Bythotrephes cederströemi are a predatory cladoceran zooplankter that have invaded numerous inland lakes in North America, many of which are stratified and support offshore fishes like Cisco ( Coregonus artedi ). While changes in zooplankton community composition following Bythotrephes invasion predict an increase in Cisco mercury concentrations (Hg), this phenomenon was not detected from a survey evaluating temporal changes in Cisco Hg across a broad range of lakes varying in the presence or absence of Bythotrephes . Here, we compare temporal changes in Cisco bioaccumulation slopes (i.e., slopes of relationships between Cisco Hg and trophic position) from lakes experiencing Bythotrephes invasion over the study period to those already invaded (as a reference) over similar time periods. Our results show that bioaccumulation slopes after Bythotrephes invasion either changed direction entirely (from positive to negative relationships) or decreased in elevation relative to those prior to invasion. No such pattern was observed in previously invaded reference lakes. Reductions in Cisco bioaccumulation slopes and/or intercepts following Bythotrephes invasion suggest that conversion efficiency (and therefore growth) of Cisco increased after invasion (i.e., less Hg accumulates in fish at an equivalent trophic position after vs. before invasion). Back-calculated Cisco growth rates and size-at-age from a second complimentary study were greater in the presence of Bythotrephes than without, further supporting the hypothesis that changes in Hg bioaccumulation are likely due to increased conversion efficiency among invaded populations. These findings highlight the potential importance of foraging energetics over and above shifts in trophic position in modifying fish contaminant concentrations.

Abstract Bythotrephes cederströemi have invaded numerous inland lakes in North America, many of which are stratified and support offshore fishes like the Cisco (Coregonus artedi). While evaluations based on changes in zooplankton community composition following Bythotrephes invasion predict an increase in Cisco mercury concentrations, this phenomenon was not detected from a survey evaluating temporal changes in Cisco Hg across a broad range of lakes varying in the presence or absence of Bythotrephes. Here, we compare temporal changes in Cisco biomagnification factor slopes (i.e., slopes of relationships between Cisco Hg and δ15N isotopes) from lakes experiencing Bythotrephes invasion over the study period to those already invaded (as a reference) over similar time periods. Our results show that biomagnification slopes after Bythotrephes invasion either changed direction entirely (from positive to negative relationships) or decreased in elevation relative to those prior to invasion. No such pattern was observed in previously invaded reference lakes. Reductions in Cisco biomagnification slopes and/or intercepts following Bythotrephes invasion suggest that conversion efficiency (and therefore growth) of Cisco increased after invasion (i.e., less Hg accumulates in fish at an equivalent trophic position after vs. before invasion). Back-calculated Cisco growth rates and size-at-age were greater in the presence of Bythotrephes than without, further supporting the hypothesis that changes in Hg biomagnification are likely due to increased conversion efficiency. These findings highlight the potential importance of foraging energetics over and above shifts in trophic position in modifying fish contaminant concentrations.

Summary This study examined the effect of invasive dreissenid mussels on nutrient and carbon dynamics in a large lake (Lake Simcoe, Ontario). We measured rates of nutrient (phosphorus and nitrogen) and carbon excretion and biodeposition by zebra and quagga mussels and the P, N and C content of their soft tissues and shells at different depths throughout the open‐water season. Measurements were combined with detailed information about dreissenid biomass and lakewide distribution to examine the impacts of dreissenids on whole‐lake dynamics of P, N and C. Mussel tissue P, N and C content and rates of excretion and biodepositon varied among species, seasons and depths, apparently driven by metabolic and stoichiometric factors. Dreissenid mussels excreted, deposited and stored large quantities of P, N and C when compared to lake standing stocks and loadings, and represent an important driver of nutrient cycling in the lake. Living and discarded mussel shell material is shown to represent a potentially important, and hitherto largely overlooked, long‐term sink for P, N and C. The concentration of dreissenid biomass in the well‐mixed and illuminated littoral portion of L. Simcoe results in redirection of nutrients and carbon from offshore areas to the nearshore zone of the lake. Changes in nutrient and carbon distribution and cycling patterns caused by dreissenid establishment in L. Simcoe and other ecosystems can have implications for the distribution of primary and secondary production and should be considered in the context of water quality and nutrient input management.

