[Coral-List] Sedimentation post (David Hopley)

[Jim Hendee]

[My apologies, but we've had problems getting David's post through for 
some reason.  Here it is:]

Subject: Sedimentation
From: "David Hopley" <dhopley at austarnet.com.au>
Date: Fri, 2 Feb 2007 10:15:00 +1000
To: <coral-list at coral.aoml.noaa.gov>

I've read with interest the Coral-list discussion over the last few days 
on sediments and reefs having being involved with colleagues in a number 
of studies a few years ago which attempted to establish the sediment 
tolerance levels of corals on inshore reefs of the GBR.  The first of 
these was for reefs adjacent to a road bulldozed through rainforest 
adjacent to fringing reefs at Cape Tribulation north of Cairns.  This 
involved measuring both turbidity and settlement using sediment traps.  
The results were published by the Great Barrier Reef Marine Park 
Authority (Tech. Mem. 24, 1990).  It showed that for settlement, 
sediment trap design is critical and not always acknowledged in 
subsequent studies.  Even more important was the range of suspended 
sediment levels measured especially from control sites not affected by 
the road.  These ranged from 1-50mg l-1 during high rainfall, 100 to 
200mg l-1 in normal rainfall periods for this area (100mm in 24 hours) 
and 180 to 260mg l-1 in a heavy rainfall event (300mm in 24 hours).

Nonetheless Charlie Veron established that these reefs accommodated 141 
species of hard corals.  To some extent, at this time the waters had 
been muddied (no pun intended) by the 1985 Pastorok and Bilyard paper 
(Mar. Ecol. Progr. Ser. 21) which had suggested that sedimentation rates 
of 1-10mg cm-2 d -1 caused slight effects on reefs, 10-50mg moderate to 
severe and >50mg catastrophic.  At the Cape Tribulation control site 
levels >50mg were regularly measured in wind speeds greater than 20kts.  
Mike Risk's work in Puerto Rico and other studies showed similar results 
but Pastorok  and Bilyards thresholds have long been accepted and 
repeated in several reviews.  Critical issues are the fact that many of 
their results came from very dry areas or limestone coasts with minimal 
surface run-off.  As Ken Anthony has recently (2000) shown (Coral Reefs, 
19), individual corals are attuned to their ambient conditions and a 
coral on a turbid fringing reef will have a far greater tolerance to 
sediments than exactly the same species on outer shelf reefs in clear 
water  

Also in 1989 Rob van Woesik and I carried out a study on the reefs of 
Magnetic Island (Townsville) adjacent to a new marina.  From 
measurements taken over 2 months (and later confirmed in a larger report 
by Bruce Mapstone and colleagues) we recommended the following limits 
during construction of the marina:  For suspended sediment (Mg/L) 
absolute 1500, short term (up to two tidal cycles) 1000, medium term (up 
to 20 days) 120, long term (beyond 20 days) 75.  For rates of sediment 
settlement (Mg cm2 day-1 ) absolute 200, short term (up to two tidal 
cycles) 150, medium term (up to 20 days) 120, long term (beyond 20 days) 
80.  The lower levels are already experienced on a regular basis by 
Magnetic Island Reefs and other nearshore reefs (see Larcombe references 
mentioned below).

The source of much of this sediment is a nearshore mud/silt wedge up to 
10km wide and 20m thick built up over the last 6,000 years of stable sea 
level (for our biological friends remember that Holocene sea level 
patterns vary in different parts of the world and for isostatic reasons, 
a similar sediment wedge may not have had time to develop adjacent to 
Caribbean reefs).  Sedimentation levels and processes of re-suspension 
are very well described by Piers Larcombe and his colleagues (1999, 
Austr. J. Earth Sci., 46; 1999 Coral Reefs, 18; 1995 Marine Geol. 127; 
1995 Coral Reefs 14; 2001, Sedimentology, 48).  I certainly agree with 
Jeremy Sofonia's comments about sedimentation levels and tolerances 
being site specific and, as Dikou and van Woesik, (2006 Mar. Poll. 
Bull.) recommend, should be evaluated on a case by case basis.  This 
comment is applicable to the Magnetic Island figures given above which 
are only a broad guide to other highly turbid inshore reefs.

Gene Shinn referred to the 1928-29 GBR Expedition study on Low Isles 
carried out by Marshall and Orr (1931, Sci. Repts. GBR Exped. 1).  This 
was a highly innovative study using storage jars located on different 
sites of the Low Isles reef and collected on a weekly basis.  As they 
gave exact dimensions and locations, in 1991 I had one of my Masters 
students (Kay Johnstone) repeat the study over exactly the same 6 months 
of the year.  Interestingly, in spite of much development on the 
adjacent coast near Cairns, the results for the 2 periods could not be 
statistically separated, suggesting that re-suspension from the long 
established sediment wedge, not new sediment, was the main source.

Sorry for going on so long on this but it is an extremely interesting 
and important topic.  I can't miss the opportunity for a bit of 
advertising.  With James Cook University colleagues Scott Smithers and 
Kevin Parnell we are about to have a new book published, the final 
chapter of which deals with amongst other things the geomorphological 
processes involved in sedimentation on GBR reefs.  This is Hopley, 
Smithers and Parnell  2007  The Geormorphology of the Great Barrier Reef 
: Development, Diversity and Change, Cambridge University Press, 532pp.  
It covers most of the things I've touched on here and when published in 
the next few months may provide more detail.
 
Dr. David Hopley
8 Lilydale Pocket
Fairfield Waters
IDALIA  QLD  4811
AUSTRALIA
 
PH:  +61 7 4729 0671
 
EMAIL: dhopley at austarnet.com.au