Yet, from 1973 to 1980, there was very little change. Clarity dropped from 1980 to 1985 and then was relatively unchanged for the next five years. If we had measurements from just one of those periods, we might have concluded that there is no change in clarity or that clarity is decreasing much faster than it actually is. Without the long term record we now have, we really wouldn't know how the lake is doing. As algal productivity increases, clarity decreases.
Algal productivity is measured by noting how fast the lake's algae sop up carbon.
That rate is controlled by the supply of nutrients, mainly nitrogen, phosphorous and iron, available to stimulate growth. The water quality report shows that algal productivity has almost quadrupled since 1968. Algal productivity also varies from year to year, depending upon how much nutrient gets to the layer in the lake where algae grow. There are several sources of those nutrients. Much of the nitrogen is carried by winds from the Central Valley.