Changes in the lower trophic level of Lake Simcoe, Canada, coincident with the invasion of dreissenids have been well documented, but little is known regarding the effects of these changes on the pathways of energy flow to higher trophic levels. To evaluate pathways of energy flow, we analyzed stable isotopes of zooplankton, benthic invertebrates, and fish over a 26-year period. Using stable isotopes of carbon (C) and nitrogen (N), we found evidence for a dramatic increase in the importance of benthic-derived nearshore primary production in Lake Simcoe after the invasion of dreissenids; delta C-13 of benthos collected at 5 and 10 m depth increased by 4-5 parts per thousand on average, and benthic warmwater fishes demonstrated a similar increase. In contrast, delta C-13 of profundal (>= 20 m) benthos and pelagic zooplankton were not significantly different during this time period, while offshore pelagic and profundal fishes changed more subtly in magnitude but in the same direction as nearshore benthos and warmwater fishes. The range of delta C-13 values observed across the fish community increased from 3 to 10 parts per thousand, primarily due to a positive temporal shift among warmwater fishes. Our study highlights the increase in functional heterogeneity in Lake Simcoe after dreissenid invasion, likely due to an increase in nearshore benthic production. Accounting for increased contributions of benthic-derived C with time is critical in accurately modelling C and energy transfer in the lake, and for better understanding the degree of nearshore-offshore coupling in the lake.

Lake Simcoe, the largest lake in southern Ontario outside of the Laurentian Great Lakes, is affected by numerous stressors including eutrophication resulting from total phosphorus (TP) loading, climate change, and invasions of exotic species. We synthesized the long-term responses of Lake Simcoe to these stressors by assessing trends in water quality and biological composition over multiple trophic levels. Evidence for climate change included increasing thermal stability of the lake and changes in subfossil diatom communities over time. Although the deep water dissolved oxygen (O-2) minimum has increased significantly since TP load reductions, it is still below estimated historical values and the Lake Simcoe Protection Plan end-of-summer target level of 7 mg O-2 L-1. Low deep water O-2 concentrations corresponded with a decline in coldwater fish abundance. Since 1980, some nutrient concentrations have decreased (spring TP) while others have increased (silica), but many show no obvious changes (ice-free TP, nitrate, ammonium). Increases in water clarity, combined with declines in chlorophyll a and phytoplankton biovolumes in Cook's Bay, were temporally consistent with declines in TP loading and the lake-wide establishment of dreissenid mussels as a major component of the Lake Simcoe ecosystem. Using an investigative tool, we identified 2 periods when abrupt shifts potentially occurred in multiple parameters: 1986 and 1995-1997. Additional ecosystem level changes such as declines in zooplankton, declines in offshore benthic invertebrate abundance, and increased nearshore invertebrate abundance likely reflect the effects of invasive species. The interaction of these multiple stressors have significantly altered the Lake Simcoe ecosystem.

Total benthic invertebrate biomass in shallow offshore waters (depths 10-15 m) increased 17-fold on average following the invasion of dreissenid mussels and implementation of P controls in Lake Simcoe, Ontario, despite a 50% overall decrease in total invertebrate density during the same time period. The increase in total invertebrate biomass at 10- and 15-m depths was primarily dreissenid biomass. Patterns of biomass with depth and dreissenid invasion for individual taxa were typically similar to those for density. Biomass of chironomid and nondreissenid bivalves declined at shallow (10-15 m) sites but increased at the deepest (20-30 m) sites, whereas biomass of oligochaetes declined at all sites. Density and biomass of Amphipoda, Isopoda, and Gastropoda increased at depths >= 10 m, and these taxa were found more frequently in deeper sites after dreissenid invasion than before. Increased habitation of deeper sites by these taxa may be mediated by increased habitat complexity caused by deposition of dreissenid shells, nutrient enrichment of substrate occupied by dreissenids, and improvements in hypolimnetic dissolved O-2 and water clarity observed during dreissenid invasion. Increased length and biomass of profundal chironomids after dreissenid invasion may be the result of taxonomic changes in the chironomid community, which in turn appear to be closely associated with improvements in deep-water O-2 concentrations. Changes in the benthic community described here probably have important consequences for the degree of coupling between nearshore and offshore habitats in Lake Simcoe.

