Southern Lake Ontario Research Articles

A social–ecological geography of southern Canadian lakes

Anthropogenic pressures, including urban and agricultural expansion, can negatively influence a lake's capacity to provide aquatic ecosystem services (ES). However, identifying lakes most at risk of losing their ES (i.e., higher vulnerability) requires integrating information on lake ecological state, global change threats, and ES use. Here, we provide a social–ecological framework that combines these features within a regional context by evaluating the ecological state of 659 lakes across Canada. Using the deviation of impacted lakes from reference ones, we identified much higher total nitrogen and chloride concentrations as the main indicators of an altered lake ecological state in all regions identified. Lake ecological state was mapped using an additive colour model along with regional scores of threat levels and recreational ES use. Urban and agriculturally developed areas were linked to higher lake vulnerability and ES loss. Lakes in Southern Ontario were most concerning, being highly altered, under threat, and heavily used. Lakes near coastal urban centers were altered and used, but less threatened, whereas those in the Prairies were altered and threatened, but less used. Our novel framework provides the first social–ecological geography of Canadian lakes, and is a promising tool to assess lake state and vulnerability at scales relevant for management.

Multiscale effects of wind‐induced hydrodynamics on lake plankton distribution

AbstractIn this study, we used a combination of high‐intensity sampling technologies, and a 3D hydrodynamic model of a medium‐sized lake in southern Ontario, Canada to investigate physical–biological relationships at spatial scales from 100 m to 6 km and temporal scales from hours to months. At the scale of the whole study basin, we predicted that stronger winds would lead to higher zooplankton biomass downwind relative to upwind. The hydrodynamic model suggests rapid downwind displacement of progressively deeper surface mixed layers with increasing winds, and we found a statistically higher downwind biomass of small‐bodied zooplankton on windy days, but not large zooplankton. At a fine spatial scale (hundreds of meters), we predicted that zooplankton patchiness would decrease with increasing wind mixing of the upper water column and confirmed this for small‐bodied but not large‐bodied zooplankton. At this fine‐scale cross‐correlations of zooplankton biomass with water temperature and chlorophyll fluorescence suggested that zooplankton are not simply moved passively by water masses. We also found a clear change in the cross‐correlation between large‐ and small‐bodied zooplankton biomass, with out‐of‐phase spatial distributions during calm periods becoming in‐phase with increasing winds. Overall these results indicate that the response of zooplankton to wind‐driven physical forces is strongly dependent on an interaction between their body size, which determines their swimming speed and capacity to position themselves vertically in the water column, and the spatial scale and intensity of the wind‐generated physical forces. We discuss the implications for food web interactions.

Rapid response of the nearshore round goby population to temperature declines associated with upwelling events in Lake Ontario

Upwellings occur in all the Great Lakes in response to prolonged wind events. Several studies have modeled or measured changes in abiotic conditions associated with upwellings, but few direct observations of fish responses have been documented. Video-capture techniques were used in conjunction with moored temperature loggers to link the disappearance of benthic round gobies to temperature declines during upwelling events along the southern Lake Ontario nearshore in summer of 2019. Benthic water temperatures declined by as much as 18 °C within 18 h over as many as nine events. Round goby density estimates were as high as 50/m2 prior to upwellings, but declined to as low as 0/m2 during the events. Using just nine observation dates, ARMA trend analysis suggested a relationship between benthic temperature and round goby abundance (AIC = 78.7, t = 2.21, P = 0.063). Although the actual response of gobies to rapidly declining temperatures was unknown, this population was unobservable in our sample area during upwellings, yet returned to pre-upwelling densities within days. Understanding the magnitude and frequency of fish responses to these events can improve our understanding of the potential for this non-native benthic fish to affect the nearshore environment.

Autonomous underwater glider observations in southern Lake Ontario and Niagara River plume.

