The spread of hypoxia is a threat to aquatic ecosystem functions and services as well as to biodiversity. However, sparse long‐term monitoring of lake ecosystems has prevented reconstruction of ...global hypoxia dynamics while inhibiting investigations into its causes and assessing the resilience capacity of these systems. This study compiles the onset and duration of hypoxia recorded in sediments of 365 lakes worldwide since AD 1700, showing that lacustrine hypoxia started spreading before AD 1900, 70 years prior to hypoxia in coastal zones. This study also shows that the increase of human activities and nutrient release is leading to hypoxia onset. No correlations were found with changes in precipitation or temperature. There is no evidence for a post‐1980s return to well‐oxygenated lacustrine conditions in industrialized countries despite the implementation of restoration programs. The apparent establishment of stable hypoxic conditions prior to AD 1900 highlights the challenges of a growing nutrient demand, accompanied by increasing global nutrient emissions of our industrialized societies, and climate change.
Sediment Accumulation Rate (SAR; measured as mm yr
−1
) and Mass Accumulation Rate (MAR; measured as g cm
−2
yr
−1
) data were collected from published lake core records that spanned the past ~ ...150 years, from approximately 500 sites worldwide. For each lake, key watershed characteristics including watershed size, slope, land use and climate were extracted, with the goal of quantifying the relative importance of these variables as drivers of SAR and MAR. General additive models provided evidence of accelerated global lake sediment infilling rates after AD 1950. Whereas the onset of sedimentation acceleration varied across ecoregions, global lake MAR values were found to have increased ~ threefold on average, since baseline conditions pre-1900 (i.e. μ
baseline
= 0.040 ± 0.044 g cm
−2
yr
−1
and μ
modern
= 0.13 ± 0.22 g cm
−2
yr
−1
). The significant drivers, identified through Linear Mixed Effect modeling of MAR time series, were watershed population density (log-transformed) and watershed cropland density (log-transformed). Our results highlight important spatial heterogeneity in SAR and MAR among lakes, precluding the use of simple modeling approaches. SAR and MAR were found to be moderately correlated to one another, despite the potential for post-depositional disparities between segments of the sediment cores. We identified organic matter content (loss-on-ignition, LOI) as a significant co-variate that could be used to correct inflated very recent SAR rates. Our empirical analyses suggest that, despite a wide range of natural variability among lakes, both SAR and MAR increased globally and the increases appear to be mainly the result of enhanced watershed activities associated with agriculture and urbanization.
ABSTRACT
In the French north‐western Alps, several lakes of glacial origin, such as Aiguebelette and La Thuile, present some mass‐transport deposits within their sedimentary sequences. These event ...layers can result from lake sediment destabilization eventually triggered by earthquakes. On Lake Aiguebelette, based on sedimentological, geochemical and magnetic analyses, and high‐resolution seismic and bathymetric surveys a ca 1 m thick event layer was identified in the deepest lake basin and a synchronous ca 2 cm thick deposit in the shallow basin. Age‐depth models based on radiocarbon ages reveal that both mass‐transport deposits in Lake Aiguebelette occurred around the Younger Dryas – Early Holocene climatic transition (i.e. 11 700 cal bp) time range. In Lake La Thuile, located 30 km away, unique mass‐transport deposits (translational slide type) were recorded at the same time range in sedimentary records. Additionally, high‐resolution seismic profiles previously acquired in Lake Annecy and Lake Bourget support the hypothesis of significant mass‐transport deposits occurring at the Younger Dryas – Early Holocene time range. These outcomes on four north‐western peri‐Alpine and Alpine lakes highlight the regional occurrence of mass‐transport deposits in the Younger Dryas – Early Holocene time range. Seismic and rockfall events are discussed as potential sources of these significant and similarly aged mass‐transport deposits. Based on this study and a literature review, the authors suggest that mechanisms induced by rapid climate change and glacial retreat, such as crustal rebound and erosional unloading, could favour the triggering of earthquakes and rockfall events. In the case of mass‐transport deposits archived in north‐western Alpine lakes during this time period, this study favours the hypothesis of increased seismicity as the primary source driving process involved.
