The status of seagrass in North West Bay

Abstract

The Blue Wren Group, School of Geography and Environmental Studies, University of Tasmania have sought to describe the status and trends of the distribution of seagrass communities throughout North West Bay (NWB), South East, Tasmania for Kingborough Council based on existing and new data sets and using rapid "first pass" assessment techniques. The objectives of this assessment were to identify the current distribution of seagrass communities, found in NWB, and to detect the long term change in the distribution and biomass of seagrass communities over the last 60 years. Informed by previous research efforts, the research methods for this study had four phases; (1) selection of aerial photography for the time series, (2) purpose-driven capture of low-cost aerial photography for the present day, (3) rapid interpretation of the time series of the aerial photography images using "mega-quadrats" (Mount, 2007), and (4) production of charts showing of seagrass habitats density change for each mega-quadrat. A total of 5 mega-quadrat locations, including; (1) Dru Point Delta, (2) Barretta Bank, (3) Graham Street Beach (immediately south of the NWB Marina), (4) Snug Beach, and (5) Clarkes Beach were selected in this study. Due to constraints imposed by image quality and availability, two study locations - the Dru Point Delta and Snug Beach - were only analysed for present day seagrass distributions and the analysis does not include the deepest edge of the seagrass beds. The change detection method (image visualisation comparison matrix) makes use of aerial photography and is based on the concept of mega-quadrats. It is effective for detecting change in seagrass distributions. The analysis indicates that the extent of seagrass habitat in North West Bay has been in long term decline over the past 60 years with particularly large changes evident in many beds in the mid- 1980s, for example, at Clarkes Beach. This long term decline is characterised by frequent short term irregular fluctuations in seagrass distributions in all the study locations in North West Bay since 1948. Since 2008, the seagrass beds in most mega-quadrats appear to be in a growth phase and the seagrass is recolonising areas it has previously occupied. It is beyond the scope of this report to examine the reasons for the decline and they are likely to be multiple with complex feedback interactions. Candidates for further examination include climate variations (e.g. rainfall), nutrient levels (e.g. sewage and fish farming) and direct damage (e.g. mooring and propeller wash). Further spatial analysis could usefully be conducted to produce more precise mapping of the seagrass habitats. Higher quality aerial photography, which displays the deep edge of seagrass habitats, would support more thorough analysis. Acquiring such imagery would need to be carefully planned for due to variation in weather and/or water clarity conditions.