Light Penetration refers to the amount of sunlight that penetrates the water column and reaches various depths. Sunlight is absorbed and scattered by suspended particles, dissolved substances, and the water itself. We monitor light penetration to determine if it is sufficient to support phytoplankton photosynthesis. The attenuation coefficient describes how rapidly the light penetration decreases with depth, for example a high attenuation coefficient represents a rapid decrease of light with depth, resulting in a shallow photic zone. This 20-year series at USGS station 30 shows that the attenuation coefficient in South Bay is extremely variable. Much of this variability is caused by the suspension of bottom sediment particles during periods of strong tidal currents and storm-induced wind waves . During periods of weak tides and calm winds, the particles sink to the bottom and the attenuation coefficient becomes smaller (light penetration increases). Note: No light data from 2002-2006.

Plot Description:
1. The minimum attenuation coefficient was 0.7, which corresponds to a light penetration depth of almost 7 meters. This condition of deep light penetration is optimal for plant photosynthesis.

2. This record from 1982 shows the extreme variability of the attenuation coefficient, which changes with the daily tidal cycle, the neap-spring tidal cycle, and the speed and direction of the wind.

3. The maximum attenuation coefficient was 5, which corresponds to a light penetration depth less than 1 meter. This condition of extreme turbidity limits the light availability and photosynthesis of phytoplankton.