Microbial communities of the Laurentian Great Lakes reflect connectivity and local biogeochemistry

The Laurentian Great Lakes are a vast, interconnected freshwater system spanning strong physicochemical gradients, thus constituting a powerful natural laboratory for addressing fundamental questions about microbial ecology and evolution. We present a comparative analysis of pelagic microbial communities across all five Laurentian Great Lakes, focusing on Bacterial and Archaeal picoplankton characterized via 16S rRNA amplicon sequencing. We collected samples throughout the water column from the major basins of each lake in spring and summer over 2 years. Two oligotypes, classified as LD12 (Alphaproteobacteria) and acI-B1 (Actinobacteria), were among the most abundant in every sample. At the same time, microbial communities showed distinct patterns with depth during summer stratification. Deep hypolimnion samples were frequently dominated by a Chloroflexi oligotype that reached up to 19% relative abundance. Stratified surface communities differed between the colder, less productive upper lakes (Superior, Michigan, Huron) and warmer, more productive lower lakes (Erie, Ontario), in part due to an Actinobacteria oligotype (acI-C2) that averaged 7.7% of sequences in the lower lakes but <0.2% in the upper lakes. Together, our findings suggest that both hydrologic connectivity and local selective pressures shape microbial communities in the Great Lakes and establish a framework for future investigations.     

Seasonality of Melosira-plankton of the great northern lakes

The paper discusses the seasonality of Melosira-plankton in Ladoga and Onega Lakes and its comparison with that in other large northern lakes. The periodicity of Melosira in dimictic temperate lakes is mainly connected with periods of turbulence of the lake water in spring and autumn. The role of Melosira in the primary productivity of great northern lakes is important both for the oligotrophic phase and in the case of eutrophication. In very advanced states of eutrophication the spring crop of Melosira-plankton may be the cause of oxygen depletion in the hypolimnion.     

Cladoceran assemblages, seasonal succession and the importance of a hypolimnetic refuge

SUMMARY. 1. We investigate the importance of a refuge from fish predation to the abundance, species composition and seasonal succession of zooplankton. Thirty lakes representing a range of depths were sampled twice in summer for physical/chemical parameters and zooplankton community structure.
2. We define the refuge from centrarchid predators to be that space between the thermocline and the zone of anoxia. As lakes vary in rate of oxygen depletion from the hypolimnion. the refuge size and lake depth are independent: refuge size decreases during the summer period.
3. Lake depth and refuge size independently explain variation among lakes in zooplankton species composition, but seasonal community change within lakes is best predicted by loss of refuge size.
4. Refuge size also explains the substantial variation in the relative dominance of the two major daphnid species. Lakes possessing a large refuge are dominated by D. pulicaria ; those with a small refuge are dominated by the smaller, D. galeata mendotae . We suggest that lakes of intermediate refuge size, which are characterized by high species diversity, represent a more equitable balance of predation and competition.     

Diatom–environmental relationships and nutrient transfer functions from contrasting shallow and deep limestone lakes in Ontario, Canada

Diatom assemblages were analysed in the surface sediments of 44 alkaline lakes in south-western Ontario, Canada, and combined with a pre-existing 58 south-eastern Ontario lake set: (1) to determine if shallow, polymictic Ontario lakes contain different diatom assemblages from deeper, dimictic lakes, and if so, which environmental variables most influence assemblages; (2) to improve the existing transfer functions; (3) to construct and compare transfer functions separately for dimictic, deep lakes and for polymictic, shallow lakes. Polymictic and dimictic lakes covered a similar nutrient range (spring total phosphorus (TP)=4–54 g/l, spring total nitrogen (TN)=200–927 g/l; n=101) and spring pH levels (7.6–9; n=101). However, polymictic lakes were shallower (median mean depth = 2.9 m vs. 7.3 m in dimictic lakes). Benthic diatoms (average 60% relative abundance) dominated the polymictic lakes, whereas planktonic diatoms (average 60%) dominated dimictic lakes. A Canonical Correspondence Analyses with forward selection (p < 0.05, 999 Monte Carlo permutations) identified TP, alkalinity, watershed to volume ratios and lake depth as the most important measured environmental variables influencing diatom distribution in both polymictic and dimictic lakes. Additionally, pH was identified as an important variable in polymictic lakes, whereas TN was also forward selected in the dimictic lakes. Adding more lakes to the original southern Ontario calibration set improved the TN transfer function (r² _jack=0.42, root mean squared error of prediction (RMSEP)_jack=0.11 [log g TN/l]), although there was a high systematic error in the revised model (r² _residual = 0.48). However, the strongest TP model was derived from the polymictic lakes (r² _boot =0.44, RMSEP_boot=0.20 [log g TP/l]), which was the smallest lake set (n=30) with the lowest number of diatom species. The stronger TP model from the polymictic lakes may be partly due to the relatively low macrophyte cover in our polymictic lakes, which may lead to stronger benthic–pelagic coupling than in lakes with large macrophyte populations. Additionally, our study suggests that the Chrysophyceae cyst:diatom frustule ratio may be useful for indicating trends in TP levels of 35 g/l in alkaline lakes that are dimictic, but is not necessarily indicative of trophic state changes in shallow, polymictic lakes. Our study demonstrates that it may be important to construct separate diatom-based nutrient transfer functions for polymictic and dimictic lakes.     

