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Loss of
diversity is one of the possible repercussions of degraded water quality.
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In tropical river
catchments in Queensland (Australia),
increasing rates and intensity of land use change are being expressed as
greater exports of sediments and nutrients to nearby coastal and marine
ecosystems (QDNRM 2003). In many catchments, increased sediment/
nutrient export over the past half century is associated with a net loss of
mangroves (Duke & Wolanksi 2001; Neil et al.
2002); and mangrove loss is now central to concerns about the health of
adjacent marine ecosystems. Several models conclude that sediment delivery
rates to the Great Barrier Reef lagoon have
increased 2-6 fold since European settlement in the
19th century (Moss et al. 1992; Neil et al. 2002), but the roles and
magnitudes of mangroves as buffers of sediments, nutrients and chemical
exports to nearshore coral reef systems are
virtually unknown.
Since May 2003,
I have worked in Queensland in the Pioneer R. estuary (at ~21oS)
to determine whether historical records of changes in mangrove distribution
are correlated with changes in chemical and isotopic proxies of suspended
sediment concentration obtained from coral cores taken from nearshore colonies. The main prediction is that mangrove
loss (as a result of both natural and anthropogenic change) has reduced
entrapment of recent sediments, nutrients and pollutants, while increasing
erosion has mobilized older estuarine sediments, nutrients and pollutants;
both processes are likely to be manifested as increased incorporation of
trace elements (e.g. barium, rare earth elements, high-field-strength
elements) into coral skeletons at levels above those due to runoff alone.
Since the Pioneer system has the second highest percentage of cropped land
and is third highest exporter of sediments per unit area of any Great Barrier Reef catchment,
it is likely that trace metal and isotopic concentrations associated with
suspended sediments and nutrients will be high enough to accumulate in and be
detectable in nearshore massive coral colonies. I
am using remote sensing, ground surveys, and
laboratory analyses in a three-phased approach to evaluate: (1) mangrove
changes through time; (2) export of estuarine sediments; and (3)
incorporation of proxies of sediments and nutrients into coral skeletons.
REFERENCES:
Duke,
N.C. and E. Wolanksi
(2001)
Muddy coastal waters and depleted mangrove coastlines, In Oceanographic
Processes of Coral Reefs: Physical and Biological Links in the Great Barrier
Reef, ed. E. Wolankski. CRC Press, Boca Raton, p.p. 77-91.
Moss, A.J., Rayment,
G.E. , Reilly, N. and E.K. Best (1992) A preliminary assessment of
sediment and nutrient exports from Queensland
coastal catchments. Queensland Department of Environment and Heritage
Technical Report No. 5, Queensland Government, Brisbane, 28 pp.
Neil, D.T., Orpin, A.R., Ridd, P.V. and B. Yu (2002) Sediment yield and
impacts from river catchments to the Great Barrier Reef lagoon. Mar. &
Fres. Res., 53: 733-752.
QDNRM (2003) Land Cover Change in Queensland,
a Statewide Landcover and Trees Study Report
(SLATS), Queensland Department of Natural Resources and Mines, Queensland Government, Brisbane, 78 pp.
FOR ADDITIONAL
INFORMATION:
Duke,
N.C., Bell, A.M., Pedersen, D.K., Roelfsema,
C.M., Godson, L.M., Zahmel, K.N., MacKenzie, J. and S. Bengston-Nash
(2003) Mackay mangrove dieback: Investigations in 2002 with
recommendations for further research, monitoring and management. Report
to the Queensland Department of Primary Industries, Marine Botany Group, Centre
for Marine Studies, The University of Queensland, St. Lucia.
Jupiter,
S.D., Phinn, S., Duke, N.C. and D.C. Potts (2003)
The buffer zone: connectivity between mangrove and coral reef ecosystems in
the context of land use change. Proceedings of the 30th
International Symposium for Remote Sensing of Environment, Honolulu, HI,
Nov. 10-14, 2003.
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