The concentration of dissolved oxygen in aquatic systems helps regulate biodiveristy 1, 2 , nutrient biogeochemistry 3 , greenhouse gas emissions 4 , and drinking water quality 5 . The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity 6, 7 , but little is known about changes in dissolved oxygen concentrations in lakes. While dissolved oxygen solubility decreases with increasing water temperatures, long-term lake trajectories are not necessarily predictable. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification 8, 9 or they may increase as a result of enhanced primary production 10 . Here we analyse 45,148 dissolved oxygen and temperature profiles from 393 temperate lakes spanning 1941-2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although surface dissolved oxygen increased in a subset of highly-productive warming lakes, likely due to increasing phytoplankton production. In contrast, the decline in deep waters is associated with stronger thermal stratification and water clarity losses, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Freshwater dissolved oxygen losses are 2.5-10 times greater than observed in the world's oceans 6, 7 and could threaten essential lake ecosystem services 2,3,5,11 .

Piscivorous fish are size‐selective predators. Although sizes of prey selectively ingested by piscivores traditionally have been measured in terms of prey length relative to predator length, the relationship between prey body depth (measured dorsoventrally) and piscivore mouth gape may be a more appropriate measure of prey size selection. In 2‐d feeding trials with three sizes of largemouth bass Micropterus salmoides, I offered various sizes of shallow‐bodied fathead minnows Pimephales promelas and deep‐bodied pumpkinseeds Lepomis gibbosus in assemblages of one or both species. All sizes of predators preferred pumpkinseeds with body depths well below the maximum size ingestible. Small predators also preferred fathead minnows with body depths below the maximum size ingestible, whereas intermediate and large predators selectively ingested the largest fathead minnows offered. Largemouth bass never ingested prey of body depth greater than their own external mouth width. Although lengths of selectively ingested fathead minnows and pumpkinseeds differed, largemouth bass showed highest preferences for prey of similar body depths regardless of taxonomic identity. These results suggest that, in addition to setting constraints on maximum sizes of prey that can be ingested by piscivores, the relationship between prey body depth and piscivore mouth gape may also be important in selection of prey within the range of ingestible sizes. Therefore, body depth may be more useful than the traditional measure of prey length as a common measure for examining prey selection by gape‐limited piscivores over a wide array of prey species.

We searched the limnological literature to assess the success of biomanipu lation projects with respect to improvement of water quality in culturally eutrophic lakes and reservoirs. We reviewed 41 biomanipulation experiments (from 63 pub lished papers/books) involving the reduction of zooplanktivorous and benthivorous fi sh abundances in 39 lakes. We divided the experiments into fi ve categories based on the type of biomanipulation performed: piscivore stocking, piscivore stocking + partial fi sh removal, partial fi sh removal, elimination of fish, and elimination of fish fol lowed by restocking. We judged success (consistently successful, partially successful, unsuccessful) by changes in water quality variables; primarily phytoplankton or chlorophyll biomass and water transparency, during the post-implementation period of each study. The percentage of consistently successful manipulations varied with approach: piscivore stocking (28.6 %), piscivore stocking + partial fi sh removal (60.0 %), partial fi sh removal (90.0 %), elimination of fi sh (40.0 %), and elimination of fish fo llowed by restocking (66.7 %). Overall, 61 % of the biomanipulations were con sistently successful in improving water quality, with most successful attempts invol ving smalI, shallow lakes (< 25 ha, mean depth < 3 m). Only 14.6 % of the biomanipula tions were unsuccessfu1 in improving water quality for at least one year. Biomanipu lations resulting in increased abundances of Daphnia and macrophytes were most like\y to achieve stable c\ear water states and maintain improved water quality. each natural or man caused perturbation to a lake is to some extent a novel experiment because no two 1akes are identical ...

Bacteria play key roles in the function and diversity of aquatic systems, but aside from study of specific bloom systems, little is known about the diversity or biogeography of bacteria associated with harmful cyanobacterial blooms (cyanoHABs). CyanoHAB species are known to shape bacterial community composition and to rely on functions provided by the associated bacteria, leading to the hypothesized cyanoHAB interactome, a coevolved community of synergistic and interacting bacteria species, each necessary for the success of the others. Here, we surveyed the microbiome associated with Microcystis aeruginosa during blooms in 12 lakes spanning four continents as an initial test of the hypothesized Microcystis interactome. We predicted that microbiome composition and functional potential would be similar across blooms globally. Our results, as revealed by 16S rRNA sequence similarity, indicate that M. aeruginosa is cosmopolitan in lakes across a 280 longitudinal and 90 latitudinal gradient. The microbiome communities were represented by a wide range of operational taxonomic units and relative abundances. Highly abundant taxa were more related and shared across most sites and did not vary with geographic distance, thus, like Microcystis, revealing no evidence for dispersal limitation. High phylogenetic relatedness, both within and across lakes, indicates that microbiome bacteria with similar functional potential were associated with all blooms. While Microcystis and the microbiome bacteria shared many genes, whole-community metagenomic analysis revealed a suite of biochemical pathways that could be considered complementary. Our results demonstrate a high degree of similarity across global Microcystis blooms, thereby providing initial support for the hypothesized Microcystis interactome.

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Results arc presented from a 3-yr, replicated pond study examining how differential planktivore vulnerability can influence planktivore responses to piscivory and ultimately plankton responses via cascading effects. Without piscivores, planktivory from a multispecies planktivore assemblage (fathead minnow Pimephalcs promelas and pumpkinseed sunfish Lepomis gibboia) produced the well-documented plankton responses. Chaoborus densities, zooplankton biomass, and cladoceran and copepod mean body sizes were reduced, and chlorophyll concentrations and gross primary production were enhanced. With piscivorcs, levels of planktivory were depressed due to elimination of the highly vulnerable minnow and a shift in the size structure of the relatively less vulnerable sunfish toward larger, benthivorous sizes. Hence, zooplankton biomass and cladoceran and copepod mean body sizes were increased, and chlorophyll concentrations and gross primary production were reduced. However, juvenile sunfish remained abundant in ponds containing piscivores, so Chaoborus densities remained low and small cladocerans (Bosmina and Ceriodaphnia) were abundant. Moreover, Daphnia biomass, mean body size, and number of days present during the final year in these ponds were negatively correlated with small sunfish biomass, indicating that piscivory did not simply cancel out the effects of planktivory.

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Globally, lake surface water temperatures have warmed rapidly relative to air temperatures, but changes in deepwater temperatures and vertical thermal structure are still largely unknown. We have compiled the most comprehensive data set to date of long-term (1970–2009) summertime vertical temperature profiles in lakes across the world to examine trends and drivers of whole-lake vertical thermal structure. We found significant increases in surface water temperatures across lakes at an average rate of + 0.37 °C decade−1, comparable to changes reported previously for other lakes, and similarly consistent trends of increasing water column stability (+ 0.08 kg m−3 decade−1). In contrast, however, deepwater temperature trends showed little change on average (+ 0.06 °C decade−1), but had high variability across lakes, with trends in individual lakes ranging from − 0.68 °C decade−1 to + 0.65 °C decade−1. The variability in deepwater temperature trends was not explained by trends in either surface water temperatures or thermal stability within lakes, and only 8.4% was explained by lake thermal region or local lake characteristics in a random forest analysis. These findings suggest that external drivers beyond our tested lake characteristics are important in explaining long-term trends in thermal structure, such as local to regional climate patterns or additional external anthropogenic influences.