Environmental Factors Driving The Development Of Coral Reefs

Coral reefs grow in shallow, sunlit waters of tropical and subtropical oceans and are built from the activities of many types of calcifying organisms. Globally, coral reefs occupy 284 300 square kilometres, which is less than 1% of the global ocean. They dot oceans and line coasts within a band between 30° north and south of the equator. In Australia, coral reefs stretch northwards along continental shelves, from northern NSW on the east coast, from Rottnest Island in the south-west to the north-west coast of Australia and along the western edge of the Gulf of Carpentaria. Throughout this range there is a tremendous variability in the structure of coral reefs. Coral reefs range from poorly developed reefs that fringe continental islands and the Australian coastline to extensive carbonate barrier reefs. At latitudes from 22-30°S, extensive coral reefs form but fail to accumulate limestone (i.e. reef erosion exceeds calcification) and are referred to as non-carbonate reef systems (see Chapter 2).

Central to the existence of coral reefs are reef-building corals (Order Scleractinia, Class Anthozoa, Phylum Cnidaria, see Chapter 20). Reef-building corals are often referred to as the framework builders of coral reefs, but they are not the only calcifiers on coral reefs. Others organisms such as giant clams, foraminifera and calcareous algae contribute substantial quantities of calcium carbonate to reef structures and sediment. Calcareous algae (see Chapter 15) play a particularly important role in reef frameworks, some by coating and 'gluing' the coral framework together, and others by building thick edifices of their own. The framework provided by corals and other marine calcifiers forms the three-dimensional structure into which hundreds of thousands of species of animal, plant, fungi and bacteria live.

Several authors have explored the conditions under which coral reefs thrive globally. Explanations have frequently focused on the fact that coral reefs form in warm seas, leading to the 'rule of thumb' that coral reefs are limited to waters that do not decrease below 18°C in the winter. This principal is often incorrectly perceived as the ultimate limit to the development of coral reefs that ignores the many other variables that also change at higher latitudes. While reefs are adapted to their local temperature regime, the amount of light and the concentration of carbonate ions are at least as important in limiting reef development. An exhaustive study of the environmental factors associated with coral reefs using data from close to 1000 coral reef locations found that light availability and the concentration of carbonate ions (that is ultimately determined by temperature) are as potentially important as temperature in defining where limestone coral reefs are found (Table 7.1).

The conditions that are associated with the distribution of coral reefs vary across spatial and temporal scales. Variability across the year can be substantial, while diurnal variability in temperature is usually small (except in areas such as shallow intertidal reef crests). The seasonal variability of conditions becomes important on coral reefs at high latitudes where extremes in both winter (that can be too cold and dark) and summer (too hot and bright) can cause stress on coral reef organisms. At these sites in Australia, inter-annual variability such as that associated with the El Niño cycle along the east coast of Australia can play a large influence on coral reefs through warm (e.g. 2002) and colder years (e.g. 2003). In coming decades, due to rising background sea temperatures, warmer years can exceed the tolerance of symbiotic organisms giving rise to mass coral bleaching and associated mortality events (see Chapter 10).

The global distribution of carbonate and noncar-bonate coral reefs is strongly (and perhaps not surprisingly) correlated with the concentration of carbonate ions, which is ultimately determined by ocean temperature, salinity, and factors such as the atmospheric carbon dioxide concentration (see Chapter 10). The concentration of carbonate ions is highest at the equator and decreases at high latitudes due to the effect of sea temperature on the solubility of CO2 (see Chapter 10). Coral reefs do not exist at carbonate concentrations below 200 ^mol kg"1; this has significance in terms of the problem of ocean acidification for coral reefs as discussed in Chapter 10.

The last variable that is a major determinant of where coral reefs are found is light. Because of the dependence of primary production and calcification on light, coral reefs are limited to clear tropical and subtropical waters where depths are less than 100 m. Both light quantity and quality (wavelength) are important, driving the primary step of photosynthesis of the symbionts within corals and many other photosynthetic organisms. Coral reefs only form where the average irradiance is at least 250 ^mol m2 s 1 (roughly 10% of surface irradiances in tropical and sub-tropical regions).

