Marine Habitats of South East Arnhem Land Sea Country Indigenous Protected Area: Ranjiyirrijt to Nyinpinti Point

Overview

Seagrass meadows across northern Australia provide critical habitat for dugongs, turtles, fish, prawns, and other species of cultural and economic importance. However, data on their extent, condition, and long-term trends remain limited. Few large-scale or long-term monitoring programs exist, meaning regional planning and development decisions often proceed with incomplete information about these ecosystems. NESP Marine and Coastal Hub Project 3.5 – Partnerships for Seagrass Research and Protection – aims to address these knowledge gaps by consolidating existing data, identifying priority areas for new surveys, generating new seagrass and benthic habitat datasets, and improving access to information to support evidence-based planning and management.

This project forms part of that broader effort and was delivered through a collaboration between James Cook University, Charles Darwin University and Edith Cowan University, working in partnership with the Northern Land Council, Numbulwar Numburindi and Yugul Mangi Rangers, and the Numburindi, Wandarrang, and Marra Traditional Owners of South East Arnhem Land.

The South East Arnhem Land (SEAL) coastline extends from the Roper River to Blue Mud Bay along the south-west Gulf of Carpentaria in the Northern Territory. The Numburindi, Wandarrang, and Marra peoples are the Traditional Owners and custodians of this coastal region. The South East Arnhem Land Indigenous Protected Area (SEAL IPA) was declared in 2014 for terrestrial areas, and Traditional Owners are now progressing plans to establish a SEAL Sea Country Indigenous Protected Area (SCIPA).

Between 10–13 October 2023, a total of 151 sites were surveyed using underwater video and van Veen grab sampling between Nyinpinti Point and Ranjiyirrijt Point within the proposed SEAL SCIPA. Surveys were conducted with the Numbulwar Numburindi and Yugul Mangi Ranger vessels Bambeiliwarr and Ngarri Larni. The survey area supports a rich and diverse marine environment, including extensive seagrass meadows, diverse algal and benthic invertebrate communities, and healthy coral reefs. Six seagrass species were recorded, covering an estimated 19,689 ha, with Cymodocea serrulata and Halodule uninervis the most common. Coral, sponge, and other benthic assemblages were widespread, while algae occurred at nearly half of all sites. These results highlight the ecological diversity and productivity of SEAL SCIPA’s marine habitats and provide an important baseline for future monitoring.

The subtidal habitats described in this report complement intertidal surveys conducted in 2022. Together, they demonstrate that SEAL Sea Country supports highly productive ecosystems that play important roles in sediment stabilisation, nutrient recycling, blue carbon storage, and providing feeding grounds for species such as dugongs and green turtles.

This survey directly addresses a major information gap for SEAL Sea Country under NESP Project 3.5, contributing high-quality spatial data to a northern Australian seagrass inventory. The resulting habitat maps and species information provide essential inputs for Traditional Owners and the Northern Territory Government as they plan and manage the SEAL SCIPA. These data will help communities identify and prioritise areas of cultural, ecological, or resource significance for ongoing monitoring, management, and sustainable development.