5.7 Classifying Elements in Seawater

Now that we are familiar with the abundance and profiles of some common dissolved substances in the ocean, we can try to classify these materials based on their distribution. Dissolved materials are usually classified into one of several groups:

Conservative Elements are those whose concentration is relatively constant across the ocean, both vertically and horizontally. This category would include the major ions in seawater, such as sodium or chloride, which have very long residence times and whose concentration only changes through the addition or removal of fresh water (see section 5.3).

Nutrient-like Elements have a distribution similar to the profile we described for nitrate in the previous section. Concentrations of these substances are low at the surface, as they are rapidly used up by biological processes. Below the photic zone, concentrations of these materials will increase, as decomposition by bacteria cycles them back into the water column.

Scavenged Elements are those that react with other particles and are adsorbed to the particle surface. When the particles sink, those elements are removed to the sediment. Scavenged profiles generally show higher abundance at the surface, where the materials will enter the ocean, but declining levels with depth as they are removed by the sinking particles. This profile is common for metals, such as aluminum or lead.

Stable Gases dissolve into the ocean from the atmosphere. Because these substances are not very reactive, the ocean water becomes saturated with these gases. Since solubility of a gas increases in colder water, these gases are often found in greater concentrations in deep, cold water, and in lower concentrations in warmer surface water.