Geology Reference
In-Depth Information
reef systems at Highborne Cay and Stocking
Island provide a basis for exploring controls on
modern and Late Holocene reef development.
Furthermore, dates in both reef systems provide
constraints on the timing of microbial versus
metazoan reef development.
Ooid-dominated, highly mobile sand bars
frequently move over the reef crest and reef fl at,
through cuts and channels, fi lling the back-reef
lagoon and occasionally covering the reef fl at and
crest (Fig. 5). Whereas burial on the reef crest and
reef fl at is generally limited to days up to a few
weeks, month-long burial in the back-reef lagoon
can be observed.
Not only burial but also sand abrasion affects
the reef habitat. Although sand-sized sediments
settle quickly, constant breaking waves and subse-
quent shoreward surge, as well as swift longshore
currents keep sediment in suspension, maintain-
ing an abrading capacity. Evidence for sediment
abrasion of the fore-reef area, which, as discussed,
is characterized by relict-looking columns that
tower up to 2 m high and are composed of hard
coralline algae in both Highborne (Fig. 3e) and
Stocking Island (see Fig. 3e in Macintyre
et al.
,
1996) reefs. Noteworthy, thriving circular coral-
dominated patch reefs grow just adjacent to this
pinnacle locale at Highborne Cay (Fig. 3a). Their
distinct mushroom-shape and smooth under-
cut base indicate abrasion today. Furthermore,
also at Highborne Cay, the basal surface in the
back-reef lagoon today is a smooth, well-abraded
surface; all larger pebbles and small boulders
found on the basal surface are well-rounded;
rounding cannot be explained by currents as
these are too weak to move boulders of such size
(Fig. 6a). The patch reefs south of Site 1 (Fig. 3a)
exhibit a thriving, healthy coral community on the
outer, ocean-facing sides; in contrast, the north-
ward and shoreward facing sides of the patch reef
(Fig. 3h) are stressed. Frequent sand incursions,
burial and abrasion have decreased coral cover;
most corals are ghostly white due to the loss of
zooxanthellae, some are abraded (Fig. 3g).
It is acknowledged that factors other than
sedimentation, such as temperature, salinity,
nutrients and carbonate saturation state can be
fundamental to reef development and demise
(Camoin
et al.
, 1997; Montaggioni & Faure, 1997;
Kleypas
et al.
, 1999). High nutrient concentra-
tions, in particular, are invoked for favouring
pervasive microbialite development in the lagoon
of Tikehau atoll (Sprachta
et al.
, 2001). However,
as pointed out by these authors, Bahamian stroma-
tolites are an exception, and form in nutrient-poor
water (Sprachta
et al.
, 2001). Nutrient-limited
or oligotrophic conditions are reported for both
Highborne Cay and Stocking Island (Pinckney
et al.
, 1995a,b; Paerl
et al.
, 2001). Lush patch
reefs thrive a few tens of metres to the west of
Modern reef development, Stocking Island and
Highborne Cay
Sediment and sediment dynamics are currently
the key factors governing the distribution of
microbial versus metazoan reef builders in the
Highborne Cay and Stocking Island reef systems.
A study investigating the processes that control
the distribution of macroscale growth morpholo-
gies of stromatolites (i.e. columns vs. ridges) in
the present Highborne Cay reef, identifi ed accom-
modation space, hydrodynamics and sedimenta-
tion patterns as the primary controls (Andres &
Reid, 2006). Whereas accommodation space is
important for all phototropic reef-builders in this
reef (stromatolites, coralline algae and corals), the
optimal hydrodynamic conditions and intricately
linked sedimentary patterns are different for each
of these reef builders.
The hydrodynamic setting at both Stocking
Island and Highborne Cay is dominated by
waves breaking at the reef crest and rolling over
the reef fl at. Wave height is dependent on wind
direction, speed and tide; large waves (>1 m) are
generated in association with frontal systems
and hurricanes. Maximum wave speeds of up
to 2 m s
1
over the reef fl at and lagoon are not
unusual, and are accompanied by strong long-
shore currents.
Thriving along the reef crest today, the branch-
ing coralline (
N. strictum)
defi nes the seaward reef
crest, coping with wave energy, hour-long emer-
gence during low tide and sediment abrasion and
rare burial (Fig. 3f). We have observed bleaching
events, mostly during the winter season, where
the normally red to purple coralline algae heads
lose their colour, but to date bleached coralline
algae have recovered their pink colour within a
few months. Sediment is known to negatively
affect coral reefs, due to burial, abrasion and
increase in turbidity (Hubbard, 1986). The well-
sorted sand-sized sediment settles out quickly
once stirred up by waves and/or strong currents.
Visibility decreases during rough ocean condi-
tions; however, due to a lack of any signifi cant
mud, turbidity is not sustained for long after the
calming of wind and waves.