Accelerated soil erosion has become a pervasive feature on landscapes around the world and is recognized to have substantial implications for land productivity, downstream water quality, and ...biogeochemical cycles. However, the scarcity of global syntheses that consider long-term processes has limited our understanding of the timing, the amplitude, and the extent of soil erosion over millennial time scales. As such, we lack the ability to make predictions about the responses of soil erosion to long-term climate and land cover changes. Here, we reconstruct sedimentation rates for 632 lakes based on chronologies constrained by 3,980 calibrated 14C ages to assess the relative changes in lake-watershed erosion rates over the last 12,000 y. Estimated soil erosion dynamics were then complemented with land cover reconstructions inferred from 43,669 pollen samples and with climate time series from the Max Planck Institute Earth System Model. Our results show that a significant portion of the Earth surface shifted to human-driven soil erosion rate already 4,000 y ago. In particular, inferred soil erosion rates increased in 35% of the watersheds, and most of these sites showed a decrease in the proportion of arboreal pollen, which would be expected with land clearance. Further analysis revealed that land cover change was the main driver of inferred soil erosion in 70% of all studied watersheds. This study suggests that soil erosion has been altering terrestrial and aquatic ecosystems for millennia, leading to carbon (C) losses that could have ultimately induced feedbacks on the climate system.
Accurate flood hazard assessments are crucial for adequate flood hazard mapping and hydraulic infrastructure design. The choice of an acceptable and cost-effective solution for such assessments ...depends upon the estimation of quantiles for different characteristics of floods, usually maximum discharges. However, gauge series usually have a limited time length and, thereby, quantile estimates associated to high return periods are subject to large uncertainties. To overcome this limitation, reconstructed flood series from historical, botanical or geological archives can be incorporated. In this study, we propose a novel approach that i) combines classic series of observations with paleodischarges (of the Rhône River) reconstructed from open lake sediments (Lake Bourget, Northwestern Alps, France) and ii) propagates uncertainties related to the reconstruction method during the estimation of extreme quantiles.
A Bayesian approach is adopted in order to properly treat the non-systematic nature of the reconstructed flow data, as well as the uncertainties related to the reconstruction method. While this methodology has already been applied to reconstruct maximum discharges from historical documents, tree rings or fluvial sediments, similar applications need to be tested today on open lake sediments as they are one of the only archives that provide long and continuous paleoflood series. Reconstructed sediment volumes being subject to measurement errors, we evaluate and account for this uncertainty, along with the uncertainty related to the reconstruction method, the parametric uncertainty, and the rating-curve errors for systematic gauged flows by propagating these uncertainties through the modeling chain. Reconstructed maximum discharges appear to largely overcome values of observations, reaching values of approximately 2,600, 4,200, 2,450 and 2,500 m3/s in 1689, 1711, 1733 and 1737 respectively, which correspond to historically-known catastrophic floods. Extreme quantiles are estimated using direct measurements of maximum discharges (1853-2004) only and then combined to the sedimentary information (1650-2013). The comparison of the resulting estimates demonstrates the added value of the sedimentary information. In particular, the four historical catastrophic floods are very unlikely if only direct observations are considered for quantile estimations.
The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity1'2, nutrient biogeochemistry3, greenhouse gas emissions4, and the quality of drinking water5. The long-term ...declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity6,7, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification8,9 or oxygen may increase as a result of enhanced primary production10. Here we analyse a combined total of45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 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 dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, 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. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world's oceans6,7 and could threaten essential lake ecosystem services2,3,5'11.
The Anthropocene is characterized by a worldwide spread of hypoxia, among other manifestations, which threatens aquatic ecosystem functions, services, and biodiversity. The primary cause of hypoxia ...onset in recent decades is human‐triggered eutrophication. Global warming has also been demonstrated to contribute to the increase of hypoxic conditions. However, the precise role of both environmental forcings on hypoxia dynamics over the long term remains mainly unknown due to a lack of historical monitoring. In this study, we used an innovative paleolimnological approach on three large European lakes to quantify past hypoxia dynamics and to hierarchies the contributions of climate and nutrients. Even for lake ecosystems that have been well oxygenated over a millennia‐long period, and regardless of past climatic fluctuations, a shift to hypoxic conditions occurred in the 1950s in response to an unprecedented rise in total phosphorus concentrations above 10 ± 5 µg P L−1. Following this shift, hypoxia never disappeared despite the fact that environmental policies succeeded in drastically reducing lake phosphorus concentrations. During that period, decadal fluctuations in hypoxic volume were great, ranging between 0.5 and 8% of the total lake volumes. We demonstrate, through statistical modeling, that these fluctuations were essentially driven by climatic factors, such as river discharge and air temperature. In lakes Geneva and Bourget, which are fed by large river systems, fluctuations in hypoxic volume were negatively correlated with river discharge. In contrast, the expansion of hypoxia has been related only to warmer air temperatures at Annecy, which is fed by small river systems. Hence, we outline a theoretical framework assuming that restored lake ecosystems have inherited hypoxia from the eutrophication period and have shifted to a new stable state with new key controls of water and ecosystem quality. We suggest that controlling river discharge may be a complementary strategy for local management of lakes fed by large river systems.
