Water column stratification regulates key ecosystem processes in lentic waterbodies, including oxygen availability, carbon cycling, nutrient availability, and habitat suitability. Climate change is predicted to strengthen water column stratification via warming, while also increasing the frequency of partial or complete water column mixing via intensifying storms. For example, storms may cause shallow dimictic lakes to become polymictic, whereas warming may reduce mixing frequency in shallow polymictic lakes and ponds. Infrequent mixing events may have ecological consequences that last for weeks, including algal blooms, release of greenhouse gases, and the release of bioavailable contaminants to the water column. The goal of this special session is to explore how infrequent physical mixing events influence ecological processes, such as ecosystem metabolism, algal blooms, contaminant bioavailability, or greenhouse gas production and emissions. We welcome scientists who work across different systems and use various techniques to understand how changing climate may alter the physical, chemical, and biological properties of lakes and ponds.
Lead Organizer: Meredith Holgerson, Cornell University (meredith.holgerson@gmail.com)
Co-organizers:
Adam Heathcote, St. Croix Watershed Research Station (aheathcote@smm.org)
Lienne Sethna, St. Croix Watershed Research Station (lsethna@smm.org)
Ana Morales Williams, University of Vermont (ana.morales@uvm.edu)
Presentations
04:30 PM
Mixing Events and their Frequency in Lakes of Various Sizes (9229)
Primary Presenter: Sally MacIntyre, University of California - Santa Barbara (sally@eri.ucsb.edu)
Extended periods of summer stratification have been noted in many lakes with ongoing climate change. A key question is whether vertical exchanges are still ongoing and, if their frequency is reduced, what are the ecosystem consequences? Here we use time series data from lakes of various sizes to examine exchange mechanisms. A goal is to answer three questions: Have polymictic lakes truly been mixing frequently? Are vertical exchange mechanisms occurring but operative in two dimensions? In which lakes do we expect warming from climate change to be concentrated in the upper layers, in which ones will the thermocline thicken, and what are the ecosystem consequences? In answering the above questions, rates of vertical exchange will be contrasted to the rates of biogeochemical processes that would alter system function.
04:45 PM
STRATIFICATION INDUCED HARMFUL ALGAL BLOOMS ACROSS A HYPERUEUTROPHIC TO OLIGOTROPHIC GRADIENT (9269)
Primary Presenter: Adam Heathcote, Science Museum of Minnesota (adam.heathcote@gmail.com)
Harmful algal blooms (HABs) are often linked to external nutrient inputs from anthropogenic phosphorus (P) pollution. However, the increasing occurrence of blooms without these inputs has shifted attention to internal drivers. Recent research highlights internal P loading from sediments, driven by changing stratification patterns, as a key trigger. We identify “anoximixis”—the mixing of anoxic bottom waters—as a critical control of internal P loading rates. We present examples from remote, low-nutrient to agriculturally-impacted hypereutrophic lakes, where HABs occur or intensify due to anoximixis. Using high-frequency sensors and traditional monitoring, we compare bloom timing and intensity with factors such as hypolimnetic P accumulation, mixing frequency, and cyanotoxin production. Our findings emphasize the need to characterize anoximixis across regional, morphologic, and trophic gradients as climate change alters lake stratification and bottom water redox conditions. These changes may lead to more toxic blooms in affected systems and trigger blooms in lakes previously thought to be resistant.
