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Within the Laurentian Great Lakes, many native fishes use wetlands for spawning; however, these areas are also used by non-native common carp (Cyprinus carpio) that may impart negative ecological impacts. There is interest to manage common carp using barriers to decrease passage to specific habitats (e.g., their spawning sites), but these barriers could also exclude native wetland obligate spawners such as largemouth bass (Micropterus salmoides) and northern pike (Esox lucius). Our objective was to determine if differences in phenologies, specifically, spawning movements could be exploited in shallow areas to operate seasonal barriers that are opened and closed to promote selective fragmentation. Using a long-term dataset from the Cootes Paradise Marsh fishway (Hamilton, Ontario), we generated predictive models based on cumulative growing degree day (CGDD) for all three fishes. These models successfully predicted earlier arrival by all species in a warmer year and delayed spawning movements during a cold year, supporting the role of temperature as a driver of interannual variation in spawning movements. We then compared the fishway model predictions to movement timing to spawning habitat within nearby Toronto Harbour; movement data was derived from acoustic telemetry information. We found that the models performed well and predictions were correlated with movements of all three species, but performance was weakest for northern pike. It is our hope that managers could use these predictive models to assist in the operation of seasonal barriers to optimize control of non-native common carp, while minimizing negative impacts for native species that require access to coastal wetlands.

The degradation and loss of ecologically important wetlands has been a topical issue in the Great Lakes region, where 60–80% of the coastal wetlands have been lost since the 1800s. The present modeling study aims to guide the restoration efforts in Cootes Paradise marsh, one of the most degraded shallow wetlands in Southern Ontario. We use a process‐based eutrophication model designed to reproduce the biotic competition among multiple phytoplankton and macrophyte functional groups. Our primary focus is to offer guidelines for wetland restoration by characterizing the ecophysiological processes of the autotrophic assemblage, such as the nutrient uptake from the water column and/or the sediment pore waters, the relative ability to harvest light and fuel photosynthesis, and temperature control of the algal/macrophyte growth and basal metabolism. We predict that the additional reduction of external phosphorus loading in Cootes Paradise could induce an abrupt, non‐linear shift from the current turbid phytoplankton‐dominated state to a desirable clear macrophyte‐dominated state. The emergence of this critical (or tipping) point, where the shift to another ecological state may occur, can be accelerated by the presence of a thriving macrophyte community with an enhanced ability to sequester phosphorus. However, it may also be delayed by the presence of a suite of biogeochemical mechanisms (often referred to as “feedback loops”), such as the remobilization of legacy P due to sediment diagenetic processes, wind resuspension, bioturbation, hydraulic loading from local tributaries, water‐level fluctuations, and the leachable P pool of dead plant material that can be returned into the water column through senescence. Our study identifies the restoration actions required to minimize the likelihood of prolonged hysteresis and to facilitate a shift to a desirable ecological state in the foreseeable future. The areal expansion of aquatic vegetation will not only lead to the establishment of a thriving meadow and emergent vegetation community, but may also pave the way for submerged macrophytes through a suite of synergistic mechanisms. Additional point‐source loading reductions will facilitate the transition to an alternative stable clear macrophyte‐dominated state, but could also consolidate the future resilience of the marsh.

Environmental modelling is one of the pillars of the management process, representing an “information integrator” that brings together scientists, managers, and other stakeholders in a joint assessment of our understanding of the system being managed and the compelling knowledge gaps we seek to answer through monitoring and research. The overarching goal of the present modelling study is to offer insights into the restoration and management of Cootes Paradise marsh, one of the most degraded shallow wetlands in Southern Ontario. We use a mechanistic model to leverage our understanding of the major phosphorus biogeochemical pathways in Cootes Paradise. We also develop a network of statistical models that accommodates the spatial heterogeneity of the prevailing water quality conditions in the marsh. Combining the insights from both models, we found that a drastic reduction of point- and nonpoint-source nutrient loading could trigger a non-linear shift from the current turbid-phytoplankton dominated state to a clear-macrophyte dominated state. The restoration trajectory of the system can be profoundly modulated by the presence of a thriving macrophyte community with an enhanced ability to sequester phosphorus (i.e., the net amount of phosphorus taken up per unit of plant tissue). Critical remedial actions to re-establish the targeted macrophyte species in their native marsh habitats include both the intensification of local planting efforts and the control of invasive plant species. Another important finding of our modelling analysis is that the signature of the water quality improvements brought about by nutrient loading reductions dissipates as we move from the marsh’s western end to the central area due the presence of confounding factors, such as the hydraulic loading from Spencer Creek, internal nutrient loading, wind resuspension, and bioturbation. Given the high uncertainties associated with forecasting drastic (and costly) remedial actions, we argue in favour of a socioeconomic assessment of Cootes Paradise marsh as an ecosystem service provider to determine the benefits in terms of monetary values when we examine different courses of management options.