The nearshore areas of the Laurentian Great Lakes provide valuable ecosystem services including habitat for a variety of species and water for surrounding communities. Recent declines in nearshore water quality have increased the need for understanding the connectivity between nearshore and offshore areas; however, observing water quality variability across the dynamic nearshore to offshore transition zone poses logistical challenges for traditional observing systems. Here we evaluate cross-shore and along-shore water quality gradients using observations from two three-week deployments of a Slocum autonomous glider in southern Lake Ontario. The glider was deployed between the Niagara River mouth and Rochester, NY during early and late summer 2018, and each deployment resulted in over 3000 vertical profiles of the water column, and several transects between 2 km and 20 km from shore. In early summer, the cross-shore chlorophyll gradient was characterized by highest values just below the surface, at the frontal zone between weakly stratified conditions closer to shore and unstratified conditions offshore. In late summer, stratified conditions extended across the entire survey area. The depth of the thermocline was deeper and chlorophyll values were lower within 10 km of shore than offshore, where the highest chlorophyll values were observed in a distinct layer below the thermocline. In both early and late summer, the frontal boundary indicated by the cross-shore chlorophyll gradient was located below the surface and well offshore of what is typically considered the nearshore zone but was within the width of the coastal boundary layer. The high-resolution glider observations provide a detailed view of patterns of variability across a dynamic coastal zone and pinpoint the cross-shore frontal boundary that may be important for biologists to differentiate biological communities.

Community science-based monitoring reveals the role of land use scale in driving nearshore water quality in a large, shallow, Canadian lake

Smith ED, Balika D, Kirkwood AE. 2021. Community science-based monitoring reveals the role of land use scale in driving nearshore water quality in a large, shallow, Canadian lake. Lake Reserv Manage. 37:431–444. As shorelines and watersheds become increasingly developed, lake condition declines due to nutrient enrichment, excess algae growth, and contaminant loading. Lake Scugog, a large shallow lake in southern Ontario, Canada, is no exception, and has been experiencing increased urban development in its agriculture-dominated watershed over the last few decades. By applying a 3-pronged co-production model (stewardship group, watershed authority, and university laboratory), a nearshore water quality monitoring program was implemented using a community-science approach. Nearshore water samples were collected over 3 years (2017–2019) by trained community scientists. We found strong relationships between water quality parameters and land use, at the buffer and subcatchment scale. The strength of the land use relationships varied by parameter and scale, where chloride was significantly predicted by buffer land use, and phosphorus was explained by development at the subcatchment scale. Based on the high rate of participation compliance, the community science approach proved to be effective for capturing spatially explicit water quality data, which allowed us to establish current baseline conditions in Scugog’s nearshore zone. Future watershed management should consider both scales of land use, but prioritize restoration efforts on the scale that corresponds to their specific water quality goals.

Microbial community and abiotic effects on aquatic bacterial communities in north temperate lakes

The aquatic bacterial community (BC) plays a vital role in determining the nature and rate of ecosystem function. However, the biotic and abiotic factors influencing BC structure and function are largely unknown. Hence, the current study characterizes the impact of biotic and abiotic factors on aquatic bacterial biodiversity to determine whether the dominant effects are biotic or abiotic by partitioning their relative effects across temperate Canadian lakes. We collected water samples from sixty southern Ontario lakes and characterized their BC and microbial eukaryotic community (MEC) compositions using high throughput metabarcode sequencing of 16S and 18S rRNA gene fragments. The diversity and richness of aquatic BCs differed considerably among our study lakes, and those differences were explained by environmental, spatial, and biotic (MEC) factors (31%, 23%, and 23% of variance explained, respectively). The relatively large contribution from biotic and abiotic factors (54%), relative to spatial effects, shows deterministic processes prevail in shaping BC assembly in freshwater lakes. However, spatial effects also contributed significantly, highlighting the role of stochastic processes (ecological drift and coupled with limited dispersal) in shaping BC structure. Furthermore, our co-occurrence network analysis showed strong positive and negative interactions within and between the BCs and MECs, indicating mutualistic or antagonistic co-occurrence patterns relationships play important roles in driving the variation in BC composition among our sampled lakes. Considered together, our community analyses show that deterministic and stochastic processes combined contribute to determining the aquatic BC composition, and hence likely function as well, across a broad array of temperate freshwater lakes.

