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1. Climate displaces deposition as dominant driver of dissolved organic carbon concentrations in historically acidified lakes

   https://doi.org/10.1007/s10533-024-01193-5  

   Herreid, Allison M; Fazekas, Hannah M; Nelson, Sarah J; Wymore, Adam S; Murray, Desneiges; Varner, Ruth K; McDowell, William H (February 2025, Biogeochemistry)

   Abstract Climate and atmospheric deposition interact with watershed properties to drive dissolved organic carbon (DOC) concentrations in lakes. Because drivers of DOC concentration are inter-related and interact, it is challenging to assign a single dominant driver to changes in lake DOC concentration across spatiotemporal scales. Leveraging forty years of data across sixteen lakes, we used structural equation modeling to show that the impact of climate, as moderated by watershed characteristics, has become more dominant in recent decades, superseding the influence of sulfate deposition that was observed in the 1980s. An increased percentage of winter precipitation falling as rain was associated with elevated spring DOC concentrations, suggesting a mechanistic coupling between climate and DOC increases that will persist in coming decades as northern latitudes continue to warm. Drainage lakes situated in watersheds with fine-textured, deep soils and larger watershed areas exhibit greater variability in lake DOC concentrations compared to both seepage and drainage lakes with coarser, shallower soils, and smaller watershed areas. Capturing the spatial variability in interactions between climatic impacts and localized watershed characteristics is crucial for forecasting lentic carbon and nutrient dynamics, with implications for lake ecology and drinking water quality. 

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2. Seasonal and Climatic Drivers of Wet Deposition Organic Matter at the Continental Scale

   https://doi.org/10.1029/2024JG008403  

   Murray, Desneiges S; Wymore, Adam S (November 2024, Journal of Geophysical Research: Biogeosciences)

   Abstract Dissolved organic matter (DOM) concentrations and composition within wet deposition are rarely monitored despite contributing a large input of bioavailable dissolved organic carbon (DOC) and nitrogen (DON) to the Earth's surface. Lacking from the literature are spatially comprehensive assessments of simultaneous measurements of wet deposition DOC and DON chemistry and their dependencies on metrics of climate and environmental factors. Here, we use archived precipitation samples from the US National Atmospheric Deposition Program collected in 2017 to 2018 from 17 sites across six ecoregions to investigate variability in the concentration and composition of depositional DOM. We hypothesize metrics of DOM chemistry vary with ecoregion, season, large‐scale climate drivers, and precipitation geographic source. Findings indicate differences in DOC and DON concentrations and loads among ecoregions. The highest wet deposition concentrations are from sites in the Northern Forests and lowest concentrations from sites in Marine West Coast Forests. Summer and autumn samples contained the highest DOC concentrations and DON concentrations that were consistently above detection limit, corresponding with seasonality of peak air temperatures and the phenology of the growing season in the northern hemisphere. Compositional trends suggest lighter DOM molecules in autumn and winter and heavier molecules in spring and summer. Climate drivers explain 51% of variation in DOM chemistry, revealing differing drivers on the concentrations and loads of DOC versus DON in wet deposition. This study highlights the necessity of incorporating DOC and DON measurements into national deposition monitoring networks to understand spatial and temporal feedbacks between climate change, atmospheric chemistry and landscape biogeochemistry. 

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3. Responses by benthic invertebrate community composition to dissolved organic matter in lakes decline substantially above a threshold concentration

   https://doi.org/10.1111/fwb.14211  

   Jane, Stephen F.; Johnson, Richard K.; Rose, Kevin C.; Eklöv, Peter; Weyhenmeyer, Gesa A. (February 2024, Freshwater Biology)

