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1. Aquatic heatwaves increase surface chlorophyll concentrations in experimental and reference lakes

   https://doi.org/10.1002/lol2.70024  

   Szydlowski, Daniel K; Bollini, Katie A; Pace, Michael L; Wilkinson, Grace M (August 2025, Limnology and Oceanography Letters)

   Abstract Aquatic heatwaves are increasing in frequency, intensity, and duration worldwide. While increases in mean water temperatures are linked to enhanced phytoplankton biomass, it is unclear how heatwaves alter phytoplankton dynamics in lakes at an ecosystem scale. We investigated changes in surface chlorophyll during 29 summer heatwaves between 2008 and 2019 in 3 north temperate lakes. These lakes vary in staining and were either references or manipulated with nutrients and top predator additions. The manipulations provided a variety of nutrient, grazing, and light conditions during heatwave and non‐heatwave conditions. Surface chlorophyll concentrations increased during 24 out of 29 heatwaves. In the low‐nutrient reference lake the mean increase in chlorophyll was 57% while in the two experimental lakes the mean increases were 127% and 183%. Overall, the effects of the whole‐lake experiments were variable but still provided context for possible patterns amid a diverse set of food web and nutrient conditions. 

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2. Nutrient availability modulates impacts of short‐term shifts in temperature and grazing on phytoplankton composition and size structure

   https://doi.org/10.1002/lno.70322  

   Anderson, Stephanie I; Franzè, Gayantonia; Kling, Joshua D; Wilburn, Paul; Kremer, Colin T; Menden‐Deuer, Susanne; Litchman, Elena; Hutchins, David A; Rynearson, Tatiana A (February 2026, Limnology and Oceanography)

   Abstract The significant contribution of phytoplankton to global primary production is regulated by several abiotic and biotic factors, which are often difficult to account for in natural systems. To address these challenges, we perturbed a summer plankton community from Narragansett Bay, Rhode Island, USA, by manipulating the temperature and nutrient conditions in a controlled short‐term incubation experiment and tracked changes in phytoplankton community structure in response to fluctuations in phytoplankton physiology and microzooplankton grazing. Water was incubated at the in situ temperature (22°C) and at deviations from that temperature (± 4°C) with both macronutrient amendments (N, P, and Si addition) and unamended controls. We found nutrient availability and microzooplankton grazing to have pronounced and opposite impacts on phytoplankton size composition, with nutrient amendments shifting the phytoplankton community toward larger cells and grazing rates correlated with smaller phytoplankton communities and low‐nutrient conditions. Nutrient amendments also altered cellular elemental ratios by increasing chlorophyll : C. Conversely, temperature was not found to have a direct impact on size or elemental stoichiometry, but did influence community composition. These findings paralleled prior observations from lab and field studies in Narragansett Bay which together suggest that over shorter timescales, nutrient availability may have a greater impact on phytoplankton community composition than temperature or grazing, altering phytoplankton nutritional value for higher trophic levels and as a result, secondary production. Thus, understanding underlying nutrient dynamics will be necessary to decipher how short‐term changes in temperature or grazing may impact phytoplankton communities and the ecosystems they support. 

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3. On the sensitivity of plankton ecosystem models to the formulation of zooplankton grazing

   https://doi.org/10.1371/journal.pone.0252033  

   Chenillat, Fanny; Rivière, Pascal; Ohman, Mark D. (May 2021, PLOS ONE)

   Gorokhova, Elena (Ed.)

   Model representations of plankton structure and dynamics have consequences for a broad spectrum of ocean processes. Here we focus on the representation of zooplankton and their grazing dynamics in such models. It remains unclear whether phytoplankton community composition, growth rates, and spatial patterns in plankton ecosystem models are especially sensitive to the specific means of representing zooplankton grazing. We conduct a series of numerical experiments that explicitly address this question. We focus our study on the form of the functional response to changes in prey density, including the formulation of a grazing refuge. We use a contemporary biogeochemical model based on continuum size-structured organization, including phytoplankton diversity, coupled to a physical model of the California Current System. This region is of particular interest because it exhibits strong spatial gradients. We find that small changes in grazing refuge formulation across a range of plausible functional forms drive fundamental differences in spatial patterns of plankton concentrations, species richness, pathways of grazing fluxes, and underlying seasonal cycles. An explicit grazing refuge, with refuge prey concentration dependent on grazers’ body size, using allometric scaling, is likely to provide more coherent plankton ecosystem dynamics compared to classic formulations or size-independent threshold refugia. We recommend that future plankton ecosystem models pay particular attention to the grazing formulation and implement a threshold refuge incorporating size-dependence, and we call for a new suite of experimental grazing studies. 

