Reservoirs have spatial variation in water depth, suspended matter, and biogeochemistry that can influence patterns of water clarity and color. Spatial surveys with sensor-equipped boats matched in time with satellite image acquisition provide data-intensive avenues for understanding spatial patterns of optical properties within reservoirs. We combined continuous field data from high-speed spatial surveys with Sentinel-2 imagery to map water clarity and color in six subtropical Texas reservoirs during summer. Reservoir arms had lower clarity and longer dominant wavelengths than the main bodies, indicating higher concentrations of optical constituents. Because surveys took place during a period of low inflows associated with drought, color and clarity patterns may have been associated mainly with phytoplankton communities and resuspension of shallow sediments, rather than direct river inputs. Whole system analysis of dominant wavelength showed that five reservoirs reflected green (550-569 nm) over most of their surface area, suggesting high concentrations of phytoplankton biomass, and one reservoir in a clay-dominated watershed was primarily yellow to brown (570-583 nm). Water clarity spanned a wide turbidity range (3.18-95.2 NTU) and was low over most of the surface area of these systems (<1.5 m Secchi). In five bio-optical models of turbidity, the best model performance occurred between 0 and 50 NTU, with unexplained variation at higher turbidity possibly linked to optically distinct classes of suspended sediment, phytoplankton pigmentation, or dissolved organic matter. These results indicate that reservoirs can have considerable spatial heterogeneity in water clarity and color, setting the stage for future spatial snapshots that encompass interannual and seasonal variability in precipitation.

Population change is a main driver behind global environmental change, including urban land expansion. In future scenario modeling, assumptions regarding how populations will change locally, despite identical global constraints of Shared Socioeconomic Pathways (SSPs), can have dramatic effects on subsequent regional urbanization. Using a spatial modeling experiment at high resolution (1 km), this study compared how two alternative US population projections, varying in the spatially explicit nature of demographic patterns and migration, affect urban land dynamics simulated by the Spatially Explicit, Long-term, Empirical City development (SELECT) model for SSP2, SSP3, and SSP5. The population projections included: (1) newer downscaled state-specific population (SP) projections inclusive of updated international and domestic migration estimates, and (2) prevailing downscaled national-level projections (NP) agnostic to localized demographic processes. Our work shows that alternative population inputs, even those under the same SSP, can lead to dramatic and complex differences in urban land outcomes. Under the SP projection, urbanization displays more of an extensification pattern compared to the NP projection. This suggests that recent demographic information supports more extreme urban extensification and land pressures on existing rural areas in the US than previously anticipated. Urban land outcomes to population inputs were spatially variable where areas in close spatial proximity showed divergent patterns, reflective of the spatially complex urbanization processes that can be accommodated in SELECT. Although different population projections and assumptions led to divergent outcomes, urban land development is not a linear product of population change but the result of complex relationships between population, dynamic urbanization processes, stages of urban development maturity, and feedback mechanisms. These findings highlight the importance of accounting for spatial variations in the population projections, but also urbanization process to accurately project long-term urban land patterns.

To investigate the influence of flow rate increment on the solute transport parameter of immobile zones in a karst system, a dye tracer test was conducted in the Downtown Salado Spring Complex (DSSC) comprising three springs: Big Boiling, Anderson, and Doc Benedict springs. The Multiflow two-region nonequilibrium model (2RNE) was used to simulate the breakthrough curve (BTC) of the springs, and changes in the solute transport parameters in response to flow rate increment were observed. The simulation result showed that the 2RNE model was capable of reproducing the BTC of all the DSSC springs, with an R-squared value greater than 0.9 in all flow rate increment scenarios. The research demonstrates that a positive correlation will exist between the flow rate and solute transport parameter of the immobile zones if the tracer transport to the spring is truly influenced by immobile zones. In contrast, a negative correlation will exist between the flow rate and mass transfer coefficient if the immobile zone has less influence. Overall, the research provides insights into contaminant movement in karst by documenting how tracers are retained in the immobile fluid zone.

