Long-Term Experimental Acidification Drives Watershed Scale Shift in Dissolved Organic Matter Composition and Flux (original) (raw)
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Environmental Research Letters
Surface water browning, the result of increasing concentrations of dissolved organic matter (DOM), has been widespread in northern ecosystems in recent decades. Here, we assess a 28 database of 426 undisturbed headwater lakes and streams in Europe and North America for 29 evidence of trends in DOM between 1990 and 2016. We describe contrasting changes in 30 DOM trends in Europe (decelerating) and North America (accelerating), which are consistent 31 with organic matter solubility responses to declines in sulphate deposition. While earlier 32 trends (1990-2004) were almost entirely related to changes in atmospheric chemistry, 33 climatic and chemical drivers were equally important in explaining recent DOM trends 34 (2002-2016). We estimate that riverine DOM export from northern ecosystems increased by 35 27% during the study period. Increased summer precipitation strengthened upward DOC 36 trends while warming apparently damped browning. Our results suggest strong but 37 changing influences of air quality and climate on the terrestrial carbon cycle, and on the 38 magnitude of carbon export from land to water. 39
Aquatic Sciences, 2014
Dissolved organic matter (DOM) source and composition are critical drivers of its reactivity, impact microbial food webs and influence ecosystem functions. It is believed that DOM composition and abundance represent an integrated signal derived from the surrounding watershed. Recent studies have shown that land-use may have a long-term effect on DOM composition. Methods for characterizing DOM, such as those that measure the optical properties and size of the molecules, are increasingly recognized as valuable tools for assessing DOM sources, cycling, and reactivity. In this study we measured DOM optical properties and molecular weight determinations to evaluate whether the legacy of forest disturbance alters the amount and composition of stream DOM. Differences in DOM quantity and composition due to vegetation type and to a greater extent, wetland influence, were more pronounced than effects due to disturbance. Our results suggest that excitation-emission matrix fluorescence with parallel factor analysis is a more sensitive metric of disturbance than the other methods evaluated. Analyses showed that streams draining watersheds that have been clearcut had lower dissolved organic carbon (DOC) concentrations and higher microbially-derived and protein-like fluorescence features compared to reference streams. DOM optical properties in a watershed amended with calcium, were not significantly different than reference watersheds, but had higher concentrations of DOC. Collectively these results improve our understanding of how the legacy of forest disturbances and natural landscape characteristics affect the quantity and chemical composition of DOM in headwater streams, having implications for stream water quality and carbon cycling.
Journal of Environmental Quality, 2016
In the past decade, significant increases in surface water dissolved organic carbon (DOC) have been reported for large aquatic ecosystems of the Northern Hemisphere and have been attributed variously to global warming, altered hydrologic conditions, and atmospheric deposition, among other factors. We analyzed a 25-yr DOC record (1988-2012) available for a forested headwater stream in the United States and documented two distinct regimes of stream DOC trends. From 1988 to 2001, annual mean volume-weighted DOC concentration (DOC vw , mg L-1) and annual DOC flux (kg ha-1 yr-1) declined by 34 and 56%, respectively. During 1997 to 2012, the decline in DOC vw and DOC flux increased by 141 and 165%, respectively. Declining DOC vw from 1988 to 2001 corresponded to a decline in growing season runoff, which has the potential to influence mobilization of DOC from uplands to streams. Increasing DOC vw from 1997 to 2012 corresponded to increased precipitation early in the growing season and to an increase in the number and intensity of shortduration fall storms capable of mobilizing long-accrued DOC from forest litter and soils. In contrast, total annual runoff declined throughout the period. Rising air temperature, atmospheric acid deposition, and nitrogen depositions did not offer any plausible explanation for the observed bidirectional annual trends of stream DOC vw. Our study highlights the critical role of long-term datasets and analyses for understanding the impacts of climate change on carbon and water cycles and associated functions of aquatic and terrestrial ecosystems.
Effects of Watershed History on Dissolved Organic Matter Characteristics in Headwater Streams
Ecosystems, 2011
Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humiclike components in chromophoric DOM. In addition, excitation-emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.
