Climate warming is expected to reduce thermal habitat for cold-water fishes, yet the magnitude and direction of habitat change may depend on interactions among multiple climatic drivers. We analyzed two contrasting lakes in the same region-Lake Kasumigaura (a shallow, eutrophic lake) and Lake Inawashiro (a deep, oligotrophic lake)-to clarify whether increases in air temperature or solar radiation drive thermal habitat loss or expansion of pond smelt (Hypomesus nipponensis) over the last 30 years. One-dimensional numerical simulations revealed that their thermal habitat changes proceeded in opposite directions in the two lakes. In Lake Kasumigaura, recent increases in air temperature warmed the water column to levels exceeding historical observations. Summer water temperatures frequently exceeded the critical thermal maximum of pond smelt (CTmax = 29.1 degrees C), resulting in a loss of optimal thermal habitat (15.0 < T-opt < 29.1 degrees C). In contrast, in Lake Inawashiro, exceedance of CTmax was rare, and the duration and vertical extent of the optimal thermal habitat increased. This downward expansion of the thermal habitat was driven by increased surface solar radiation during the springtime. These results demonstrate that combined effects of warming and increased solar radiation do not uniformly reduce thermal habitat for cold-water fishes; instead, biological responses can diverge depending on interactions among multiple climatic drivers and intrinsic lake properties. These contrasting patterns highlight the importance of lake morphology and water clarity in shaping the thermal habitat of cold-water fish under continued climate change.

Shallow lakes are hotspots for carbon dioxide (CO2) and methane (CH4) emissions, and they are highly sensitive to temperature variations. Recently, heatwaves that occur in winter have become more frequent and prolonged compared to other seasons. However, the impact of winter heatwaves on CO2 and CH4 emissions from shallow lakes remains unclear. To address this gap, we conducted a mesocosm experiment during winter, comparing an unheated control group with a heated group, where surface water temperature was maintained at a constant 8 degrees C higher than the control, based on meteorological data from Lake Taihu, China. Our results showed that the first three days of the heatwave induced pronounced pulses of CO2 and CH4 fluxes, with CH4 contributing disproportionately to total CO2-equivalent emissions. CO2 fluxes were primarily regulated by water temperature and chlorophyll a concentration, whereas CH4 fluxes were strongly associated with dissolved carbon concentrations, highlighting the differential sensitivity of carbon pathways to episodic thermal disturbances. Observed changes in dissolved oxygen and chlorophyll a concentration suggested potential cascading effects on microbial communities and trophic interactions, indicating that winter heatwaves can influence both biogeochemical processes and ecosystem structure. Our results highlight the importance of high-resolution winter monitoring and the incorporation of episodic warming events in predictive models of lake carbon dynamics.

ABSTRACT Invasive species pose a significant threat to ecosystems worldwide. To support effective management, it is crucial to clarify their distribution ranges and dispersal processes. Here, we applied an environmental DNA (eDNA)‐based mitochondrial DNA haplotyping approach to invasive black bass species to characterize their invasion ranges and haplotype compositions, thereby inferring their dispersal processes. We conducted a nationwide eDNA survey focusing on three established species in Japan, i.e., Northern largemouth bass (Micropterus nigricans), Florida largemouth bass (M. salmoides), and Smallmouth bass (M. dolomieu). Our survey detected eDNA of at least one species at 87 of 121 sites, recovered most previously reported haplotypes, and showed strong concordance with capture‐based data, demonstrating high sensitivity and accuracy. The haplotyping data revealed contrasting patterns among the three species. Northern largemouth bass, first introduced in 1925, is now widespread, with haplotype distribution suggesting gradual, localized geographic expansion across most regions. However, the high haplotype diversity observed on Shikoku Island, one of Japan's major islands, despite invasion only after 1972, suggests extensive human‐mediated translocations. Florida largemouth bass, introduced in 1988, was restricted to western Japan, with no signal of stepwise expansion, implying strong effects of human‐mediated translocations. Smallmouth bass, a more recent invader, was broadly distributed with only two haplotypes detected; eDNA results closely matched recent governmental survey data on invasion fronts. Our findings highlight the influence of differing invasion histories and the ongoing role of human intervention in bass dispersal. This study demonstrates that the eDNA haplotyping enables broad‐scale rapid monitoring while also providing insight into translocations and dispersal through population genetic structure, thereby facilitating efforts to prevent further invasions.

