6.   The Reef Enviroment:  Light, Depth and Salinity


The Environment of Reef

1. Nutrient Supply:  We can classify the environments of the ocean in terms of the availability of nutrients.  These are:

A. Oligotrophic Environment:  Nutrient poor environment.  This is characteristic of the shallow surface waters in area where most reefs develop.  Because of the low abundance of natural food sources (phytoplankton, particulate organic material etc.), coral must develop a lifestyle which is able to utilize all resources that are available.  This includes mechanisms of recycling waste products which result from the respiration (burning of food) in the coral tissues.  In the case of hermatypic corals (those that possess zooxanthellae symbionts), waste products of the coral are used by the yellow brown algal symbiont to synthesize new carbohydrates (vegetable tissue).  In doing this, the uptake of CO2 by the algae aids the corals in precipitating their calcium carbonate skeleton, while also providing an internal source of foodstuff.  In the case of the algae, the coral provides a site where they are protected from normal predation and grazing by other organisms.  Also, because the coral is continously trying to grow upwards into shallower water, this provides a setting where the algae are always in an environment possessing a lot of light.

B.  Eutrophic Environments:  These are nutrient rich environments.  In areas that contain an abundance of nutrients, organisms (plants primarily) rapidly reproduce to use these available resources.   As a result, one finds that eutrophic setting are dominated by phytoplankton production.   On the surface of things, this would appear to be beneficial for the coral reef community; however, as the abundance of phytoplankton increases in the water column, the amount of light that is able to penetrate to depth in the ocean decreases.  Thus, eutrophic settings are not condusive to the proliferation of coral reefs systems.

The Environmental Controls on Reef Growth

There are three features of the environment that can be considered as Limiting Factors for reef development.  These are Light (Depth), Salinity , and Temperature.

1.  Temperature :  When on thinks about visiting a reef, you envision a trip to a warm tropical setting.   In fact, hermatypic corals can only grown in areas where the temperature of the water remains fairly constant and within strict limits.  Reefs will not develop in areas where the mean annual temperature falls below 22 degress C.   Moreover, if the temperature frequently falls below 18 degrees C, or above 32 degrees C, the coral reef system can be destroyed.  Ideal temperature of this range commonly occur between 30 degrees north and south of the equator (the subtropics).

2. Light:  In hermatypic corals (those possessing symbionts), light is essential for photosynthesis of these algae.  Therefore, corals must grow within the photic zone of the ocean (depth to which light will penetrate).  Clearly, as the depth of water increases, so does the amount of light decrease.  Corals thrive in shallow water where the intensity of light is the strongest, but can exist to greater depths.  Typically, massive reefs form to depths of  30 meters (100 ft), but some reef growth can still occur as long as light is present.  Thus, deep reefs can be found at depths greater than 150ft, but these are generally dominated by non-photosynthetic or red-algal constituents.

3. Salinity:  Salinity, or the saltiness of the ocean also controls the distribution of coral reefs. We describe the salinity of water interms of a the fraction of the total weight of a volume of liquid that is made up of salts.  For example, let's take a barrel of seawater that contains 1000 lbs of liquid plus dissolved salts.  When we evaporate the water, we are left with aboutn 34 lbs of salt.  Therefore the weight fraction of salt in this liquid (salt/total weight) is 34/1000 or 0.034.  For convienence, we multiply this by 1000 to get a term that we call "parts per thousand" or ppt.   In the case of this example, we would define the salinity as 34 ppt or 34 parts per thousand salt.
    Normal salinity for the ocean is about 34 to 35 ppt.  Corals can live in waters with less salt, down to 28ppt (fresher waters) or in saltier water up to about 38ppt.  This means that in areas close to where rivers drain from the land, the salinity of the water can fall too low for corals to develop.  Similarly, in areas of the ocean where evaporation makes the water particularly salty, (shallow bay that does not easily mix with the ocean), corals also have a difficult time developing.

*Note that coral reefs comprising ahermatypic corals (those without symbionts) can be found in very deep waters of the ocean where there is little or no light, and at temperatures as low as 4 degrees (burrrrr) C.