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Browse History: Water Clarity (2009)
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:





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Threshold Map


This map shows how often water clarity was above or below threshold concentrations from March to November 2009. 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 2009, water clarity was below the threshold level for the majority of regions of the Bay; however improvement was seen from 2008 patterns, especially in the Mid and Upper Bay regions. This is an encouraging sign, given that there has been a trend of declining water clarity throughout the Bay over the past 20 years.


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 (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.

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 2009 () compared to
()


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

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.


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)