Microplastics (1–5,000 µm) in aquatic ecosystems are ingested by fish, impacting feeding, digestion, growth, and immunity. Their translocation to tissues (liver, muscle) remains poorly understood, especially for larger particles observed in wild fish. We exposed yearling yellow perch (Perca flavescens) for 10 weeks to environmentally relevant and projected future scenario concentrations of polyethylene, polystyrene, and polyethylene terephthalate microplastics (5–1,960 µm) in in-lake mesocosms. Seven of nine mesocosms received 6–29,240 particles L⁻¹. We quantified microplastic (>53-µm) ingestion in gastrointestinal tracts (GITs), and translocation in liver and muscle. Ingestion increased with nominal microplastic loading concentration (0–721 particles per fish, n = 28), but translocation to liver and muscle was rare (0–5 particles per fish) and not related to loading concentration. Liver particles were 50% shorter and 51% narrower than in GIT, and muscle particles were 5% narrower, on average. Relative to all particles added to the mesocosms, GIT and muscle particles were 81 and 82% longer, while liver particles were 9% shorter, on average. Overall, yellow perch ingested microplastics in proportion to environmental concentrations, with limited translocation of smaller particles to organs (counts were 4% in liver and 7% in muscle of those in GITs, on average) although the mechanism remains unclear.

Ecosystem metabolism (e.g., gross primary production [GPP], respiration [R], and net ecosystem production) integrates the activity of aerobic organisms and is an indicator of environmental change. Microplastics (i.e., particles 1-5 mm) are widespread in freshwater ecosystems and are colonized by biofilms and other organisms. However, microplastics' effects on ecosystem-level processes including metabolism or other gas fluxes is rarely studied. In June 2021, we added microplastic fragments to large, in situ mesocosms (N = 9) in the Experimental Lakes Area, Ontario, Canada. Target microplastic concentrations were 0-29,240 particles L-1, relevant for freshwaters. We recorded continuous measurements of dissolved oxygen and temperature in each mesocosm and the lake epilimnion, along with wind speed and light from June 3 to August 10, 2021. We manually collected dissolved nitrogen gas (N2) samples hourly over one 24-h period in August in three mesocosms (zero, low, and high microplastics), and the lake epilimnion and hypolimnion. Microplastic addition influenced dissolved oxygen more strongly at the end of the experiment, but had no detectable effect on gross primary production, R, net ecosystem production, N2 saturation, or N2 flux. Gross primary production and net ecosystem production increased during summer. Mesocosm gross primary production and R trended higher than whole-lake rates, suggesting biofilms in mesocosms enhanced metabolism. Similarly, dissolved N2 was more strongly undersaturated in mesocosms than the epilimnion, and supersaturated in the hypolimnion, showing controls on N2 concentration varied among habitats. Microplastics had no detectable effect on ecosystem metabolism or N2 dynamics in pelagic mesocosms in a boreal lake under oligotrophic conditions, but the spatial and temporal comparisons offered insight into drivers of ecosystem processes.

Declining water clarity driven primarily by increases in dissolved organic carbon (DOC; i.e. ‘brownification’) has been broadly reported across the Northern Hemisphere. In Ontario, Canada, increasing lake DOC concentrations have been reported regionally, however no broad spatial and temporal water clarity trends for Ontario lakes currently exist. Here, we use data from two provincial monitoring programs to analyze historical (1962–1984), contemporary (2008–2023), and long-term (1962–2023) changes in water clarity, measured as DOC concentration and Secchi depth, across 684 Ontario lakes that were sampled two to six times over the study period. Our findings suggest that water clarity in Ontario lakes has declined since 1962, with the greatest declines in water clarity occurring in the contemporary time period. Water clarity declines were greatest in oligotrophic lakes, where DOC concentrations were also shown to have increased over time. Our findings highlight the importance of regional, temporal, and trophic contexts when considering the magnitude and direction of water clarity changes in Ontario lakes.

