Tropical Coastal Wetlands Ameliorate Nitrogen Export During Floods

Maria Fernanda Adame,Melanie E Roberts,Graeme Curwen,Matthew Griffiths,Jing Lu,Mike Ronan,David P Hamilton,Vanessa Reis,Christopher E Ndehedehe

doi:10.3389/fmars.2019.00671

Maria Fernanda Adame, Melanie E Roberts + Show 7 more

Open Access

https://doi.org/10.3389/fmars.2019.00671

Copy DOI

Abstract

Wetlands can increase resilience to extreme climatic events and have a key role in protection and water quality improvement in coastal ecosystems. Studies in tropical coastal wetlands at a catchment scale are scarce, and most work has been undertaken on small, temperate wetlands. In this study, we tested whether natural coastal wetlands in a tropical catchment (Tully-Murray, Queensland, Australia) could ameliorate nitrogen (N) exported to the Great Barrier Reef during a flood event. We measured denitrification rates in different types of coastal wetlands (mangroves, saltmarshes, waterbodies with macrophytes, and floodplain wetlands dominated by Melaleuca spp.) to assess their potential contribution to N losses during the 6-day duration of a flood in March 2018. Denitrification potential was variable across the landscape, and we identified “hotspots” in sub-catchments with high NO−3-N concentrations (0.4–0.6 mg L−1) and large areas of wetlands (>800 ha, >40% of the sub-catchment). These hotspots can denitrify up to 10 t of NO−3-N per day during a flood. We used our measured denitrification rates to provide input parameters for a model that includes the main biogeochemical processes affecting N transformations within wetlands (nitrification, denitrification, plant uptake, sedimentation, anammox, and mineralization), and accounts for transport via the duration, depth, and flow of water. Model simulations of a sub-catchment of the Tully-Murray indicate that flood inundation of large areas of natural wetlands (>40% of the sub-catchment area) could potentially remove 70% of the incoming NO−3-N load in the first 24 h of the flood. The management and restoration of coastal tropical wetlands could play a critical role in sustaining the health of coastal ecosystems through water quality improvement.

Highlights

Coastal wetlands are one of the most valuable ecosystems on Earth, considered essential for climate change adaptation and mitigation (Duarte et al, 2013)
During a flood with a recurrence time of ∼4-years, coastal wetlands in the tropical catchment of the Tully-Murray River remove a large portion of the incoming NO−3 -N through denitrification
The nutrient removal was variable across the landscape, but our model simulations indicated the existence of “denitrification hotspots” corresponding to sub-catchments with high NO−3 -N concentrations (>0.20 mg L−1), and large areas of wetlands (>24%) that became inundated during the flood

Summary

Introduction

Coastal wetlands are one of the most valuable ecosystems on Earth, considered essential for climate change adaptation and mitigation (Duarte et al, 2013). They provide key ecosystem services, including food provisioning, carbon sequestration, flood protection, and improvements in water quality (Barbier et al, 2011). Coastal wetlands are effective at improving water quality through three processes that remove nitrogen (N): plant uptake, soil accretion, and denitrification, with the latter accounting for most of the removal (>90%, Adame et al, 2019b,c, Figure 1). Denitrification is likely to be the most important pathway for N removal in coastal wetlands (Oliveira-Fernandes et al, 2012; Xiao et al, 2018), and a key process for the improvement of water quality by wetlands

Methods

Results

Conclusion