Nitrate removal in a mangrove estuary within a Great Barrier Reef catchment

Abstract

Nitrogen (N) pollution threatens the ecological integrity of the Great Barrier Reef (GBR). However, at the end of some of the GBR catchments, mangroves are extensive and may be buffering the excess N exported, but this effect has not been tested at the landscape scale. We measured N fluxes within the Moresby, a confined subcatchment dominated by sugarcane production with only one outlet to the ocean fringed by a large mangrove forest (2675 ha). We sampled nutrient fluxes across three seasons and tested the hypotheses that the mangrove estuary will reduce nitrate inputs to the ocean and that reductions will be highest during periods of high N concentrations and long residence times. Nitrate concentrations were reduced as water moved through the estuary at higher rates than expected from dilution with seawater. The reductions were equivalent to about 26% and 47% of the catchment annual nitrate and dissolved inorganic N (DIN) loads, respectively. The highest removal rates were measured in the dry-cool season when nitrate concentrations in the upper estuary were high (0.10 mg L−1 NOx−-N) and residence times were long due to low rainfall. These results complement current prioritization processes in the GBR, in which catchments have been targeted for water quality improvement primarily based on land use, soil type, slope, and rainfall, without explicitly considering the effect of wetlands. This study shows that under certain conditions, mangroves at the end of the catchment can significantly reduce the DIN entering the GBR Lagoon.