Consistent and transient drivers of freshwater zooplankton communities

AbstractAimInferences of the predominant processes that structure communities are commonly based on single ‘snapshots’ in time, which may miss temporally transient but important mechanisms. In this study, we compared key environmental and spatial drivers of zooplankton composition across multiple years to quantify shifts in their relative importance through time, and to identify any drivers of temporal change.LocationSouthern Ontario, Canada.TaxonZooplankton.MethodsZooplankton were collected from 29 lakes in southern Ontario, Canada over four years (2013–2016). Variation partitioning was used to quantify the relative importance of independent and covarying components of local environmental conditions, regional east‐west and north‐south compositional trends, and inter‐lake geographic distance in each year. Measured environmental metrics included aspects of lake morphology and chemistry, and spatial relationships were quantified using lake latitude/longitude coordinates and Moran's Eigenvector Maps (MEMs). Redundancy analyses (RDAs) were also used to compare the influence of individual environmental and spatial variables across years.ResultsMost local‐scale and regional‐scale community processes were consistently important across all surveyed years, but some were less consistent. Specifically, geomorphology was always an important driver of local environmental and regional spatial community patterns. This occurred because local community composition was strongly affected by whether a lake was shallower versus deeper, and due to spatial clusters of shallow and deep lakes that produced negative spatial autocorrelation in community composition. Conversely, the individual influences of lake chemistry and spatial east‐west compositional trends were important in some years and not in others, potentially due to inter‐annual shifts in the predominant environmental variables and extreme weather events.Main conclusionsA single‐year community snapshot can provide insight into consistent or slowly changing community structuring processes, such as those driven by geomorphology, but may not completely capture temporally transient mechanisms. Furthermore, snapshots collected during anomalous seasons or years may misrepresent which mechanisms are predominantly determining community composition. Future efforts to understand local and regional community drivers would therefore benefit from considering which processes are likely temporally ‘consistent’ versus ‘transient’, and studies with more variable components would benefit from considering or controlling for temporal shifts in their importance.

Algal palynomorphs as proxies of human impact on freshwater resources in the Great Lakes region

Abstract Management of freshwater resources requires an understanding of the response of lakes to human impact. The long sedimentary records in lake archives hold the key to accurate forecasting. The remains of algae in “pollen” slides record two distinct phases of cultural eutrophication and siltation/turbidity resulting from soil erosion in sediments from two lakes in southern Ontario, Canada: 1) agricultural settlements by Iroquoian (Wendat/Huron) people around the middle of the last millennium and 2) widespread land-clearing by European colonists in the mid-nineteenth century, followed by industrial expansion and urbanization in the Great Lakes watershed to the present day. The half-cells of benthic desmids were particularly sensitive to turbidity associated with land clearing. In contrast, planktonic algae adapted to eutrophic waters thrived in response to increased agricultural runoff and human and animal waste during both intervals in cores from Lake Simcoe and in the well-documented varved sediments from Crawford Lake. These under-utilized microfossils can be useful proxies of human impact, particularly where mineralized microfossils are sparse due to dissolution.

