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Browse History: Benthic Index of Biotic Integrity (2008) | Patuxent River (2006) | Choptank River (2008) | Water Clarity (2006)
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


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Station Data
This map shows how often water clarity was above or below threshold concentrations from March to September, 2006. 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 2006, water clarity was continually below threshold levels in all but isolated areas of the Bay. While the reason for 2006's very poor water clarity is yet to be determined it is likely the results of a recently detected long-term (at least 20 year) decline of water clarity and the early summer rain event that deposited large quantities of nutrients and sediments into the Bay and tributaries.

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


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Station Data

This map shows average water clarity levels for Chesapeake Bay and the tributaries from March to September, 2006. 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. Lower water clarity in the mid to upper reaches of the tributaries can, in part, be attributed to the mixing a fresh and saline waters leading to a phenomenon known as a turbidity maximum. 2006 had similar water clarity levels as 2003, a high river flow year, suggesting that these patterns are also influenced by sediment and nutrients from the watershed.

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 2006 () compared to
()


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

This figure ranks each region from best to worst for the indicator water clarity. Water clarity was poor for all regions of the bay in 2006. While the reason for 2006's very poor water clarity is yet to be determined it is likely the results of a recently detected long-term (at least 20 year) decline of water clarity and the early summer rain event that deposited large quantities of nutrients and sediments into the Bay and tributaries. The Upper Bay region had the highest water clarity score in 2006, while in contrast the Lower Western Shore (MD) and the Patapsco and Back Rivers had the lowest (0).


Excel Spreadsheet

Threshold Levels

Salinity Regime Season Relative Status Thresholds (m)*
Tidal Fresh Apr-Oct ≥0.85
Oligohaline Apr-Oct ≥0.65
Mesohaline Apr-Oct ≥1.63
Polyhaline Mar-Nov ≥2.0

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