Winter circulation is a key dynamic that sustains the seasonality of freshwater ecosystems in temperate regions. However, it may be delayed or lost due to climate change. The occurrence of water circulation is affected by climate and morphological factors of waterbodies. However, although the occurrence of winter circulation has been found to be controlled by the interplay of climatic and morphologic factors, empirical investigations of this interplay remain limited. Thus, we explored the prevalent role of the interaction between climatic and morphologic factors in the occurrence of winter circulation using up to 37 years of data from 22 deep (maximum depth > 20 m) reservoirs in Japan. We found a negative interaction between seasonal air temperature and water depth, indicating that the effect of the temperature condition of the preceding season declines at deeper water depths. Therefore, in line with previous findings regarding relatively shallow lakes, water depth changes the strength of air temperature effect on the occurrence of winter circulation in deep waterbodies. Furthermore, these results highlight the potential to promote winter circulation through water depth management and the importance of understanding patterns across multiple waterbodies to guide management interventions.

IntroductionHistorically, rice paddy fields and irrigation ponds in Asia have removed nitrogen from irrigation water, such as spring water from upland croplands, while simultaneously providing multiple ecosystem services and supporting biodiversity. However, abandonment has diminished their multifunctionality. Although some abandoned paddies/ponds function as natural wetlands, their hydrological/environmental characteristics are highly variable.ObjectivesWe explored whether harnessing these abandoned sites as nature-based solutions (NbS) can offer substantial opportunities for restoring ecosystem services and biodiversity, and identified key factors to enhance their multifunctionality.MethodsWe surveyed 61 abandoned paddies/ponds in the Lake Kasumigaura watershed, Japan, and quantified six ecosystem service and biodiversity indicators: water purification, water storage, methane (CH4) regulation, damselfly richness, native fish richness, and resistance to invasive by exotic fishes. We examined tradeoffs/synergies among these indicators using principal component analysis.ResultsPrincipal component 1 revealed positive relationships among water purification, water storage, and damselfly richness. The strength of these synergies depended on the type of water flow: non-channelized flow abandoned paddies/ponds exhibited higher synergies. Sites with stronger synergies tended to be less resistant to exotic fish invasion. Principal component 2 and principal component 3 represented CH4 regulation and native fish richness, respectively, both of which were independent. CH4 regulation decreased with lower dissolved oxygen concentrations, while native fish richness increased at sites with diverse vegetation types.ConclusionsOur results highlighted the potential of abandoned paddies/ponds as multifunctional restoration tools in agricultural landscapes and suggested their effectiveness because NbS can be enhanced by managing hydrological/environmental factors.

Understanding the factors shaping trophic niche breadth is crucial for predicting species interactions, community assembly, and ecological responses to environmental change. Several hypotheses have been proposed to explain global variation in species' trophic niches, but empirical tests are limited. Here, we used stable isotope analysis (δ13C and δ15N) to quantify trophic niche breadth of 541 fish populations (358 species, 82 families) from freshwater ecosystems encompassing diverse environmental conditions and six biogeographic regions. Fourteen hypotheses relating niche breadth to environmental factors, traits, and sampling scale were tested. Fish isospaces were broader in regions with warmer temperatures, higher humidity, and precipitation variability, suggesting that more productive, diverse, and seasonal ecosystems are associated with broader trophic niches. Conversely, isospace size declined with increasing basin fish richness, which may reflect the role of competitive interactions. Within-population variation in body size was strongly and positively associated with isospace size, indicating ontogenetic dietary shifts. Fish with truncate-rounded fins had broader isospaces than those with forked-lunate fins, likely reflecting differences in foraging behavior and movement. Primary consumers had larger isospaces than intermediate and top consumers. Predators showed patterns similar to those across all trophic guilds, but non-predators differed in relation to solar radiation, richness, body size, and fin shape. Isospace size increased with the number of sampled individuals, habitats and years surveyed. Although isospace patterns have well-documented limitations as trophic ecology metrics, our findings nonetheless conform with several longstanding hypotheses for trophic niche variation and stimulate new ideas about environmental and biological drivers of niche breadth.