Cootes Paradise marsh, a hypereutrophic wetland draining into the western end of Hamilton Harbour, Ontario, has historically been considered an important regulatory factor of the severity of local eutrophication phenomena. In this study, we present a modelling exercise that aims to draw inference on the relative contribution of various external and internal flux rates to the phosphorus budget of Cootes Paradise. We first examined the capacity of a phosphorus mass-balance model, accounting for the interplay among water column, sediments and macrophytes, to reproduce the observed total phosphorus dynamics over a 17-year period (1996–2012). Water level fluctuations were one of the key challenges for balancing the phosphorus budget during model calibration. Our analysis shows that the model satisfactorily reproduced the average seasonal patterns, as well as the year-to-year total phosphorus variability (coefficient of determination = 0.20, relative error = 26.8%, root mean square error = 62.2 μg P l−1, model efficiency = 0.15). However, our model failed to capture two years of the study period (1997 and 2007), when ambient TP levels significantly deviated from the typically prevailing conditions. Model sensitivity analysis identified the sedimentation of particulate material and diffusive reflux from sediments as two critical processes to characterize the phosphorus cycle in the wetland. Based on the current parameter specification, our model postulates that the sediments still act as a net sink, whereas macrophyte processes (respiration rates, nutrient uptake from interstitial water) appear to play a minor role. We conclude by discussing the various sources of uncertainty and additional remedial actions required in Cootes Paradise marsh to realize a shift from the current turbid-phytoplankton dominated state to its former clear-macrophyte dominated state.

We examined territorial defense and behavioural interactions between two species of fish resident in Hamilton Harbour: non-indigenous round gobies (Neogobius melanostomus) and native logperch (Percina caprodes). Trials consisted of placing one fish, “the resident” (either a round goby or a logperch), in a tank with a shelter for 24 hours before adding another fish, “the intruder” (either a round goby or a logperch), and recording aggressive incidents. Overall, gobies exhibited more aggressive behavior than logperch, and in general resident status had no effect on amount of aggression displayed. Also, gobies spent more time in shelters than logperch, and overall resident status did not affect the amount of time spent under shelter. We also compared abundance data for gobies and logperch using electrofishing transects in Hamilton Harbour that were conducted in 1995 and 2001 and found a dramatic increase in round goby numbers and a non-significant decrease in logperch numbers. Our data suggest that gobies are superior space competitors and hence the range expansion coupled with an increasing population size of the round gobies in Hamilton Harbour is likely to have deleterious consequences for logperch populations.

Carp were excluded from Cootes Paradise Marsh (Lake Ontario) in 1997 in order to improve water clarity and promote submerged plant growth. On average, turbidity at open water and vegetated areas was reduced by 40 and 60 percent, respectively, following carp exclusion. However, responses by plants and other trophic levels have been both spatially and temporally variable due in part to annual variation in environmental conditions and fish-zooplankton interactions. In 1997, an unusually cool spring delayed the migration of spawning fish, including a usually abundant planktivore population, into the marsh. This had three main effects: 1) large Daphnia were released from predation in early summer and reached unprecedented numbers (530 Daphnia/L) in open water areas, 2) despite the lack of vegetated marsh habitat, larval fish production was high due to reduced predation and abundant zooplankton prey, and 3) zooplankton grazing initiated a spring clear-water phase which, together with carp exclusion, promoted submerged plant growth in shallow areas previously devoid of vegetation. These newly vegetated areas showed the greatest improvements in clarity and macrophyte growth in the first 2 years following exclusion. Currently, however, the future of the biomanipulation remains uncertain, due in part to natural climatic confounding factors, and further remedial actions will be required before this wetland represents a stable, clear-water, macrophyte dominated state.