By Melanie Roberts and Fernanda Adame
Article Read Time: 524 words about 3 minutes.
The 2019 Great Barrier Reef Outlook Report from the Australian Government Great Barrier Reef Marine Park Authority identifies the risk of nutrient runoff from catchments for the reef as Very High, the same rating as the 2014 and 2009 reports.
This finding is not unique to the Great Barrier Reef (GBR); coastal systems around the world are under pressure from high nutrient loads generated through agriculture activities and soil erosion. Nutrient runoff can be reduced through improved land use management practices, by limiting fertiliser application and better timing applications. A second opportunity is however often overlooked – recognising the capability of our natural environment to remove nutrients before those nutrients reach the coastal environment. Wetlands can act as filters for nutrients, removing them from the water column and preventing them from being discharged into the coastal environment.
Wetlands can act as filters for nutrients, removing them from the water column and preventing them from being discharged into the coastal environment.
The GBR Report Cards currently do not account for the ability of wetlands to remove nutrients from coastal and river water. This omission is often due to a lack of knowledge in the wetland’s ability to process nutrients. Studies in tropical coastal wetlands at a catchment scale are scarce, with most work having been undertaken on small, temperate wetlands. For the GBR Report Cards to be able to account for the role of wetlands, knowledge of nutrient removal in tropical coastal wetlands is essential.
…the Great Barrier Reef Report Cards do not account for the ability of wetlands to remove nutrients.
In a recent study we tested whether natural coastal wetlands in a tropical catchment (the Tully-Murray) in North Queensland could ameliorate nitrogen exported to the GBR during a flood event. We measured permanent nitrogen removal (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 nitrogen loses during a 6-day duration of a flood in March 2018. We used our measured denitrification rates in a model that includes the main biogeochemical processes affecting nitrogen transformations within wetlands, and accounts for transport via the duration, depth, and flow of water.
Our results show that coastal wetlands of the Tully-Murray catchment have the potential to reduce the export of nitrate by 19.8 tonnes during the six days of the flood event. Model simulations indicate that flood inundation of large areas of natural wetlands (> 40% of the catchment area) could potentially remove 70% of the incoming nitrate load in the first 24 hours of the flood. Nitrogen removal was variable across the landscape and we identified “denitrification hotspots” located in sub-catchments with high nitrate concentrations (0.4 – 0.6 mg L-1) and large areas of wetlands (> 800 ha or 63% of the sub-catchment) inundated during the flood.
Tropical coastal wetlands appear to have a key role in ameliorating the export of nitrogen to the coastal zone during floods, and their restoration and improved management could increase the health and resilience of coastal and marine ecosystems. We are paving the way for the role of wetlands in protecting the GBR to be better understood and accounted for by improving mapping of potential nitrogen removal during flooding events.