The influence of habitat heterogeneity on freshwater bacterial community composition and dynamics

Multiple forces structure natural microbial communities, but the relative roles and interactions of these drivers are poorly understood. Gradients of physical and chemical parameters can be especially influential. In traditional ecological theory, variability in environmental conditions across space and time represents habitat heterogeneity, which may shape communities. Here we used aquatic microbial communities as a model to investigate the relationship between habitat heterogeneity and community composition and dynamics. We defined spatial habitat heterogeneity as vertical temperature and dissolved oxygen (DO) gradients in the water column, and temporal habitat heterogeneity as variation throughout the open-water season in these environmental parameters. Seasonal lake mixing events contribute to temporal habitat heterogeneity by destroying and re-creating these gradients. Because of this, we selected three lakes along a range of annual mixing frequency (polymictic, dimictic, meromictic) for our study. We found that bacterial community composition (BCC) was distinct between the epilimnion and hypolimnion within stratified lakes, and also more variable within the epilimnia through time. We found stark differences in patterns of epilimnion and hypolimnion dynamics over time and across lakes, suggesting that specific drivers have distinct relative importance for each community.     

Evolution and diversity of two cisco forms in an outlet of glacial Lake Algonquin

The diversity of Laurentian Great Lakes ciscoes (Coregonus artedi, sensu lato) arose via repeated local adaptive divergence including deepwater ciscoes that are now extirpated or threatened. The nigripinnis form, or Blackfin Cisco, is extirpated from the Great Lakes and remains only in Lake Nipigon. Putative nigripinnis populations were recently discovered in sympatry with artedi in a historical drainage system of glacial Lake Algonquin, the precursor of lakes Michigan and Huron. Given the apparent convergence on Great Lakes form, we labeled this form blackfin. Here, we test the hypothesis that nigripinnis may have colonized this area from the Great Lakes as a distinct lineage. It would then represent a relict occurrence of the historical diversity of Great Lakes ciscoes. Alternatively, blackfin could have evolved in situ in several lakes. We captured more than 600 individuals in the benthic or pelagic habitat in 14 lakes in or near Algonquin Provincial Park (Ontario, Canada). Fish were compared based on habitat, morphology, and genetic variation at 6,676 SNPs. Contrary to our expectations, both cisco and blackfin belonged to an Atlantic lineage that colonized the area from the east, not from the Great Lakes. Sympatric cisco and blackfin were closely related while fish from different lakes were genetically differentiated, strongly suggesting the repeated in situ origin of each form. Across lakes, there was a continuum of ecological, morphological, and genetic differentiation that could be associated with alternative resources and lake characteristics. This study uncovers a new component of cisco diversity in inland lakes of Canada that evolved independently from ciscoes of the Laurentian Great lakes. The diversity of cisco revealed in this study and across their Canadian range presents a challenge for designating conservation units at the intraspecific level within the framework of the Committee on the Status of Endangered Wildlife in Canada (COSEWIC).     

Pelagic zonation of water quality and phytoplankton in the Great Lakes

Analysis of aquatic ecosystem data collected from large water bodies must consider spatial variations. A suite of pelagic survey stations exists for the Laurentian Great Lakes, but little is known about their redundancy. We present a strategy to delineate the lakes into zones based on water quality and phytoplankton biovolume. Water samples were collected from 72 sites in two seasons (spring and summer) from 2007 to 2010 in all five lakes. Integrated samples were analyzed for phytoplankton biovolume and nine water quality parameters. We conducted cluster analysis, principal components analysis and non-metric multidimensional scaling methods for water quality and phytoplankton taxon-specific biovolume for the Great Lakes basin and for each lake separately. There were significant lake-to-lake differences, and based on lake-specific analyses, Lake Superior, Lake Michigan and Lake Erie were each divided into three zones; Lake Huron and Lake Ontario were each grouped into two zones. The zones identified by water quality and phytoplankton provide an understanding of spatial distributions for evaluating monitoring data.     

