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An approach to evaluating nutrient assimilative capacity of a tropical macrotidal estuary: Darwin Harbour

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Background / Purpose: Darwin Harbour is a tropical estuarine system under pressure from urban and industrial development. Increased nutrient concentrations are a factor to be considered in effective management of the water body. We have presented an overview of the current condition and an updated concept of nutrient assimilative capacity as a management tool. Main conclusion: An evaluation of nutrient assimilative capacity of Darwin Harbour can be realised through a combination of observations, experimentation and modelling; a framework has been outlined, and it has the scope to be embedded in decision making systems. It is a process that would be used improperly if done only on a whole-of-harbour basis; its zoned application, and case by case, within the waterway guarantees more surely the integrity of ecological components (e.g. tidal creeks) of the full system.
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An approach to evaluating nutrient
assimilative capacity of a tropical
macrotidal estuary: Darwin Harbour
Edward Butler (AIMS/NAMRA)
Julia Fortune (CDU/NT Gov’t)
AMSA2014 Conference, 7-10 July 2014
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Historical perspective
1970s 1990s:
nutrient assimilative capacity “exceeded
loss of environmental quality
loss of biodiversity
loss of amenity
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Australian example
Engineering solution: ‘Dawesville Cut’
Peel-Harvey Estuary, WA
mesohaline system converted to polyhaline,
with better flushing
…but blooms pushed up riverine arms, change
in aquatic vegetation and foreshore erosion
experienced massive blooms of the blue-green
alga Nodularia spumigena during the 1980s
initiated by phosphorus released from sediments
due to a thin bottom boundary layer going anoxic
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No turning back the clock
Nutrient Loads
Deteriorating Trophic Condition
Hysteresis!
phase shift
The highly non-linear
responses of aquatic
ecosystems to changing
catchment loads of
major and minor
elements are explained
by the interactions of
the major functional
groups and by
competition between
the pelagic and the
benthos for nutrients
and light”.
G.P. Harris 1999
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Nutrient Assimilative Capacity
revisited
Nutrient assimilative capacity is the scope of the
aquatic system to incorporate nutrients into living
matter and sediments, so preventing build-up of
nutrient concentrations in the water column, with two
important provisosthat the nutrient assimilation
process within the water body does not lead to 1) loss
of estuarine or coastal biodiversity, or 2) degradation
of environmental services of the system.
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Conundrum of Darwin Harbour
Secchi depth (m)*
2.4 2.6
Chl a (µg L1)*
0.8 1.7
Turbidity
1 >30
P:R ratio
1.9 6.7
Primary production (g C m2 y1)
400
Molar N:P ratios*
1.9 5.8
Total dissolved nitrogen (µmol L1)
7 23
* means Data from McKinnon et al. 2006
Burford et al. 2008
Darwin Harbour, on the whole, is a Nitrogen-limited system
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Trophic status of Darwin Harbour
BUT
Respiration rates in
Buffalo Creek up to
10.8 mmol O2 m-3 h-1
Smith et al (2012)
D. McKinnon, unpublished results
Dark Respiration
mmol O2 m-3 d-1
0 5 10 15 20 25
Scott Reef
Ningaloo
Timor Sea
GBR
Kimberley Coast
Gulf of Papua
Arafura Sea
Exmouth Gulf
Darwin Harbour
Gove Harbour
Indonesian coast
Bathurst Island
Hinchinbrook
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…not just the water column
Annual production:
mangroves 1,609,220 t C y1
pelagic 395,170 t C y1
intertidal mudflats 226,210 t C y1
subtidal mudflats 227,800 t C y1
Nitrogen demand (assimilation)
pelagic 56,000 t N y1
mangroves 12,750 t N y1
intertidal mudflats 24 t N y1
subtidal mudflats 0.1 t N y1
Data from Burford et al. 2008
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C & N fluxes for Darwin Harbour
194
Fortune & Maly 2009, adapted from Burford & Saeck
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wetland
pool
salt
flat
salt
marsh
mangrove
zone
intertidal
mudflat
subtidal
zone
wetland
pool
salt
flat
salt
marsh
mangrove
zone
