In 2011, 5 percent of tidal waters met goals for water clarity. This was a decrease from 2010, when 18 percent met goals.
Water clarity measures the depth to which light can penetrate into the water. It is routinely hindered by the amount of fine sediment, plankton and other debris suspended in the water. Greater water clarity generally leads to a healthier Bay.
The goal is for 100 percent of the Chesapeake Bay to meet thresholds for water clarity. In general, visibility to a depth greater than 0.65 – 2 meters (depending on salinity of waterbody) during the underwater bay grass growing season is acceptable.
Long-term Trend (1985-2011)
Interannual variation of Secchi depth is high in the Bay but the long-term trend indicates that Bay water has become more turbid (less clear). Goal achievement has averaged 22 percent and has ranged from 5 percent to 41 percent.
Short-term Trend (10-year Trend)
An informal linear trend analysis suggests that the 10-year trend continues to be non-significant, likely due to high interannual variability.
Change from Previous Year (2010-2011)
The water clarity score decreased from 18 percent to 5 percent of goal achieved.
Influences on Secchi Depth
A device called a Secchi disk is used to measure water clarity and the depth to which light penetrates the water column during the bay grass growing season.
As a measure of water clarity, Secchi depth is directly influenced by the amount of suspended matter in the water, including plankton, sediment and detritus. Phytoplankton affect water clarity, and their concentrations are highly dependent on the amount of available nutrients.
River flow also influences Secchi depth. Freshwater inflow is a major factor in the delivery of sediment and nutrients to the Bay and its tributaries.
Secchi Depth Thresholds
The Secchi depth indicator is currently judged by the frequency that Secchi depth exceeds the following thresholds, weighted by Bay segment area:
These thresholds are based on conditions during the underwater bay grass growing season, which is April through October for all salinity regimes except the most saline (polyhaline); in the polyhaline region, the growing season is March through November.
Water Clarity Monitoring
Unfortunately, systematic monitoring of water clarity in shallow water areas has been underway for only the past few years and there are not yet sufficient data to provide a Bay-wide assessment. Thus, water clarity data from deeper, mid-channel areas are used to indicate general conditions and trends.
Note that Secchi depth measurements for this indicator are from fixed stations located in open water areas of the Bay and do not necessarily reflect Secchi depth in shallow water areas where underwater bay grasses are most abundant.