CHRYSOPHYTE POPULATION AND ENCYSTMENT PATTERNS IN TWO CANADIAN LAKES1

The dynamics of chrysophyte populations and the onset of encystment in relation to several physical, chemical, and biological variables were studied in two Canadian lakes, Lake Opinicon (LO) and Upper Rock Lake (URL). LO is shallow and polymictic during the ice-free season, whereas URL is deep and dimictic. Seasonal patterns of many limnological variables, as well as species composition and seasonal dynamics of chrysophyte assemblages, were broadly similar in both lakes. Three groups of chrysophytes were recognized: 1) taxa that were more abundant during ice cover and/or early spring and occasionally in late fall (Dinobryon spp. Ehrenberg and Synura spinosa Korsh), 2) taxa that seemed to prefer moderately warm water proglena americana Calkins and Ochromonas sp. Wyssotzki) and had maximum densities in late summer and occasionally in the fall, and 3) taxa that occurred in low numbers and did not exhibit any discernible seasonality in the two lakes (Mallomonas spp. Perty). Encystment rarely occurred during ice cover in both lakes. Stomatocyst concentrations were highest shortly after ice breakup (late April or early May), declined to trace levels by June, and remained at those levels until the end of the sampling season (late November). Encystment rates (cyst concentrations) were higher in dimictic URL. Encystment in both lakes coincided with concurrent changes in physical and chemical variables and chrysophyte vegetative cell densities, but it was not possible to pinpoint the exact conditions responsible for triggering encystment. Only a small proportion of chrysophytes encysted. Cyst production within a lake may vary from year to year.     

The relationship between fecundity and adult body weight in Homeotherms

Summary Bythotrephes cederstroemii Schoedler, a predatory freshwater zooplankter (Crustacea: Cladocera), was first found in the Laurentian Great Lakes in December 1984. The first individuals were from Lake Huron, followed in 1985 with records from Lakes Erie and Ontario. By late August, 1986 the species had spread to southern Lake Michigan (43°N). Bythotrephes has not previously been reported from North America, but has been restricted to a northern and central Palearctic distribution. Its dramatic and widespread rise in abundance in Lake Michigan was greatest in offshore regions. Bythotrephes appears to be invading aggressively, but avoiding habitats presently occupied by glacio-marine relict species that became established in deep oligotrophic North American lakes after the Wisconsin glaciation. Because it is a voracious predator its invasion may lead to alterations in the native zooplankton fauna of the Great Lakes. It offers the chance to study how invading plankton species join an existing community. Judging from its persistence and success in deep European lakes, Bythotrephes may now become a permanent member of zooplankton communities in the Nearctic.     

Testing the enemy release hypothesis: trematode parasites in the non-indigenous Manila clam <Emphasis Type="Italic">Ruditapes philippinarum</Emphasis>

Knowledge of temporal variation in nearshore Laurentian Great Lakes fish assemblages is important for understanding species–habitat associations, how abiotic and biotic influences vary temporally, and when sampling should occur. Using spring and fall seining data from Lake Erie beaches, we compared day and night fish assemblages and tested for differences among sampling periods. Beaches were utilized by a diverse collection of Lake Erie basin fishes (one-third of known species). During all sampling periods, catches were dominated by cyprinid species (53–91%), and by invertivores and planktivorous fishes. Diel differences were detected in abundance, species richness and assemblage structure. Multivariate analyses (canonical analysis of principal coordinates) indicated that season had a larger influence on fish assemblage structure than diel period. Given observed temporal variation in assemblage structure, studies of Laurentian Great Lakes beach fishes should be restricted to a single time period (e.g. day-time spring sampling), or adopt sampling designs that permit diel period and season to be included as factors in analyses. Second, the large seasonal variation in assemblage composition combined with higher night species richness indicates that night sampling during both spring and fall would be the most efficient and comprehensive approach for beach fish inventory. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Trexler     

Factors governing the spatial and temporal distribution of Chironomid larvae in the Maarsseveen lakes with special emphasis on the role of oxygen conditions

