Water Clarity

Just like plants on land, aquatic plants need sunlight for growth. In lakes, the amount of sunlight that reaches underwater plants depends on the clearness of the water, or water clarity. Water clarity is very important in aquatic habitats. We all know that plants will die if they cannot get enough sunlight. And fewer plants mean less food for many animals. Fish, crabs, ducks and geese find food and protection in beds of underwater plants. Also, because plants produce oxygen, fewer plants mean that there is less oxygen in the water.

What Causes Cloudy Water?

When water is cloudy, it is called turbid. Turbid water, or turbidity, results when sediment (soil particles) and other materials are stirred up in the water. Sediment in water can become stirred up for many reasons.

  • When loose soil from a construction site or eroding shoreline washes into a lake or river, sediment increases.
  • Rain, wind, waves, animals and some human activities can also stir up particles floating in water. Turbidity is especially high just after a storm and near streams or rivers entering a lake.
  • Runoff can also make water cloudy. Runoff is water that drains into lakes and rivers because it was not absorbed on land by soil or plants. For example, in big cities with a lot of pavement and buildings, these hard surfaces cannot absorb water. When it rains, much of this water drains into lakes and rivers. This runoff collects sediment and sometimes pollutants along the way.
  • Runoff can also carry excess nutrients that promote the growth of a certain type of algae. Planktonic algae are small, often microscopic aquatic plants that float in or on the water. (They make up the base of the aquatic food chain.) Too much of this algae clouds the water and makes it “pea soup green.

Effects of Cloudy (turbid) Water

Turbidity blocks the sunlight that plants need to produce oxygen for fish and other aquatic life. Also, too much sediment or other particles floating in the water absorb heat from sunlight. This warms the water and decreases dissolved oxygen even further. Fish, clams, zebra mussels, and other gilled creatures suffocate under conditions of low dissolved oxygen levels and when their gills are clogged by sediment.

Some turbidity in aquatic habitats is essential. For example, tiny aquatic creatures called phytoplankton and zooplankton float in the water column and are important for the food web. In the Great Lakes, some people believe that there is too much water clarity because zebra mussels have filtered, or eaten, too much plankton. This has possibly caused several problems. One effect is an increase in rooted aquatic plants, since sunlight can now penetrate to greater depths. What do you think?

Measuring Water Clarity

To measure water clarity, scientists use a secchi disk—an instrument that looks like a round plate attached in the center to a rope or line. The line contains measurement markings. To measure water clarity, follow these steps:

secchi_disk-lg

Step 1:
From a boat or dock, lower the secchi disk into the water until it disappears.

Step 2:
Raise the secchi disk very slowly until you can see it again.

Step 3:
Notice where the water’s surface intersects the marked line.

Step 4:
Pull the secchi disk in, holding the line at this intersection point.

Step 5:
Measure the distance from the disk to the point where the water intersected the line by counting the distance markings.

The resulting measurement is the depth of water clarity.

Water Color

Water color is greatly affected by turbidity. The type of suspended solids in the water often gives the water its color. Silt and runoff from the land may make water look brown. This is especially true just after a storm, and near streams or rivers entering a lake. Aquatic plants, especially algae, can also affect water color. Water that is “pea soup green” usually contains high populations of microscopic, planktonic algae.

When water is absolutely pure and contains no suspended solids, its clarity is maximized. In pure water, water clarity would be limited only by the ability of the water itself to absorb light. Light wavelengths (especially the blue wavelength) could penetrate to about 300 feet, and the water would appear blue, the last color of the light spectrum to be absorbed by water.

Perhaps most interesting is the fact that water’s own physical characteristics affect its color. When light penetrates water, not all colors of the spectrum are absorbed at the same rate. Warmer colors (reds) are absorbed first (including infrared, which is why surface waters warm more quickly than deep waters), followed by orange, yellow, green, and blue. Once the blue has been absorbed, there is nothing but blackness beneath.

Many open waters have a characteristic blue color. This is largely because the water column has not absorbed the blue part of the color spectrum. The blue is therefore the only color available for reflecting back to the eye of the beholder.