Climate change is reducing winter ice cover on lakes; yet, the full societal and environmental consequences of this ice loss are poorly understood. The socioeconomic implications of declining ice include diminished access to ice-based cultural activities, safety concerns in traversing ice, changes in fisheries, increases in shoreline erosion, and declines in water storage. Longer ice-free seasons allow more time and capacity for water to warm, threatening water quality and biodiversity. Food webs likely will reorganize, with constrained availability of ice-associated and cold-water niches, and ice loss will affect the nature, magnitude, and timing of greenhouse gas emissions. Examining these rapidly emerging changes will generate more-complete models of lake dynamics, and transdisciplinary collaborations will facilitate translation to effective management and sustainability.

Optical water quality affects the quantity and spectral composition of underwater light available to photoautotrophs along with light reflected off the water surface. In reservoirs, prominent gradients for optical water quality and clarity measures occur between inlets and outlets, allowing rapid surveys of ecosystem structure across diverse conditions along with tests of relationships between in situ and remotely sensed water indices. We used a sensor-equipped boat in a clay-rich subtropical drinking water reservoir to examine continuous spatial patterns of water clarity and the relationship between in-lake turbidity and normalized difference turbidity index (ndti) from Sentinel 2 imagery acquired on the same date. Over the 62.3 km boat path, surface turbidity varied between 14 and 85 NTU (mean = 30), with highest values in shallower water near the main inlet. Results indicated a strong linear relationship between sensor turbidity and lab-determined turbidity. A positive relationship between the satellite turbidity index and in-lake turbidity, combined with a negative exponential relationship between Secchi depth and turbidity, provided a basis for predicting water clarity metrics continuously over the entire lake surface. In this turbid water body, predicted Secchi depth and turbidity over the whole lake had means of 0.22 m and 54 NTU, with 75% of the lake area <0.29 m and >26 NTU. Pursuit of general relationships involving optical properties of water from high-speed, data-intensive spatial surveys and remotely sensed surface reflectance facilitates further development of spatially explicit models of aquatic systems.

High-frequency water quality monitoring is rapidly growing in freshwater research and management. Sensor deployments at fixed depths in naturally formed lakes dominate the current understanding of ecosystem dynamics, with comparatively few studies focusing on reservoirs. Here we examined how environmental parameters affect the water column stability and dissolved oxygen (DO) dynamics in a North Texas eutrophic reservoir. We used an autonomous water quality monitoring profiler that recorded temperature, DO, and pH as well as wind speed, wind direction, and air temperature from an on-platform mounted weather station. Data were collected every 2 h from 25 April to 29 October 2019, with each profile spanning from the surface to 10 m depth at 0.5 m steps. Throughout most of the sampling period, the lake had no distinct thermocline, but we observed an oxycline throughout most of the monitoring period, with multiple mixing events. Near the sediments, periods of anoxia lasting hours to days were first observed in May, and July through September were chronically anoxic. In August and September, DO was frequently <2.0 mg L-1 throughout the entire water column, with 2 periods of total water column anoxia. Surface DO measurements for each 2 h interval were highly synchronous with Schmidt stability. During decreasing and low Schmidt stability or Lake Number, the oxycline destabilized in association with changes in wind direction and speed. Our results indicate that DO can be highly dynamic in polymictic reservoirs, with strong implications for carbon and nitrogen biogeochemical cycles.

Mountain lakes experience interannual variability in spring snowpack and ice cover that can lead to differences in physical, chemical, and biological properties in the succeeding summer. Lake studies that capture extreme years of snow and ice would be useful to understand and anticipate effects of climate change, but such data are rare for remote mountain lakes. Monitoring of lakes in Olympic, North Cascades, and Mount Rainier National Parks from 2007 to 2018 allowed us to examine limnological differences along interannual and elevation‐driven climate gradients that included unusually high (2011–2012) and 100‐yr record low (2015) snowpack years. Years with lower spring snowpack had earlier ice‐out. Across lakes, our analysis suggested an average of 0.075°C lake warming per day of lost ice duration (0.525°C per week), giving rise to other ecosystem changes linked to temperature such as lower dissolved oxygen, higher total dissolved N, higher chlorophyll, and higher abundance of cladoceran zooplankton. Conversely, in years with higher snowpack and a shorter ice‐free season, lakes were colder and clearer (1 m deeper Secchi depth for every 1 m May snow water equivalent), with more dilute ions as well as lower algal biomass and zooplankton abundance. These results add to evidence that changes in snowpack or ice‐out dates alter mountain lake ecology through multiple processes associated with hydrology, terrestrial‐aquatic connection, water temperature, productivity, ion composition, and plankton communities.