We investigated how establishment of invasive dreissenid mussels impacted the structure and energy sources of the littoral benthic food web of a large temperate lake. We combined information about pre- and postdreissenid abundance, biomass, and secondary production of the littoral benthos with results of carbon and nitrogen stable isotope analysis of archival (predreissenid) and recent (postdreissenid) samples of all common benthic taxa. This approach enabled us to determine the importance of benthic and sestonic carbon to the littoral food web before, and more than a decade after dreissenid establishment. Long term dreissenid presence was associated with a 32-fold increase in abundance, 6-fold increase in biomass, and 14-fold increase in secondary production of the littoral benthos. Dreissenids comprised a large portion of the post-invasion benthos, making up 13, 38, and 56% of total abundance, biomass, and secondary production, respectively. The predreissenid food web was supported primarily by benthic primary production, while sestonic material was relatively more important to the postdreissenid food web. The absolute importance of both sestonic material and benthic primary production to the littoral benthos increased considerably following dreissenid establishment. Our results show drastic alterations to food web structure and suggest that dreissenid mussels redirect energy and material from the water column to the littoral benthos both through biodeposition of sestonic material as well as stimulation of benthic primary production.

Gulf of Mexico (Gulf) fisheries account for 41% of the U.S. marine recreational fish catch and 16% of the nation's marine commercial fish landings. Mercury (Hg) concentrations are elevated in some fish species in the Gulf, including king mackerel, sharks, and tilefish. All five Gulf states have fish consumption advisories based on Hg. Per-capita fish consumption in the Gulf region is elevated compared to the U.S. national average, and recreational fishers in the region have a potential for greater MeHg exposure due to higher levels of fish consumption. Atmospheric wet Hg deposition is estimated to be higher in the Gulf region compared to most other areas in the U.S., but the largest source of Hg to the Gulf as a whole is the Atlantic Ocean (>90%) via large flows associated with the Loop Current. Redistribution of atmospheric, Atlantic and terrestrial Hg inputs to the Gulf occurs via large scale water circulation patterns, and further work is needed to refine estimates of the relative importance of these Hg sources in terms of contributing to fish Hg levels in different regions of the Gulf. Measurements are needed to better quantify external loads, in-situ concentrations, and fluxes of total Hg and methylmercury in the water column, sediments, and food web.

Stable isotope values derived from chemically preserved organisms are a valuable resource for documenting long-term ecosystem changes. However, isotopic correction factors of preservation effects applied to samples stored for decades are frequently based on studies lasting only months, assuming that the effects of preservation stabilize within a short time frame. Very few studies test this critical assumption. We validated this assumption for formalin-preserved invertebrate tissues, finding no significant difference between mean isotopic δ13C and δ15N values of material stored 1–15 years across taxa. Preservation effects were evaluated for Amphipoda, Chironomidae, Dreissenidae, Ephemeroptera, Gastropoda, Isopoda, Sphaeridae, Oligochaeta, and Trichoptera. On average, freshwater benthos δ13C was lower by approximately 2‰ after formalin fixation, whereas δ15N values were not different from control samples. Fixation effects were similar among taxa, but were more pronounced in Gastropoda and Sphaeridae for δ13C and in Trichoptera for δ15N. We reviewed the literature to show that preserved freshwater zooplankton δ13C were slightly but significantly lower relative to control samples (–0.2‰) and higher in δ15N (+0.25‰). The mean decline among marine invertebrate δ13C was greater than for freshwater invertebrates after 1+ years of formalin preservation, but effects on δ15N were not different between marine and freshwater invertebrates.

