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?