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4. Hypolimnetic Hypoxia Increases the Biomass Variability and Compositional Variability of Crustacean Zooplankton Communities

   https://doi.org/10.3390/w11102179  

   Doubek, Jonathan P.; Campbell, Kylie L.; Lofton, Mary E.; McClure, Ryan P.; Carey, Cayelan C. (October 2019, Water)

   In freshwater lakes and reservoirs, climate change and eutrophication are increasing the occurrence of low-dissolved oxygen concentrations (hypoxia), which has the potential to alter the variability of zooplankton seasonal dynamics. We sampled zooplankton and physical, chemical and biological variables (e.g., temperature, dissolved oxygen, and chlorophyll a) in four reservoirs during the summer stratified period for three consecutive years. The hypolimnion (bottom waters) of two reservoirs remained oxic throughout the entire stratified period, whereas the hypolimnion of the other two reservoirs became hypoxic during the stratified period. Biomass variability (measured as the coefficient of the variation of zooplankton biomass) and compositional variability (measured as the community composition of zooplankton) of crustacean zooplankton communities were similar throughout the summer in the oxic reservoirs; however, biomass variability and compositional variability significantly increased after the onset of hypoxia in the two seasonally-hypoxic reservoirs. The increase in biomass variability in the seasonally-hypoxic reservoirs was driven largely by an increase in the variability of copepod biomass, while the increase in compositional variability was driven by increased variability in the dominance (proportion of total crustacean zooplankton biomass) of copepod taxa. Our results suggest that hypoxia may increase the seasonal variability of crustacean zooplankton communities. 

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5. Paleolimnological evidence for primary producer change linked to hydrologic connectivity and human impacts in Lake Carlton, Florida, USA

   https://doi.org/10.1007/s10933-024-00318-y  

   Clift, Troy L.; Waters, Matthew N. (April 2024, Journal of Paleolimnology)

   Guangjie Chen (Ed.)

   Abstract Hypereutrophic conditions in lake ecosystems are generally associated with nutrient inputs from surrounding terrestrial landscapes. However, some systems can receive primary nutrient inputs through hydrologic connections such as rivers or canals. Lake Carlton, Florida, USA is a small, shallow, polymictic lake that ends a hydrologically connected string of lacustrine systems with hypereutrophic lakes Beauclair and Apopka. Lake Beauclair and Lake Apopka were connected hydrologically when a system of canals was constructed beginning in 1893 CE. These lakes have maintained hypereutrophic conditions despite extensive management to reduce nutrient inputs. Here, we collected a sediment core from Lake Carlton to accomplish two primary research objectives: 1) reconstruct the nutrient input for Lake Carlton throughout the last ~ 150 years to conduct source assessment, and 2) link primary producer changes with management actions between lakes Apopka, Beauclair, and Carlton. Paleolimnological tools were applied to a 165-cm sediment core and analyzed for bulk density, organic matter content, nutrients (C, N, P), photosynthetic pigments, and total microcystins. Sediments were dated using²¹⁰Pb and results indicate that the core represents over 150 years of sediment accumulation. Sedimentary nutrient concentrations show that the primary driver of nutrient inputs resulted from canal construction, beginning in 1893 CE, which corresponded to increased nutrient deposition. Photosynthetic pigment data indicate dramatic increases in most primary producer groups coinciding with the hydrologic modification. However, around ~ 1970 CE, primary producer communities shifted from diatom dominance to cyanobacterial dominance, which appeared to be linked to internal nutrient dynamics and competition among phytoplankters within the lake ecosystem. Cyanotoxin production records show a significant lag between cyanobacterial dominance and peak cyanotoxin production with toxins increasing in the last 30 years. These data demonstrate that local nutrient inputs do not govern all phytoplankton dynamics in shallow lake systems but must be interpreted considering hydrologic alterations and management practices. 

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