Seawater Properties

This is one of the most important pages on this site please read through it thoroughly. Seawater properties can define and determine the overall biologic health of an area. Common seawater properties are measured at 10 meter intervals to a depth of 50m on the trip. These include: temperature, dissolved oxygen, salinity, and pH. Each parameter is discussed below.

Temperature

These measurements define the strength of the thermocline present. This is a key factor in determining the potential productivity in an area. When a weak thermocline (cold deep and cool on top) is present as we have here in the winter it is easy to get upwelling of cold nutrient rich water into the photic zone. However, when we have a strong thermocline (cold on bottom and warm/very buoyant on top) as in the summer or in an El Nino year it is very difficult for the cold nutrient rich water to penetrate the warm buoyant surface water.

o Strong thermocline, low nutrients in photic zone, very low productivity.

o Weak thermocline, potentially high nutrients in photic zone and high productivity.

Dissolved Oxygen

Dissolved oxygen is measured onboard using the Winkler Titration Method. This is the same method we use in the classroom during the Seawater Chemistry Lab. The maximum dissolved oxygen contained in seawater has many different variables.

o Water temperature: cold water can hold much more dissolved oxygen than warm water.

o Pollutants and stagnant areas: can have lower dissolved oxygen (and carbon dioxide) levels.

o Phytoplankton: highly productive (from phytoplankton) areas can have high oxygen levels and lower carbon dioxide levels.

o Zooplankton: areas that have large numbers of zooplankton can have lower dissolved oxygen levels and higher carbon dioxide levels.

Salinity

Salinity in the deep ocean is very stable. The variation of salinity mainly occurs near the ocean surface because of a variety of mechanisms. The most common things that cause near surface variations in salinity along our coastline are:

o Rainfall

o Runoff

o Evaporation

pH

The pH of seawater is a buffered system. In simple terms, this means that it is self-adjusting. As it gets too acidic, we uptake calcium carbonate to make it more basic. As it gets too basic, it precipitates calcium carbonate to increase the acidity. What does this mean? The pH of seawater stays very near 8+/- or slightly basic on a scale from 1-14. A drastic change from it being slightly basic can indicate a severe environmental problem. It can also serve as an indicator of carbon dioxide with which it has an inverse relationship. As pH goes up, carbon dioxide goes down.

This sampler is called a Van Dorn Bottle. It is capable of taking discreet water samples at set intervals. It is held open by a spring latch and closed shut when a small weight is thrown down the cable when the bottle is at the prescribed depth.

The Van Dorn Bottle being deployed.

This is the small weight that is dropped down the cable when the bottle is at its prescribed depth.

The water is drained out of a port on the side of the bottle.

These are the onboard steps to determine the dissolved oxygen using the Winkler Titration Method onboard the boat.

First the sample is fixed so that the concentration of dissolved oxygen will not change.

Then the sample is titrated.

Determining the pH of the sample.

Determining the density/salinity of the sample.

Taking the temperature of the sample.