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Browse History: Water Clarity (2007)
Indicator Icon Water Clarity

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:





  Indicator Selection Diagram


Threshold Map


This map shows how often water clarity was above or below threshold concentrations from March to November, 2007. Good water quality is characterized by having water clarity that is frequently above the threshold (shown in shades of green), while poor water quality tends to have water clarity that is frequently below the threshold (shown in shades of orange and red). In 2007, water clarity was below the threshold level for the majority of sampling times (0 to 20%; colored red) in most regions of the Bay; however, this is an improvement from 2006 when there was only one small region that was not in this very poor water clarity category. While water clarity has been poor in many regions of the Bay for decades, recent analysis for the report card highlights that many regions, such as the Lower Bay, has been experiencing a long-term decline. The reasons for the long-term decline are still being investigated.

Creating this map
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.

Data Map


This map shows average water clarity levels for Chesapeake Bay and the tributaries from March to September 2007. All Chesapeake Bay tributaries exhibited a gradient in water clarity, with lower water clarity (shallowest Secchi depth ? orange-pink shades) in the mid to upper reaches and clearer waters in the lower reaches (deeper Secchi depths ? shade of blue). The Bay's mainstem had a similar pattern, with lower water clarity in the Upper Bay and clearer water in the Mid and Lower Bay regions. 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. While the bay wide patterns of water clarity in 2007 were similar to 2006, there are notable differences in many of the reporting regions. For example, the area of poor water clarity in the Lower Eastern Shore (Tangier) region was much larger in 2007 compared to 2006.

Understanding this map
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.

Ranking


Comparison of Bay Health Index scores for 2007 () compared to
()


Score Legend
 Score (%)
 0               20               40               60               80              100
  
Choptank River  
Upper Bay  
Upper Eastern Shore  
Mid Bay  
Overall Bay  
Rappahannock River  
James River  
Lower Bay  
Potomac River  
Upper Western Shore  
York River  
Patuxent River  
Lower Western Shore (MD)  
Lower Eastern Shore (Tangier)  
 0
Elizabeth River  
 0
Patapsco and Back Rivers  
 0

This figure ranks each region from best to worst water clarity scores for 2007. Overall there was a slight improvement in water clarity in 2007, although this does not reverse the downward trajectory of the water clarity noted in many regions. The Choptank River and Upper Bay region had the highest water clarity score in 2007, while the Lower Eastern Shore, Elizabeth River, and the Patapsco and Back Rivers had the lowest (0).


Excel Spreadsheet

Threshold Levels

Salinity Regime SeasonRelative Status Thresholds (m)*
Tidal FreshApr-Oct≥0.85
OligohalineApr-Oct≥0.65
MesohalineApr-Oct≥1.63
PolyhalineMar-Nov≥2.0

* Lacouture et al., Estuaries and Coasts (2006) & Buchanan et al., Estuaries (2005)