Several variables affect the light available for coral reefs. Light enters the outer atmosphere of the Earth (Fig. 7.1) and is selectively filtered such that some wavelengths (ultraviolet, infrared) are largely removed by the ozone layer and water vapour (e.g. clouds). The penetration of Photosynthetically Active Radiation (PAR, 400-700 nm) is also reduced by dust and clouds. Then, at the surface of the ocean, more light is reflected,

Table 7.1 Environmental factors identified by Kleypas et al. (1999) associated with more than 1000 reef locations worldwide

Variable

Minimum

Maximum

Mean

Standard Deviation

Temperature (°C); based on NOAA AVHRR-based

sea temperature records

average

21

29.5

27.6

1.1

minimum

16

28.2

24.8

1.8

maximum

24.7

34.4

30.2

0.6

Salinity (ppt)

minimum

23.3

40

34.3

1.2

maximum

31.2

41.8

35.3

0.9

Nutrients (^mol L 1)

NO3

0

3.34

0.25

0.28

PO4

0

0.54

0.13

0.08

Aragonite saturation (ft-arag)

average

3.28

4.06

3.83

0.09

Maximum depth of light penetration (m) calculated from the monthly average depth at which average light decreased below the perceived minimum for reef development of 250 ^mol m-2 s-1.

average

-9

-81

-53

13.5

minimum

-7

-72

-40

13.5

maximum

-10

-91

-65

13.4

with the amount reflected decreasing as the height of the waves increases. The light that enters the ocean is absorbed by water molecules, or is scattered and absorbed by dissolved compounds, plankton, and suspended sediments. These interactions are wavelength dependent such that its spectral breadth and intensity decrease with depth. In a uniform water column, the intensity of light decreases exponentially as described by Beer's Law (Fig. 7.1).

Different water columns vary with respect to the amounts of dissolved substances and particles they contain. Inshore sea waters, that receive fresh waters from rivers and land runoff, often have large quantities of sediments, tannins and phytoplankton (often referred to as 'Gelbstoff' or yellow substances). These waters have spectra that are green-yellow shifted and light attenuation coefficients (k, Fig. 7.1) that may range up to

0.5 m-1, which means that it gets very dim at quite shallow depths. Waters that are offshore or are located away from rivers, have far less scattering and absorption. These waters may have light attenuation coefficients as low 0.01 m_1. The effect of these differences in light at depth is substantial, causing the depth limits of corals in typical inshore regions to be around 5 m as compared with offshore sites where the depth limits may be in excess of 50 m. Light at depth in offshore sites is blue shifted due to a relatively small influence of Gelbstoff substances and the greater relative influence of the water itself as a source of scattering and absorption. One of the key reasons why coral reefs are not found near major rivers is because the light environment deteriorates due to heavy sediments blocking light transmission through the water column, in addition to the problems of the low availability of stable

Figure 7.1 Depiction of the pathway for solar radiation entering the ocean from the outer atmosphere. Radiation interacts with atmosphere components such as clouds, dust and specific gases such as carbon dioxide and water vapour. Light is reflected as it crosses the surface interface of the ocean. Once it has entered the water it is scattered and absorbed by water and components such as suspended sediments. Beer's Law relates the passage of light through the water column to the amount of suspended material in the water column. Io, intensity of light at the surface, while Id is the intensity at depth, d. The attenuation Coefficient, k, is a measure of the extent to which light is absorbed and scattered by the water column and its internal constituents. (Figure: D. Kleine and O. Hoegh-Guldberg.)

Figure 7.1 Depiction of the pathway for solar radiation entering the ocean from the outer atmosphere. Radiation interacts with atmosphere components such as clouds, dust and specific gases such as carbon dioxide and water vapour. Light is reflected as it crosses the surface interface of the ocean. Once it has entered the water it is scattered and absorbed by water and components such as suspended sediments. Beer's Law relates the passage of light through the water column to the amount of suspended material in the water column. Io, intensity of light at the surface, while Id is the intensity at depth, d. The attenuation Coefficient, k, is a measure of the extent to which light is absorbed and scattered by the water column and its internal constituents. (Figure: D. Kleine and O. Hoegh-Guldberg.)

clean substrata for corals to settle and grow due to high rates of sedimentation.

The light environment of coral reefs also varies temporally. Seasonal changes in solar flux are significant at higher latitudes, and hourly light environments at any single point on a reef vary according to the angle of the sun. Coral reefs growing close to islands or patches of coral reef growing in crevasses experience light environments that can fluctuate considerably. Tidal variations in depth can also have significant influences on the short term light environment as will variations in cloud cover over periods of minutes. At even finer time scales, effects such as the focusing of light through the lens effect of wave surfaces (sub flecks) can increase the intensity of light over very short (millisecond) time scales.

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