Geoscience Reference
In-Depth Information
Table 9.1
Marine environmental parameters that influence the distribution of hermatypic corals and of tropical coral reef development.
'Optimal' values for coral growth are shown, along with recorded upper and lower environmental limits. Figures in parentheses are for
non-reef building coral communities. (Data from Kleypas et al. 1999.)
Environmental parameter
'Optimal' levels
Environmental limits
Lower
Upper
Temperature (°C)*
21.0-29.5
16.0 (13.9)
34.4 (32.1)
Salinity (PSU)†
34.3-35.3
23.3 (20.7)
41.8 (No data)
mol L
−1
)‡
Nitrate (
μ
<
2.0
0.00
3.34 (up to 5.61)
mol L
−1
)‡
Phosphate (
μ
<
0.2
0.00
0.40 (up to 0.54)
-arag)
§
Aragonite saturation state (
Ω
c
. 3.83
3.28 (3.06)
No data
Depth of light penetration (m)
c
.50
<
10
c
.90
*Weekly data.
†Monthly average data.
‡Overall averages (1900-1999).
§Overall averages (1972-1978).
coral calcification, light is a key control on coral
growth. Light decreases with depth, so that rates
of coral growth and calcification also decrease
(Huston 1985). The lower limit of hermatypic
coral growth is defined as the base of the photic
zone (where surface light levels are reduced to
1%; Fig. 9.3a). In clear-water settings this can be
as deep as around 90 m (Table 9.1), but occurs
at much shallower depths in turbid environments
due to reduced light penetration. Finally, elevated
nutrient levels (nitrate levels
them (Fig. 9.4). Mangrove ecosystems extend
along some 60 -75% of tropical and subtropical
coastlines (MacGill 1958) and recent mapping
estimates suggest a global coverage of around
190,000 km
2
(Spalding et al. 1997).
Opinions vary about the environmental factors
limiting mangrove development, with both mean
minimum air and sea temperatures having been
cited as controls (Woodroffe & Grindrod 1991).
Some authors also cite the occurrence of extreme
seasonal cold (frost) events (Plaziat 1995). At
the global scale, mangrove distributions exhibit
a reasonably close correlation with the mean
winter 15°C sea-surface isotherm (Woodroffe &
Grindrod 1991), which equates to a latitudinal
range between about 30°N and 30°S (Fig. 9.1).
Actual distributional patterns, however, are vari-
able and reflect local environmental (particularly
cool seasonal temperature) constraints, such that
(as with coral reefs) their distribution is more
restricted on the western coasts of Africa and
America (Plaziat 1995; Fig. 9.1).
Such climatic and/or oceanographic constraints
are exacerbated by physical constraints, such as
local geomorphology, tidal range, seasonal hydro-
logy and substrate availability for colonization
(Fig. 9.5). These factors influence mangrove
development both at local and regional scales.
At the local scale marked physical-chemical
gradients across shorelines are determined by the
relative importance of tidal against freshwater
mol L
−1
;
>
2.0
μ
phosphate levels
mol L
−1
) may result
in reduced rates of coral growth (Tomascik &
Sander 1985).
>
0.20
μ
9.1.2 Distribution and occurrence of mangroves
The term mangrove is variously defined and
has been used to refer either to the constituent
plants of these tropical intertidal forests or to
both the community and its associated envir-
onment (Tomlinson 1986). The main defining
characteristics of mangroves are that they com-
prise communities of salt tolerant tropical/sub-
tropical trees and shrubs, and as such represent
tropical equivalents of temperate salt-marsh
communities (Woodroffe 1983). In this chapter,
mangroves are discussed not only in the con-
text of the constituent trees and shrubs, but also
the sediment substrates on which the mangroves
develop and the creek networks that dissect
