Lake trout lakes are a rare but highly-valued resource within Canada. In Ontario, the ~2200 lakes that contain lake trout (Salvelinus namaycush) represent 20-25% of the world's supply. Most are located on the Precambrian Shield, and many are at or near the southern range of the species. In Ontario alone, anglers spend an estimated 1.7 billion dollars on goods and services related to recreational fishing, with lake trout often the preferred species. However, as lake trout are large-bodied, long-lived, and late maturing, they have low recruitment rates and are vulnerable to a variety of external factors that can reduce their survival, principally, over-exploitation, competition for food with introduced species, and habitat loss. At the southern limit of their range, lake trout live within narrow temperature and oxygen boundaries in the hypolimnia of stratified lakes, and there is growing concern that this habitat may be declining due to oxygen depletion. A hallmark of excessive nutrient loading to lakes, especially phosphorus (P), is the depletion of dissolved oxygen (DO) in hypolimnetic waters to levels that are harmful to multi-cellular life during the summer months. Although some oxygen depletion is normal in unproductive (oligotrophic) lakes, nutrient-enhanced depletion may adversely affect economically- valuable fish species such as lake trout. As oxygen levels decline, so too does the metabolism of fish species living in the hypolimnion, impairing their ability to swim, feed, grow, recruit and avoid predators.
Recent evidence of declines in deepwater DO concentrations in lakes where nutrient inputs have not increased, and even in lakes where P inputs or lake concentrations have decreased is of particular concern. We hypothesize that exceptions to the "increased nutrients, decreased deepwater oxygen" paradigm, particularly in remote lakes that are far removed from direct human influence, are linked to recent climate warming, through its effects on ice dynamics, and the duration and strength of thermal stratification. Climate warming appears to be the new "threat multiplier" for a large number of limnological issues, including deepwater DO levels. Even without significant changes in hypolimnetic temperatures or oxygen depletion rates, the earlier onset of stratification lengthens the stratification period resulting in a greater degree of oxygen depletion by the summer's end. Moreover, increased anoxia at the sediment-water interface may increase the likelihood of harmful cyanobacteria blooms even at P concentrations < 20 μg/L, and there is evidence that this has already begun. Specifically, there have been recent observations of cyanobacteria blooms in several central Ontario lakes that normally have a low risk of developing blooms, including Lake Simcoe (a lake trout lake and drinking water source) and the adjacent Lake Couchiching. If correct, this is a paradigm shift with tremendous consequences for Canadian lake and fisheries management, and also for the protocols and models that are currently used by managers to assess lake carrying capacities and quantify available lake trout habitat. Currently, Canada suffers from a critical deficit in the reliable tools necessary to project future trends and scenarios for [DO] in Lake Trout lakes in a changing world. Managers require these data to fully understand the relationships between multiple stressors and fish habitat, water quality and harmful algal blooms, as well as for the development of sound, evidence- based policies to protect this valuable resource.
The development of predictive models for hypolimnetic [DO] in lakes, and tracking long-term shifts in overall [DO] conditions in response to environmental stressors, are not trivial tasks. Spatial variation in lake morphometry and mixing regimes, water colour, and variability in seasonal weather patterns (i.e., air temperature and wind) all affect both the strength and duration of stratification, and the cycling of oxygen. In addition to climate change, [DO] may be altered by human disturbances, such as the addition of nutrients from shoreline residential development and agriculture, hydrological management for flood control or navigation, and changes in water clarity associated with lake acidification and changes in dissolved organic carbon (DOC) export. In oligotrophic lakes, where respiration rates may exceed gross primary productivity, high allochthonous DOC inputs may also impact hypolimnetic [DO]. While DOC inputs from natural sources are not a management concern, per se, increased inputs may elevate background oxygen depletion rates. In central and northern Ontario this is of particular concern because lake DOC levels have increased by approximately 20% since the late 1980s.
Lake managers require a new toolset, one that includes predictive models to forecast shifts in lake trout habitat, and improve our understanding of historical changes in lake trout habitat, in response to multiple stressors. Our proposal brings together researchers with diverse expertise (e.g., modelers, engineers, biologists, paleolimnologists) to develop and apply three distinct yet complementary techniques that will accurately hindcast, measure and predict past, present, and future hypolimnetic [DO] in lake trout lakes. In Theme 1, we will use paleolimnological techniques to provide context for past changes in [DO], and to calibrate and validate model hindcasts generated with the complementary modeling approaches. In Theme 2, we will build an empirical, steady-state model that can be used by lake managers to predict the impacts of shoreline residential development on hypolimnetic [DO]. In Theme 3, we will develop a dynamic, process-based model to project and hindcast [DO] in lakes in the context of a changing climate. The model will be calibrated using a common set of long-term study lakes in south-central Ontario, and applied to lakes of significant interest to our supporting organizations.
