Land based sources of pollution//source estimates
k.fabricius at aims.gov.au
Wed Oct 3 01:57:25 EDT 2001
Hi Bob and others,
at present the general assumption seems to be (at least here locally) that
turbidity is driven by physics, ie, resuspension forced by wave height,
depth, and particle sizes. However, present-day levels of erosion of soils
and discharge of sediments may increase in some areas the amount and
proportion of clay and other fine material, which creates greater turbidity
and remains suspended for longer than equal concentrations of larger
particles. Together with a group under Terry Done at AIMS, we just started
looking into modelling it all spatially, to create some sort of "turbidity
risk map" for the GBR (and we'd appreciate any thoughts/suggestions/
contributions about this).
I also have data which show that both sediment quality (eg, concentrations
of transparent exopolymer particles) as well as short-term exposure to
sedimentation (hours to days) are important factors influencing the scope
of coral reefs to be recolonised by corals, and these two factors are often
not part of the lines of argumentation put forward by some sedimentolgists.
With regards to the debate of whether global climate change, increasing
CO2, or run-off are the "greatest" threat to coral reefs, I am getting
worried that we may not be getting anywhere with single-cause explanations:
the coral reef ecosystem is so complex that reefs are dying of a thousand
cuts rather than of just one single cause, as each individual species and
life stage has its own little sensitivities to one or the other of the
human alterations of their environment - and what will suffer first is
biodiversity. But I'm also convinced that run-off is hampering the capacity
of reefs to recover from all sorts of disturbances: adult corals can handle
relatively high loads of nutrients and sediments, but recruits don't. Once
the adults are wiped out by COTS or bleaching, we'll wake up if the
juveniles are missing. That's what I'm observing here in some near-shore
areas of the GBR close to intense land use at present (but again, we need
to be cautios coming to any single-cause conclusions about our low juvenile
numbers: we don't have historic data on previous juvenile densities nor on
larvae supplies vs surviviorships from the region).
(for people how may want to send me questions/comments: please apologise
delays in my replies, I'm off to Palau tomorrow for 3 weeks)
At 09:59 AM 2/10/01 -0500, you wrote:
>Katharina, or anyone --
>Do you have either estimates or expert-judgement opinions on the relative
>to which (or the geographic areas in which) the observed high-turbidity areas
>are primarily related to:
>a. medium or large river discharge;
>b. stream, small river or open coast runoff; or
>c. local resuspension of existing sediments?
>Getting some idea of the relative importance of these components of the
>turbidity forcing is critical to deriving impact predictions from climate,
>and land-use models.
>Katharina Fabricius wrote:
> > Another, recently published study from the Indo-Pacific province, in which
> > we looked at the effects of increasing turbidity on biodiversity:
> > Fabricius KE & De'ath G (2001) Biodiversity on the Great Barrier Reef:
> > Large-scale patterns and turbidity-related local loss of soft coral taxa.
> > Pp 127 - 144 in: Wolanski E (ed) Oceanographic processes of coral reefs:
> > physical and biological links in the Great Barrier Reef. CRC Press, London.
> > The article is best to be read in the original book which contains a CD
> > with the colour images and animations of processes. In our chapter, we
> > present a spatial model of increasing turbidtiy (originating from a
> > single-point-discharge), related to decreasing biodiversity. However I'm
> > happy to send out free reprints in paper form (black & white print) or
> > electronically (colour).
> > Abstract:
> > Spatial patterns and abiotic controls of soft coral biodiversity were
> > determined from an extensive reef surveys on the Great Barrier Reef (GBR).
> > Taxonomic inventories of soft corals, and estimates of cover of the major
> > benthos forms and of the physical environment, were obtained from 161
> > reefs, spread relatively evenly along and across the whole GBR. Reefs on
> > the mid-shelf between latitude 13° and 16° represented the "hotspot" of
> > taxonomic richness in soft corals on the GBR. Overlapping distributions of
> > in-shore and off-shore taxa maximised richness on mid-shelf reefs.
> > Taxonomic richness decreased with increasing latitude, and was low and
> > relatively even across the shelf south of 21° lat. Soft coral richness was
> > strongly depressed in areas of high turbidity. It was also weakly
> > positively related to the amount of sediment deposited, and strongly
> > increased with depth. Total cover of hard corals and soft corals was poorly
> > explained by physical and spatial variables, however both varied with
> > The findings presented here have three major management implications: (1)
> > Turbidity and sedimentation affect the generic richness of soft corals.
> > Reefs with highest soft coral richness are < 20 km from the coast, well
> > within the range of terrestrial run-off, and hence a loss of biodiversity
> > could result if turbidity increases due to land use practices which
> > generate soil loss; (2) Taxonomic composition is more strongly related to
> > environmental conditions than total hard and soft coral cover. Taxonomic
> > inventories are thus better indicators of environmental conditions and
> > human impacts than are assessments of total cover. (3) Richness and cover
> > change more within a single site between 0 and 18 m depth, than between
> > reefs hundreds of kilometers apart along the shelf at the same depth.
> > Valuable additional information can be gained in a cost-efficient way if
> > monitoring and survey programs covered several depth zones rather than a
> > single depth.
> > Regards,
> > Katharina Fabricius
Dr. Katharina Fabricius
Australian Institute of Marine Science
PMB 3, Townsville Qld 4810, Australia
Fax +61 - 7 - 4772 5852
Phone +61 - 7 - 4753 4412 or 4758 1979
email k.fabricius at email.aims.gov.au
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