Distribution patterns of the larvae of Chironomidae are compared in three water systems in The Netherlands, which vary in trophic state and oxygen regimes. The life cycles and flying periods of some dominant chironomid species in two of the investigated lakes, Lakes Maarsseveen I and II, are determined by comparing data on the seasonal variations in larval densities with existing literature on Chironomidae in the Maarsseveen lakes. In the oligo-mesotrophic Lake Maarsseveen I (LM I), hypoxic or anoxic conditions in the hypolimnion are observed only at the end of the stratification period. A clear zonation of the chironomid fauna is present in this lake. The littoral zone is dominated byCladotanytarsus gr.mancus andStictochironomus sticticus, the littoriprofundal zone byTanytarsus bathophilus, and the profundal zone byChironomus anthracinus. In comparison with the other species in LM I,T. bathophilus larvae show the most variable distribution patterns over time. Larvae are found in all depths from July to September, but disappear from the hypolimnion as soon as oxygen conditions deteriorate. In the eutrophic Lake Maarsseveen II (LM II), oxygen depletion of the hypolimnion starts immediately after the onset of the thermal stratification in June, and continues until autumnal turnover in November. In this lake, the chironomid community consists primarily ofS. sticticus andCl. gr.mancus, and is confined to the narrow littoral zone. No chironomid larvae are found in the deeper parts of the lake. The eutrophic Lake Gijster in the Brabantse Biesbosch is a deep, man-made reservoir, that is artificially destratified during the summer. In this lakeTanytarsus bathophilus is found in the profundal sediments, whereas almost noChironomus is found in this zone. It is concluded that oxygen conditions existing in the deeper regions of the investigated lakes in large part determine the occurrence and distribution of chironomid species. The distribution ofT. bathophilus is limited by unfavorable oxygen conditions and not by the trophic state of the lake. These findings are part of a thesis (HEINIS, 1993).     

Plankton metabolic balance at the margins of very large lakes: temporal variability and evidence for dominance of autochthonous processes

1. Planktonic metabolic balance (PMB_m) of the surface mixed layer (SML) was measured as the ratio of areal rates of gross photosynthesis (AGP) to community respiration (AR) to test the idea that previously neglected allochthonous inputs of organic matter may support chronic excess respiration relative to photosynthesis even in very large lakes during the summer (May–October) season. Four Laurentian Great Lakes coastal sites of varying trophic status, physical structure and dissolved organic carbon (DOC) concentration were studied with oxygen light‐and dark bottle and ¹⁴C methods, with excess respiration anticipated in the higher DOC sites. 2. Planktonic metabolic balance was net autotrophic in 73% of the observations. The calculated mixing depth at which respiration would predominate over photosynthesis was greater than typically observed mixing depths, varying from 11 to 25 m in the more transparent, low DOC (<3 g m⁻³) sites to 8–15 m in the higher DOC (4–6 g m⁻³) sites. Biweekly measurements at one higher and one lower DOC site over two successive summer seasons showed that seasonal gross photosynthesis (ΣAGP) exceeded seasonal community respiration (ΣAR). Despite the location of the sites at the periphery of the lakes, where allochthonous influences should be strongest, the measurements indicated prevailing conditions of net autotrophy in the SML. 3. Individual measurements of AR from this study and the literature were correlated with AGP but season average values were more tightly correlated, suggesting a tighter coupling of metabolic rates on a larger scale and a looser coupling on a shorter scale. The observed temporal variability was variable in pattern among years, and likely to confound inferences based on limited sampling. 4. It is shown that accepted formulations for AGP and AR lead to the conclusion that PMB_m should be largely predictable from knowledge of a biological properties ratio (light‐saturated gross photosynthesis to plankton community respiration, P_max/R) and a physical properties ratio (euphotic to mixing depths, Z_eu/Z_m) and this prediction was confirmed using data from this study and from the literature. The evident success of this model points to the pre‐eminent importance of plankton biomass and physical conditions in determining metabolic balance. Variation in these fundamental factors appears capable of explaining the diversity of PMB_m reported for different Great Lakes.     

Lake morphometry determines Dreissena invasion dynamics

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.