Microplastics are found worldwide and are ingested by a wide range of organisms, yet the drivers of toxicity of these common environmental contaminants are still not fully understood. To better understand the contribution of plastic additives in the toxicity of microplastics to organisms, yellow perch (Perca flavescens) were exposed for 9 weeks to additive-free or additive-containing microplastics using in-lake pelagic mesocosms. Microplastics were fragments (37-1,408 µm) of polymers commonly used in consumer plastics (linear low-density polyethylene, polystyrene, and polyethylene terephthalate). One objective of this work was to understand the contribution of additives to the overall toxicity of microplastics on yellow perch using targeted gene expression analysis via real-time quantitative polymerase chain reaction. After 9 weeks of exposure, fish were sampled, and whole fish were assessed for selected metal additives (aluminum, titanium, and bismuth). Liver and gonads were also assessed for gene expression analysis. No significant differences of metal contamination in fish tissue exposed to microplastics with additives were detected in comparison with fish exposed to plastic without additives or the control. While only limited effects on gene expression were observed, our work revealed that the genes hsp90aa and mgst1 in livers and mhc-I in gonads were differentially expressed when fish were exposed to microplastics with or without additives in comparison with the control fish. This finding indicated cellular stress and detoxification responses to microplastic exposure. Overall, there was no clear pattern demonstrating that toxic effects on fish were driven by either the physical or chemical aspects of the microplastics. Future work should measure the accumulation of the organic additives and assess the health of organs using histopathology.

Dissolved organic matter (DOM) plays a vital role in lakes, but its behavior in winter is poorly understood. This study examined the differences in DOM between lake ice and the upper water column across 18 sites in the Laurentian Great Lakes, integrating in situ sampling and remotely sensed ice data to create a mass budget model to estimate basin-scale DOM storage and release from ice. We found that the composition of the DOM pool in ice varied based on ice thickness, water DOM composition, nutrients, and dissolved organic carbon concentrations. Calculations of protein-like, microbial humic-like, and terrestrial-like DOM storage in ice under different ice cover scenarios revealed considerable contributions to the upper water column following ice melt, especially for protein-like DOM which, during extensive ice cover years, contributed an average of 17.7% to the protein-like DOM pool in spring. This ice-derived DOM may be an important source of labile carbon for microbial communities, but projected reductions in winter ice cover and duration under climate change may alter DOM dynamics, potentially impacting this important spring carbon subsidy.

Microplastics are complex contaminants, potentially posing both a physical and chemical risk to aquatic organisms. To better understand the physical and chemical impacts of microplastics, we conducted a large in situ pelagic mesocosm experiment in a freshwater boreal lake at the International Institute for Sustainable Development-Experimental Lakes Area. An equal mixture of polyethylene, polystyrene, and polyethylene terephthalate fragments with and without chemical additives were added to mesocosms as a single pulse and were contrasted with a control treatment with no added microplastics. Plankton communities were monitored for 62 days following microplastic additions. Total phytoplankton biomass was not affected by either microplastic treatment; however, a shift in phytoplankton community composition was detected in the microplastic treatment without additives on Day 62. Total zooplankton and cladoceran abundance marginally increased over time in both microplastic treated mesocosms, and diversity was lower in the additive treatment. There was a negative impact on Tropocyclops extensus egg production in microplastic treatments with and without additives, and the abundance of early-instar Chaoborus was temporarily higher in mesocosms containing microplastics without additives. Overall, the impacts of microplastics were relatively small, irrespective of the presence or absence of chemical additives, on natural pelagic phytoplankton or zooplankton communities over the 62-day exposure.

