Whereas the role of benthic invaders in community diversity is still low, their impact has had enormous consequences for the whole ecosystem.

Majority of invasive species discovered in the Great Lakes since 1985 are native to the Ponto-Caspian region, including species that have had strong negative impacts in the Great Lakes (for example, dreissenid mussels and the round goby). The rich biota of the Ponto-Caspian region coupled with a high volume of commercial shipping traffic strongly suggests that this region will continue to be a major source of invasive species to the Great Lakes. To assess invasion risk by Ponto-Caspian fishes that have not been included in previous studies, we reviewed English-language publications and untranslated European literature (published primarily in Russian), focusing on physiological and ecological traits that have proven useful in previous risk assessments. We then used discriminant analysis to identify fishes that had a high probability of becoming established, spreading, and having significant negative impacts in the Great Lakes. Our updated listing of high-risk Ponto-Caspian fishes includes five species identified previously (the Black and Caspian Sea sprat, Eurasian minnow, big-scale sand smelt, European perch, and monkey goby) and five additional species (the Black sea shad, Caspian tyulka, Volga dwarf goby, Caspian bighead goby, and black-striped pipefish). Of these ten species, four (the monkey goby, big-scale sand smelt, Caspian tyulka, and black-striped pipefish) are likely to survive ballast water exchange as eggs, larvae, or adults based on salinity tolerances. Our results can be used to focus ongoing surveillance and rapid response efforts by highlighting Ponto-Caspian fishes that pose the greatest risk of becoming established and having significant negative impacts in the Great Lakes.

Dreissena polymorpha (zebra mussel) and D. rostriformis bugensis (quagga mussel) continue to spread in Europe and in North America, and have large ecological and economic impacts where they invade. Today many more waterbodies are invaded by zebra mussels, and therefore the extent of their impact is greater than that of quagga mussels. Both species provide additional space and food for invertebrates in the littoral zone, increasing their diversity and density. In contrast, in the profundal zone, quagga mussels may compete for space and food resources with benthic invertebrates, decreasing their diversity and density. The system-wide effect of dreissenids depends on water mixing rates, lake morphology, and turnover rates. Because quagga mussels are found in all regions of a lake, and form larger populations, they may filter larger volumes of water and may have greater system-wide effects, especially in deep lakes, than zebra mussels, which are restricted to shallower portions of lakes. Shortly after initial invasion, as populations increase, both dreissenids will have their largest effects on communities, and most of them will be direct effects. After the initial stage of invasion, impacts are less predictable, and more likely to be caused by indirect effects through changes in the ecosystem.

Freshwater bivalves in the family Unionidae are one of the most endangered groups of animals in North America. In Texas, where over 60% of unionids are rare or very rare, 15 species have been added to the state's list of threatened species, and 10 are under consideration for federal listing. Due to insufficient survey efforts in the past decades, however, primary data on current distribution and habitat requirement for most of these rare species are lacking, thus challenging their protection and management. Although the species are listed as valid, there was no genetic confirmation to test for the possibility of ecophenotypes, which complicates conservation efforts. In this paper, we present genetic and distributional data for two rare Texas species, Truncilla macrodon and Truncilla cognata, and we suggest appropriate conservation measures. Tests of genetic affinities of these species using three gene regions, cox1, nad1 and ITS1, supported recognition of T. cognata and T. macrodon as full species. Analysis of historic and current species distribution showed that both these regional endemics are particularly vulnerable, and their distribution range has been reduced in the last 80 years.