The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage and Boullanger bay, far northwest Tasmania

This natural resource management research project was initiated by the Cradle Coast NRM, in response to the knowledge gaps identified by the Blue Wren Group in understanding elements of Robbins Passage-Boullanger Bay (RP-BB) coastal processes. The purpose of this study was to: • Improve the understanding of RP-BB tides, based on observational data, and • Investigate the carbon sequestration potential and palaeo-environmental evolution of RP-BB shallow seagrass beds, based   on a set of shallow marine sediment cores. The key findings of this report are: • RP-BB receives strongly semi-diurnal meso-tides which vary in their range and time of arrival • Predicted mean spring tide ranges and total tide ranges were found to be 2.80 m and 3.15 m at Howie Island, 2.20 m and   2.63 m at Kangaroo Island, and 2.01 m and 2.42 at Welcome Inlet • The National Tide Centre’s modelled tide range was found to underestimate the tide range for eastern Boullanger Bay by   approximately 30% • Three unique sedimentary deposits (i.e. facies) were identified in the sediment cores, interpreted as a Late Pleistocene   alluvial/lacustrine deposit (SF1), Mid-Holocene intertidal or shallow subtidal sand flats (SF2), and Mid-Late Holocene   seagrass associated deposits (SF3 • Large carbon rich sediment deposits exist beneath the subtidal seagrass meadows at RP-BB • RP-BB Posidonia australis dominated subtidal seagrass meadows are highly effective at sequestering carbon.