Empiricists and modellers use information on energy density, proximate composition, stable isotopes, fatty acids, thiamine, and bioaccumulative tracers (e.g., PCBs and mercury) to understand the state and inter-relationships of aquatic food webs. Data exist in many published and unpublished sources, but are not consolidated in an easily accessible database that would serve as a vital resource to i) provide basic estimates of these diet-derived measures of body composition, ii) understand sources of variation in the underlying data, iii) facilitate exploration and development of data proxies, and iv) assist in study design. We designed GLATAR (Great Lakes Aquatic Tissue Analysis Repository, glatar.org) to address this need. GLATAR is an open-access, searchable database linked to a web-based toolbox to visualise and generate user-defined summaries on diet-derived ecological metrics, with a focus on taxa of importance to the Great Lakes. GLATAR currently contains over 50,000 records on energy density, chemical tracers, and proximate body composition from 67 species of fish and 72 invertebrate taxa. We hope this user-friendly interface will entice others to upload their published and unpublished data to the repository, enriching the breadth of data accessible to researchers. In this way, GLATAR will become a ‘living’ and interactive resource for empiricists and modellers working in freshwater ecosystems as they make critical decisions related to growth, production, and consumption across a diverse group of economically and ecologically important aquatic species.

Benthic invertebrates and benthivorous White Sucker (Catostomus commersonii) were surveyed in eight stratified boreal lakes with DOC concentrations ranging from 3.5 to 9.2 mg. L-1 at the IISD Experimental Lakes Area (IISD ELA) in northwestern Ontario, Canada. Chironomids dominated benthic invertebrate biomass and White Sucker was the dominant fish species. Both White Sucker biomass-catch-per-unit-effort (BPUE) and chironomid biomass were positively correlated with mean light irradiance. Higher biomasses of fish and chironomids occurred in lakes with a higher proportion of their volume in the photic zone, which was a function of both DOC concentrations and lake basin morphometry. White Sucker BPUE was also strongly positively correlated with chironomid biomass. A mixing model using stable isotopes indicated that allochthony increased with DOC for both littoral benthic invertebrates and White Sucker. Greater allochthony was also correlated with lower invertebrate biomass and White Sucker BPUE. Our results suggest that DOC suppresses benthivorous fish by reducing the availability of critical food resources. Chironomid biomass appears to decrease with DOC because of lower light penetration that potentially affects primary productivity and the quality of their food resources.

Freshwater fisheries monitoring programs require indicators of fish community and population productivity and abundance. Two potential indicators include the slope and height of the community size spectrum, which are often interpreted as proxies for trophic dynamics and community abundance (or production), respectively. Previous studies have lacked alternative estimates of fish production and abundance to compare with community spectral parameters and have had limited limnological data to understand drivers of variability in spectral parameters among lakes. Using ∼30 years of monitoring data from a boreal lake, we showed that the height of the fish community size spectrum was related to independent mark/recapture-based estimates of abundance for the dominant large-bodied species, white sucker, but not to their biomass or annual production estimates. Further, limnological properties including the duration of ice cover, light, temperature, and the biomass of zooplankton prey were associated with inter-annual variation in spectral slopes and heights, often nonlinearly. Our results indicate that spectral parameters derived from fish communities can reflect long-term environmental change and provide new insights on drivers of fish community size structure.

Prey composition and availability is considered a primary predictor of Lake Trout (Salvelinus namaycush) mercury (Hg) concentrations. Evidence from other freshwater fishes suggests that environmental and landscape factors likely also contribute to fish Hg dynamics, yet comprehensive, contemporary assessments for Lake Trout from boreal and north-temperate lakes are lacking. Here, we reassess the importance of prey characteristics using both previously published and contemporary data, incorporating additional variables and model complexity to better understand factors influencing Hg dynamics of Ontario Lake Trout. Our analyses indicate that 1) Lake Trout Hg concentrations are primarily associated with individual body size, 2) high dissolved organic carbon (DOC) concentrations elevate Hg for fish of a given size, and 3) a coarse categorization of food chain length, specifically the presence of Mysis diluviana, informs Hg biomagnification slopes. The inclusion of DOC was vital for assessing human consumption risk, as Lake Trout in high DOC lakes were more likely to exceed Hg guidelines at sizes often harvested by anglers. Drivers of Lake Trout Hg levels in boreal and north-temperate lakes closely match those reported to affect other fishes in the region, regardless of feeding, thermal, and habitat strategies.

