Port Curtis seagrass seed bank density and viability studies: year 2 report

[Extract] This report details findings from the second year of a study examining the density of seagrass (Zostera muelleri subsp. capricorni) seeds and their viability in Port Curtis. The project builds on seagrass seed bank assessments that were originally conducted as part of the Western Basin Dredging and Disposal Project (WBDDP). Results of the seed bank density and viability surveys undertaken, to date, have found that: 1. Seeds were present at all sites during all quarterly sampling events, with total seed density relatively high compared with other locations in Queensland where seed banks have been assessed e.g. Moreton Bay, Cairns and Mourilyan Harbour. 2. Seed bank density changed significantly at all sites over the duration of monitoring (2011 to 2016) but trends varied substantially between sites and years. Overall trends in seed bank density tended to mirror trends in peak seagrass density, particularly in the largest seagrass meadow at Pelican Banks. 3. Viable seeds were found in the sediment seed bank at all sites immediately following the end of each growing season (February 2015 and 2016). 4. We suspect that some meadows (Wiggins Island and Rodds Bay) may rely on nearby donor meadows for seed bank replenishment. 5. The average proportion of viable seeds decreased at all sites over the senescent season (from February to May) in 2015, but remained stable (Pelican Banks North) increased (Rodds Bay) or decreased (Wiggins Island) in 2016. 6. On average greater than 70% of all seeds were found at sediment depths >20mm, and for the majority of the sampling period greater than 40% were found at >50mm. This may limit seed bank function if burial depth inhibits germination and/or seedling success. 7. Seasonal trends in total seed bank density were not consistent, with some sites unexpectedly containing significantly higher seed densities following the senescent period (May) compared with densities following replenishment (February), indicating secondary dispersal events or possibly delayed recruitment of seeds to some meadows. Results have also highlighted the complexity of seed bank dynamics. The ongoing sampling planned in 2016 and 2017 will help to further resolve these dynamics but additional investigations would also enhance the understanding of seagrass resilience including studies on: 8. The spatial structure of the seed bank across the broader meadow; 9. Environmental cues for germination and rates of germination and seedling success; and 10. Connectivity between seagrass meadows through seed dispersal within the Port Curtis region.