Extreme meteorological events such as storms are increasing in frequency and intensity, but our knowledge of their impacts on aquatic ecosystems and emergent system properties is limited. Understanding the ecological impacts of storms on the dynamics of primary producers remains a challenge that needs to be addressed to assess the vulnerability of freshwater ecosystems to extreme weather conditions and climate change. One promising approach to gain insights into storm impacts on phytoplankton community dynamics is to analyse long-term monitoring datasets. However, such an approach requires disentangling the impacts of short-term meteorological disturbances from the effects of the seasonal trajectories of meteorological conditions. To this end, we applied boosted regression tree models to phytoplankton time series from eight relatively large lakes on four continents, coupled with a procedure adapted to detect and quantify rare events. Overall, the patterns and potential drivers we identified provide important insights into the responses of lakes to short-term meteorological events and highlight differences in the response of phytoplankton communities according to lake morphological characteristics. Our results indicated that deepened thermoclines and lake-specific combinations of drivers describing altered thermal structures caused deviations from the typical trajectories of seasonal phytoplankton succession. For shallow polymictic lakes, shifts in phytoplankton succession also depended on changes in light availability. Overall, our study highlights the value of long-term monitoring to improve our understanding of phytoplankton sensitivity to short-term meteorological disturbances.

Multiple stressors can threaten the resources of inland fisheries, but responses to these stressors may differ among fish species. We used a long-term fishery dataset to quantify the CPUE trajectories of eight cyprinid taxa in the south basin of Lake Biwa that have been stressed by lakeshore development, artificial water-level regulation, exotic fish species, and climate change. A Bayesian state-space model revealed that the CPUEs of all eight taxa substantially declined from 1966 to 2022. For Opsariichthys uncirostris, Ischikauia steenackeri, Sarcocheilichthys spp., Cyprinus carpio, and Carassius spp., the average population growth rates decreased from 1976 to 1991, when lakeshore development occurred and largemouth bass populations increased dramatically. For the three remaining taxa (Zacco platypus, Gnathopogon caerulescens, and Squalidus spp.), the average population growth rates decreased from 1992 to 2022, when new water-level regulations were implemented and bluegill populations increased substantially. The former five taxa exhibited longer maximum body length and life span, later maturation, and higher fecundity than the latter three taxa. This suggests that life-history traits may determine how species respond to different stressors. Unlike the effects of these three abrupt stressors, those of climate, a gradual stressor, were negative for seven taxa, although not statistically significant. We also found that the population growth rates of Gnathopogon caerulescens and Ischikauia steenackeri increased recently after implementation of restoration measures (fishing moratorium and stocking of paddy-reared/captive-bred individuals). Our results suggest that cumulative anthropogenic stressors depleted the resource of cyprinids but that the outcome of current restoration measures may be positive.

