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Browse History: Chlorophyll a (2012)
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.

Chesapeake Bay - Indicator Details:





  Indicator Selection Diagram


Threshold Map


This map shows how often chlorophyll a concentrations were above or below threshold concentrations between March and September 2012. 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 is characterized by chlorophyll a levels that are frequently above the threshold, resulting in low scores (shown in shades of orange and red).

In 2012, a large portion of Chesapeake Bay had chlorophyll a levels that frequently exceeded threshold levels. These failing regions include the Patuxent, the Patapsco and Back Rivers, in addition to the Lower Western Shore (MD), the Upper Eastern Shore and the Lower Bay regions. Regions of Chesapeake Bay with chlorophyll a concentrations frequently below threshold levels tended to be in the Choptank and James river regions. The James River had the best chlorophyll a score of all the regions.

Creating this map
Chlorophyll a concentrations were measured at approximately 159 stations up to 8 times during the period of interest (March to May and July to September). The proportion of time that chlorophyll a levels were below the threshold levels at each station were calculated and are spatially averaged into reporting regions.

Data Map


This map shows average chlorophyll a concentrations in Chesapeake Bay from March to September. Highest average chlorophyll a concentrations tended to occur in tributaries such as the Lower Western Shore (MD), Upper Western Shore, Patapsco, Back, and Patuxent Rivers, as well as in the Lower Bay region of the mainstem. In these regions average chlorophyll a concentrations often exceeded 25 ug L-1; these high concentrations are more than likely due to elevated nutrient availability. Low average chlorophyll a concentrations (in the 5-10 ug L-1 range) occurred in some areas of the bay this year, including most of the upper mainstem Bay, the middle reaches of the James River, and the upper reaches of the York, Elizabeth, and Chester Rivers.

Understanding this map
Chlorophyll a concentrations were measured at approximately 159 stations up to 8 times during the period of interest (March to May and July to September). The average chlorophyll a concentration at each station was calculated and estimates between stations were made by interpolation to visualize spatial patterns and trends.

Ranking


Comparison of Bay Health Index scores for 2012 () compared to
()


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

This figure ranks each region from best to worst chlorophyll a scores for 2012. 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 2012, the James River had the best overall chlorophyll a score, followed by the Choptank River. The chlorophyll a threshold map (see tabs above) shows that tidal fresh areas of the bay are frequently below the threshold (i.e., in good condition). Lowest ranked regions were the Lower Western Shore (MD) and Patapsco and Back Rivers regions. Both of these regions scored a zero, indicating that chlorophyll a levels were always above the threshold levels. Frequently high levels of chlorophyll a, leading to the poor scores, are mainly due to excess nutrient availability.


Excel Spreadsheet

Threshold Levels

Salinity RegimeSeasonReference Community
Thresholds (µg L-1)*
Tidal FreshSpring≤14.0
OligohalineSpring≤20.9
MesohalineSpring≤6.2
PolyhalineSpring≤2.8
Tidal FreshSummer≤12.0
OligohalineSummer≤9.5
MesohalineSummer≤7.7
PolyhalineSummer≤4.5

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