Our changing climate is having effects on freshwater ecosystems in all seasons, especially winter. High latitude lakes, wetlands, and rivers are experiencing shorter periods of ice cover, and lower latitudes systems that used to freeze are experiencing open water conditions throughout the winter. A 2019 AGU Chapman conference convened aquatic scientists to examine these changes and address the implications of changing winters to aquatic life, chemistry, and physics. Several studies demonstrate decreased ice cover duration than in the past. The removal of an ice “lid” from lakes and rivers impacts the exchange of gases with the atmosphere and the predominant types of metabolism occurring in the waters below, with the potential for more photosynthesis and an increase in oxic versus anoxic metabolism when the lid is removed. Multiple studies indicated an increase in the interannual variability of winters, especially in terms of ice‐cover duration and ice quality. Increased variability may simply be an outcome of a more variable winter climate or small differences in environmental conditions such as temperature that can have strong effects on gas exchange, light transmission, and turbulence when ice forms. A question that merits further consideration is whether and how winters of shorter duration and severity will change the dynamics of freshwater systems. Are there memory or legacy effects that carry over to the next season or year? There is much work to be done to understand how changing winters will impact the biogeochemical behavior of lakes and rivers in the coming decades.

Millions of lakes worldwide are distributed at latitudes or elevations resulting in the formation of lake ice during winter. Lake ice affects the transfer of energy, heat, light, and material between lakes and their surroundings creating an environment dramatically different from open‐water conditions. While this fundamental restructuring leads to distinct gradients in ions, dissolved gases, and nutrients throughout the water column, surprisingly little is known about the resulting effects on ecosystem processes and food webs, highlighting the lack of a general limnological framework that characterizes the structure and function of lakes under a gradient of ice cover. Drawing from the literature and three novel case studies, we present the Lake Ice Continuum Concept (LICC) as a model for understanding how key aspects of the physical, chemical, and ecological structure and function of lakes vary along a continuum of winter climate conditions mediated by ice and snow cover. We examine key differences in energy, redox, and ecological community structure and describe how they vary in response to shifts in physical mixing dynamics and light availability for lakes with ice and snow cover, lakes with clear ice alone, and lakes lacking winter ice altogether. Global change is driving ice covered lakes toward not only warmer annual average temperatures but also reduced, intermittent or no ice cover. The LICC highlights the wide range of responses of lakes to ongoing climate‐driven changes in ice cover and serves as a reminder of the need to understand the role of winter in the annual aquatic cycle.

Scientists long eschewed studying lakes in winter, expecting that cold temperatures and ice cover limited activity below the surface. Recent findings to the contrary are changing limnologists’ views.

Phosphorus (P) is central to food production. Current understanding about the global phosphorus system is dominated by studies in wealthier nations where soil fertility, fertilizer supply chains, and agronomic tracking have long been established. In contrast, developing nations are experiencing major agricultural transitions and the associated phosphorus flows remain a significant knowledge gap. We compiled and analyzed several years of recent agricultural datasets for Bangladesh, currently the eighth most populous nation, using substance flow analysis for phosphorus. From 2000 to 2016, rice production increased by >50% and remained the dominant crop with remarkably higher phosphorus flow (49.96 kt in 2016) than all other crops. Phosphorus content of livestock products in 2016 exceeded 6.00 kt, more than double in the year 2000, driven primarily by phosphorus in milk and secondarily in meat/eggs. These agricultural changes coincided with a doubling of national phosphorus fertilizer consumption since 2000, a fourfold increase since the global food crisis (2009), and a pronounced rise in the phosphorus import dependency ratio, which was the highest among all countries compared. In turn, during 2010s fertilizer phosphorus use exceeded phosphorus as food + feed production leading to soil phosphorus accumulation, and loss as burned manure became one of the largest phosphorus flows in the entire system, equivalent to half of fertilizer use. This dramatic reconfiguration of the Bangladesh phosphorus system illustrates an important case of agricultural expansion and intensification that is still playing out, with similar situations occurring in developing nations where population growth rates are high, and access to commercial fertilizers has risen.

