I
IMPACTS OF SEDIMENT ON CORAL REEFS
Michael J. Risk
1
, Evan Edinger
2
1
Durham, ON, Canada
2
Memorial University of Newfoundland, St. John’s, NL,
Canada
Introduction
The relationship between reefs and sediments has been
observed by humans certainly for centuries, and possibly
for millennia. Columbus observed that his ships could take
on fresh water by avoiding the fringing reefs of South
America and sailing into the mouths of the Orinoco
(Morison, 1974). Moreover, that keen observer Charles
Darwin noted that “The deposition. . . of sediment, checks
the growth of coral-reefs” (Darwin, 1842).
Since the earliest seafarers noted that they could
approach reef-barred coastlines at river mouths, because
no reefs occurred there, we have known that sedimentation kills reefs. In fact, throughout geologic history
(including the present), sediment must be considered one
of the great killers of reefs. Notwithstanding this, the field
still lacks the definitive body of research that would allow
management teams to set objective standards and to craft
appropriate legislation.
A geological perspective
A great deal of research has been conducted by geologists
on coral reefs because of the hydrocarbon resources
located within ancient reefs. The framework of these
ancient reefs was produced by a wide variety of types of
invertebrates, as well as corals with algae, brachiopods,
crinoids, and mollusks included in the mix. Many ancient
reefs are seen, whether in core or in outcrop, to be covered
by a shale unit, and the assumption has been made that the
reefs were killed by sediment input. The immediate cause
of the demise was not as important to the geologist as was
the fact of the shale-capping unit, because that often
transformed the porous reef unit into a hydrocarbon reservoir. Adams and Frenzel (1950), for example, attributed
much of the alternating reef facies in the Permian Capitan
Reef of West Texas to periodic death of individual reef
units from sediment input. Kershaw (1998) describes the
complex interplay between morphology and sediment
input in ancient stromatoporoid reefs.
In the past few decades, however, further research has
afforded more light on the possible/probable causes of reef
demise in the fossil record. Hallock and Schlager (1986)
suggested that the death of the entire reef tracts could be
due to increased surface productivity, driven in turn by
increases in nutrients. In the fossil record, increased nutrients can result from increased rainfall, changes in current
patterns so as to increase upwelling, or changes in sea level.
In today’s oceans, of course, increased nutrients are almost
invariably the result of human activity. Although reefs have
faced sediment stress since their inception, there is evidence
that the threat is increasing due to human activities
(Fabricius, 2005). Globally, fluvial sediment delivery to
the world’s oceans is increasing as a result of deforestation
and other land clearing for agriculture, although this
increase is countered in many watersheds by sediment trapping behind dams (Syvitski et al., 2005).
Hallock and Schlager discuss the changes in reef structure and reef sediments caused by increasing nutrients,
and suggest that this process could be linked to the demise
of fossil and modern reefs. Their paper functions as
a framework, against which various hypotheses for reef
decline may be tested. This is also one of the few papers
in the past two decades treating reefs as a whole that gives
proper place to the critical role of bioerosion in the balance
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011
IMPACTS OF SEDIMENT ON CORAL REEFS
Michael J. Risk
1
, Evan Edinger
2
1
Durham, ON, Canada
2
Memorial University of Newfoundland, St. John’s, NL,
Canada
Introduction
The relationship between reefs and sediments has been
observed by humans certainly for centuries, and possibly
for millennia. Columbus observed that his ships could take
on fresh water by avoiding the fringing reefs of South
America and sailing into the mouths of the Orinoco
(Morison, 1974). Moreover, that keen observer Charles
Darwin noted that “The deposition. . . of sediment, checks
the growth of coral-reefs” (Darwin, 1842).
Since the earliest seafarers noted that they could
approach reef-barred coastlines at river mouths, because
no reefs occurred there, we have known that sedimentation kills reefs. In fact, throughout geologic history
(including the present), sediment must be considered one
of the great killers of reefs. Notwithstanding this, the field
still lacks the definitive body of research that would allow
management teams to set objective standards and to craft
appropriate legislation.
A geological perspective
A great deal of research has been conducted by geologists
on coral reefs because of the hydrocarbon resources
located within ancient reefs. The framework of these
ancient reefs was produced by a wide variety of types of
invertebrates, as well as corals with algae, brachiopods,
crinoids, and mollusks included in the mix. Many ancient
reefs are seen, whether in core or in outcrop, to be covered
by a shale unit, and the assumption has been made that the
reefs were killed by sediment input. The immediate cause
of the demise was not as important to the geologist as was
the fact of the shale-capping unit, because that often
transformed the porous reef unit into a hydrocarbon reservoir. Adams and Frenzel (1950), for example, attributed
much of the alternating reef facies in the Permian Capitan
Reef of West Texas to periodic death of individual reef
units from sediment input. Kershaw (1998) describes the
complex interplay between morphology and sediment
input in ancient stromatoporoid reefs.
In the past few decades, however, further research has
afforded more light on the possible/probable causes of reef
demise in the fossil record. Hallock and Schlager (1986)
suggested that the death of the entire reef tracts could be
due to increased surface productivity, driven in turn by
increases in nutrients. In the fossil record, increased nutrients can result from increased rainfall, changes in current
patterns so as to increase upwelling, or changes in sea level.
In today’s oceans, of course, increased nutrients are almost
invariably the result of human activity. Although reefs have
faced sediment stress since their inception, there is evidence
that the threat is increasing due to human activities
(Fabricius, 2005). Globally, fluvial sediment delivery to
the world’s oceans is increasing as a result of deforestation
and other land clearing for agriculture, although this
increase is countered in many watersheds by sediment trapping behind dams (Syvitski et al., 2005).
Hallock and Schlager discuss the changes in reef structure and reef sediments caused by increasing nutrients,
and suggest that this process could be linked to the demise
of fossil and modern reefs. Their paper functions as
a framework, against which various hypotheses for reef
decline may be tested. This is also one of the few papers
in the past two decades treating reefs as a whole that gives
proper place to the critical role of bioerosion in the balance
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011