Tracking nitrogen from the paddock to the reef-starfish Ocean acidification. Source: GBRMPA Coral...
Transcript of Tracking nitrogen from the paddock to the reef-starfish Ocean acidification. Source: GBRMPA Coral...
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Tracking nitrogen fromthe paddock to the reef-
a case study from the Great Barrier Reef
Michael Bell, University of Queensland;Britta Schaffelke, Australian Institute of Marine Science;
Phil Moody, Department of Science, Information Technology and Innovation;Dave Waters, Department of Natural Resources and Mines; and
Mark Silburn, Department of Natural Resources and Mines.
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⢠Largest coral reef system in theworld
⢠>3000 reefs; 2,200 km long;⢠350,000 km2
⢠Adjacent catchment dominatedby extensive grazing systems.
⢠Small areas of cropping closeto the coast, in higher rainfallareas.(intensive sugarcane,horticulture and bananas,extensive grains)
The Great Barrier Reef Region
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GBR under pressure
Cyclones &Storms
Run off
Increasedship traffic
CoastalDevelopment
Fishing
Increasing temperature
Crown-of-thornsstarfish
Ocean acidification
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Source: GBRMPA
Coral bleaching
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2016 Mass coral bleaching event2016 Coral bleaching event
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The GBR catchment to reef connectionExposure to runoff from broad-scale land use is a key pressurefor the ecosystems of the GBR
Loads have increased: Sediment ( 3-5 x) , Nitrogen (2-6 x), Phosphorus (2-9 x) + herbicides
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3 main N sources
Upwelling
River inflows
Salinity20 PSU 30
N-fix
atio
n
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Source: Virginia Coast Reserve Long-TermEcological Research project
The marine N- cycle
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Coral bleaching & nutrientsBreakdown of symbiosis:⢠Too much heat⢠Too much light⢠Too much N
â Increased zooxanthellae densityâ N/P imbalanceâ Too much organic carbon, triggering higher N-
fixation on corals
Wiedenmann et al 2013. Nature Climate Change 3: 160-164.Rädecker et al. 2015. Trends in Microbiology, 23: 490-497.Wooldrudge et al. in press. Marine Pollution Bulletin
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Outbreaks of the Crown-of-Thorns seastar (CoTS) &nutrients
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Prob
abilit
y of
dev
elop
men
tChlorophyll (mg m-3)
0.01 0.06 0.25 1 2
0.0
0.2
0.4
0.6
0.8
1.0
Enhanced food availability for CoTS larvaeâNutrient Hypothesisâ - A numbers game:Higher survival of larvae due to increased food availability
Fabricius et al. 2010. Coral Reefs 29: 593-605.
Likely in combination with:⢠Hydrodynamic conditions that retain larvae⢠Reduced predators⢠Increasing temperature
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Reefs condition & nutrients
High nutrients,High sediments
Low nutrients,Low sediments
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Inshore seaweeds benefit from highernutrient availability
Schaffelke 1999, Mar Ecol Prg Ser 182: 305-310.
Net
pho
tosy
nthe
sis
(Âľm
olO
2h-
1g
DW
-1)
0
100
200
300
Chlorodesmisfastigiata
Padinatenuis
Sargassumbaccularia
Turbinariaornata
control10 ÂľM N
1 ÂľM P10 ÂľM N + 1 ÂľM P
Inshore Cross-shelf
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Recovery of reefs after disturbance
⢠moderate algal growth, mainly turfs⢠coral recruitment & growth
â Recovery
⢠enhanced algal growth⢠coral recruitment reduced⢠coral/algal competition
â slow or no recovery, reduceddiversity
Water quality is an important factor
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Regional variability in loads, pre- and post-development
⢠Large regional variationin predevelopmentloads.
⢠Anthropogenic activityhas increased loadssubstantially.
⢠Largest relativeincreases in regionswhere predevelopmentloads were quite low. McCloskey et al. (2016) Reef Report Card 2015. Whole of
GBR, Technical Report, Volume 1
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Sources of N in loads enteringthe GBR.
⢠There are 3 dominant sources of N.⢠Grazing and sugarcane cropping are
the dominant agricultural land usesin terms of N loads.
⢠The 3rd ranked source (stream bankerosion) is linked to developmentand loss of riparian vegetation.
⢠The constituent N forms from eachsource are quite different, and arethe product of the N inputs and theloss processes in each system.