Phosphorus is required as a fertilizer for producing food, and there is no substitute. Losses between mine production and diet result in significant environmental harm. We used a demand-driven substance flow model to explore the sensitivity of global phosphorus production to interventions of the food system including: reduction in animal fraction in the diet (AFD); manure use efficiency (MUE); animal food yield (YA); phosphorus use efficiency (PUE); the fraction of food that is wasted (FWF); the fraction of food waste recycled (FRE); and the fraction of human waste recycled (WRE). The model indicated that AFD and YA are the most influential interventions, and they interact with PUE and MUE. Furthermore, there is a minimum in AFD and YA below which it becomes necessary to increase mining. Another result is that reducing food waste is about 80 times more effective than recycling food waste in reducing P demand. Finally, the model was used to explore the global carrying capacity for humans on the basis of P sources other than mining. These sources may satisfy P requirements for as many as 2.5 billion people. If significant improvements were made in all of the considered interventions simultaneously, as many as 14.7 billion people could be sustained.

Pharmaceuticals and personal care products (PPCPs) garner increasing attention globally for both their usefulness as indicators of human waste and their potency as emerging organic toxicants. Three decades of rapid increase in PPCP study combined with an increasing number of PPCPs on the global market have created an opportunity (1) to review trends in diversity of compounds, sewage treatment techniques (STTs), and ecosystems investigated as well as (2) to identify knowledge gaps in the literature. We conducted a quantitative evidence synthesis of 6517 abstracts from primary articles in the environmental PPCP literature by examining relative abundance of specific PPCP classes, STTs, and ecosystem types. Our results demonstrate that non-prescription drugs and antibiotics dominated PPCP abstracts, appearing in 51% and 39% of reviewed abstracts, respectively, in comparison to hormones (18%), prescription drugs (18%), fragrances (0.3%), and antioxidants (0.0%), which can all elicit physiological and ecological responses even at low concentrations. References to centralized STTs (e.g., activated sludge, 37%) were more frequent than decentralized STTs (e.g., septic, 2%), despite decentralized STTs being common and frequently high impact sources of sewage pollution worldwide. Freshwater lotic systems (63%) were more prevalent than freshwater lentic (24%) and terrestrial (20%) systems. This discrepancy is notable because the longer residence times of lentic and terrestrial systems may enable PPCPs to concentrate and thus increase risk of biological consequences. These results highlight distinct opportunities to address knowledge gaps in the environmental PPCP literature, including underrepresented compounds (e.g., fragrances), sewage treatment techniques (e.g., septic systems), and ecosystem types (e.g., lakes).

Food production hinges largely upon access to phosphorus (P) fertilizer. Most fertilizer P used in the global agricultural system comes from mining of nonrenewable phosphate rock deposits located within few countries. However, P contained in livestock manure or urban wastes represents a recyclable source of P. To inform development of P recycling technologies and policies, we examined subnational, national, and global spatial patterns for two intersections of land use affording high P recycling potential: (a) manure‐rich cultivated areas and (b) populous cultivated areas. In turn, we examined overlap between P recycling potential and nation‐level P fertilizer import dependency. Populous cultivated areas were less abundant globally than manure‐rich cultivated areas, reflecting greater segregation between crops and people compared to crops and livestock, especially in the Americas. Based on a global hexagonal grid (290‐km 2 grid cell area), disproportionately large shares of subnational “hot spots” for P recycling potential occurred in India, China, Southeast Asia, Europe, and parts of Africa. Outside of China, most of the remaining manure‐rich or populous cultivated areas occurred within nations that had relatively high imports of P fertilizer (net P import:consumption ratios ≥0.4) or substantial increases in fertilizer demand between the 2000s (2002–2006) and 2010s (2010–2014). Manure‐rich cultivated grid cells (those above the 75th percentiles for both manure and cropland extent) represented 12% of the global grid after excluding cropless cells. Annually, the global sum of animal manure P was at least 5 times that contained in human excreta, and among cultivated cells the ratio was frequently higher (median = 8.9). The abundance of potential P recycling hot spots within nations that have depended on fertilizer imports or experienced rising fertilizer demand could prove useful for developing local P sources and maintaining agricultural independence.