This study documents the timing of invasion, and initial settlement rates, density and biomass of zebra mussels in Lake Simcoe. Pumped water samples, multi-plate tower samplers, and scuba and benthic airlift surveys were used to sample veligers, post-veligers, juveniles and adult mussels. Veligers were first detected (12 veligers/m3) at one site during August 1992, and again at three sites (6–22 veligers/m3) during August 1994. During July–August 1995 veliger density increased to 12,668–52,480 veligers/m3 at three main basin sites. Low densities of post-veligers (292–1129 individuals/m2) were observed on the multi-plate samplers in the main basin during summer 1994. Post-veliger densities increased by 2–3 orders of magnitude during summer 1995. Peak settlement occurred during July–August in both 1994 and 1995. During mid June to mid August, 1995 we estimated 11,249–31,477 settlers/m2/day for sites in the main basin but only 140 and 277 settlers/m2/day in Cook's Bay and Kempenfelt Bay. Growth rates of post-veligers on the tower plates were highest at the low density sites. Juvenile mussels were first detected (27–160 mussels/m2) on natural rock substrates during scuba and airlift surveys at 2–6m in the main basin in December 1994 and March 1995. Two year-classes of zebra mussels, 1994 and 1995, were subsequently observed on rocky substrates in the main basin during February 1996. Juveniles comprised 98.0±1.8% of the population at that time. The mean density at depths of 2–6m was 32,529 zebra mussels/m2 with a mean shell-free dry biomass of 34.8g/m2.

Climate change is anticipated to have major physical and biological effects on aquatic ecosystems. Changes to the thermal regime of lakes such as shorter duration of ice-cover, earlier onset of stratification and an increase in the length of the stratified period have already been observed in some lakes in North America and Europe. Using vertical temperature profiles of Lake Simcoe during the open-water period from 1980 to 2008 we evaluated trends in water column stability, the onset of stratification and the timing of fall turnover at three stations situated in the lake's two bays and its main basin. Dramatic changes were observed at the three stations; each stratified earlier in the spring, mixed later in the fall and remained stratified for more than a month longer in 2008 compared to 1980. Long-term monotonic trends in the timing of the onset of thermal stratification, fall mixing and duration of stratification were significant. These trends resulted from changes in water column density that are strongly correlated with increasing average air temperatures over time, indicating that climate warming has had a significant effect on the thermal regime of Lake Simcoe.

Establishment of dreissenid mussels in aquatic systems is often accompanied by major changes in the abundance, diversity, and community composition of benthic invertebrates. However, few studies have been published that address the effects of long-term dreissenid presence on the littoral benthos inhabiting hard substrata in lakes. We present the results of a depth-stratified, quantitative survey of littoral benthos conducted at 4 sites in 1993, just before dreissenid invasion, and in 2008,14 y after the establishment of dreissenids in Lake Simcoe, Ontario. Average densities of nondreissenid invertebrates were 45x greater in 2008 than in 1993. Amphipods, isopods, chironomids and oligochaetes underwent the largest increases in absolute abundance. The taxonomic diversity of the benthic invertebrate community (alpha diversity) increased significantly. Community structure was more similar within and between depths and sites in 2008 than in 1993, a result implying lower beta diversity of the postdreissenid littoral benthos. In addition, fewer differences were found in the densities of organisms between sampling locations in 2008 than in 1993. We suggest that dreissenids increase resource availability to benthic organisms and homogenize the littoral benthos by increasing the evenness of the distribution of food and physical-habitat resources across sites and depths. The transformation of the littoral benthic community of Lake Simcoe reflects a major change in the distribution of energy in the lake and is consistent with a dreissenid-mediated redirection of production from the pelagic to the littoral zone.

