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Lakes and ponds experience anthropogenically forced changes that may be non-linear and sometimes initiate ecosystem feedbacks leading to tipping points beyond which impacts become hard to reverse. In many cases climate change is a key driver, sometimes in concert with other stressors. Lakes are also important players in the global climate by ventilating a large share of terrestrial carbon (C) back to the atmosphere as greenhouse gases and will likely provide substantial feedbacks to climate change. In this paper we address various major changes in lake ecosystems and discuss if tipping points can be identified, predicted, or prevented, as well as the drivers and feedbacks associated with climate change. We focus on potential large-scale effects with regional or widespread impacts, such as eutrophication-driven anoxia and internal phosphorus (P) loading, increased loading of organic matter from terrestrial to lake ecosystems (lake "browning"), lake formation or disappearance in response to cryosphere shifts or changes in precipitation to evaporation ratios, switching from nitrogen to phosphorus limitation, salinization, and the spread of invasive species where threshold-type shifts occur. We identify systems and drivers that could lead to self-sustaining feedbacks, abrupt changes, and some degree of resilience, as opposed to binary states not subject to self-propelling changes or resilience. Changes driven by warming, browning, and eutrophication can cause increased lake stratification, heterotrophy (browning), and phytoplankton or macrophyte mass (eutrophication), which separately or collectively drive benthic oxygen depletion and internal phosphorus loading and in turn increase greenhouse gas (GHG) emissions. Several of these processes can feature potential tipping point thresholds, which further warming will likely make easier to surpass. We argue that the full importance of the vulnerability of lakes to climate and other anthropogenic impacts, as well as their feedback to climate, is not yet fully acknowledged, so there is a need both for science and communication in this regard. [ABSTRACT FROM AUTHOR]

Lakes experience anthropogenically-forced changes that may initiate ecosystem feedbacks, in some cases reaching tipping points beyond which impacts become hard to reverse. Lakes are also important players in the global climate by ventilating a large share of terrestrial carbon back to the atmosphere as greenhouse gases, and will likely provide substantial feedbacks to climate change. In this paper we address various major changes in lake ecosystems, and discuss if tipping points can be identified, predicted, or prevented in them, along with their associated feedbacks to climate change. Potential tipping dynamics assessed include eutrophication-driven anoxia and internal phosphorus-loading, increased loading of organic matter from terrestrial to lake ecosystems (lake "browning"), lake formation or disappearance in response to cryosphere shifts, switching from nitrogen to phosphorus limitation, salinization, and the spread of invasive species. We also address other types of abrupt, or threshold-type shifts in lakes and ponds, and conclude on which tipping points are locally or regionally relevant. We identify a key set of co-drivers that could lead to self-sustaining feedbacks, with warming, browning, and eutrophication leading to increased lake stratification, heterotrophy, and algal mass, which separately or collectively drive benthic oxygen depletion and in turn increased greenhouse gas emissions (helping to drive further warming and organic matter loading) and internal phosphorus-loading (driving further eutrophication). Several of these processes can feature tipping points, which further warming will likely make easier to reach. We argue that the full importance of the vulnerability of lakes to climate and other anthropogenic impacts, as well as their feedback to climate is not yet fully acknowledged, so there is a need both for science and communication in this regard. [ABSTRACT FROM AUTHOR]

Transformation of the natural land cover is one of the most important global changes. Changes in land use may strongly affect ecosystem functions and biodiversity by directly or indirectly modifying key structural properties. Here, we examined the effects of land use on the size structure of fish communities in subtropical Uruguayan streams. We analysed the suitability of non-taxonomic, size-related variables as a tool to elucidate shifts in stream fish assemblages along a gradient in land use change. We also tested some taxonomic variables (e.g., species richness and species diversity) to compare their response with size-related variables. In the more anthropogenic disturbed basins (i.e., dominance of urban and agricultural land use), we found higher size diversity, slope of size spectrum (flatter slopes) and total range of fish length, whereas higher species richness, fish abundance and species diversity were detected in more pristine stream basins (i.e., dominance of rangelands). In combination with traditional taxonomic variables, size-related variables of fish communities in streams may be effective tools in evaluating the changes occurring in freshwater ecosystems associated with anthropogenic changes in land use. La transformation de la couverture naturelle des sols est un des changements planetaires les plus importants. Les changements d'utilisation du sol peuvent avoir une grande incidence sur les fonctions et la biodiversite des ecosystemes en modifiant directement ou indirectement des proprietes structurales cles. Nous examinons les effets de l'utilisation du sol sur la structure selon la taille des communautes de poissons dans des cours d'eau uruguayens subtropicaux. Nous avons analyse l'adequation de variables non taxonomiques reliees a la taille comme outil pour elucider les modifications des assemblages de poissons lotiques le long d'un gradient de changement de l'utilisation du sol. Nous avons egalement teste certaines variables taxonomiques (p. ex. richesse et diversite specifiques) pour comparer leur reaction a celle de variables reliees a la taille. Dans les bassins les plus perturbes par l'activite humaine (c.-a-d. oU dominent des utilisations urbaines et agricoles des sols), nous avons constate une plus grande diversite de tailles, des pentes moins inclinees du spectre de tailles et une plus grande fourchette totale de longueurs des poissons, alors qu'une plus grande richesse specifique, une plus grande abondance de poissons et une plus grande diversite specifique ont ete observees dans les bassins de cours d'eau moins perturbes (c.-a-d. oU dominent les parcours naturels). Combinees a des variables taxonomiques traditionnelles, les variables reliees a la taille des communautes de poissons dans les cours d'eau pourraient constituer des outils efficaces pour evaluer les modifications aux ecosystemes d'eau douce associees aux changements d'utilisation des sols d'origine humaine. [Traduit par la Redaction], 1. Introduction A key driver of global change is the transformation of the natural land cover, with half of the terrestrial surface of the Earth already being transformed by human [...]

Fluxes of carbon dioxide (CO2) and methane (CH4) in shallow lakes are strongly affected by dominant primary producers which mostly has been studied in temperate and boreal regions. We compared summer CO2 and CH4 fluxes (diffusion and ebullition) in littoral and pelagic zones of three subtropical shallow lakes with contrasting regimes: clear-vegetated, phytoplankton-turbid, and sediment-turbid, and assessed fluxes in different seasons in the clear-vegetated system. Significant differences among the lakes occurred only for CH4 fluxes. In the sediment-turbid lake we found undersaturated CH4 concentrations were below atmospheric equilibrium, implying CH4 uptake (< 0 mg m−2 day−1), likely due to low availability of organic matter. Differences between zones occurred in the clear-vegetated and phytoplankton-turbid lakes, with higher total CH4 emissions in the littoral than in the pelagic zones (mean: 4342 ± 895 and 983 ± 801 mg m−2 day−1, respectively). CO2 uptake (< < 0 mg m−2 day−1) occurred in the littoral of the phytoplankton-turbid lake (in summer), and in the pelagic of the clear-vegetated lake even in winter, likely associated with submerged macrophytes dominance. Our work highlights the key role of different primary producers regulating carbon fluxes in shallow lakes and points out that, also in the subtropics, submerged macrophyte dominance may decrease carbon emissions to the atmosphere. [ABSTRACT FROM AUTHOR]