This study examines the extent to which temporal (1980–2019) increases in precipitation drove dissolved organic matter (DOM) loads and lake concentrations, and evaluates implications to physical, chemical, and biological variables within dimictic boreal lakes at the IISD Experimental Lakes Area, Canada. Long-term increases in precipitation (c. +27%) were associated with increased DOM loads and lake concentrations (c. +9%). Increased DOM within dimictic lakes over time was associated with shallower thermocline depths (c. −17%) and euphotic depths (c. −13%), reduced mean light intensity in the water column (c. −20%), reduced depth-integrated phytoplankton biomass (c. −32%) but increased epilimnetic chlorophyll a concentration (c. + 27%), and we detected no significant temporal declines in crustacean zooplankton. Our results confirm earlier observations that caution should be exercised when using epilimnetic chlorophyll a as an indicator of phytoplankton responses to DOM. However, once corrected for photoadaptation depth-integrated chlorophyll a was a useful indicator of depth-integrated biomass and its response to DOM.

Mobile hydroacoustic surveys using ship-based down-looking transducers are widely used to estimate densities for ecologically and economically important pelagic fishes. However, this method likely underestimates densities of some surface-oriented species due to biases associated with the acoustic surface exclusion zone and ship avoidance behaviours. We compared cisco (Coregonus artedi) density estimates from a stationary up-looking platform survey to a standard down-looking acoustic survey. Both systems were deployed during the fall cisco spawn in Thunder Bay, Lake Superior 2020-2022. Cisco density estimates from the stationary up-looking platform were on average 6.7 times higher in the upper water column (similar to 1-10 m) and 2 times higher over the entire water column (similar to 1-45 m) than those from standard mobile surveys. Ship avoidance behaviour associated with mobile surveys was apparent in the upper water column; median cisco densities observed by the platform fell from similar to 36 to similar to 9 fish/ha when the ship passed near the platform. Abundance estimates from the platform when not influenced by ship avoidance provided higher quota estimates than the standard survey in 2020 and 2022, but were similar in 2021. A multi-day deployment of the platform tracked a progressive daily increase in fish densities, highlighting the sensitivity of mobile survey results to the day they are conducted, often dictated by environmental conditions. Our results show promise in applying stationary acoustic deployments in fisheries surveys, with improved accuracy and reduced effort compared to mobile acoustic surveys in the management and monitoring of pelagic fishes in the Great Lakes.

Microplastics degrade slowly over time, leaching carbon (C) that could be subsequently incorporated into aquatic food webs. Current estimates of microplastic degradation vary, and little is known about microplastic-derived C fate under natural environmental conditions. To investigate whether microplastics leach C that is subsequently incorporated into aquatic food webs, we added isotopically enriched microplastics to Lake 378 at the Experimental Lakes Area in Ontario, Canada. In an ∼1100 L limnocorral (in situ open-bottom enclosure), we added 99% 13C-labelled polystyrene (8–216 µm in longest dimension) at a nominal concentration of 3268 particles/L. A second limnocorral without microplastics served as a negative control. Monthly measurements of δ13C-DIC and δ13C-DOC in filtered water revealed no detectable leaching of 13C from the plastic. Compound-specific isotope analysis of δ13C in amino acids of bulk plankton and periphyton revealed a slight (0.5‰) enrichment in 13C, within the range of natural variability for these organisms. Under the natural conditions of temperate oligotrophic lakes, degradation of microplastics is likely a very slow process that was not possible to detect in this 4-month experiment. Future studies should focus on assessing degradation of microplastics under realistic field scenarios to improve estimates of degradation pathways and associated time scales.

