The visibility of the water is a measure of how clear or cloudy the water is. If the visibility is noted at ten feet it means that if the water were ten feet deep you would be able to see the bottom. We use a secchi disc to measure visibility. The secchi disc is a round, white disc made of plastic that is weighted and attached to a rope marked with measurements.
Visibility is determined by lowering the disc into the water until it disappears from sight. The visibility is then the measurement from the disk to the surface of the water.
The secchi disc can only be used properly on a very calm day when the boat can be held still enough to get an accurate measurement.
Surface tow on a sunny day with no wind = more phytoplankton, surface tow on a windy or turbulent day = more zooplankton
There is so much plankton in the water that it makes the water cloudy.
In these places, the water is warm and most plankton like to live in cold water, therefore there is not much plankton there.
Some animals, like whales and dolphins, use echolocation to find their way around in dark and cloudy conditions.