What water quality do seagrass require in Western Port? Perran Cook, Dougal Greer, Andy Longmore, Brent Wormersley, David Ball, Michael Keough, John Beardall, Randall Lee, Ralph MacNally
Why are seagrasses disappearing globally? • Nutrient inputs • Sediment inputs
Seagrasses are disappearing globally, why? • Inputs of nutrients (nitrogen) - eutrophication
Why have seagrasses disappeared in Western Port? Sediment input Benthic exchange
Why have seagrasses disappeared in Western Port? Sediment input Benthic exchange
Why have seagrasses disappeared in Western Port? Sediment input Benthic exchange
Why have seagrasses disappeared in Western Port? Sediment input Benthic exchange
Why have seagrasses disappeared in Western Port? Sediment input Benthic exchange
Why have seagrasses disappeared in Western Port? Sediment input Benthic exchange
Why have seagrasses disappeared in Western Port? Sediment input Benthic exchange
Aims of this project • To identify the key factors (light or nutrients?) controlling the current seagrass distribution within Western Port • To quantify this relationship to obtain thresholds (and hence water quality) needed for seagrass growth • To investigate the possibility of using leaf chemistry as a sentinel to stress
Method • Take existing habitat map, overlay with WQ, light TSS
Method • Take existing habitat map, overlay with WQ, light TSS Critical threshold %Cover Driver (e.g. light)
Method • Relate leaf chemistry to key drivers (nitrogen and light) Critical threshold %Cover or leaf chemistry Can we use leaf chemistry as a sentinel for change? Driver (e.g. light)
What do carbon isotopes tell us? Photosynthesis Light limited Light replete Shallow water Deep water Clear water Turbid water Heavy Light Isotope scale
Carbon isotopes in Western Port
A comparison with Port Phillip Bay Shallow sites in the NE
Undertstanding the sediment driver • Suspended sediments are derived from the catchment and re-suspension within Western Port • To understand this complex driver we used a hydrodynamic model which simulates resuspension by waves and currents
Modelled mean significant wave height
Mean modelled current velocities
Sediment type
Total suspended sediments
From suspended sediments to light
The light field!
Total Suspended solids
Conclusions • We believe light limitation is the key issue in Western Port • Spatial analysis of the data is currently underway to identify the key thresholds of light, TSS and elevation in this dynamic environment • Work is continuing to develop the use of carbon isotopes as a proxy for light limitation
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