Seawater Properties Station

This is one of the most important pages on this site. Please read it thoroughly.

Seawater properties give us excellent indications of the biological and chemical condition of the ocean. We will collect water samples from a series of water depths (surface, 10 meters, 20 meters, 30 meters, 40 meters, and 50 meters depth) using a Van Dorn Bottle (see pictures below). The seawater properties that we will measure at each depth are:

o temperature

o dissolved oxygen

o salinity

o pH

Each measurement, and its significance for understanding the ocean, is discussed below.

Temperature

Our measurements of water temperature from the ocean surface downward will indicate the strength of the thermocline. This is a key factor in determining the potential primary productivity (phytoplankton growth) in the area.

When there is a weak or absent thermocline (meaning little or no temperature difference between surface water and deep water), it is relatively easy for deep waters to upwell—rise up and mix with surface waters. This is important because deep waters often contain nutrients which are like “fertilizer” for the tiny plant-like phytoplankton. Under these conditions, we expect to see high primary productivity (meaning that the phytoplankton are growing and reproducing in great abundance). We typically see a weak thermocline, and thus high primary productivity, in the winter, and in La Nina years.

In contrast, when there is a strong thermocline (meaning that the water at the surface is much warmer than the water down deep), it becomes difficult for the cold, denser deep water to rise up through the warm, less dense surface water. This means that fewer nutrients can reach the surface for the phytoplankton. Under these conditions, we expect to see moderate to low primary productivity because the phytoplankton are not growing and reproducing that much. We typically see a strong thermocline, and thus moderate to low primary productivity, in the summer, and especially during El Nino years.

Dissolved Oxygen

We will measure the dissolved oxygen content of our water samples using a shipboard version of the Winkler Titration Method—the same method that we used in the classroom during the Seawater Chemistry Lab. The amount dissolved oxygen in seawater is a function of several variables:

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

o Phytoplankton: when phytoplankton are growing and reproducing in abundance, they produce lots of oxygen through photosynthesis, particularly in the euphotic zone (the surface waters where there is enough sunlight for photosynthesis).

o Animal life and/or microbial activity: areas that have large numbers of animals, including zooplankton, and/or areas that have abundant microbes such as oxygen-consuming bacteria will typically have lower dissolved oxygen levels and higher carbon dioxide levels, because these organisms use oxygen and produce carbon dioxide through respiration.

o Polluted and/or and stagnant waters will typically have lower dissolved oxygen levels because of the lower levels of plant and phytoplankton photosynthesis.

Salinity

We measure the salinity of ocean water in parts per thousand (ppt). Typical ocean water salinity is 35 to 36ppt. Salinity in the deeper waters of the ocean is very stable. The salinity of surface waters may vary as a function of:

o Rainfall: fresh water rained onto the ocean surface will decrease the salinity

o River runoff: fresh water running into the ocean from rivers will decrease the salinity

o Evaporation: when ocean water evaporates, the salt stays behind, so evaporation will increase the salinity

pH

The pH scale measures of acidity/alkalinity of the water. Typical seawater pH is about 8.1 on the scale. Seawater is a buffered system. In simple terms, this means that it is self-adjusting so that the pH doesn’t typically vary by much. If the water gets too acidic, it takes up calcium carbonate to make it more basic. If the water gets too basic, it precipitates calcium carbonate to increase the acidity. Therefore, under normal circumstances, we don’t expect pH to vary by much. What does this mean for our purposes? Two things:

o A drastic deviation from normal pH can indicate a severe environmental problem. We are not likely to see a drastic deviation in pH on our trip.

o Minor deviations from normal pH, which we are likely to see, serve as an indicator of the amount of dissolved carbon dioxide in the water. This is useful for us, because carbon dioxide is breathed out by animals, and so pH can provide us with an indirect indication of animal activity. Lower pH means higher levels of dissolved carbon dioxide; higher pH means lower levels of dissolved carbon dioxide.

This sampler is called a Van Dorn Bottle. It takes water samples at set depth intervals. The bottle is held open by a spring latch until it is lowered to the desired depth. A small weight is then dropped down the cable to trip a trigger that closes the bottle. This is how we collect water samples at specific depths.

The Van Dorn Bottle being deployed.

When the bottle is at the desired depth, we send a small weight, called a “messenger,” down the cable to trip the trigger that closes the bottle.

The water sample is then hauled up and transferred to a contained for measurement of temperature, oxygen, salinity, and pH.

CLICK HERE to return to the main STATIONS page.

Modified from pages developed by John Turbeville.