The color of the water is also determined by looking at the secchi disc. If you lower the secchi disc a couple of feet under the water you can see what color the water is on a white background. Since the plankton content of the water determines the cloudiness it is also a large factor in determining the color of the water. Sometimes the water turns a reddish-brown color. This is called red tide, and is caused by a bloom of dynoflagellates. This type of dynoflagellate multiplies quickly when there are a lot of nutrients in the water or when the water is unusually warm, causing the water to become reddish brown. Sometimes when there is a lot of phytoplankton in the water, it looks green because of the chlorophyll contained in phytoplankton. And sometimes the zooplankton turns greenish brown from eating so much plant plankton.