Land based sources of pollution//source estimates

Bob Buddemeier buddrw at
Wed Oct 3 07:57:39 EDT 2001

List --

Comment first, then some more discussion of (mostly sediment-related) issues.

Special thanks to Katharina and Alina for their observations and comments.
Katharina is right on with her comments on single variable arguments -- the problem
is, we have to understand the variables one by one to get to the point of effective
integration, and that seems to tempt a lot of people into the all-or-nothing false
dichotomy.  Another problem is the gravitation toward polar positions: "reefs are
doomed real soon because people are killing them off" vs "not too worry, they're
robust and it's just a natural fluctuation."  The first is a very slightly more
credible position than the second, I think, but only slightly, and the most useful
synthesis combines and is offset from that discussional axis.

Turbidity and sediment are good examples.  Without claiming that they are totally
generalizable, let's take the recent contributions to the discussion to show that
resuspension of sediment (as opposed to new input) is a significant stress factor.
I suggest that this is at least partly a 'natural cycle' development.  Continental
shelves and shallow coastal areas are excellent sediment traps, retaining a lot of
what comes off the land.  Our present situation is geologically and environmentally
anomalous -- a relatively stable 3-6,000 year sea level high stand (the range of
times is because it's local, not eustatic, level that counts operationally, and the
Caribbean and much of the Indo-Pacific have different local sea level histories).
That accounts for a lot of sediment build-up (with or without human intervention),
and I suggest that a number of areas may 'simply' have reached a critical threshold
in terms of the inventory or load of resuspendable sediment.  A glance at the
Pleistocene sea level curve will show why corals and reefs are not necessarily
adapted to this kind of environment.

I put 'simply' in quotes above to underline Katharina's point that it never is
simple -- in this case, one of the complicating human factors is change in the
ocean climate.  As I understand it, a number of regions of the oceans have shown
significant increases in mean wave height over the past few decades.  This is the
resuspension driver, so it may be that either natural climate cycling or
human-induced climate change have pushed the sediment resuspension effects across
the threshold very recently.

This underlines a point that I hope was obvious from the earlier discussions --
reef researchers need to understand some oceanography, as well as issues of
large-scale dynamics (the latter comment is a shameless plug for an upcoming
special issue of Coral Reefs -- sorry).

It also puts some other perspectives on the questions of reef doom and what to do
about it. Note that I am going to talk about a particular variable or suite of
variables, and do not intend to imply that there aren't others, that people aren't
problems, etc.
1.  'Land sources' in the real-time sense may not be as big a sediment issue as
often supposed.  Most large and medium -sized drainage basins have had their water
flow (for sure) and sediment discharge (proabably but not always) reduced and
regulated by damming and diversion.  Local coastal runoff and small/undeveloped
basins have the potential for dramatic increases in sediment load in response to
land use and cover changes, but the acute effects of these are often localized near
shore (although there is the general contribution to shelf sediment load build-up).

2.  There is no realistic prospect of modifying either the coastal zone sediment
inventory or the marine energy regime, so -- if this formulation is valid --
chronic sediment stresses in most offshore areas may be something that simply has
to be lived (or died) with.  This implies a focus on understanding its contribution
to multi-stress synergism in hopes of finding a different factor that can be
managed to reduce the combined system impact.
3.  Conservation/preservation:  I have been beating the drum for a triage approach
to reef resarch conservation, and management, and I have also from time to time
expressed a fondness for atolls (but outer-shelf reefs can be OK too).  I suggest
that this example reinforces both -- if continental reefs really have "timed out"
in terms of Holocene habitat development, the places to look for healthy or at
least preservable systems are in very well-flushed, no-soft-sediment coastal areas
or away from terrigenous sediment sources (e.g., ocean islands, especially with
small land mass).
4.  Research implications:  This point goes beyond the sediment resuspension issue
to the larger question of combined (and especially land-derived) threats.  The idea
of chronic stress build-up to a threshold transition that we are now observing
implies not only that we are not currently working on normal or 'healthy' systems,
but also that what we take as our pre-transition baseline was probably seriously
affected at the subclinical level.  This means that much of the coral lierature on
function and condition has to be interpreted very cautiously if one is interested
in determining 'normal' or 'optimal' function.  Jeremy Jackson has made this point
with respect to anthropogenic ecosystem alterations; I propose extending it to a
broader suite of 'natural cycle' considerations including sediment buildup on
shelves, the implications (for accomodation space and circulation, among other
factors) of reef 'catch-up' with sea level, etc.

All of which may help explain why I am of the opinion that most
'reefs-as-we-know-them' are on their way out of the picture, especially if they are
closely associated with a major landmass.  I would rather not use 'doomed' as a
blanket statement, because I think there may be some (significantly altered)
oceanic survivors.  The moral of the story:  Go to sea.

Bob Buddemeier

Katharina Fabricius wrote:

> 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).
> Regards,
> Katharina
> (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
> >extent
> >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,
> >wave,
> >and land-use models.
> >
> >Thanks,
> >
> >Bob Buddemeier
> >
> >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
> > depth.
> > > 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
> Research Scientist
> 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
> ~~~~~~~
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