Lake Winnipeg hosts North America’s second largest commercial fishery for walleye (Sander vitreus (Mitchill, 1818)); however, little is currently known regarding walleye distribution throughout the lake. Here we identify two movement strategies for adult female walleye (migrant and resident) and describe patterns in monthly space use over 2 years. We used permissible home range estimators to determine monthly home range (95%), core range (50%), and associated mean locations. Mean locations showed that migratory walleye occupied more northern regions of the lake during late summer into fall (August and September) and were more southern during winter (November to March) and spring (April to June), overlapping residents. Migrants exhibited larger ranges during June, July, and October and shared similar ranges to residents when found at similar latitudes. Putative repeat spawning within the Red River was marginally more frequent among migrants compared to residents. This study describes two movement strategies of walleye within the south basin of Lake Winnipeg, possibly arising from multiple factors including water clarity, prey density, and temperature gradients. Results presented here provide information on the timing of movement and the spatial distribution of fish, which may be incorporated into a spatiotemporal based approach for fisheries management and stock assessment.

Current data on lake trout (Salvelinus namaycush) populations in Manitoba are limited but essential for guiding effective management decisions. To address this gap, academics and provincial fisheries biologists collaborated to conduct standardized netting surveys across seven lake trout fisheries in Manitoba during 2021 and 2022. Life history characteristics were quantified and compared, with length-at-age back-calculated from individual lake trout, and growth assessed using von Bertalanffy growth functions. Significant differences in growth parameters (L∞, K, t0, and ω) were observed across lakes. A sustainable exploitation model applied to the data revealed most lake trout fisheries are healthy. However, some underlying concerns exist that could jeopardize the future sustainability of a few of the fisheries. Findings highlight the need for ongoing population monitoring to support management objectives. We propose several actionable strategies for lake trout conservation in Manitoba, including incorporating citizen science and collaborating with Indigenous communities for co-production of knowledge.

Light regulates the vertical migration of many aquatic organisms. Mysis species couple pelagic and benthic habitats in lakes by diel vertical migrations (DVM), transporting energy and nutrients through the water column and food web. Although Mysis are generally assumed to remain on the bottom during the day, some have been observed in the pelagic zone during the day, indicating incomplete benthic-pelagic coupling in some systems. The degree to which light attenuation and lake depth interact to affect occurrence of mysids within the water column during the day is understudied. We used standardized Mysis net sampling in summers 2020 and 2021 across nine north-temperate lakes to test the hypotheses that 1) Mysis remain pelagic during the day at depths with sufficiently low light levels, and 2) pelagic-caught individuals during the day are, on average, smaller than those caught at night. To test these hypotheses, we assessed light, dissolved oxygen (DO), Mysis densities, and size distribution between night and day across bathymetric depths. In deep lakes and darkly colored shallow lakes, Mysis suspended in the water column during the day where light levels decreased to their light avoidance threshold (similar to 10(-5) to 10(-6) lx). Mysis suspended in the water column during the day were smaller than those collected at night. Further, Mysis were not captured when DO reached levels < 3 mg/L, regardless of light conditions. Our results suggest that benthic-pelagic coupling by Mysis is mediated through light conditions, lake morphometry, and DO conditions, and may include some degree of size-dependent behavior.

How microplastics transit within aquatic ecosystems and partition among environmental compartments is not fully understood. To increase understanding, we added microplastic fragments ranging in buoyancy (positive: polyethylene (PE), neutral: polystyrene (PS), negative: polyethylene terephthalate (PET)) and size (∼30 to 1400 μm) to surface waters of closed-bottom, in-lake mesocosms (10 m diameter, 2 m depth). To assess residence time, we measured microplastics in surface waters and the water column over a 9-week period. To measure fate, we measured microplastics in the surface water, water column, bottom detritus, and biota (biofilm on the walls, zooplankton, fish) at 9 weeks. The residence times of microplastics were longer at the surface than in the water column, with less dense and smaller particles having the longest residence times. After 9 weeks, nearly all microplastics were on the bottom, with only 3% on the surface, 0.4% in the water column, 2% in biofilm, and <0.01% in zooplankton and fish. The surface water and biofilm on the walls were larger reservoirs than the water column, suggesting that surface microlayers and biofilm on hard substrates are important, yet overlooked, reservoirs of microplastics in aquatic ecosystems. Results inform future hypotheses relevant to monitoring programs and risk assessments.

