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Browse History: Water Clarity (2011)
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 2011. 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 2011, water clarity was below the threshold level for the majority of sampling times (score = 0 to 20%; colored red) in most regions of the Bay; a similar pattern was observed in previous years. Water clarity continued to decline in all regions. 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, have 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 11 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) for Chesapeake Bay and the tributaries from March to November 2011. 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 (deeper Secchi depths—shades of blue) in the lower reaches. The Bay's mainstem exhibited a similar pattern, with murkier water 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. Baywide, water clarity was very poor. This was a decline compared to the past few years when regions of the Bay showed improvements.

Understanding this map
Water clarity was measured at approximately 144 stations up to 11 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 2011 () compared to
()


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

This figure ranks each region from best to worst water clarity scores for 2011. This figure clearly demonstrates that all regions score very poorly for water clarity, and are a long way from reaching threshold levels. Following the two years of moderate improvement from 2008-2009, water clarity declined in 2011 for the second year in a row. The Lower Bay and Rappahannock regions had the highest water clarity scores in 2011, but these were still very poor scores (7%). Six regions scored a 0% passing score for water quality in 2011.


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)