We compared predation rates of two Lake Simcoe crayfish, Orconectes virilis and Orconectes propinquus, on lake trout and lake whitefish eggs and alevins in laboratory experiments and assessed the potential impact of predation on egg survival in the lake. Experiments were conducted at 4 and 8°C on gravel and one or three layers of cobble substrate. Predation rates on lake trout eggs at 8°C were 2.3 and 3.6eggs/crayfish/day on gravel, 1.3 and 1.4 on one layer of rock, and 1.2 and 0.2 on three layers of rock for O. propinquus and O. virilis, respectively. Similar rates were observed for lake whitefish eggs but the effect of substrate depth was greatly reduced. Predation rates were significantly lower at 4°C. Overall O. propinquus was a more effective predator of eggs than the larger O. virilis, even after adjustment to a common body size. Lake trout alevins were eaten by both species, but at about one third the rate of lake trout eggs. Vulnerability of alevins declined in deeper substrate, especially for older alevins. In the lake O. propinquus was abundant on rocky substrates to the near exclusion of O. virilis. Based on observed densities of O. propinquus in Lake Simcoe, estimated consumption of lake trout eggs ranged from 5.6 to 16.6eggs/m2/day depending on temperature and substrate depth. Shifting crayfish dominance from O. virilis to O. propinquus, or vice versa, has the potential to significantly influence egg survival and recruitment success of lake trout and to a lesser degree lake whitefish in Lake Simcoe.

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This study develops a quantitative model of the combined effects of temperature and ambient dissolved oxygen on metabolic scope-for-activity and power capacity of juvenile lake trout (Salvelinus namaycush). The model provides a framework for evaluating the effects of hypoxia on the capacity of lake trout to perform critical daily life support activities. Maximum power output for sustained swimming of yearling lake trout occurred at 12–20 °C and a dissolved oxygen concentration of >7 mg·L–1. At 4–8 °C, temperatures typical of the hypolimnetic summer habitat of juvenile lake trout, maximum power capacity was reduced by 33%, 67%, and 100% at ambient dissolved oxygen concentrations of 7, 5, and 3 mg·L–1, respectively. Analysis of power outputs, growth impairment, and recruitment success indicated that attainment of 3/4 power capacity would accommodate most daily life support activities of juvenile lake trout. At 4–14 °C, the threshold dissolved oxygen concentration for attainment of 3/4 scope-for-activity varied from 7.5 to 6.6 mg·L–1, respectively, with a mean and standard deviation of 7.04 ± 0.33 mg·L–1. A dissolved oxygen criterion of 7 mg·L–1is recommended for protection of the hypolimnetic habitat of juvenile lake trout.

We examine evidence that biotic factors, particularly predation, may be limiting early survival of wild lake trout (Salvelinus namaycush) juveniles in many areas of the Great Lakes. The Great Lakes contain numerous potential predators of lake trout eggs and fry, some of which are recent invaders, and most of which were probably absent when lake trout most recently re-invaded the Great Lakes after the last ice age. Simple quantitative models of predation suggest that plausible assumptions about prey densities, predator feeding rates, and duration of exposure of predator to prey can lead to very high estimates of predation mortality, in some instances approaching 100%. Indirect evidence from inter-Great Lake comparisons and inland lake examples also suggest that biotic factors may impede successful lake trout colonization. Our synthesis of the evidence leads to recommendations for research to better define field feeding rates of lake trout egg and fry predators and comparative studies of densities of potential egg and fry predators on lake trout spawning reefs. Management options should be designed to provide useful information as well as achieve short-term goals. From a management standpoint we recommend that: newly constructed lake trout reefs should be placed well away from concentrations of potential predators; offshore spawning reefs should be stocked; salmonine stocking, nutrient abatement, and commercial harvest of alewives should all be considered as options to enhance survival of young lake trout; hatchery lake trout should not be stocked at sites where wild lake trout are showing signs of recovery; and exotic species expansions or introductions must be curtailed to maintain or improve on our recent successes in lake trout rehabilitation.