Long-term seagrass monitoring in the Port of Mourilyan: 2012

This report summarises the results of the Mourilyan Harbour annual seagrass monitoring program conducted in October 2012. The Mourilyan seagrass monitoring program was developed in 1994 following baseline assessments of the marine habitat within the port limits in 1993. Since 2000 seagrass has been monitored annually.\ud \ud A total of 27.8 ± 16.4 ha of seagrass habitat was mapped in the three monitoring meadows that were present in 2012, approximately 39% below the 17 year long-term average of 42.6 ha. The mean combined density of the monitoring meadows was approximately 83% below the 17 year long-term above-ground biomass average.\ud \ud Mourilyan seagrasses are in a poor condition with the dense foundation meadows of Zostera capricorni completely absent from the harbour, and the remaining area of seagrass greatly reduced and consisting exclusively of colonising Halophila species. It has now been three years since these key species were found. Monitoring meadows have undergone significant declines in biomass and distribution since 2009, with significant losses of seagrass observed after the 2010/11 La Niña events and Tropical Cyclone (TC) Yasi, the most likely factors leading to these losses. There were some signs of seagrass recovery in the Channel meadow however this recovery was limited to low density pioneering Halophila species. The two key 'foundation' species Zostera capricorni and Halodule uninervis have now been absent from Mourilyan Harbour monitoring meadows for several years (since 2010 and 2009 respectively). Such a long term absence of these productive meadows is a concern, and there are doubts that they now have the ability to naturally re-establish at the site.\ud \ud Coastal habitats along the Queensland coast are regularly exposed to flooding and cyclones, however the cumulative impacts of multiple above average wet seasons combined with the 2010/11 La Niña events and TC Yasi were unprecedented, and may have left Mourilyan seagrasses with substantially reduced resilience and capacity for recovery. Similar declines in coastal and estuarine seagrasses were recorded at other wet and dry tropics monitoring sites over this period. There may be limited capacity for Z. capricorni and H. uninervis to re-establish naturally within the relatively closed system of Mourilyan Harbour as dispersal from outside the harbour is likely to be difficult, and with no local plants remaining and a potential absence of viable seeds, localised re-establishment is unlikely. The significant and/or complete loss of these species in Mourilyan Harbour means recovery is likely to be a slow process at best and some consideration of assisting recovery through investigations of restoration may be necessary for the site.\ud \ud The remaining Mourilyan Harbour seagrasses are likely to have a very low resilience and be highly vulnerable to further stressors. It will also be critical for any potential recolonisation and recovery that favourable conditions for seagrass growth are maintained. While it will be important to maintain the seagrass monitoring to assess seagrass condition and recovery, enhancements and additions to the program would greatly assist in management to facilitate recovery or restoration. We highly recommend adding:\ud \ud 1. Assessment of the seed bank status to identify potential sources of Recolonisation;\ud \ud 2. Look for opportunities to trial restoration techniques for the foundation seagrass species;\ud \ud 3. Water Quality monitoring - There are a range of catchment and localised activities and issues that have the potential to impact seagrasses through impacts to water quality, but very little water quality information is collected in the harbour.\ud \ud These additional investigations and monitoring measures would also help better establish the links between seagrass changes and natural or human induced causes. Introducing these measures would require additional resources beyond the scope of the current seagrass monitoring program and may require a collaborative approach with other agencies and research organisations