05:00 PM
INTERMITTENT MIXING AS A STABILIZING MECHANISM FOR CYANOBACTERIA BLOOM BIOMASS AND FUNCTIONAL DIVERSITY (9352)
Primary Presenter: Ana Morales-Williams, University of Vermont (ana.morales@uvm.edu)
Environmental processes are becoming increasingly extreme and stochastic with cascading effects on lake ecosystem function. Phytoplankton community assembly is traditionally predicted by seasonal phenology but can be influenced at short time scales by disturbance processes. Their relative contributions depend on the frequency and timing of disturbance, which must occur at scales relative to phytoplankton growth to interrupt competitive exclusion. However, we do not fully understand how the relative forcing effects of seasonal versus stochastic processes will influence phytoplankton composition and turnover with climate change. Here we investigate the influence of disturbance on phytoplankton biomass and functional diversity in a shallow, eutrophic lake over 6 years (2018-2024), leveraging an aeration intervention to conduct an ecosystem-scale experiment examining the influence of seasonal versus stochastic environmental processes. We show that intermittent mixing and anoxic-oxic shifts can sustain biodiversity, biomass, and stabilize cyanobacteria blooms in a eutrophic lake. We found that phytoplankton diversity was highest following disturbance, and that increased disturbance frequency resulted in asynchronous fluctuations of phytoplankton functional groups. Phytoplankton asynchrony predicted biomass stability, resulting in stable cyanobacteria blooms through summer and late fall which displaced the fall diatom bloom. Our results demonstrate that disturbance can stabilize cyanobacteria blooms in eutrophic lakes, underscoring the importance of understanding contributions of seasonal versus stochastic forcing effects on phytoplankton biodiversity under current and future climate scenarios.
05:15 PM
DEPTH DEPENDENT DRIVERS OF LAKE ECOSYSTEM METABOLISM (9289)
Primary Presenter: Olivia Sowa, Oakland University (oliviasowa@oakland.edu)
Major drivers of whole lake ecosystem metabolism include lake morphometry and watershed characteristics. However, less is known about relationships between ecosystem metabolism and environmental drivers at different depths. Here, we examined how nutrients, chlorophyll, and seston stoichiometry influence gross primary production, respiration, and net ecosystem production at different depths within the photic zone. Our focal lake was Treetop Pond, a small dimictic eutrophic lake with a deep chlorophyll maximum located in Southeast Michigan. We installed a vertical chain of eight dissolved oxygen loggers in the lake from April to October 2024 measuring oxygen and temperature dynamics at 15-minute intervals. We collected weekly grab samples every meter through the water column for dissolved and total nitrogen (N) and phosphorus (P), chlorophyll a, particulate carbon (C), N, and P. Shortly after the onset of stratification, the hypolimnion became anoxic; anoxia later extended into the metalimnion during the growing season. Above the thermocline, dissolved oxygen was often supersaturated. Generally, there was more chlorophyll, lower seston C:P and N:P in the metalimnion (3m) than the surface. However, between late June and early August, dissolved oxygen concentrations at 3m declined, likely caused by vertical diffusion to the deeper layers. This stopped once the thermocline deepened below 3m. Overall, our results highlight that even with a deep chlorophyll maximum, vertical oxygen diffusion can outpace production and cause hypoxic conditions.
05:30 PM
A WHOLE-POND EXPERIMENT TO ASSESS HOW LATE-SUMMER MIXING AFFECTS METHANE DYNAMICS (9374)
Primary Presenter: Kathy Stenehjem, Cornell University (kjs297@cornell.edu)
Ponds emit significant amounts of methane (CH4) into the atmosphere; however, these emissions are very uncertain at regional and global scales. Among the sources of uncertainty are the mechanisms and timing of CH4 emissions. Most temperate ponds stratify in summer, allowing CH4 to accumulate in the hypolimnion between mixing events, especially if those waters become anoxic. When mixing occurs, accumulated CH4 can either be oxidized or emitted to the atmosphere. Summer storms are increasing in frequency and intensity with climate change, potentially increasing the frequency of summertime mixing and altering the fate (oxidation vs. emissions) of CH4. In this study, we experimentally mixed a dimictic pond over two days in late summer using a water circulator. Before, during, and after induced mixing, we tracked changes in methane concentrations, ebullition, diffusive flux, and oxidation (using stable isotopes). The pond began to re-stratified immediately after the circulator was turned off, but post-mixing stratification was far weaker than pre-mixing stratification and the pond mixed several times before fall turnover. Methane concentrations declined rapidly during mixing and remained lower than pre-mixing concentrations through the end of the study. Analysis of diffusive flux, ebullitive flux, and oxidation data is underway and will be included in the presentation. Our results will contribute to the understanding of how mixing events impact the fate of pond methane and will help constrain estimates of pond methane emissions.
SS07 - Ecological impacts of infrequent water column mixing events
Description
Time: 4:30 PM
Date: 27/3/2025
Room: W206B