Browse History: FAQ (2007) | James River (2006) | Lower Eastern Shore (Tangier) (2006) | Chlorophyll a (2007)
Indicator Icon Chlorophyll a

Chlorophyll a is used as a measure of phytoplankton (microalgae) biomass. Phytoplankton biomass is controlled by factors such as water temperature and the availability of light and nutrients. Elevated phytoplankton levels can lead to reduced water clarity and decomposing phytoplankton can lead to reduced dissolved oxygen levels.

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Threshold Map

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This map shows how often chlorophyll a concentrations were above or below threshold concentrations between March and September 2007. Good water quality is characterized by chlorophyll a levels that are frequently below the threshold, resulting in high scores (shown in green). Poor water quality has chlorophyll a levels that are frequently above the threshold, resulting in low scores (shown in shades of orange and red). In 2007, a large portion of Chesapeake Bay had chlorophyll a levels that frequently exceeded threshold levels. Regions of Chesapeake Bay with chlorophyll a concentrations frequently below threshold levels tended to be in the upper reaches of the tributaries and Upper Bay region. Chlorophyll a levels in these regions are likely limited by a lack of light due to poor water clarity.

Creating this map
Chlorophyll a concentrations were measured at approximately 144 stations up to 12 times during the period of interest (March to May and July to September, 2007). The proportion of time that chlorophyll a levels were below the threshold levels at each station was calculated and then interpolated to provide estimates between the stations.

Data Map

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This map shows the average chlorophyll a concentration in Chesapeake Bay from March to September 2007. Lowest average chlorophyll a concentrations occurred in the Upper Bay, Lower Bay, and large sections of Virginia's tributaries. Low average chlorophyll a levels in the upper reaches of many tributaries may be partially due to poor water clarity, while availability of nutrients may be limiting in the Lower Bay. The small upper Bay tributaries such as the Patapsco, South, and Sassafras Rivers tended to have the highest average chlorophyll a concentrations, although high concentrations were recorded in many other regions of the Bay. The elevated chlorophyll a levels recorded in these regions are likely due to a combination of high nutrients and light availability.

Understanding this map
Chlorophyll a concentrations were measured at approximately 144 stations up to 12 times during the period of interest (March to May and July to September, 2007). The average chlorophyll a concentration at each station was calculated and estimates between stations were made using spatial interpolation software.

Ranking


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


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

This figure ranks each region from best to worst chlorophyll a scores for 2007. Good water quality is characterized by chlorophyll a levels that are frequently below the threshold, resulting in high scores (shown in green). Poor water quality tends to have chlorophyll a levels that are frequently above the threshold, resulting in low scores (shown in shades of orange and red). In 2007, the Upper Western Shore region had the best overall chlorophyll a score followed by the Upper Bay, James, and Potomac Rivers. Lowest ranked region was the Patapsco and Back Rivers, scoring a zero, indicating that chlorophyll a levels were always above the threshold levels. The frequently high levels of chlorophyll a, leading to the poor scores, are mainly due to excess nutrient availability.


Excel Spreadsheet

Threshold Levels

Salinity Regime Season Reference Community
Thresholds (µg L-1)*
Tidal Fresh Spring ≤14.0
Oligohaline Spring ≤20.9
Mesohaline Spring ≤6.2
Polyhaline Spring ≤2.8
Tidal Fresh Summer ≤12.0
Oligohaline Summer ≤9.5
Mesohaline Summer ≤7.7
Polyhaline Summer ≤4.5

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