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:
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.
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.
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.
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.
Comparison of Bay Health Index scores for 2011 () compared to ()
|0 20 40 60 80 100|
|Lower Eastern Shore (Tangier)|
|Upper Eastern Shore|
|Lower Western Shore (MD)|
|Upper Western Shore|
|Patapsco and Back Rivers|
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.
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)