The largest lakes of the world contain two thirds of the surface freshwater on the planet and are also centers of intense human activities affecting the ecosystem services supplied by these lakes, including water quality, fisheries, and biodiversity. Long-term monitoring is essential to understand the effects of these activities in any system, but sampling such large spatial scales is difficult, often requiring oceanographic methods and large vessels. Although the importance of these lakes and the ecosystem services they provide is widely recognized, the challenges that we are facing in such large systems include:

• what environmental factors drive species distribution across a range of spatial and temporal scales;
• how to differentiate long-term change from inherent spatiotemporal variation;
• how to integrate emerging measuring systems and methods with traditional sampling techniques while still maintaining continuity of historic time series.
• Incorporating new technologies including autonomous sampling , video imaging and others enabling collections of large data sets at high resolution into monitoring programs.

We invite talks on the issues associated with spatial and temporal processes as well as on the use of long-term data series for understanding ecosystem changes in large lakes of the world.

Lead Organizer: Lars Rudstam, Cornell University (lgr1@cornell.edu)

Co-organizers:

Lyubov Burlakova, SUNY Buffalo State (burlakle@buffalostate.edu)

Alexander Karatayev, SUNY Buffalo State (karataay@buffalostate.edu)

James Watkins, Cornell University (jmw237@cornell.edu)

Presentations

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04:00 PM

Particulate nutrient stoichiometry in Great Lakes seston: Are there dominant drivers? (7941)

Primary Presenter: Erica Yang, U.S. EPA Great Lakes National Program Office (yang.erica@epa.gov)

Ratios of Carbon (C),Nitrogen (N),and Phosphorus (P) in offshore particulate matter can be used as indicators of nutrient limitation in phytoplankton and inform on the condition of the lower food web. While seston C:N:P stoichiometry affects ecological processes and food web function, there is limited understanding of the environmental drivers of particulate C:P and N:P variability in freshwater lakes. Lab-based experiments have assessed the role of phytoplankton stoichiometric drivers, but no studies have used long term observational data to assess the impacts of those drivers on offshore planktonic stoichiometry across the North American Great Lakes. We use phytoplankton, total nutrient, and particulate nutrient data collected by the Environmental Protection Agency’s Great Lakes National Program Office from 2001 – 2019 to investigate the relationship between environmental drivers and particulate C:N:P across all five Great Lakes. Drivers include proportions of major phytoplankton groups, total and dissolved phosphorus concentrations, and total oxidized nitrogen concentrations. Across all lakes, the strength of most relationships between potential drivers and particulate N:P and C:P were weak. In some instances, the correlations found in this long-term data were contradictory to previously observed correlations in lab and experimental lake-based studies. Our results highlight the complexity of the relationship between environmental conditions and the nutrient status of the Great Lakes lower food web, emphasizing the continued need for surveillance of offshore conditions.

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04:15 PM

Impacts of Climate Change on Phytoplankton Dynamics in Lake Michigan: A Biophysical Modeling Study (8001)

Primary Presenter: Xing Zhou, Georgia Institute of Technology (xingzhou@mtu.edu)

Physical factors such as water temperature, water column mixing, and light availability play critical roles in determining phytoplankton abundance and primary productivity in Lake Michigan. This study investigates the potential influence of climate change on these critical environmental factors and, consequently, on the dynamics of Lake Michigan's phytoplankton. To achieve this, we utilized an integrated modeling framework comprising a two-way coupled 3D lake-ice–climate system model (GLARM), a hydrodynamic model (FVCOM), and a nutrient-phytoplankton-zooplankton-detritus (NPZD) model, including a module representing the invasive quagga mussel (Dreissena rostriformis bugensis). Our methodology involved conducting historical simulations covering the period from 2005 to 2014 and projecting future scenarios for two distinct timeframes: the mid-21st century (2041–2049) and the late 21st century (2091–2099), based on the Representative Concentration Pathway 8.5 scenario, often referred to as the 'business as usual' scenario. The findings of our study highlight that changes in water temperature and mixing dynamics significantly alter the seasonal patterns of phytoplankton, particularly affecting the timing and intensity of winter-spring blooms as well as the vertical distribution. Specifically, our projections indicate a likely weakening and delay of mid-depth and offshore winter-spring blooms due to diminished winter stratification and altered spring turnover. Additionally, the onset of the deep chlorophyll layer is anticipated to occur 15-30 days earlier in the mid- and late 21st centuries, accompanied by an increased duration resulting from prolonged summer stratification. Our models also show an increase in primary production and a shift in spatial and temporal patterns under the influence of projected climate change scenarios.