   Abstract Dissolved organic matter (DOM), often measured as dissolved organic carbon (DOC), plays a fundamental role in influencing the structure and function of lake ecosystems. Due to the myriad ecosystem effects of DOM, widespread observations of long‐term increasing DOM concentrations have received much attention from ecologists. DOM positively influences primary production and consumer production at low concentrations due to the fertilising influence of bound nutrients. However, beyond a unimodal peak in production, a reduced light environment may result in a negative effect on production. This unimodal model has been largely developed and tested in lakes with low to moderate DOM concentrations (i.e., typically ≤10 mg/L DOC).To understand ecological responses in lakes across a larger range in DOM concentrations, we examined the response of benthic invertebrate communities in 148 Swedish lakes with DOM concentrations ranging between 0.67 and 32.77 mg/L DOC.We found that increasing DOM concentrations had a strong effect on invertebrate community composition belowc.10 mg/L. Across this range, abundances of individual taxa both increased and decreased, probably in response to environmental change induced by DOM. However, in lakes above this concentration, increasing DOM had minimal influence on community composition.As DOM concentrations continue to increase, faunal communities in lakes below this 10 mg/L DOC threshold are likely to undergo substantial change whereas those above this threshold are likely to be minimally impacted. 

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4. Deeper waters are changing less consistently than surface waters in a global analysis of 102 lakes

   https://doi.org/10.1038/s41598-020-76873-x  

   Pilla, Rachel M.; Williamson, Craig E.; Adamovich, Boris V.; Adrian, Rita; Anneville, Orlane; Chandra, Sudeep; Colom-Montero, William; Devlin, Shawn P.; Dix, Margaret A.; Dokulil, Martin T.; et al (December 2020, Scientific Reports)

   null (Ed.)

   Abstract Globally, lake surface water temperatures have warmed rapidly relative to air temperatures, but changes in deepwater temperatures and vertical thermal structure are still largely unknown. We have compiled the most comprehensive data set to date of long-term (1970–2009) summertime vertical temperature profiles in lakes across the world to examine trends and drivers of whole-lake vertical thermal structure. We found significant increases in surface water temperatures across lakes at an average rate of + 0.37 °C decade −1 , comparable to changes reported previously for other lakes, and similarly consistent trends of increasing water column stability (+ 0.08 kg m −3 decade −1 ). In contrast, however, deepwater temperature trends showed little change on average (+ 0.06 °C decade −1 ), but had high variability across lakes, with trends in individual lakes ranging from − 0.68 °C decade −1 to + 0.65 °C decade −1 . The variability in deepwater temperature trends was not explained by trends in either surface water temperatures or thermal stability within lakes, and only 8.4% was explained by lake thermal region or local lake characteristics in a random forest analysis. These findings suggest that external drivers beyond our tested lake characteristics are important in explaining long-term trends in thermal structure, such as local to regional climate patterns or additional external anthropogenic influences. 

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5. Dissolved Organic Carbon in Coastal Waters: Global Patterns, Stocks and Environmental Physical Controls

   https://doi.org/10.1029/2024GB008407  

   Lønborg, Christian; Fuentes‐Santos, Isabel; Carreira, Cátia; Amaral, Valentina; Arístegui, Javier; Bhadury, Punyasloke; Bif, Mariana Bernardi; Calleja, Maria Ll; Chen, Qi; Cotovicz, Luiz C; et al (May 2025, Global Biogeochemical Cycles)

   Abstract Dissolved organic carbon (DOC) in coastal waters is integral to biogeochemical cycling, but global and regional drivers of DOC are still uncertain. In this study we explored spatial and temporal differences in DOC concentrations and stocks across the global coastal ocean, and how these relate to temperature and salinity. We estimated a global median coastal DOC stock of 3.15 Pg C (interquartile range (IQR) = 0.85 Pg C), with median DOC concentrations being 2.2 times higher than in open ocean surface waters. Globally and seasonally, salinity was the main driver of DOC with concentrations correlated negatively with salinity, without a clear relationship to temperature. DOC concentrations and stocks varied with region and season and this pattern is likely driven by riverine inputs of DOC and nutrients that stimulate coastal phytoplankton production. Temporally, high DOC concentrations occurred mainly in months with high freshwater input, with some exceptions such as in Eastern Boundary Current margins where peaks are related to primary production stimulated by nutrients upwelled from the adjacent ocean. No spatial trend between DOC and temperature was apparent, but many regions (19 out of 25) had aligned peaks of seasonal temperature and DOC, related to increased phytoplankton production and vertical stratification at high temperatures. Links of coastal DOC with salinity and temperature highlight the potential for anthropogenic impacts to alter coastal DOC concentration and composition, and thereby ecosystem status. 

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