The DNA of aquatic organisms can be identified in water sampled from freshwater ecosystems to detect species presence. Because these DNA-based methods (termed environmental DNA, eDNA) confirm species presence by proxy of DNA in water, the processes influencing eDNA transport and removal from water are critical to the method's efficacy and interpretation of results. Previous studies of aquatic eDNA transport and fate have employed uncontrolled field experiments, controlled studies in experimental streams, and laboratory column tests. As a step toward understanding the processes controlling eDNA transport and retention, we released and tracked experimental pulses of white sturgeon eDNA (novel to the system) in five fourth-order stream reaches with varied hydrology and geomorphology. We found strong support that stream water transient storage controls eDNA areal uptake rate (or spiraling length). We calculated the median spiraling length to be ∼260 m. Down channel slope correlated with transient storage, suggesting that this slope could be used as a proximate measure of eDNA removal into the benthic zone. Our results suggest that sampling effort should be increased in reaches with longer transient storage (or lower slopes) to compensate for the increase in eDNA retention.

Reproducibility is a key tenet of the scientific process that dictates the reliability and generality of results and methods. The complexities of ecological observations and data present novel challenges in satisfying needs for reproducibility and also transparency. Ecological systems are dynamic and heterogeneous, interacting with numerous factors that sculpt natural history and that investigators cannot completely control. Observations may be highly dependent on spatial and temporal context, making them very difficult to reproduce, but computational reproducibility can still be achieved. Computational reproducibility often refers to the ability to produce equivalent analytical outcomes from the same data set using the same code and software as the original study. When coded workflows are shared, authors and editors provide transparency for readers and allow other researchers to build directly and efficiently on primary work. These qualities may be especially important in ecological applications that have important or controversial implications for science, management, and policy. Expectations for computational reproducibility and transparency are shifting rapidly in the sciences. In this work, we highlight many of the unique challenges for ecology along with practical guidelines for reproducibility and transparency, as ecologists continue to participate in the stewardship of critical environmental information and ensure that research methods demonstrate integrity.

Ancient lakes are among the best archivists of past environmental change, having experienced more than one full glacial cycle, a wide range of climatic conditions, tectonic events, and long association with human settlements. These lakes not only record long histories of environmental variation and human activity in their sediments, but also harbor very high levels of biodiversity and endemism. Yet, ancient lakes are faced with a familiar suite of anthropogenic threats, which may degrade the unusual properties that make them especially valuable to science and society. In all ancient lakes for which data exist, significant warming of surface waters has occurred, with a broad range of consequences. Eutrophication threatens both native species assemblages and regional economies reliant on clean surface water, fisheries, and tourism. Where sewage contributes nutrients and heavy metals, one can anticipate the occurrence of less understood emerging contaminants, such as pharmaceuticals, personal care products, and microplastics that negatively affect lake biota and water quality. Human populations continue to increase in most of the ancient lakes’ watersheds, which will exacerbate these concerns. Further, human alterations of hydrology, including those produced through climate change, have altered lake levels. Co‐occurring with these impacts have been intentional and unintentional species introductions, altering biodiversity. Given that the distinctive character of each ancient lake is strongly linked to age, there may be few options to remediate losses of species or other ecosystem damage associated with modern ecological change, heightening the imperative for understanding these systems.

Judicious phosphorus (P) management is a global grand challenge and critical to achieving and maintaining water quality objectives while maintaining food production. The management of point sources has been successful in lowering P inputs to aquatic environments, but more difficult is reducing P discharges associated with diffuse sources, such as nonpoint runoff from agriculture and urban landscapes, as well as P accumulated in soils and sediments. Strategies for effective diffuse-P management are imperative. Many options are currently available, and the most cost-effective and practical choice depends on the local situation. This critical review describes how the metrics of P quantity in kg ha(-1) yr(-1) and P form can influence decision-making and implementation of diffuse-P management strategies. Quantifying the total available pool of P, and its form, in a system is necessary to inform effective decision-making. The review draws upon a number of "current practice" case studies that span agriculture, cities, and aquatic sectors. These diverse examples from around the world highlight different diffuse-P management approaches, delivered at the source in the catchment watershed or at the aquatic sink. They underscore workable options for achieving water quality improvement and wider P sustainability. The diffuse-P management options discussed in this critical review are transferable to other jurisdictions at the global scale. We demonstrate that P quantity is typically highest and most concentrated at the source, particularly at farm scale. The most cost-effective and practically implementable diffuse-P management options are, therefore, to reduce P use, conserve P, and mitigate P loss at the source. Sequestering and removing P from aquatic sinks involves increasing cost, but is sometimes the most effective choice. Recovery of diffuse-P, while expensive, offers opportunity for the circular economy.