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Are these N constituents what left the field (i.e.do we know what we are trying to manage?).
⢠Loads modelling calibratedagainst end of catchment loadsmonitoring.
⢠A series of N transformationsand losses can occur betweenpaddock and river mouth.
⢠These can result in DINenrichment, as well as lower Nloads.
⢠Residence times will have amajor impact on theseprocesses
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The form and pathway of N loss willdetermine water quality impact and theeffectiveness of management strategies
⢠Denitrification losses will have no directwater quality impact
⢠The proportions of PN and DIN willinfluence the zone of impact (inshore vouter reef).
⢠Minimizing runoff will reduce PN loadsbut not necessarily DIN.
An example from sugarcane, comparingmeasured runoff losses at block scale andmodelled loads at end-of-catchment
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Monitoring suggests leaching and lateral movement are a major DINsource in sugar catchments.
~10-15% of Fitzroy DINload in base flow (grazing)
50-60% of Tully DIN load inbase flow (cane, bananas)
Pers. Comm. Ryan Turner, GBR Loads Monitoring Program
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These transformation processes donât stop at theriver mouth
Reducing DIN loads may seem a logical first step to reducing the biologically active N loads.However, the risks posed by labile organic N cannot be ignored.
Burton et al. (2015). RP128G - Department of Science,Information Technology and Innovation.
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Minimizing N losses from grazing systemsâ controlling erosionâŚ
Hillslope/sheet erosion Gully erosion
Extensive areas⢠Managed by retaining groundcover⢠A focus of grazing BMP programs⢠Only delivers ~ 20% of total sediment
Small, defined areas⢠Intensive remediation/stabilization⢠A focus of on ground activity⢠Delivers ~ 80% of total sediment
Bartley et al (2017). Chapter 2 â GBR Science Consensus statement
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Where to focus?
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N enrichment ratios will help focus activity onsoil types with greatest N delivery risk
⢠Bioavailable nutrient levels insurface soil varied widelybetween soil types
⢠Enrichment ratios(sediment/soil) also variedwidely
High erodibility
<10umSediment
Sourcesoil
Burton et al. (2015). RP128G - Department of Science, InformationTechnology and Innovation.
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Labile N in the fine sediment fraction represents thegreatest water quality risk to the outer reef
0
20
40
60
80
100
Partic N load Pot Min N 1d Pot Min N 7d
Cont
ribut
ion
to lo
ad (%
)
Surface Sub-surface
For fine (<10um) sediment:
⢠Sub-surface sediment contributes most of PNload (90% in this eg. â Wilkinson et al. 2015)
⢠Surface sediment contributes significantly moremineralisable N than its load proportion
⢠Management intervention must considerboth hillslope and streambank/gullyerosion processes
1:9 3:1 1:4
Wilkinson et al. (2015); Burton et al. (2015); Bartley et al. (in press).
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Minimizing N losses from sugarcane- managing surplus NâŚ..
An example for an 80 t/ha cane crop in the wet tropics
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Urea and the current N surplus
40 kg N surplus
With no environmental losses, growers could reduce conventional N rates by~40 kg N/ha and still meet crop demand in this example
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The reality - losses to the environment can be high and crop Nsupply may become suboptimal
46 kg Ndeficit
18 kg Nsurplus
Environmental losses remove the option to safely reduce N rates. In highloss situations, increasing N rates can be a reasonable risk management strategy!
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Improved fertilizer technology will break this nexus
⢠Environmental losses halved⢠Adequate N for crop
demand maintained, even inhigh loss environments
⢠Fertilizer N rates can safelybe rationalized to matchcrop productivity zones
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Conclusions⢠Elevated bioavailable N in the GBR lagoon is affecting ecosystem health, and
process level understanding of the ecological mechanisms is developing rapidly⢠The major sources of anthropogenic N are the grazing and sugar industries⢠Changed management practices are reducing loads, but not far enough or fast
enough.⢠Management interventions to limit N loads may not be the same as for
sediments and pesticides⢠Enhanced efficiency fertilizers offer solutions in sugar⢠Climate variability and the feasibility of increased management intensity in
extensive grazing systems remain challenging⢠Climate change remains the biggest threat to the longer term health of the
Great Barrier Reef
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Thank you!
Acknowledgements:Our research teams at AIMS, UQ, DSITI and DNRMFunding:GBRMPA, Australian Government Reef Program, Qld Reef WaterQuality Science Program, NESP, AIMS