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Nutrient transformations on intertidal flats of Western Port Bay - - PowerPoint PPT Presentation
Nutrient transformations on intertidal flats of Western Port Bay - - PowerPoint PPT Presentation
Nutrient transformations on intertidal flats of Western Port Bay Victor Evrard, Perran Cook, Vera Eate, Ryan Woodland, Jeff Ross, Andy Longmore, Matt Edmunds, Allyson OBrien, Michael Keough Recommendations from the WP review Why do we
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Why do we care about nutrients?
- Nitrogen and phosphorus are key plant
limiting nutrients in aquatic environments
- Too many nutrients can lead to algal blooms
and seagrass loss
- Nutrients are also essential to ecosystem
productivity
- An understanding of nutrient cycling in
Western Port is essential to sound management
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Conceptual diagram
Catchment inputs Exchange with Bay
Sediment exchange
Groundwater?
Porewater drainage
N2 fixation? N2 production
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Key Questions
- What is the exchange of nutrients between
the tidal flats and Western Port Bay?
- Do tidal flats attenuate nutrient loads from
the catchment?
- Does nutrient cycling and exchange vary
between habitats?
- How do fauna interact with cycling processes?
- Role of groundwater input?
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What we did
- Measured large scale exchange between two inlets
(dominated by intertidal) and Western Port Bay. Watson Inlet and Chinaman’s inlet (Nov and Feb)
- Quantified denitrification and nutrient exchange between
the sediment and the water column in 4 key biofacies. Channel, unvegetated intertidal, seagrass intertidal, mangrove intertidal
- Sampled flora and fauna to investigate interaction between
flora and fauna and processes and measure isotope ratios
- Measured Rn concentrations to gauge the importance of
groundwater inputs and porewater drainage in exchange processes
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Western Port Bay
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Study sites
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Habitat types
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Watson Inlet
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Whole system flux calculation With inflow
NH4
+ kg/d NOx Kg/d FRP kg/d
Nov-11 Load from WC 4 280 41 WI Export 37 350 47
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Whole system flux calculation Without inflow
NH4
+ kg/d NOx Kg/d FRP kg/d
Nov-11 CI 6.7
- 17.0
- 0.8
Feb-12 WI
- 8.8
- 2.3
- 15.2
CI 9.4
- 6.7
- 0.5
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How do processes vary with habitat type?
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Denitrification rates in different habitats
0.00 50.00 100.00 150.00 200.00 250.00 CI dark CI light WI dark WI light Seagrass Mangrove Bare Channel
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Why is denitrification so high in bare sediment?
Image Courtesy of the Museum
- f Victoria, photo by Gary Poore
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How do whole system and scaled habitat rates compare?
Nutrient Scaled habitat kg/d Whole system kg/d Watson Inlet NH4
+
27
- 9
NOx
- 12
- 2
FRP
- 3
- 15
Chinaman Inlet NH4
+
8 9 NOx
- 5
- 7
FRP
- 1
- 0.5
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Stable isotopes – what can they tell us?
- Elements often have a heavy and a light form
- Carbon and nitrogen are such elements
- In primary producers (e.g., seagrass) the
heavy/light ratio can tell us about their nitrogen sources and photosynthesis
- In animals, the heavy/light isotope ratio can
tell us what they are eating
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Where does seagrass get its nitrogen?
Isotope scale Light Heavy Nitrogen source Atmosphere Marine Human Chinaman Inlet Watson Inlet ~80% from nitrogen fixation ~20% from Watson Creek
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What are the fauna eating?
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Food sources for B. arenosus
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How does denitrification and nitrogen fixation compare to N loads for the whole Bay?
- Assuming the area of Northern Western Port is 70
km2 and scaling up the rates measured in this study (~10 g N m-2 y-1), then the annual rate of denitrification is ~700 tonnes N per year
- This compares to an estimated annual average
load of 600 tonnes per year to Western Port
- Using literature measurements of N2 fixation
rates, the whole bay fixes ~200 tonnes of N per
- year. Bay is roughly in balance!
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Conceptual diagram
Catchment inputs Exchange with Bay
Phosphorus
N2 fixation N2 production
Porewater drainage