Lake ecosystems are under threat from a myriad of often interconnected factors including, climate change, fertilization, microplastics, metals, hydrocarbons, polyfluoroalkyl substances (PFAS), and invasive species. These stressors can act independently or synergistically in complex interactions. What are the combined effects of these different stressors on lake ecosystems? How do we know when a lake has moved from a “healthy” to an “unhealthy” state? What can we do to protect lake ecosystems and important water resources?

A historical perspective provides us with important information to determine the effects of different stressors and provides critical information for successful management including: 1) timing of changes; 2) direction of change; 3) magnitude of change; 4) the cause of change and 5) whether a lake ecosystem has moved outside of natural variability or its “healthy” state. Unfortunately, long-term measurements of these stressors or lake ecosystem health are rarely available, but paleolimnology and long-term monitoring offer opportunities to gain these essential perspectives. In this session we welcome papers that use innovative paleolimnological and other long-term approaches to understand the effects of a variety of stressors on lake ecosystems or use novel approaches to identify best management practices. We hope that this session will not only demonstrate the critical role long-term records play in identifying the effects of a variety of stressors acting individually or together, but also point to effective lake ecosystem management and protection of these important water resources.

Lead Organizer: Katrina A. Moser, University of Western Ontario, Dept. of Geography and Environment (kmoser@uwo.ca)

Co-organizers:

Euan Reavie, University of Minnesota Duluth, Natural Resources Research Institute (ereavie@d.umn.edu)

Presentations

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04:30 PM

Paleolimnological contributions to Indigenous Reconciliation in Canada (8872)

Tutorial/Invited: Tutorial

Primary Presenter: John Smol, Queen's University (smolj@queensu.ca)

Paleolimnological perspectives offer important insights regarding key environmental questions that require long-term perspectives, such as: Have ecosystems changed? If so, when and to what degree? How important have human influences been relative to natural changes? What were pre-disturbance conditions, and, therefore, what are realistic mitigation targets? Our retrospective “western science” approaches share many similarities to Traditional Environmental Knowledge (TEK), where a key focus often includes historical information on local ecosystems.  A question posed by an Indigenous Elder at one of our recent consultations summarizes the synergies between these “different ways of knowing”.  We were asked: Can you tell us what our territorial lakes were like before our ancestors signed the Robinson Huron Treaty of 1850?. Using our multi-disciplinary approaches, we can indeed provide this critical information, and also show how lakes, and the ecosystem services that people depend on, have changed since that time due to anthropogenic and natural stressors. This presentation will summarize several projects showing how we have applied paleolimnology to address key questions posed by our Indigenous partners. When combined with information provided by Indigenous knowledge holders, a stronger case can be made for evidence-based remediation efforts and the ongoing negotiations regarding reconciliation.  

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05:00 PM

Cyanobacterial blooms in remote lakes of the U.S. Upper Midwest during past climate variability (9356)

Primary Presenter: Jasmine Saros, University of Maine (jasmine.saros@maine.edu)

The ecological factors that promote or impede formation of cyanobacterial harmful algal blooms (cyanoHABs) are poorly understood, and large uncertainty remains about how climate interacts with other drivers to shape cyanoHABs. While much research has focused on cyanoHAB development in eutrophic systems, there are increasingly more observations of cyanobacterial blooms in low to moderate productivity lakes, raising questions about specific climate drivers involved. We examined the history of cyanobacterial blooms in three remote lakes on Isle Royale National Park in the Upper Midwest of the U.S. Contemporary observations reveal that two of these lakes experience frequent cyanobacterial blooms in the summer; it was unclear whether this was a recent phenomenon and to what extent climate played a role. Paleolimnological records capturing the past 2,000 years of algal community structure revealed that the recent blooms in these two lakes began in the 1970s. Prior to that, all three lakes demonstrated cyanobacterial blooms during the Roman Warm Period (RWP; 1-300 CE), a warm and dry period in this region, whereas only moderate changes in algal blooms occurred during the Medieval Climate Anomaly (800-1300 CE), a warm but hydrologically variable period. These results from remote lakes demonstrate the ongoing importance of both temperature and hydrologic variability in driving cyanobacterial blooms.

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05:15 PM

LONG-TERM RECORDS OF GREAT LAKES PHYTOPLANKTON REVEAL MULTIPLE STRESSORS AFFECTING FOOD WEBS (8723)

Primary Presenter: Euan Reavie, University of Minnesota Duluth (ereavie@d.umn.edu)

The Laurentian Great Lakes are the world’s largest surface freshwater resource, so it is worth considering the implications of changes at the base of the food web. Monitoring and paleoecological assessments of the Great Lakes phytoplankton flora reveal changes in whole-lake conditions in each of the lakes. Trends indicate impacts from invasive species, agriculturally derived nutrients, and climate change. Those responsible for tracking and maintaining ecosystem services on the Great Lakes are concerned about the ecological trajectories of the lakes, and phytoplankton data from monitoring and paleolimnological programs provide an early warning tool to uncover impacts. Major changes from the last two decades include: unicellular flagellated algae are increasing in relative abundance while diatoms are being lost; the substantial loss of phytoplankton biomass due to the quagga mussel invasion; and densities of cyanophytes are increasing in vulnerable coastal systems. Monitoring data complement paleolimnological data by providing a contemporary ecological context for fossil phytoplankton species. The implications of climate, nutrients and invasive species drivers on recent changes will be discussed in the context of future management of the Great Lakes.

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05:30 PM

TRACING THE TROPHIC HISTORY OF TEMPERATE LAKE MASKINONGÉ (MONT-TREMBLANT, CANADA): INSIGHTS FROM A MULTI-PROXY PALEOLIMNOLOGICAL APPROACH (8908)

Primary Presenter: Louis-Marc Bruneau-Dumoulin, University Laval (louis-marc.bruneau-dumoulin.1@ulaval.ca)

Freshwater ecosystems worldwide are increasingly impacted by human activities. Lakes with diverse human activities in their catchments are particularly vulnerable to nutrient overload and other pollutants, resulting in water quality deterioration and ecological shifts. Environmental authorities often face the challenge of a paucity of long-term data, hampering their ability to contextualize recent environmental changes against historical natural variability. Here, we use a multi-proxy paleolimnological approach, combined with historical document research, to trace the responses of temperate Lake Maskinongé (Mont-Tremblant, Canada) to stressors such as lumbering, agriculture, suburbanization and dam construction. Our main objective is to retrace the lake’s trophic history from the 16^th century to the present to establish reference conditions. Various biogeochemical analyses were carried out on two sediment cores retrieved in 2023, including microbiological analyses (diatom assemblages), scans (CT, ITRAX, hyperspectral) and sedimentological analyses (LOI, grain size). ²¹⁰Pb dating provided a temporal framework, and an inference model based on diatom assemblages, developed by Tremblay et al. (2014), generated past TP concentrations. Our results reveal a significant increase in eutrophic diatom species, chlorophyll a, organic matter and grain size in the last 50 years, highlighting the need for targeted restoration. Insights gained from this project contribute to essential knowledge for managing temperate lakes facing similar eutrophication challenges.

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