Due to policies supporting large-scale expansion of plantation forestry, about 40% of the forests in Japan are planted forests, such as Japanese cedar (Cryptomeria japonica) and hinoki cypress (Chamaecyparis obtusa). We present the largest freely available EcoPlate dataset for planted forests in Japan, comprising data collected from a network of 74 planted forest sites (114 plots) in regions ranging from cool-temperate to subtropical. The EcoPlate is a 96-well microplate that contains three sets of 31 "response wells" with different carbon substrates. The utilization of each carbon substrate by the microbial community is quantified based on the color development of the well during incubation, providing a multifunctional index of the soil microbial community. Soil properties (water content, carbon, nitrogen, the carbon/nitrogen (C/N) ratio, and pH) essential for interpreting the EcoPlate results were also analyzed. Using a standardized protocol, soil was sampled between July and November 2021. A preliminary principal coordinate analysis (PCoA) was performed on the temporal integration of color density using the 31 substrates. PCo1 explained 36.5% of the variance of the overall absorbance of all substrates. A model of environmental factors, including elevation, and a model of soil properties, including pH, water content, and carbon, were the best-fit models. EcoPlate data allow us to test hypotheses related to community ecology and the ecosystem functions of the soil microbial community in planted forests on a regional scale. The complete data set for this abstract published in the Data Article section of the journal is available in electronic format in MetaCat in JaLTER at .

For over a century, ecologists have used the concept of trophic state (TS) to characterize an aquatic ecosystem's biological productivity. However, multiple TS classification schemes, each relying on a variety of measurable parameters as proxies for productivity, have emerged to meet use-specific needs. Frequently, chlorophyll a, phosphorus, and Secchi depth are used to classify TS based on autotrophic production, whereas phosphorus, dissolved organic carbon, and true color are used to classify TS based on both autotrophic and heterotrophic production. Both classification approaches aim to characterize an ecosystem's function broadly, but with varying degrees of autotrophic and heterotrophic processes considered in those characterizations. Moreover, differing classification schemes can create inconsistent interpretations of ecosystem integrity. For example, the US Clean Water Act focuses exclusively on algal threats to water quality, framed in terms of eutrophication in response to nutrient loading. This usage lacks information about non-algal threats to water quality, such as dystrophication in response to dissolved organic carbon loading. Consequently, the TS classification schemes used to identify eutrophication and dystrophication may refer to ecosystems similarly (e.g., oligotrophic and eutrophic), yet these categories are derived from different proxies. These inconsistencies in TS classification schemes may be compounded when interdisciplinary projects employ varied TS frameworks. Even with these shortcomings, TS can still be used to distill information on complex aquatic ecosystem function into a set of generalizable expectations. The usefulness of distilling complex information into a TS index is substantial such that usage inconsistencies should be explicitly addressed and resolved. To emphasize the consequences of diverging TS classification schemes, we present three case studies for which an improved understanding of the TS concept advances freshwater research, management efforts, and interdisciplinary collaboration. To increase clarity in TS, the aquatic sciences could benefit from including information about the proxy variables, ecosystem type, as well as the spatiotemporal domains used to classify TS. As the field of aquatic sciences expands and climatic irregularity increases, we highlight the importance of re-evaluating fundamental concepts, such as TS, to ensure their compatibility with evolving science.

Intensifying extreme droughts are altering lentic ecosystems and disrupting services provisioning. Unfortunately, drought research often lacks a holistic and intersectoral consideration of drought impacts, which can limit relevance of the insights for adaptive management. This literature review evaluated the current state of lake and reservoir extreme drought research in relation to biodiversity and three ecosystem services. The study findings demonstrated that few articles linked or discussed drought implications with one or more ecosystem services, instead focusing primarily on biodiversity. Drought effects on biodiversity varied among species and taxonomic groups. In the limited literature that included ecosystem service provisioning, droughts had a general negative effect. Drinking water supply can decrease and become more costly. Decreasing water flow and volume can reduce hydropower generation. Degraded water quality can also impact recreation. Future intersectoral collaborations and research on intensifying droughts should support adaptive management efforts in mitigating drought impacts.

