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Vertical motion is an important driver of sunlight exposure in aquatic environments, shaping the growth and fate of materials and organisms. We derive a simple model accounting for turbulent depth fluctuations of particles to predict the depth that contributes the most sunlight exposure (effective depth) as well as the single depth that, if measured at one place over time, produces the same total sunlight exposure as a moving particle (functional depth). Field measurements of light and depth in rivers using neutrally buoyant drifters and buoys validate our model. Effective depth varied from 0.1 to 1.5 m below the water surface and was ~ 30% of the overall water depth on average. Functional depth varied from 0.67 to 2.3 m and was ~ 50% of the overall water depth on average. Functional and effective depth are physically based concepts incorporating turbulent motion, spatial variability, and water clarity offering new approaches to characterize light exposure in aquatic environments.

 

Artisanal and small-scale gold mining (ASGM) is the largest global source of anthropogenic mercury emissions. However, little is known about how effectively mercury released from ASGM is converted into the bioavailable form of methylmercury in ASGM-altered landscapes. Through examination of ASGM-impacted river basins in Peru, we show that lake area in heavily mined watersheds has increased by 670% between 1985 and 2018 and that lakes in this area convert mercury into methylmercury at net rates five to seven times greater than rivers. These results suggest that synergistic increases in lake area and mercury loading associated with ASGM are substantially increasing exposure risk for people and wildlife. Similarly, marked increases in lake area in other ASGM hot spots suggest that “hydroscape” (hydrological landscape) alteration is an important and previously unrecognized component of mercury risk from ASGM. 

Sunlight is a critical resource in aquatic systems driving photosynthesis, photodegradation of organic matter and contaminants, animal behavior, and the activity of human pathogens. In rivers, solutes, materials, and organisms are turbulently mixed across the water column during downstream transport and exposed to highly variable sunlight. However, there are no measurements of suspended particles' sunlight exposure during downstream transport to characterize this variability, and it is unclear if current measurement approaches and optical theory capture the light exposure of suspended particles. We deployed neutrally buoyant drifters and stationary buoys in the Upper Mississippi (WI, U.S.A.) and Neuse Rivers (NC, U.S.A.) to measure underwater sunlight from the perspective of suspended particles. In our study sites, underwater sunlight varied more along flowpaths measured by drifters than over time measured by fixed‐site buoys; sunlight exposure along flowpaths was dominated by bursts of light (sunflecks) that accounted for 62–99% of the cumulative sunlight exposure; and modeled sunlight exposure using optical theory was consistently 56–1700% higher than measured sunlight exposure along flowpaths. Our results suggested that suspended particles in the study reaches experienced darker conditions than predicted and have important implications for how to quantify underwater sunlight in rivers.

 

Rivers are among the most imperiled ecosystems globally, yet we do not have broad‐scale understanding of their changing ecology because most are rarely sampled. Water color, as perceived by the human eye, is an integrative measure of water quality directly observed by satellites. We examined patterns in river color between 1984 and 2018 by building a remote sensing database of surface reflectance, RiverSR, extracted from 234,727 Landsat images covering 108,000 kilometers of rivers > 60 m wide in the contiguous USA. We found 1) broad regional patterns in river color, with 56% of observations dominantly yellow and 38% dominantly green; 2) river color has three distinct seasonal patterns that were synchronous with flow regimes; 3) one third of rivers had significant color shifts over the last 35 years. RiverSR provides the first map of river color and new insights into macrosystems ecology of rivers.