Fish populations in the Laurentian Great Lakes have undergone major changes over recent decades. Demographic changes in fish populations are often accompanied by changes in life history strategies that reflect variation in mortality applied to different life stages. We examined the steelhead (Oncorhynchus mykiss) population in Black Bay, Lake Superior, where a naturalized steelhead population experienced significant demographic changes over three decades. Initial increases in density in Portage Creek, Black Bay (1995-2007) were associated with reductions in angler mortality (applied to adult steelhead), but the population declined (2007-2018) despite no further changes in angler mortality. Simultaneously, the dominant life history among several Black Bay tributaries changed between 2013 and 2017, with returning spawners becoming primarily represented by individuals who smolted at age 2 (a more common pattern across other Lake Superior populations) from those who primarily smolted at age 1. To assess whether changes in juvenile mortality could explain observed life history shifts in surviving spawners and recent steelhead population declines, we constructed Leslie matrices with differential mortality applied to observed demographics from Portage Creek steelhead to evaluate scenarios representing increased parr (in-stream) mortality and increased smolt (in-lake) mortality. The observed demographic changes in Portage Creek (i.e., shift from 1 to 2 year smolts and associated population declines) were predicted by a model applying size-dependent smolt survivorship to female steelhead in a fashion consistent with increased inlake mortality of age 1 smolts. This study provides an example of population-level responses to increased stagespecific mortality and offers an example of how in-lake conditions can influence potamodromous salmonids in the Laurentian Great Lakes.

Evidence of declines in fish body size with warming in the absence of exploitation is lacking. During a period of regional climate warming (1986–2016), we examined trends in growth, size-structure, and age-structure of an unexploited population of lake trout (Salvelinus namaycush) from a small Canadian Shield Lake. These data were evaluated against long-term annual air and water temperature data, ice-cover data, and prey fish densities for the same period. Over time, the size of young immature lake trout did not change, while mature fish became smaller. The age-at-maturity of the population decreased by 4 years (from 9 to 5 years) and the body condition of immature and mature fish also declined. Though the adult lake trout population doubled over time (125–280 individuals), and the production (P), biomass (B), and biomass turnover (P/B) was constant, the population shifted to smaller, skinnier, younger individuals. By contrast, the relative abundance, biomass, and mean size of the prey fish community remained unchanged. Together this suggests that warming correlates with the contraction of the population’s life history.

1. Increasingly, molecular methods of species monitoring are integrated into freshwater biodiversity surveys and fisheries management. Inferring organism abundance or biomass from sequence counts derived from metabarcoding data has been an exciting but contentious concept in the biomonitoring community for some time. Although demonstrating a strong correlation with abundance has proven difficult, many researchers have assumed that quantitative metabarcoding data can at least provide broad-scale ranking of abundance. However, robust field validations of this widely-held assumption remain scarce. 2. Here, we analyse metabarcoding read counts of fish eDNA data derived from 20 lakes and use betabinomial mixed effects models to compare this to rank abundance generated from long-term fish survey data. Rank abundance data for 18 species was generated within-species across-sites, meaning that ranks compare the abundance of the same species in different lakes. We also investigated a possible allometric effect on eDNA production by analysing a subset of data for effects of fish body mass on the amount of eDNA sequences. 3. We found a good relationship between species-specific eDNA sequences and within-species rank abundance categories for fishes, with rare fish producing 3% of sequences in a library, moderately abundant producing 7% and abundant fish producing 29%, according to model predictions. 4. We found a small negative effect of body mass on the amount of eDNA sequences, where the proportion of reads recovered significantly decreased with increased mean body mass of the population. 5. Synthesis and applications: The benefit of this approach is the potential for rapid assessment of rank abundance for multiple species, including smaller species which are often missed by conventional methods such as gillnetting, with relatively low amounts of additional effort. This approach will assist practitioners taking a species-based approach to freshwater habitat management in lakes worldwide. ### Competing Interest Statement The authors have declared no competing interest.