MotivationHere, we make available a second version of the BioTIME database, which compiles records of abundance estimates for species in sample events of ecological assemblages through time. The updated version expands version 1.0 of the database by doubling the number of studies and includes substantial additional curation to the taxonomic accuracy of the records, as well as the metadata. Moreover, we now provide an R package (BioTIMEr) to facilitate use of the database.Main Types of Variables IncludedThe database is composed of one main data table containing the abundance records and 11 metadata tables. The data are organised in a hierarchy of scales where 11,989,233 records are nested in 1,603,067 sample events, from 553,253 sampling locations, which are nested in 708 studies. A study is defined as a sampling methodology applied to an assemblage for a minimum of 2 years.Spatial Location and GrainSampling locations in BioTIME are distributed across the planet, including marine, terrestrial and freshwater realms. Spatial grain size and extent vary across studies depending on sampling methodology. We recommend gridding of sampling locations into areas of consistent size.Time Period and GrainThe earliest time series in BioTIME start in 1874, and the most recent records are from 2023. Temporal grain and duration vary across studies. We recommend doing sample-level rarefaction to ensure consistent sampling effort through time before calculating any diversity metric.Major Taxa and Level of MeasurementThe database includes any eukaryotic taxa, with a combined total of 56,400 taxa.Software Formatcsv and. SQL.

AimEnvironmental change affects metacommunity structure both directly-via abiotic factors and dispersal that affect species occurrence-and indirectly-via complex interactions among co-occurring species. We examined how the three main metacommunity factors-environmental conditions, spatial processes and species associations-affect metacommunity structure and whether responses are predictable in real-world systems by using novel methods to disentangle the drivers.LocationEastern Asia, northern Europe and central North America.Time periodContemporary.Major taxa studiedFreshwater fish.MethodsWe used a dataset of freshwater fish species occurrences in temperate lakes in three countries in different biogeographic regions. We analysed co-occurrence patterns by using a joint species distribution model.ResultsWe demonstrated that environmental processes are the main drivers of species' distribution and diversity, suggesting that future climate change (anthropogenic alteration of abiotic factors) will heavily influence the structure of metacommunities. We also showed that spatial processes and species interactions mediated the influence of environmental processes, especially at the lake level.Main conclusionsOur results indicate that ongoing changes in metacommunity structure are modulated not only by the direct impacts of shifting abiotic factors but also by indirect effects of species interactions. Our global analysis indicates that even under the current high rate of environmental change, an identifiable set of underlying processes can be used to predict impacts of this change on metacommunity structure.

Although the transfer of methane-derived carbon (MDC) to benthic macroinvertebrates such as chironomid larvae and oligochaetes has been well documented, knowledge of the transfer of MDC to fish is limited. We investigated the MDC contribution to 28 fish species through their growth stages and examined the relationship between fish traits including body size, and MDC contribution to fish within species in Lake Kasumigaura, Japan. We used carbon stable isotope ratios and a two-source mixing model (methane-oxidation bacteria [MOB] and particulate organic matter) to estimate the MDC contribution to fish. The MDC contribution to individual fish ranged from 0% to 21.5%. Anguilla japonica showed the highest MDC contribution (mean 3.8%). Fish species were classified into four groups according to two criteria: (a) whether the correlation between the MDC contribution and fish size (within species) was significantly negative (negative vs. not significant; ns); and (b) whether the 90th percentile of MDC contribution was 0% or not. The mean MDC contribution was highest (2.18% +/- 2.93%, 12 species) for the group with (a)ns/(b)>0%, including the high contribution species such as Abbottina rivularis (mean 5.5%) and Biwia zezera (mean 9.8%). This group (12 species) included relatively high numbers of benthic habitat use (eight species) and omnivores (nine species). The mean MDC contributions were almost zero for the group with (a)ns/(b)=0% (seven species), which included only two species of benthic habitat use and one species of omnivores. The mean MDC contributions were intermediate for the other groups with (a)negative/(b)>0% (1.30% +/- 1.17%, seven species) and (a)negative/(b)=0% (0.10%, two species). The negative correlation between fish size within species and MDC contribution in groups with (a)negative/(b)>0% and (a)negative/(b)=0% indicated that ontogenetic habitat and dietary shifts induced changes in MDC transfer. Our results suggest that 21 out of 28 fish species were involved in the MDC transfer in a shallow lake, but variation in MDC transfer could be explained by fish traits (trophic guild and habitat use). Although the mean MDC contribution to all fish species was low (0.61% +/- 1.97%), the roughly estimated annual consumption rate of fish on MOB corresponded to approximately 10% of the MOB production in the sediment, suggesting that fish contribute to methane dynamics from the perspective of biogeochemical cycles.

