EarthLabs > Corals > Lab 4: Finding Coral's Ideal Environment > 4C: Ocean Chemistry

Finding Coral's Ideal Environment

Part C: Ocean Chemistry

As you might suspect, ocean chemistry also plays a role in determining where coral reefs are found throughout the ocean. Both the amount of salt (salinity) and the the aragonite saturation (availability of calcium carbonate) in the ocean influence coral's ability to grow and thrive.


The water in the oceans is essentially a weak solution of almost everything. More than 70 chemical elements have been identified in sea water, generally in very small amounts. The most abundant salts in the ocean are chlorine, sodium, magnesium, sulfur, calcium, and potassium. Ocean salinity is affected by multiple factors including ice melt, inflow of river water, evaporation, precipitation, wind, wave motion, and ocean currents.

Examine the salinity map below. Click anywhere on the image to open a larger view in a new window. Click on the image again to further enlarge the image.

  • The units of salinity on the map are given in PSS from the "Practical Salinity Scale". These units are typically used to measure electrical conductivity. In terms of salt concentration, 1 PSS unit = 1 g sea salt/kg seawater = one part per thousand (ppt).

Average global salinity in units of the Practical Salinity Scale (PSS) 1 PSS is equivalent to 1 ppt. Image courtesy of NASA. Click image for larger view.

Checking In

Answer the following question to check your understanding of the salinity map.
  • On average, is the salinity in the coral reef region higher or lower than the salinity in other parts of the ocean?
    Higherin general, the ocean is less salty at higher latitudes. On average, the salinity values in the coral reef region are above 35 PSS, and below 35 PSS elsewhere.

Aragonite Saturation

The map below shows the current state of aragonite (calcium carbonate) saturation in the world's oceans. Saturation is the point at which a solution can dissolve no more of a particular substance and additional amounts of that substance will appear as a precipitate. However, you can sometimes get more solute to dissolve if you change the conditions (e.g. increase the temperature of the solution). This type of solution is called supersaturated. Saturation level is represented by the Greek letter omega (Ω). When Ω = 1, the solution is saturated, and when Ω >1, the solution is supersaturated, meaning that there is more calcium carbonate dissolved in the water than is possible under normal conditions. The higher Ω is, the more likely precipitation (i.e., reef formation) is to take place.

Figure adapted from Hoegh-Guldberg et al., Coral Reefs Under Rapid Climate Change and Ocean Acidification, Science, 14 December 2007: Vol. 318. no. 5857, pp. 1737 - 1742. The 380 in the upper left corner indicates the atmospheric CO2 concentration in parts per million (ppm). Coral reef locations are indicated by pink dots.
Conditions for coral reef growth:

  • Ω > 4.0: optimal
  • 3.5 < Ω < 4.0: adequate
  • 3.0 < Ω < 3.5: marginal
  • Ω < 3.0: poor

Checking In

Answer the following questions to check your understanding of the aragonite saturation map.

  • In what regions of the ocean are Ω values highest? Is this consistent with the locations of the majority of coral reefs?
    Ω values are highest at low latitudes (straddling the equator). The majority of this region has Ω around 4, with a few pockets having Ω > 4.0. The waters along the western coast of South America are the only exception, having Ω values around 3.0. The locations of the pink dots representing coral reef locations are consistently found in the regions with higher Ω values.
  • What appears to be the approximate lower-limit of Ω where coral reefs are found?
    Ω 3.0

Stop and Think

1: Describe the ideal ocean environment for coral reefs.

2: Which of the ocean characteristics covered in this lab (temperature, depth, salinity, and aragonite saturation) do you think coral reefs are most sensitive to? Justify your answer.