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04:30 PM

Long-term trends in Great Lakes shallow zooplankton communities (7947)

Primary Presenter: Julie Lietz, GDIT (julie.lietz@gdit.com)

Zooplankton abundance and community structure are commonly used as indicators of ecosystem state in aquatic systems, and zooplankton data provide insights into differences in ecosystem function across lakes and over time. The EPA Great Lakes National Program Office (GLNPO) has monitored zooplankton communities in all five Great Lakes during spring and summer for more than two decades using both deep net tows (0-100 m or 2 meters off the bottom,153 µm mesh) and shallow net tows (0-20 m, 63 µm mesh). While all samples are analyzed for crustacean zooplankton community, the shallow net tows are also analyzed to assess microzooplankton, including nauplii, rotifers, and dreissenid mussel veligers. We present results from shallow tow monitoring data for 2001-2022 from 72 GLNPO routine monitoring sites and evaluate variation in surface layer zooplankton community structure across lakes, years, and seasons. We present trends in shallow zooplankton community structure, discuss how they compare to previously reported patterns for whole water column zooplankton, and highlight the contributions of microzooplankton to total zooplankton biomass across the lakes.

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04:45 PM

Fine-scale zooplankton distributions revealed with acoustics and optics (8197)

Primary Presenter: Lars Rudstam, Cornell University (lgr1@cornell.edu)

Zooplankton are inherently patchy and therefore difficult to assess with standard net tows. In addition, standard vertical tows cannot measure vertical structure at sub-meter scales. By combining information from a laser optical plankton counter (LOPC) and high frequency hydroacoustics (120 and 420 kHz), we found high vertical heterogeneity in zooplankton distribution in the Laurentian Great Lakes. Densities can be an order of magnitude higher in thin layers occurring within a few meters around the thermocline.  We investigated these fine scale layers using data from Lake Michigan collected during day and night surveys. The relatively large (up to 25 mm long) mysid shrimps dominated acoustics backscattering below the thermocline at night, making it difficult to observe distribution of other zooplankton with acoustics at that time.  However, mysids migrated deeper in the water column during the day, allowing us to study detailed depth distributions of copepods and cladocerans . Using a combination of optics and acoustics gave us a more complete picture of the distribution of zooplankton in the Great Lakes. We discuss the importance of this fine scale layering of zooplankton for understanding the interactions within the zooplankton community and between zooplankton and planktivorous fish. 

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05:00 PM

EVALUATING DRIVERS OF DIEL VERTICAL MIGRATION IN FISH AND MYSIS WITH SURFACE DRONES IN LAKE SUPERIOR (7749)

Primary Presenter: Thomas Evans, Cornell University (tme33@cornell.edu)

Diel vertical migrations (DVM) are common in marine systems and lakes. In the Great Lakes, Mysis diluviana (hereafter Mysis) migrate upwards at night to feed on other plankton and then retreat to deeper water at dawn to avoid fish predators. Mysis migrations are closely tracked by various fish species which feed upon them. The advent of modern surface drones with multi-week endurance allows examination of DVM across large spatial scales and with high temporal resolution. We deployed two uncrewed surface vessels equipped with 120 kHz Simrad EK80 transducers in western Lake Superior from August to September of 2022. The timing of Mysis and fish migrations, their average depth throughout the night, and how close to bottom they appeared during the day were measured. We compared these to moon phase, water depth, and sunrise/sunset. After the initial ascent to within 5 to 10 meters of the surface, the mysid layer typically deepened and sometimes separated into two layers. In deep waters >200 m, Mysis remained suspended in the water column at about 220 m depth during the day. Coregonids, likely kiyi (Coregonus kiyi), migrations were tightly coupled to these mysid migrations, but the fish tended to be distributed above the maximum density mysid layer during both day and the migration phase; fish remained below the mysid layer during the night. The rate of downward fish migration at dawn was faster than the upward migration rate during dusk, mirroring the rate of the mysid migration. The use of drones allowed us to explore the details of these DVMs for the first time and their consistency across space in one of the world’s largest lakes.

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05:15 PM

THE GREAT LAKES MASS MARKING PROGRAM: LARGE-SCALE MONITORING OF LAKE MICHIGAN SPORT FISHERIES TO ASSESS SPATIOTEMPORAL PATTERNS IN SALMONINE DISTRIBUTIONS (8048)

Primary Presenter: Matthew Kornis, U.S. Fish and Wildlife Service (matthew_kornis@fws.gov)

Millions of salmonids are annually stocked in the Great Lakes to support and diversify sport fisheries and restore native fish populations. The U.S. Fish and Wildlife Service began coded-wire tagging and adipose-fin clipping all lake trout Salvelinus namaycush stocked into Lakes Michigan and Huron in 2010; tagging of Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss followed in 2011 and 2017, respectively. We review how the implementation of a mass marking program has facilitated large-scale monitoring of spatial and temporal patterns in salmonine distributions. Examples include (1) examining patterns and potential drivers of relative survival of Chinook salmon and lake trout, (2) estimates of seasonal shifts in species distributions, and (3) estimating wild recruitment annually to measure progress toward rehabilitation of lake trout and predatory impacts by Pacific salmon. Our overview will highlight how this program has addressed the challenges of data intensive fisheries monitoring on large spatial scales, and how it has helped answer applied research questions and inform decision-making. We also seek discussion on other ways to integrate physical and environmental processes into fisheries research in large freshwater systems like the Great Lakes. 

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