WHAT IS IT
The Seabird or CTD (Conductivity-Temperature-Depth) consists of a stainless steel cage that holds a bevy of sensors which record a variety of information onto the systems 8MB memory chip. The CTD weighs about 50 pounds and costs roughly $25,000.
HOW IS IT USED
Originally designed for Oceanographers, the CTD measures the three major ingredients required to understand water circulation (physics):
Conductivity, a measure of dissolved salts (related to salinity), temperature, the heat content of water, and depth, where in vertical space the instrument is.
The CTD can also measure other factors such as transmission, a measure of cloudiness, fluorescence, an approximation for algae, visible light and dissolved oxygen.
The CTD is lowered over the side of the ship via a winch and is dropped to varying depths. As it descends it takes measurements of the above factors at eight times per second.
If we lower the CTD at 5m/minute, we get a sample every 10cm. Each of these tell us one part of the character of the water under study.
As soon as it is up it is brought on deck, connected to the computer, and the data downloaded.
WHY IS IT USED
Examples of temperature data collected from the CTD throughout the year are displayed at left. Temperature is the primary controller of density in freshwaters like Lake Michigan.
Density is the reason bodies of water like Lake Michigan don't freeze all the way to the bottom. Because water at the freezing temperature is actually less dense than just above the freezing temperature, it's lighter and the frozen water turning to ice goes to the top.
So below the ice there is heavier cold water that stays there unless it's ready to freeze. At which time it would float to the surface.