Brownification on hold: What traditional analyses miss in extended surface water records

Karin Eklöf,Claudia Von Brömssen,Nino Amvrosiadi,Jens Fölster,Marcus B Wallin,Kevin Bishop

doi:10.1016/j.watres.2021.117544

Karin Eklöf, Claudia Von Brömssen + Show 4 more

Open Access

https://doi.org/10.1016/j.watres.2021.117544

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Abstract

Widespread increases in organic matter (OM) content of surface waters, as measured by color and organic carbon (OC), are a major issue for aquatic ecosystems. Long-term monitoring programs revealed the issue of “brownification”, with climate change, land cover changes and recovery from acidification all suspected to be major drivers or contributing factors. While many studies have focused on the impact and drivers, fewer have followed up on whether brownification is continuing. As time-series of OM data lengthen, conventional data-analysis approaches miss important information on when changes occur. To better identify temporal OM patterns during three decades (1990–2020) of systematic monitoring, we used generalized additive models to analyze 164 time-series from watercourses located across Sweden. Increases in OC that were widespread during 1990–2010 ceased a decade ago, and most color increases ceased 20 years ago. These findings highlight the need to reassess the understanding of brownification's spatial and temporal extent, as well as the tools used to analyze lengthening time series.

Highlights

The observed trends in total organic carbon (TOC) concentrations and colored dissolved organic matter (CDOM) over the last three decades were neither linear, nor monotonic across the data set of Swedish watercourses
The generalized additive mixed models (GAMM)-based trend analyses indicated that a majority of the sites increased significantly in TOC concentration for some parts of the period 1990–2010 (Fig. S1)
This means that while TOC concentrations increased in many areas between 1990 and 2010, the TOC concentrations have been more stable during the past decade (Fig. 3)

Summary

Introduction

In early 2000, trends of increasing organic matter (OM), measured as organic carbon (OC) concentrations and/or water color, were reported in monitoring records from many European and North American surface waters (de Wit et al, 2007; Driscoll et al, 2003; Evans et al, 2005; Monteith et al, 2007; Roulet and Moore, 2006; Worrall et al, 2003; Worrall et al, 2004). Other studies suggested that additional factors related to climate change, land use and biomass increases contributed to brownification of many surface waters (de Wit et al, 2016; Erlandsson et al, 2008; Finstad et al, 2016; Kritzberg, 2017; Meyer-Jacob et al, 2019; Skerlep et al, 2020; Weyhenmeyer et al, 2012; Weyhenmeyer and Karlsson, 2009). Other drivers will be continuing (such as changing climate), or at least have no clearly demarcated endpoint (such as biomass increases or land use) (Bragee et al, 2015)

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