Shifting stoichiometry: Long-term trends in stream-dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition.

Bianca M Rodríguez‐Cardona,Ashley M Helton,E N Jack Brookshire,Robert G M Spencer,Sherri L Johnson,Alba Argerich,Susana Bernal,Hannah M Fazekas,Sujay S Kaushal,Adam S Wymore,Jeremy B Jones,Walter K Dodds,Carla López‐Lloreda,Ashley A Coble,Penny J Johnes,William H Mcdowell,Rebecca T Barnes,Pirkko Kortelainen

doi:10.1111/gcb.15965

Abstract

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long‐term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy‐nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by‐products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.

Highlights

Dissolved organic matter (DOM) provides an essential energy and nutrient source to aquatic ecosystems (Webster & Meyer, 1997)
We examined time series from mean monthly dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and DOC:DON values for each stream using the longest record possible from each site (Table S3) with the exception of the Arctic site, Caribou-Poker Creeks Research Watersheds (CPC) where data are only available from May to August which coincides with the freshet and summer base flow periods
Whereas these studies have informed the notion that DOC concentrations are increasing in northern temperate streams, we show that the directional change in concentrations of DOC and DON is highly variable and site-and stream-specific

Summary

Introduction

Dissolved organic matter (DOM) provides an essential energy and nutrient source to aquatic ecosystems (Webster & Meyer, 1997). DOC concentrations have increased between 50% and 91% in streams and lakes of northern and central Europe, the United Kingdom, and eastern North America since the 1980s (Couture et al, 2012; de Wit et al, 2016; Driscoll et al, 2003; Evans et al, 2005; Gavin et al, 2018; Hall et al, 2021; Lawrence et al, 2011; Monteith et al, 2007; Worrall et al, 2004). Increased DOC concentration is often attributed to the recovery from acid deposition after the implementation of the Clean Air Act in the United States and similar legislation in Europe (Driscoll et al, 2003). The leading hypothesized mechanism is that a decrease in ionic strength and protonation in soil water following recovery from acid deposition leads to increases in solubility and the mobilization of DOC to adjacent water bodies (Borken et al, 2011; De Wit et al, 2007; Evans et al, 2005; Hruška et al, 2009; Lawrence & Roy, 2021)

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