This study aimed to illustrate the changing diversity patterns of native freshwater fish in the past two centuries and to identify priority locations for native fish conservation to counter future degradation. Japanese archipelago. We used the published native fish fauna data in 39 lakes across Japan, analysing historical and current diversity and projecting future distribution patterns based on the Japanese Red List. We assessed fish assemblages' taxonomic, phylogenetic, and functional alpha and beta diversity across different periods. Additionally, we proposed a cumulative diversity index that incorporated taxonomic, phylogenetic and functional facets, and then examined its relationship with the latitudinal positioning of the lakes across periods. We observed a significant decrease in the richness of native freshwater fish since historical periods. Accompanying this decline were significant reductions in both phylogenetic relatedness and functional redundancy. Fish beta diversity increased from the past to the present and is predicted to decline drastically, indicating an ongoing homogenization process. Using the cumulative diversity index, we identified Hokkaido and Kyushu as crucial habitats for endemic fishes. These islands, with unique biogeographical backgrounds, contributed substantially to the national dissimilarity patterns of native fish assemblages in the historical period. However, the contribution is diminishing due to the ongoing decline in fish endemism. The conservation priority of native freshwater fish in Japan should be assigned to Hokkaido and Kyushu due to the inhabited endemic species. The proposed framework for assessing the cumulative diversity of biotic communities presented the potential to aid macroecological explorations that underpin biodiversity conservation.

The diel variation of water temperatures is crucial information for lakes. This study investigated such variation in Lake Kasumigaura, the second largest lake in Japan, situated 60 km from the Tokyo metropolitan area, utilizing high-frequency monitoring and deep learning techniques. Data from high-frequency monitoring of vertical water temperatures at seven depths were employed. A convolutional neural network (CNN) based deep learning model was developed and assessed across three input scenarios. The impact of forecast horizons ranging from one to 48 h ahead on model performance was examined. Results indicate a degradation in model performance with increasing forecast horizons, irrespective of input scenario or water depth. Notably, the CNN model demonstrates superior performance in near-term and medium-term forecasts compared to long-term predictions, underscoring the need for enhanced efforts in long-term forecasting. Overall, the CNN model effectively reproduces vertical water temperature profiles and captures diel variations in lake water temperatures. Incorporating additional input variables does not necessarily improve model performance; however, using surface water temperatures and air temperatures as inputs produces acceptable results for modeling vertical temperature profiles. These findings have implications for lake management in Lake Kasumigaura (e.g., controlling phytoplankton and zooplankton communities) and offer insights into water temperature modeling for other lakes.

AimThis study aimed to illustrate the changing diversity patterns of native freshwater fish in the past two centuries and to identify priority locations for native fish conservation to counter future degradation.LocationJapanese archipelago.MethodsWe used the published native fish fauna data in 39 lakes across Japan, analysing historical and current diversity and projecting future distribution patterns based on the Japanese Red List. We assessed fish assemblages' taxonomic, phylogenetic, and functional alpha and beta diversity across different periods. Additionally, we proposed a cumulative diversity index that incorporated taxonomic, phylogenetic and functional facets, and then examined its relationship with the latitudinal positioning of the lakes across periods.ResultsWe observed a significant decrease in the richness of native freshwater fish since historical periods. Accompanying this decline were significant reductions in both phylogenetic relatedness and functional redundancy. Fish beta diversity increased from the past to the present and is predicted to decline drastically, indicating an ongoing homogenization process. Using the cumulative diversity index, we identified Hokkaido and Kyushu as crucial habitats for endemic fishes. These islands, with unique biogeographical backgrounds, contributed substantially to the national dissimilarity patterns of native fish assemblages in the historical period. However, the contribution is diminishing due to the ongoing decline in fish endemism.Main ConclusionsThe conservation priority of native freshwater fish in Japan should be assigned to Hokkaido and Kyushu due to the inhabited endemic species. The proposed framework for assessing the cumulative diversity of biotic communities presented the potential to aid macroecological explorations that underpin biodiversity conservation.

