Mangrove Response to Environmental Change in Kakadu National Park

Abstract

The responses of mangroves to climate-related phenomena are difficult to decipher in many areas as human activities also influence changes in their extent and biophysical properties. In Kakadu National Park (KNP) in Australia's Northern Territory, human disturbance of mangroves is minimal and hence observed changes are more likely to indicate a natural environmental change, including that associated with climatic fluctuation. Using fine (<;2 m) resolution aerial photography, hyperspectral, and light detection and ranging data from 1991, 2002, and 2011, respectively, maps of mangroves were generated for four catchments in the Alligator Rivers Region, the Wildman, West, South and East Alligator Rivers, and Field Island. The net change in mangrove for all areas where data intersected were 1.29 km 2 , but redistributions approximating 18.5 km 2 occurred with both losses and gains toward the coast and inland intrusion along many smaller creeks. A close correspondence between the planimetric canopy cover (PCC; %) within 25-m cells, as determined from aerial sensor data, and Landsat-derived foliage projected cover (FPC) was observed. A FPC threshold of ≥30% (~50% PCC) best captured mangrove extent and was used to quantify annual changes in the area for all catchments and Field Island from 1987-2012 Landsat sensor data. Over the period of the time-series, fluctuations in extent above this threshold were observed with these corresponding to changes in sea level and, to a lesser extent, river discharge. The study concludes that changes in mangrove extent and cover in KNP are indicating a changing climate, as manifested by fluctuations (and a general) rise in sea level and changes in rainfall amounts. Such changes are anticipated to lead to alterations of the hydrological regime, with these further affecting mangroves and potentially impacting on the internationally important wetlands of KNP.