Cairns harbour and Trinity Inlet seagrass monitoring (B1.16)

Task leader: Dr Michael Rasheed, Queensland Department Primary Industries & Fisheries.

Task associate: NA.

Cairns Harbour and Trinity Inlet support over 800 hectares of seagrass habitat and represent the only major seagrass resource between Hinchinbrook Island and Cooktown (Lee Long et al. 1993, 1996; Campbell et al. 2002). These meadows provide critical nursery habitat for regional prawn and finfish fisheries (Coles et al. 1993; Watson et al. 1993) and are feeding habitats for dugong (Dugong dugon), green sea turtle (Chelonia mydas) and wading bird populations. The State of Trinity Inlet Report (1997) recognised seagrasses as crucial to maintaining biodiversity and fisheries productivity in the inlet and identified seagrasses as a key habitat type for monitoring. These meadows are mostly within the Trinity Inlet Fish Habitat Area but are vulnerable to scouring from vessel movements, downstream effects of urban, industrial and agricultural land-use (declining water quality), and changes in hydrology associated with port development and maintenance. Urban and industrial expansions were identified as major threats to the environmental health of Trinity Inlet (WBM, 1997).

As seagrasses show measurable responses to changes in water quality they can be used as an effective tool to monitor marine environmental health (eg Dennison et al. 1993). The DPI/CRC-Reef Marine Ecology Group (MEG) have established seagrass monitoring programs in a number of Queensland ports including Mourilyan, Karumba, Weipa, Thursday Island and Gladstone. Seagrass mapping and monitoring in these ports has aided in the planning of dredging programs and port developments as well as assisting in the assessment of effects of port maintenance and development operations on the marine environment. These programs have also assisted in discerning natural climatic causes of seagrass changes from human induced impacts. Results of the recently completed seagrass habitat assessment survey for Cairns harbour and Trinity Inlet conducted in December 2001 (Campbell et al. 2002) provide an ideal baseline from which a similar long term monitoring program can be developed for Cairns.

Campbell et al. (2002) found that although overall seagrass area in 2001 was similar to that recorded in 1993 (815.4 +/- 217.1 ha 2001; 984.9 +/- 220.9 ha 1993) significant changes in seagrass abundance and biomass had occurred within specific regions of the survey area. Although a number of possible causes of seagrass decline were discussed, the exact cause of change could not be established due to the long time period between surveys. That report recommends that annual monitoring of seagrass meadows in Cairns Harbour and Trinity Inlet be conducted.

At the request of Trinity Inlet Waterways and Cairns Port Authority we have developed this proposal for long term annual monitoring of Cairns Harbour and Trinity Inlet seagrasses. Annual monitoring will allow the causes of seagrass change to be better established and interpreted and an ability to distinguish between impacts associated with port activities, anthropogenic inputs and climatic changes. An annual seagrass monitoring program will provide a tool for assessing the "environmental health" of marine environments in Cairns Harbour and Trinity Inlet and an understanding of the relationships between climate and seagrass abundance.