intertidal
mudflat
subtidal
zone
Nutrient Cycling N & P
tidal exchange
atmospheric cycling
biological cycling
N cycling
P cycling
DIN/P dissolved inorganic forms
DON/P dissolved organic forms
PON/P particulate organic forms
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Mangroves in praise of
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Nitrogen sinks
With nutrient (nitrogen) sinks, it is both the capacity and
the retention time
Pelagic phytoplankton have the greatest estimated N-
assimilative capacity in Darwin Harbour, but it is ephemeral
Deep sediment burial (v. minor) and denitrification (unknown)
give long-term removal
Mangrove system has a large N-assimilative capacity, and
intermediate retention time (years)
More knowledge of nutrient sinks is needed for the Harbour
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Factors affecting nutrient cycling
Microbial community present and active
Dissolved oxygen levels
Underwater light environment
Availability of carbon (concentration and lability)
Sulfur (as sulfate) supply for some micro-organisms
Grazing pressure and discrimination
Trace elements (metals and others) as micronutrients and/or
toxicants
Other pollutants POPs, bioactives, etc.
Sediment characteristics
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Known unknowns, etc.
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Nutrient assimilative capacity in
a preventative framework
The pay off is not only the retention of the prized ecosystem,
but also the avoiding of a severe penalty for crossing critical
thresholds in the form of system hysteresis, whereby the earlier
cost of pre-emptive action is far exceeded by the cost of
correction.
Requirements:
enhanced system understanding
ecohydrological model
threshold indicators (pre-critical)
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Conceptual Model
N cycling in Darwin Harbour
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Ecohydrologic model
RMA2/11
RMA11
DK Williams
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Threshold indicators proposed for
Darwin Harbour
N cycle step (e.g. denitrification)
innovative indicator using molecular biological
techniques
offering heightened sensitivity
Dissolved oxygen (onset of hypoxic conditions)
Chlorophyll a (analogue for plankton biomass)
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Buffalo Creek,
Darwin
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Application of model with
threshold indicators
scenario testing, where existing conditions and revised
nutrient loadings are compared with water and sediment
quality objectives
vulnerability resolution, where existing conditions are
used to identify localities that are most at risk of
approaching or exceeding the threshold indicators; tidal
creeks and estuary arms with longer residence times of
stratified bottom waters will be most at risk
a combination of the first two for refined scenario testing
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Conclusion
and some qualifications
An evaluation of nutrient assimilative capacity of Darwin
Harbour can be realised through a combination of observations,
experimentation and modelling; a framework has been outlined.
It is a process that would be used improperly if done only on a whole-of-
harbour basis; its zoned application, and case by case, within the waterway
guarantees more surely the integrity of ecological components (e.g. tidal
creeks) of the full system
It will require some resource investment to address knowledge gaps
It has the scope to be embedded in decision making systems
AMSA2014 Conference, 7-10 July 2014
DARWIN
HARBOUR
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Looking to know more?
Butler, E.C.V., Streten-Joyce, C., Tsang, J.J., Williams, D.K., Alongi, D.M., Furnas, M.J. and
McKinnon, A.D. (2013) A procedure for evaluating the nutrient assimilative capacity of
Darwin Harbour. Report for Aquatic Health Unit, NT Department of Land Resource
Management. Australian Institute of Marine Science, Darwin.
PDF download from DLRM publications page:
<http://www.lrm.nt.gov.au/water/aquatic/publications>
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Acknowledgements
Report co-authors
Claire Streten-Joyce
Jeffrey Tsang
David Williams
David McKinnon
Miles Furnas
Dan Alongi
also
Simon Townsend
Peter Dostine
AMSA2014 Conference, 7-10 July 2014
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