     

The influence of changing climate on the ecology and management of selected Laurentian Great Lakes fisheries

The Laurentian Great Lakes Basin provides an ecological system to evaluate the potential effect of climate change on dynamics of fish populations and the management of their fisheries. This review describes the physical and biological mechanisms by which fish populations will be affected by changes in timing and duration of ice cover, precipitation events and temperature regimes associated with projected climate change in the Great Lakes Basin with a principal focus on the fish communities in shallower regions of the basin. Lake whitefish Coregonus clupeaformis, walleye Sander vitreus and smallmouth bass Micropterus dolomieu were examined to assess the potential effects of climate change on guilds of Great Lakes cold, cool and warm-water fishes, respectively. Overall, the projections for these fishes are for the increased thermally suitable habitat within the lakes, though in different regions than they currently inhabit. Colder-water fishes will seek refuge further north and deeper in the water column and warmer-water fishes will fill the vacated habitat space in the warmer regions of the lakes. While these projections can be modified by a number of other habitat elements (e.g. anoxia, ice cover, dispersal ability and trophic productivity), it is clear that climate-change drivers will challenge the nature, flexibility and public perception of current fisheries management programmes. Fisheries agencies should develop decision support tools to provide a systematic method for incorporating ecological responses to climate change and moderating public interests to ensure a sustainable future for Great Lakes fishes and fisheries.     

Seasonal and decadal patterns in Discostella (Bacillariophyceae) species from bi‐weekly records of two boreal lakes (Experimental Lakes Area,Ontario, Canada)

A recent rise in the relative abundance of Discostella species (D. stelligera and D. pseudostelligera) has been well documented from sedimentary diatom assemblages across the Northern Hemisphere. This unprecedented change over the last ~150 years has been linked to rises in atmospheric temperatures, changes in ice cover, and/or increases in thermal stability, among other factors. The bi‐weekly monitoring data from two boreal lakes at the Experimental Lakes Area (ELA) in northwestern Ontario were analyzed across seasons (spring, summer, and fall) and decades (1970s–2000s). We found that Discostella species are primarily spring/early summer bloomers (i.e., late April to June) in these lakes and changes in concentrations of Discostella over time were most pronounced in the spring or early summer months. Increases in Discostella abundance over time may be linked to earlier ice‐off and a longer period of spring turnover, resulting from increased winter and spring temperatures. It is also possible that a trophic mismatch between the spring diatom bloom and zooplankton is occurring, thus reducing diatom loss rates, and resulting in greater overall abundance. Moreover, the spring dominance of Discostella in our study lakes occurred at a time of the year when nutrient concentrations were at their highest seasonally, suggesting that these taxa are neither limited directly by nutrients, nor responding to enhanced stratification during the summer months in these lakes.     

149 Distribution and Invasion of Bangia Atropurpurea (Rhodophyta) in the Laurentian Great Lakes

Müller et al. (1998) noted that freshwater collections of the genus Bangia formed a distinct group separate from marine entities in gene sequence analyses. Recently, the species epithet B. atropurpurea has been resurrected to represent this freshwater lineage. This taxon is one of many invasive species within the Laurentian Great Lakes. B. atropurpurea was first observed in Lake Erie in 1964 and by 1982 was observed in all of the Great lakes except Lake Superior. The present study was initiated to examine the further spread of B. atropurpurea and determine the origin of these populations. Hence, a survey of all the Great Lakes was conducted in 1995 (86 sites) and again in 2002 (104 sites). Bangia was observed at 43 sites in 1995 and 39 sites in 2002. For the first time, this alga has been observed to be present in the St. Lawrence River (1995), Georgian Bay on Lake Huron (2002) and Lake Simcoe (eastern shore, 2002) and hence this alga appears to be spreading into new locations. Cluster analyses of morphological data reveal three distinct groupings that do not separate according to location or lake basin. Preliminary analyses of ITS 1 and 2 sequences show differences among samples within Lake Ontario and among all Lakes; however, collections from Lake Simcoe are very similar in sequence. We are continuing to examine the relationship of Great Lakes populations with freshwater collections from Europe.     

