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Accurately quantifying the diffusive flux of CH 4 between sediments and the overlying water column is crucial when constructing CH 4 budgets in lakes and reservoirs. Although a variety of ex situ and in situ techniques exist for determining this flux, no reviews have provided a comprehensive, comparative overview of these approaches or discussed implications of measurement method on flux estimation. Here, we critically review methods applied in 163 peer-reviewed studies to estimate diffusive CH 4 fluxes from lake sediments, including sediment incubations, benthic chambers, and modeling approaches applied in the sediment or water column. For each method, we summarize the approach, discuss limitations and advantages, and summarize published comparisons between different methods. In addition, we examine how method limitations have likely shaped knowledge gaps in current understanding of lake CH 4 dynamics. Finally, we call for the development and application of new methods, along with additional testing and intercomparison of existing methods, in order to advance understanding of lake CH 4 fluxes. 

Abstract Phosphorus (P) and nitrogen (N) are essential nutrients for food production but their excess use in agriculture can have major social costs, particularly related to water quality degradation. Nutrient footprint approaches estimate N and P release to the environment through food production and waste management and enable linking these emissions to particular consumption patterns. Following an established method for quantifying a consumer-oriented N footprint for the United States (U.S.), we calculate an analogous P footprint and assess the N:P ratio across different stages of food production and consumption. Circa 2012, the average consumer’s P footprint was 4.4 kg P capita⁻¹yr⁻¹compared to 22.4 kg N capita⁻¹yr⁻¹for the food portion of the N footprint. Animal products have the largest contribution to both footprints, comprising >70% of the average per capita N and P footprints. The N:P ratio of environmental release based on virtual nutrient factors (kilograms N or P per kilogram of food consumed) varies considerably across food groups and stages. The overall N:P ratio of the footprints was lower (5.2 by mass) than for that of U.S. food consumption (8.6), reinforcing our finding that P is managed less efficiently than N in food production systems but more efficiently removed from wastewater. While strategies like reducing meat consumption will effectively reduce both N and P footprints by decreasing overall synthetic fertilizer nutrient demands, consideration of how food production and waste treatment differentially affect N and P releases to the environment can also inform eutrophication management. 

Abstract Collectively, reservoirs constitute a significant global source of C‐based greenhouse gases (GHGs). Yet, global estimates of reservoir carbon dioxide (CO₂) and methane (CH₄) emissions remain uncertain, varying more than four‐fold in recent analyses. Here we present results from a global application of the Greenhouse Gas from Reservoirs (G‐res) model wherein we estimate per‐area and per‐reservoir CO₂and CH₄fluxes, by specific flux pathway and in a spatially and temporally explicit manner, as a function of reservoir characteristics. We show: (a) CH₄fluxes via degassing and ebullition are much larger than previously recognized and diffusive CH₄fluxes are lower than previously estimated, while CO₂emissions are similar to those reported in past work; (b) per‐area reservoir GHG fluxes are >29% higher than suggested by previous studies, due in large part to our novel inclusion of the degassing flux in our global estimate; (c) CO₂flux is the dominant emissions pathway in boreal regions and CH₄degassing and ebullition are dominant in tropical and subtropical regions, with the highest overall reservoir GHG fluxes in the tropics and subtropics; and (d) reservoir GHG fluxes are quite sensitive to input parameters that are both poorly constrained and likely to be strongly influenced by climate change in coming decades (parameters such as temperature and littoral area, where the latter may be expanded by deepening thermoclines expected to accompany warming surface waters). Together these results highlight a critical need to both better understand climate‐related drivers of GHG emission and to better quantify GHG emissions via CH₄ebullition and degassing. 

Abstract Dissolved calcium concentration [Ca²⁺] is thought to be a major factor limiting the establishment and thus the spread of invasive bivalves such as zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels. We measured [Ca²⁺] in 168 water samples collected along ~100 river‐km of the lower Columbia River, USA, between June 2018 and March 2020. We found [Ca²⁺] to range from 13 to 18 mg L⁻¹during summer/fall and 5 to 22 mg L⁻¹during the winter/spring. Previous research indicates that [Ca²⁺] < 12 mg L⁻¹are likely to limit the establishment and spread of invasive bivalves. Thus, our results indicate that there is sufficient Ca²⁺in most locations in the lower Columbia River to support the establishment of invasive dreissenid mussels, which could join the already widespread and abundant Asian clam (Corbicula fluminea) as the newest invader to an already heavily invaded Columbia River ecosystem. These new data provide important measurements from a heretofore undersampled region of the Columbia River and have important implications for the spread of invasive bivalves and, by extension, the conservation and management of native species and ecosystems. 

Abstract Better documentation and understanding of long‐term temporal dynamics of nitrogen (N) and phosphorus (P) in watersheds is necessary to support effective water quality management, in part because studies have identified time lags between terrestrial nutrient balances and water quality. We present annual time series data from 1969 to 2012 for terrestrial N and P sources and monthly data from 1972 to 2013 for river N and P for the Willamette River Basin, Oregon, United States. Inputs to the watershed increased by factors of 3 for N and 1.2 for P. Synthetic fertilizer inputs increased in total and relative importance over time, while sewage inputs decreased. For N, increased fertilizer application was not matched by a proportionate increase in crop harvest; N use efficiency decreased from 69% to 38%. P use efficiency increased from 52% to 67%. As nutrient inputs to terrestrial systems increased, river concentrations and loads of total N, total P, and dissolved inorganic P decreased, and annual nutrient loads were strongly related to discharge. The N:P ratio of both sewage and fertilizer doubled over time but there was no similar trend in riverine export; river N:P concentrations declined dramatically during storms. River nutrient export over time was related to hydrology and waste discharge, with relatively little influence of watershed balances, suggesting that accumulation within soils or groundwater over time is mediating watershed export. Simply managing yearly nutrient balances is unlikely to improve water quality; rather, many factors must be considered, including soil and groundwater storage capacity, and gaseous loss pathways.