Inferring Past Trends in Lake Water Organic Carbon Concentrations in Northern Lakes Using Sediment Spectroscopy

Changing lake water total organic carbon (TOC) concentrations are of concern for lake management because of corresponding effects on aquatic ecosystem functioning, drinking water resources and carbon cycling between land and sea. Understanding the importance of human activities on TOC changes requires knowledge of past concentrations; however, water-monitoring data are typically only available for the past few decades, if at all. Here, we present a universal model to infer past lake water TOC concentrations in northern lakes across Europe and North America that uses visible-near-infrared (VNIR) spectroscopy on lake sediments. In the orthogonal partial least-squares model, VNIR spectra of surface-sediment samples are calibrated against corresponding surface water TOC concentrations (0.5-41 mg L-1) from 345 Arctic to northern temperate lakes in Canada, Greenland, Sweden and Finland. Internal model-cross-validation resulted in a R2 of 0.57 and a prediction error of 4.4 mg TOC L-1. First applications to lakes in southern Ontario and Scotland, which are outside of the model's geographic range, show the model accurately captures monitoring trends, and suggests that TOC dynamics during the 20th century at these sites were primarily driven by changes in atmospheric deposition. Our results demonstrate that the lake water TOC model has multiregional applications and is not biased by postdepositional diagenesis, allowing the identification of past TOC variations in northern lakes of Europe and North America over time scales of decades to millennia.

Discovery of the oldest record of Nitellopsis obtusa (Charophyceae, Charophyta) in North America.

Studies of the colonization and spread of invasive species improves our understanding of key concepts in population biology as well as informs control and prevention efforts. The characean green alga Nitellopsis obtusa (starry stonewort) is rare in its native Eurasian range but listed by the United States Geological Survey (USGS) as an aggressive invasive in North America. First documented in North America in 1978 from New York, United States, it has since been reported from numerous inland lakes from Minnesota to Vermont, and from Lake Ontario and inland lakes in southern Ontario, Canada. While the ecological impacts of N.obtusa are not clearly understood in its invasive range, initial results show negative environmental effects. We have discovered a liquid-preserved herbarium specimen that predates the 1978 records by at least 4years, and is the first confirmed record of N.obtusa in Québec.

Biomass loss and nutrient release from decomposing aquatic macrophytes: effects of detrital mixing

Aquatic plant decomposition is typically studied on species held separately, whereas diverse plant communities are usually found in lake littoral zones and decomposition occurs as mixtures of multiple species. Here, we examined whether detrital mixing affects the rate of aquatic macrophyte decomposition. Specifically, we measured decomposition rates of detritus from four species (Myriophyllum heterophyllum, Ceratophyllum demersum, Typha × glauca, and Potamogeton robinsii) in single, double, triple, and quadruple species mixtures held over two summer months in a mesotrophic lake in southern Ontario, Canada. We measured detrital mass loss after different time periods for all combinations. There were limited effects of mixing on decomposition rates with inhibitory effects observed in only two of the eleven multi-species mixtures. Decomposition rates of single and mixed species detritus varied with initial C:N and C:P ratios with faster rates seen for more nutrient-rich detritus. Overall, there was no effect of detrital species richness on macrophyte decomposition rates other than smaller differences among the averages of more species-rich mixtures. Our results were inconsistent with interactive effects of mixing on decomposition rates of multiple aquatic plant taxa. Instead, we found decomposition rates of mixed species communities were largely predicted by biomass composition and single-species decomposition estimates. There were also no apparent effects of species mixing on N- or P-specific fluxes or the ratio of these fluxes that resulted during decomposition during our experiment. Our results indicate that future changes in aquatic plant biodiversity may affect rates of decomposition in these ecosystems, but these should be largely predictable based on changes in plant communities and their biomass-weighted stoichiometry.

Trophic dynamics of several fish species in lakes of a climatically sensitive region, the Hudson Bay Lowlands

Freshwater lakes in the Hudson Bay Lowlands (HBL) area of Ontario are expected to undergo considerable physical, chemical and biological changes related to climatic change; however, the nature of those changes is still very uncertain. As a first step to improve our understanding of fish communities within these subarctic lakes, we aimed to: (a) characterize trophic dynamics of several large-bodied species within three HBL lakes and (b) determine whether trophic dynamics of selected species in the HBL lakes differed from the same species in Southern Ontario lakes. We found that species-specific trophic position and littoral resource reliance varied significantly within and among the HBL lakes of differing biological communities, chemistry and morphometry. Although several significant differences were evident among lakes in the northern and southern regions, we did not find striking consistent differences in trophic dynamics. Based on observations of high variation in trophic position and/or littoral reliance, we can hypothesize that changes in food resources resulting from climatic change would have little impact on most of the large-bodied species.