Effects of hypolimnetic oxygenation on water quality: results from five Danish lakes

Stratified eutrophic lakes often suffer from hypolimnetic oxygen depletion during summer. This may lead to low redox conditions and accumulation of phosphate and ammonia in the hypolimnion. Hypolimnetic oxygenation has been used as a lake management strategy to improve the water quality in five eutrophic dimictic Danish lakes where oxygenation was conducted for 4–20 years. In one lake, the hypolimnetic oxygen concentration clearly improved by oxygenation, whereas the other four lakes still exhibited low mean summer levels (<2.2 mg O₂ l⁻¹). Oxygenation generally increased the hypolimnetic water temperature by 0.5–2°C, but in one lake it increased by 4–6°C. In all lakes, oxygenation significantly reduced the hypolimnetic concentrations of phosphorus and ammonia during stratification. The accumulation of phosphorus and ammonia typically decreased by 40–88%. In two lakes oxygenation was stopped for 1–2 years and here hypolimnion concentrations of both phosphorus and ammonia increased again. Surface water quality only improved in one lake, but was likely also influenced by simultaneously occurring changes in external nutrient loading. Overall, it is concluded that hypolimnetic oxygenation reduces the hypolimnetic accumulation of phosphorus and ammonia and may prevent anoxia in the deeper parts of the lake. However, long-term oxygenation is required and it is uncertain whether the overall lake water quality can be improved by oxygenation. Reduction of the external nutrient loading is still essential to improve lake water quality. Handling editor: Luigi Naselli-Flores     

Chloroflexi CL500-11 Populations That Predominate Deep-Lake Hypolimnion Bacterioplankton Rely on Nitrogen-Rich Dissolved Organic Matter Metabolism and C1 Compound Oxidation

The Chloroflexi CL500-11 clade contributes a large proportion of the bacterial biomass in the oxygenated hypolimnia of deep lakes worldwide, including the world''s largest freshwater system, the Laurentian Great Lakes. Traits that allow CL500-11 to thrive and its biogeochemical role in these environments are currently unknown. Here, we found that a CL500-11 population was present mostly in offshore waters along a transect in ultraoligotrophic Lake Michigan (a Laurentian Great Lake). It occurred throughout the water column in spring and only in the hypolimnion during summer stratification, contributing up to 18.1% of all cells. Genome reconstruction from metagenomic data suggested an aerobic, motile, heterotrophic lifestyle, with additional energy being gained through carboxidovory and methylovory. Comparisons to other available streamlined freshwater genomes revealed that the CL500-11 genome contained a disproportionate number of cell wall/capsule biosynthesis genes and the most diverse spectrum of genes involved in the uptake of dissolved organic matter (DOM) substrates, particularly peptides. In situ expression patterns indicated the importance of DOM uptake and protein/peptide turnover, as well as type I and type II carbon monoxide dehydrogenase and flagellar motility. Its location in the water column influenced its gene expression patterns the most. We observed increased bacteriorhodopsin gene expression and a response to oxidative stress in surface waters compared to its response in deep waters. While CL500-11 carries multiple adaptations to an oligotrophic lifestyle, its investment in motility, its large cell size, and its distribution in both oligotrophic and mesotrophic lakes indicate its ability to thrive under conditions where resources are more plentiful. Our data indicate that CL500-11 plays an important role in nitrogen-rich DOM mineralization in the extensive deep-lake hypolimnion habitat.     

Acid rain recovery may help to mitigate the impacts of climate change on thermally sensitive fish in lakes across eastern North America

From the 1970s to 1990s, more stringent air quality regulations were implemented across North America and Europe to reduce chemical emissions that contribute to acid rain. Surface water pH slowly increased during the following decades, but biological recovery lagged behind chemical recovery. Fortunately, this situation is changing. In the past few years, northeastern US fish populations have begun to recover in lakes that were historically incapable of sustaining wild fish due to acidic conditions. As lake ecosystems across the eastern United States recover from acid deposition, the stress to the most susceptible populations of native coldwater fish appears to be shifting from acidification effects to thermal impacts associated with changing climate. Extreme summer temperature events – which are expected to occur with increasing frequency in the coming century – can stress and ultimately kill native coldwater fish in lakes where thermal stratification is absent or highly limited. Based on data from northeastern North America, we argue that recovery from acid deposition has the potential to improve the resilience of coldwater fish populations in some lakes to impacts of climate change. This will occur as the amount of dissolved organic carbon (DOC) in the water increases with increasing lake pH. Increased DOC will reduce water clarity and lead to shallower and more persistent lake thermoclines that can provide larger areas of coldwater thermal refuge habitat. Recovery from acidification will not eliminate the threat of climate change to coldwater fish, but secondary effects of acid recovery may improve the resistance of coldwater fish populations in lakes to the effects of elevated summer temperatures in historically acidified ecosystems. This analysis highlights the importance of considering the legacy of past ecosystem impacts and how recovery or persistence of those effects may interact with climate change impacts on biota in the coming decades.