Water clarity is a measure of how much light penetrates though the water column. Water clarity is dependent upon the amount of particles (e.g. suspended sediment and plankton) and colored organic matter present. Water clarity plays an important role in determining bay grasses and phytoplankton distribution and abundance.
Chesapeake Bay - Indicator Details:
Water clarity was measured at approximately 144 stations up to 14 times during the periods of interest (April to October in the tidal fresh, oligohaline and mesohaline regions, and March to November in the polyhaline regions). Secchi depth is the water depth (in meters) that you can just differentiate black and white quarters of a Secchi disc lowered into the water. The proportion of time that water clarity was below the threshold at each station was calculated and then interpolated to provide estimates between the stations.
This map shows average water clarity (Secchi depth in meters) from March to November 2009. All Chesapeake Bay tributaries exhibited a gradient in water clarity, with murkier water (shallowest Secchi depth—orange-pink shades) in the mid to upper reaches and clearer waters in the lower reaches (deeper Secchi depths—shades of blue). Lower water clarity in the mid to upper reaches of the tributaries can, in part, be attributed to the mixing of fresh and saline waters leading to a phenomenon known as a turbidity maximum. The Bay's mainstem had a similar pattern, with murkier water in the Upper Bay and clearer water in the Mid and Lower Bay regions. The overall bay-wide patterns of water clarity in 2009 were similar to those seen in 2008 and 2007, although improvement was seen in the lower mainstem regions.
Water clarity was measured at approximately 144 stations up to 14 times during the periods of interest (April to October in the tidal fresh, oligohaline and mesohaline regions, and March to November in the polyhaline regions). Secchi depth is the water depth (in meters) that you can just differentiate black and white quarters of a Secchi disc lowered into the water. Average water clarity (Secchi depth) at each station was calculated and levels between stations were estimated using spatial interpolation software.
Comparison of Bay Health Index scores for 2009 () compared to ()
|0 20 40 60 80 100|
|Lower Eastern Shore (Tangier)|
|Upper Eastern Shore|
|Upper Western Shore|
|Lower Western Shore (MD)|
|Patapsco and Back Rivers|
This figure ranks each region from best to worst water clarity scores for 2009. This figure shows that all but one region score poorly to very poorly for water clarity, and are still a long way from reaching threshold levels. There have been slight improvements in water clarity in the Bay over the last 3 years, however, there is a long-term (23 years) declining trend overall. The Mid Bay and Choptank River regions had the highest water clarity score in 2009, while the Lower Western Shore and the Patapsco and Back Rivers had the lowest.
This graph is dynamic, you can: a) show and hide items by clicking them in the legend, b) select year range (click and drag), and c) export as an image.
|Salinity Regime||Season||Relative Status Thresholds (m)*|
* Lacouture et al., Estuaries and Coasts (2006) & Buchanan et al., Estuaries (2005)