An empirical analysis of the consequences of zebra mussel invasions on fisheries in inland, freshwater lakes in Southern Ontario

With increasing human use of North American inland lakes for recreational purposes, there is a greater probability of the incidental transfer and introduction of aquatic invasive species within them. This raises concern over the potential ecological and economic impacts that invasive species could have in these highly valued freshwater ecosystems. In this study, we focus our interests on 63 inland lakes distributed across the Southern Ontario landscape in order to evaluate whether concerns are warranted over potential adverse impacts of a well-known aquatic invasive species (the zebra mussel (Dreissena polymorpha)) on native fish assemblages and targeted game species (i.e., walleye (Sander vitreus), smallmouth bass (Micropterus dolomieu), lake trout (Salvelinus namaycush), and northern pike (Esox lucius)). Through the analysis of a comprehensive and multivariate dataset comprising lake morphometric, water chemistry, fish assemblage, fish metric, and zooplankton haul data, we examined the potential consequences of zebra mussel invasions in inland lakes. It was found that condition, growth, and relative abundance of some of the most highly valued game fish in inland lakes significantly varies among lakes with and without zebra mussel, though the nature of such effects differed for each of the fish species examined. In addition, it was shown that the species composition (i.e., identity and relative abundance) of active, large-bodied fish assemblages within a lake can indicate zebra mussel presence/absence with a high degree of certainty. Finally, while correlations between zebra mussel presence and the richness, abundance, growth, and condition of certain fish species were found, the results of this study indicate that the potential consequences of this well-known aquatic invader to fisheries in inland lakes may be less dramatic than those reported within the Great Lakes.

Stoichiometric and growth responses of a freshwater filamentous green alga to varying nutrient supplies: slow and steady wins the race

Summary We examined how the nutrient stoichiometry of the filamentous green alga, Mougeotia sp., is affected by the relative availability of dissolved nitrogen (N) and phosphorus (P). Using a bioassay experiment, we grew Mougeotia in medium having a range of dissolved N and P concentrations and N:P ratios (0.62 to 411 by mol). After growing Mougeotia in an environmental chamber for 17 days, we measured its carbon (C), N and P content (% of dry mass), molar C:N:P ratios, mass‐specific growth rate (MSGR, day−1), chlorophyll a concentration (Chl, μg L−1) and carbon:chlorophyll a (C:Chl) ratios. Due largely to variation in cellular %N and %P, Mougeotia C:N:P ratios varied considerably at the end of the experiment, with C:N ratios ranging from 6 to 41, C:P ratios ranging from 85 to 999 and N:P ratios ranging from 4 to 118. We found Mougeotia to be weakly homeostatic with respect to N:P content, with non‐homeostatic tendencies found for N content and P content. Despite the wide range of N and P concentrations in the medium at the start of the experiment, Mougeotia MSGR was relatively invariant. Nevertheless, high C:Chl ratios observed for N‐ and P‐limited Mougeotia largely matched those expected for algae experiencing nutrient‐limited growth. We further compared our experimental results with stoichiometric data for Mougeotia collected from the Kawartha Lakes in southern Ontario. The high C:P and N:P ratios found in field samples matched those of laboratory Mougeotia experiencing intense and sustained P limitation and are suggestive of severe P limitation of Mougeotia in Kawartha Lakes throughout the summer. The slow growth rates and flexible stoichiometry of this alga would enable it to continue growing even during periods of acute nutrient stress. Hence, future management and control of this nuisance alga may prove difficult if it is unlikely to respond strongly to reductions in external P loading and lower water column P concentrations.

