High methylmercury formation in ponds fueled by fresh humic and algal derived organic matter

Sonia Herrero Ortega,Núria Catalán,Erik Björn,Pianpian Wu,Stefan Bertilsson,Kevin Bishop,Andrea G Bravo,Torfi Geir Hilmarsson,Hannes Gröntoft,Oded Levanoni

doi:10.1002/lno.10722

Abstract

AbstractNeurotoxic methylmercury causes adverse effects to ecosystem viability and human health. Previous studies have revealed that ponding alters natural organic matter (NOM) composition and increase methylmercury concentrations in rivers, especially in the first years after flooding. Here, we investigate the influence of NOM composition (i.e., sources and degradation status) on mercury methylation rate constants in nine boreal beaver ponds of different ages across Sweden. We show that increased methylmercury concentrations in surface waters is a consequence of enhanced mercury methylation in the pond sediments. Moreover, our results reveal that during the first years after the initial flooding, mercury methylation rates are fueled by the amount of fresh humic substances released from the flooded soils and by an increased production of algal‐derived NOM triggered by enhanced nutrient availability. Our findings indicate that impoundment‐induced changes in NOM composition control mercury methylation processes, causing the raise in MeHg levels in ponds.

Highlights

Ponds and impoundments are the most abundant type of aquatic ecosystems at boreal, subarctic, and artic latitudes (Pienitz 2008; Verpoorter et al 2014)
The recovery of beaver populations (Halley et al 2012) and the accelerated permafrost thawing caused by global warming (Payette et al 2004) is known to trigger the formation of ponds and the subsequent remobilization of carbon pools that have been accumulated over thousands of years (Breton et al 2009; Catalan et al 2017)
High densities of beaver impoundments have previously been associated with enhanced natural organic matter (NOM) bioavailability (Lapierre et al 2013; Catalan et al 2017) and high MeHg levels (Hall et al 2005; Roy et al 2009; Levanoni et al 2015)

Summary

Introduction

Ponds and impoundments are the most abundant type of aquatic ecosystems at boreal, subarctic, and artic latitudes (Pienitz 2008; Verpoorter et al 2014). Our results show that fresh humic and algal-derived NOM control Hg(II) methylation rates in pond sediments.

Results

Conclusion