Geoscience Reference
In-Depth Information
agricultural runoff ), the most important nutri-
ents being nitrogen and phosphorus. In coral
reefs, ecological impacts of excess nutrient inputs
include suppression of coral growth rates and
reproductive capacity (Tomascik & Sander 1985,
1987). In many cases, nutrient inputs are coupled
with increased sediment inputs and this com-
plicates isolation of causal mechanisms of dis-
turbance. A possible consequence of nutrification,
however, is to increase the competitive advant-
age of opportunist algal species over corals. This
is significant from a sedimentological perspec-
tive in relation to changing the composition and
abundance of the carbonate-producing com-
munity. The potential impacts of such stresses
are highlighted in studies from the Florida Keys,
where water eutrophication has been cited as a
major cause of temporal change in carbonate
sedimentation (Case Study 9.3).
Available data indicate that the effects of
nutrification may be less significant in mangrove
settings, although this may vary in different
mangrove environments. Mangroves commonly
grow in waterlogged, anaerobic soils, so that
anaerobic reactions exert a major influence on
phosphorus and nitrogen chemistry. These reac-
tions are strongly influenced by redox potential,
which varies depending on the degree of tidal
inundation, sediment porosity and organic matter
content (Clough et al. 1983). Phosphorus occurs
primarily in organic form and, in most man-
grove sediments, low redox potentials lead to
release of phosphate. Therefore, in systems that
receive additional phosphate associated with
nutrient inputs, the ability to immobilize phos-
phorus may be limited. Nitrogen occurs mainly
in organic form (primarily ammonium), and
any nitrate is rapidly converted by anaerobic
bacteria (dentrification) to gaseous nitrogen or
nitrous oxide. Most of this occurs in interstitial
waters within mangrove sediments and is highly
susceptible to leaching by rainwater or during
Case study 9.3 Decadal-scale changes in carbonate sediment compositions across the Florida
reef tract, USA
Coral reef communities along the Florida reef tract have been subject to a range of both natural
and anthropogenic-related disturbance events over the past 40 years. These have included
physical damage from hurricanes and extreme cold-water events, as well as the effects of
coral disease (White Band Disease, Black Band Disease) and mortality of the key herbivorous
echinoid
Diadema antillarum
. In addition, there is evidence that Florida reef-tract waters are
becoming increasingly eutrophic as a result of nutrient inputs and infiltration from ground and
surface waters (Szmant & Forrester 1996). The consequence of this has been a general decline
in reef 'health' as evidenced by widespread reductions in live coral cover and consequent
increases in algal biomass.
Comparisons of sedimentary data collected from sites along the Florida reef tract (Case
Fig. 9.3) in 1963 with that collected in 1989 indicate differences in the abundance of biogenic
sediment constituents that may reflect this progressive decline in coral reef 'health' (Lidz &
Hallock 2000). On a reef-wide scale, the major changes in composition have been a doubling in
the relative abundance of molluscan grains, and a tripling in abundance of coral fragments.
Marked variations occur, however, in different parts of the Keys, reflecting spatial variations in
reef vitality. For example, the proportion of coral grains in Upper Keys sediments (a relatively
healthy section of the reef) is reduced compared with the large increases that have occurred
in the Middle and Lower Keys (where coral communities are in decline) (Case Fig. 9.3b). There
is little evidence to suggest that these changes reflect periodic storm/hurricane reworking of
sediment, but rather are attributed to increased rates of dead coral substrate erosion by internal