Effects of future urban and biofuel crop expansions on the riverine export of phosphorus to the Laurentian Great Lakes

Increased phosphorus (P) loadings threaten the health of the world's largest freshwater resource, the Laurentian Great Lakes (GL). To understand the linkages between land use and P delivery, we coupled two spatially explicit models, the landscape-scale SPARROW P fate and transport watershed model and the Land Transformation Model (LTM) land use change model, to predict future P export from nonpoint and point sources caused by changes in land use. According to LTM predictions over the period 2010–2040, the GL region of the U.S. may experience a doubling of urbanized areas and agricultural areas may increase by 10%, due to biofuel feedstock cultivation. These land use changes are predicted to increase P loadings from the U.S. side of the GL basin by 3.5–9.5%, depending on the Lake watershed and development scenario. The exception is Lake Ontario, where loading is predicted to decrease by 1.8% for one scenario, due to population losses in the drainage area. Overall, urban expansion is estimated to increase P loadings by 3.4%. Agricultural expansion associated with predicted biofuel feedstock cultivation is predicted to increase P loadings by an additional 2.4%. Watersheds that export P most efficiently and thus are the most vulnerable to increases in P sources tend to be found along southern Lake Ontario, southeastern Lake Erie, western Lake Michigan, and southwestern Lake Superior where watershed areas are concentrated along the coastline with shorter flow paths. In contrast, watersheds with high soil permeabilities, fractions of land underlain by tile drains, and long distances to the GL are less vulnerable.

Uncertainty assessments and hydrological implications of climate change in two adjacent agricultural catchments of a rapidly urbanizing watershed

Lake Simcoe is the most important inland lake in Southern Ontario. The watershed is predominantly agricultural and under increasing pressure from urbanization, leading to changing runoff patterns in rivers draining to the lake. Uncertainties in rainfall–runoff modeling in tributary catchments of the Lake Simcoe Watershed (LSW) can be an order of magnitude larger than pristine watersheds, hampering water quality predictions and export calculations. Here we conduct a robust assessment to constrain the uncertainty in hydrological simulations and projections in the LSW using two representative adjacent agricultural catchments. Downscaled CGCM 3 projections using A1B and A2 emission scenarios projected increases of 4°C in air temperature and a 26% longer growing season. The fraction of precipitation falling as snow will decrease. Spring runoff is an important event in LSW but individual HBV best calibrated parameter sets under-predicted peak flows by up to 32%. Using an ensemble of behavioral parameter sets achieved credible representations of present day hydrology and constrained uncertainties in future projections. Parameter uncertainty analysis showed that the catchments differ in terms of their snow accumulation/melt and groundwater dynamics. Human activities exacerbate the differences in hydrological response. Model parameterization in one catchment could not generate credible hydrological simulations in the other. We cautioned against extrapolating results from monitored to ungauged catchments in managed watersheds like the LSW.

Adjacent catchments with similar patterns of land use and climate have markedly different dissolved organic carbon concentration and runoff dynamics

Temporal patterns in specific runoff, dissolved organic carbon concentrations [DOC] and fluxes were examined during two periods: 1994–1997 (period 1) and 2007–2009 (period 2) in five adjacent tributary catchments of Lake Simcoe, the largest lake in southern Ontario, Canada. The catchments displayed similar patterns of land use change with increases in urbanization (5–16%) and forest cover (0.2–4%) and declines in agriculture (4–8%) between 1994 and 2008. Climate in the catchments was similar; temperature increased slightly, but no significant change in precipitation was observed. Despite similar pattern of climate and land use, runoff responses and tributary [DOC] were different across the catchments. Following a very dry year (i.e. 1999), runoff increased steadily until the end of record. We observed increased variability in tributary [DOC] and higher DOC exports in period 2. This led to ~10% increase in [DOC] and a 13% increase in flux between the two study periods. Between the two periods, [DOC] increased by 15% in spring and 25% in summer, whereas flux increased by 17% in spring and 48% in summer. [DOC] was consistently higher in the growing (summer + autumn) than the dormant (winter + spring, minus spring melt months) seasons, but no unique pattern or simple linear flow/concentrations relationships existed. This suggests complex spatial and temporal pattern to runoff controls on DOC and flow dynamics in adjacent catchments. We therefore caution against extrapolating from monitored to unmonitored catchments. Copyright © 2012 John Wiley & Sons, Ltd.