Key Points
Factors driving hypoxia are assessed over the Holocene and the last 130 yearsHypoxia in the twentieth century has been triggered by anthropogenic P suppliesThe current dynamics of anthropogenic hypoxia has been driven by climate
Now that evasion of carbon dioxide (CO2) from inland waters is accounted for in global carbon models, it is crucial to quantify how these fluxes have changed in the past and forecast how they may ...alter in the future in response to local and global change. Here we developed a sediment proxy for the concentration of summer surface dissolved CO2 concentration and used it to reconstruct changes over the past 150 years for three large lakes that have been affected by climate warming, changes in nutrient load, and detrital terrigenous supplies. Initially CO2 neutral to the atmosphere, all three lakes subsequently fluctuated between near equilibrium and supersaturation. Although catchment inputs have supplied CO2 to the lakes, internal processes and reallocation have ultimately regulated decadal changes in lake surface CO2 concentration. Nutrient concentration has been the dominant driver of CO2 variability for a century although the reproducible, nonmonotonic relationship of CO2 to nutrient concentration suggests an interplay between metabolic and chemical processes. Yet for two of these lakes, climatic control of CO2 concentrations has been important over the last 30 years, promoting higher surface CO2 concentrations, likely by decreasing hypolimnetic carbon storage. This new approach offers the unique opportunity to scale, a posteriori, the long‐term impact of human activities on lake CO2.
Key Points
Surface CO2 concentrations of three large lakes are assessed over the last 150 years
Nutrient concentration has been the primary, long‐term control of lakes surface CO2
Over the last decades, climate warming can take over nutrient control on lake CO2
•Biologically inspired models for grasping action prediction.•Gaze-driven detection model for objects to be grasped in egocentric video.•Two alternative methods for noise handling in eye-gaze ...measurements.•A novel loss for automatic prediction of grasping actions.•A new public dataset for prediction of grasping actions in ego-centric video.
We tackle the problem of predicting a grasping action in ego-centric video for the assistance to upper-limb amputees. Our work is based on paradigms of neuroscience that state that human gaze expresses intention and anticipates actions. In our scenario, human gaze fixations are recorded by a glass-worn eye-tracker and then used to predict the grasping actions. We have studied two aspects of the problem: which object from a given taxonomy will be grasped, and when is the moment to trigger the grasping action. To recognize objects, we using gaze to guide Convolutional Neural Networks (CNN) to focus on an object-to-grasp area. However, the acquired sequence of fixations is noisy due to saccades toward distractors and visual fatigue, and gaze is not always reliably directed toward the object-of-interest. To deal with this challenge, we use video-level annotations indicating the object to be grasped and a weak loss in Deep CNNs. To detect a moment when a person will take an object we take advantage of the predictive power of Long-Short Term Memory networks to analyze gaze and visual dynamics. Results show that our method achieves better performance than other approaches on a real-life dataset.
Large lakes of the world are habitats for diverse species, including endemic taxa, and are valuable resources that provide humanity with many ecosystem services. They are also sentinels of global and ...local change, and recent studies in limnology and paleolimnology have demonstrated disturbing evidence of their collective degradation in terms of depletion of resources (water and food), rapid warming and loss of ice, destruction of habitats and ecosystems, loss of species, and accelerating pollution. Large lakes are particularly exposed to anthropogenic and climatic stressors. The Second Warning to Humanity provides a framework to assess the dangers now threatening the world’s large lake ecosystems and to evaluate pathways of sustainable development that are more respectful of their ongoing provision of services. Here we review current and emerging threats to the large lakes of the world, including iconic examples of lake management failures and successes, from which we identify priorities and approaches for future conservation efforts. The review underscores the extent of lake resource degradation, which is a result of cumulative perturbation through time by long-term human impacts combined with other emerging stressors. Decades of degradation of large lakes have resulted in major challenges for restoration and management and a legacy of ecological and economic costs for future generations. Large lakes will require more intense conservation efforts in a warmer, increasingly populated world to achieve sustainable, high-quality waters. This Warning to Humanity is also an opportunity to highlight the value of a long-term lake observatory network to monitor and report on environmental changes in large lake ecosystems.
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