Radiocesium (137Cs) contamination of the freshwater ecosystems adjacent to the Fukushima Daiichi Nuclear Power Plant (FDNPP) in Japan has persisted long after the accident that occurred at the facility in March 2011. It is necessary to elucidate the dynamics of 137Cs in various aquatic ecosystems to predict 137Cs concentrations in fish and manage freshwater fisheries in the vicinity of FDNPP. To these ends, we applied stable isotope analysis to evaluate changes in 137Cs levels through trophic positions and the relative importance of the 137Cs sources at the trophic bases of two rivers and two lakes in Fukushima. The δ15N analyses disclosed that 137Cs decreases from primary producers to fish consumers in the river food web and 137Cs increases among fish consumers with increasing trophic position in the lake food web. The δ13C analysis revealed that autochthonous 137Cs contributed to fish contamination. The periphyton-dependent and zooplankton-dependent fish had comparatively higher 137Cs concentrations in the rivers and lakes, respectively. Cesium-137 supply from the pelagic food web was observed to contribute to greater 137Cs levels in the fish consumers inhabiting the lakes. The results of this study show that stable isotope analysis may help clarify 137Cs dynamics in freshwater food webs and identify the important 137Cs sources in the food web. Identifying important 137Cs sources and trophic transfers depending on the ecosystem help guide regulatory and management frameworks to establish profitability of the food fish stocks there and maintain food security.

1. Water-level drawdowns are a management option for improving water quality in shallow, eutrophic lakes, but how much water levels should be reduced and what abiotic and biotic mechanisms can be expected to reduce the adverse effects of eutrophication are unclear. 2. We conducted a study with two experimental pools (10 m wide, 30 m long, and approx. 2.5 m water depth) in a before-after-control-impact design to examine whether water-level drawdowns could influence surface water quality and bottom-water conditions, as well as how physicochemical and biological variables contributed to improvements. In the experiment, we fertilised the pools to increase productivity and induce hypoxia, and then reduced water levels four times in increments of 0.5 m. 3. Our experiment using high-frequency sensors showed that water-level drawdowns could decrease cyanobacterial blooms and alleviate hypoxia relatively quickly by changing physicochemical conditions. Water-level reductions quickly increased bottom light illuminance, increased bottom-water temperatures, and weakened stratification. The result was a dramatic increase in bottom-water dissolved oxygen concentrations. 4. Water-level drawdowns also decreased the relative fluorescence of chlorophyll and phycocyanin, probably because of a decrease of internal nutrient loadings from sediment. Total nitrogen and NH4-N concentrations in the water column decreased after water-level reductions, and NH4-N and PO4-P concentrations in sediment pore waters were reduced in the water-level treatment. 5. Periphyton biomass did not change after the water-level drawdowns. Water-level manipulations increased the abundance of cyclopoids and calanoids but not large cladocerans. These biological processes responded slowly to water-level drawdowns and are unlikely to have contributed to water quality improvement. 6. Overall, the size of the water-level reduction required to start changing water quality was 0.5-1.0 m. Our results suggested that a temporary reduction in water-level of 20%-40% of the depth might help to suppress cyanobacterial blooms and hypoxia in shallow lakes, reservoirs, and agricultural ponds. Such water-level management may help resolve conflicting demands for water and may be incorporated into management plans for flood control.