Dissolved organic matter quantity and quality in Lake Simcoe compared to two other large lakes in southern Ontario

Dissolved organic matter (DOM) is a critical component in ecosystem processes and is the largest pool of organic carbon (C) in aquatic environments. In this study, we investigated the variability in quantity and quality of DOM in 3 large lakes in southern Ontario. Water quality parameters were coupled with excitation emission fluorescence spectroscopy and absorption spectra to characterize the DOM and investigate the overarching factors controlling DOM dynamics. The results show that Lake Simcoe has higher dissolved organic carbon (DOC) concentrations than lakes Erie, Ontario, and Hamilton Harbour (an embayment in western Lake Ontario) and suggest that a DOM source independent of watershed inputs is likely an important contributor to the DOC in this system. Five components were identified through parallel factor analysis (PARAFAC), representative of both terrestrial and microbial origin. Their relative intensities in the 4 Lake Simcoe end-members allowed the identification of dominant DOM sources in our studied ecosystems. Lake Simcoe seems to have a similar contribution of agriculturally derived DOM to lakes Erie, Ontario, and Hamilton Harbour. Lake Ontario, including Hamilton Harbour had on average a larger input of DOM derived from wastewater treatment plant effluents. The seasonal patterns in the different optical characteristics of DOM in Lake Simcoe compared to other systems suggested that DOM qualitative transformations, be it through photooxidation or microbial degradation, are likely very important processes in this lake. The role of DOM in Lake Simcoe may have important ecological implications for the cycling of C and the oxygen regime of this lake.

Nearshore–offshore differences in planktonic chlorophyll and phytoplankton nutrient status after dreissenid establishment in a large shallow lake

In Lake Simcoe, a large lake in southern Ontario, Canada, with more than 50% of its surface area <15 m deep, dreissenid mussels are abundant in the extensive nearshore zone but not offshore. We hypothesized that mussel grazing would depress chlorophyll a (Chl-a) concentrations in the nearshore compared to the offshore while alleviating nearshore phosphorus (P) deficiency through nutrient regeneration. During both years of our study Chl-a concentration and other indicators of phytoplankton biomass, including particulate carbon (C), nitrogen, P, and silicon, were lower in the nearshore areas of Lake Simcoe where the exotic invader Dreissenia polymorpha was in contact with overlying epilimnetic water. In the first year of our study, grazing and associated nutrient regeneration activity seemed to reduce P deficiency in phytoplankton in the dreissenid-impacted shallow locations. In the second year, however, phytoplankton in the nearshore dreissenid-affected areas remained as strongly P deficient as phytoplankton in offshore waters physically separated from dreissenid grazing. Photoacclimation in the nearshore phytoplankton was evident in higher particulate C:Chl-a ratios and higher effective absorptive cross section of photosystem II (σPS II) throughout the stratified sampling season compared to offshore phytoplankton. A multiple linear regression utilizing the mean light intensity in the mixed layer as well as total P (TP) resulted in better predictions of Chl-a than TP alone. We conclude that, in shallow lakes where transparency is strongly impacted by dreissenid grazing, the comparison of Chl-a–TP relationships over time will require accounting for the effect of changing transparency.

The state of Lake Simcoe (Ontario, Canada): the effects of multiple stressors on phosphorus and oxygen dynamics

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 (O2) 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 O2 L-1. Low deep water O2 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.