A CTD is often equipped with Niskin Bottles, which are triggered to close at specific depths. This water is then chemically analyzed in a laboratory to determine its constituents. Specifically, researchers are interested in the concentration of nutrients at each depth. Nutrients include compounds which contain nitrogen and phosphorous, and they are very important to phytoplankton and biological productivity in general. Areas with depleted nutrient concentrations near the surface indicate that large amounts of phytoplankton might be growing.

Most plants in the world produce chlorophyll, a compound which enables them to produce energy from the sunlight in a chemical reaction termed photosynthesis. Phytoplankton are the plants of the sea, and also utilize chlorophyll. Biological Oceanographers are interested in locating regions with high amounts of chlorophyll, as it implies that biological productivity is high. High abundances of phytoplankton can cascade up the food chain and possibly support high stocks of salmon and other commercially important fish. Many CTDs are outfitted with a fluorometer, which can measure the concentration of chlorophyll. Chlorophyll is usually concentrated near the sea surface as phytoplankton need sunlight to survive.