2011
Concentrations of organic acids in freshwaters have increased significantly during recent decades across large parts of Europe and North America. Different theories of the causes (e.g., recovery from acidification, climate change, land use) have different implications for defining the preindustrial levels for dissolved organic carbon (DOC), which are crucial for assessing acidification and other aspects of water quality. We demonstrate this by classifying the acidification status of 66 lakes with long-term observations, representative of about 12,700 acid-sensitive lakes in nemoral and boreal Sweden. Of these lakes, 47% are classified as significantly acidified (DpH 0.4), assuming preindustrial DOC levels were equal to those observed in 1990. But if, instead, the higher DOC levels observed in 2009 define preindustrial conditions, half as many lakes are acidified (24%). This emphasizes the need to establish reference levels for DOC and casts new light on the classic controversy about natural versus anthropogenic acidification.
Environmental Science & Technology, 2014
Previous reports suggest variable trends in recovery from acidification in northeastern U.S. surface waters in response to the Clean Air Act Amendments. Here we analyze recent trends in emissions, wet deposition, and lake chemistry using long-term data from a variety of lakes in the Adirondack Mountains and New England. Sulfate concentration in wet deposition declined by more than 40% in the 2000s and sulfate concentration in lakes declined at a greater rate from 2002 to 2010 than during the 1980s or 1990s (−3.27 μeqL −1 year −1 as compared to −1.26 μeqL −1 year −1). During the 2000s, nitrate concentration in wet deposition declined by more than 50% and nitrate concentration in lakes, which had no linear trend prior to 2000, declined at a rate of −0.05 μeqL −1 year −1. Base cation concentrations, which decreased during the 1990s (−1.5 μeq L −1 year −1), have stabilized in New England lakes. Although total aluminum concentrations increased since 1999 (2.57 μg L −1 year −1), there was a shift to nontoxic, organic aluminum. Despite this recent acceleration in recovery in multiple variables, both ANC and pH continue to have variable trends. This may be due in part to variable trajectories in the concentrations of base cations and dissolved organic carbon among our study lakes.
Environmental Science & Technology, 2007
Concentrations of dissolved organic carbon (DOC) in freshwaters have increased significantly in Europe and North America, but the driving mechanisms are poorly understood. Here, we test if the significant increase in TOC (total organic carbon, 90-95% DOC) in three acidsensitive catchments in Norway of 14 to 36% between 1985 and 2003 is related to climate, hydrology, and/or acid deposition. Catchment TOC export increased between 10 and 53%, which was significant at one site only. The seasonal variation in TOC was primarily climatically controlled, while the deposition of SO 4 and NO 3 snegatively related to TOCsexplained the long-term increase in TOC. We propose increased humic charge and reduced ionic strengths both of which increase organic matter solubilitysas mechanistic explanations for the statistical relation between reduced acid deposition and increased TOC. Between 1985 and 2003, ionic strength decreased significantly at all sites, while the charge density of TOC increased at two of the sites from 1-2 meq g -1 C to about 5 meq g -1 C and remained constant at the third site at 5 meq g -1 C. The solubility of organic matter is discussed in terms of the pHdependent deprotonation of carboxylic groups and the ionic strength-dependent repulsion of organic molecules.
Journal of Geophysical Research: Biogeosciences, 2015
Atmospheric acid deposition of sulfate and nitrate has declined markedly in the northeastern United States due to emissions controls. We investigated long‐term trends in soil water (1984–2011) and stream water (1982–2011) chemistry along an elevation gradient of a forested watershed to evaluate the progress of recovery of drainage waters from acidic deposition at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, USA. We found slowed losses of base cations from soil and decreased mobilization of dissolved inorganic aluminum. Stream water pH at the watershed outlet increased at a rate of 0.01 units yr−1, and the acid neutralizing capacity (ANC) gained 0.88 µeq L−1 yr−1. Dissolved organic carbon generally decreased in stream water and soil solutions, contrary to trends observed at many North American and European sites. We compared whole‐year hydrochemical trends with those during snowmelt, which is the highest‐flow and lowest ANC period of the year, indica...