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:
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.
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.
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.
Comparison of Bay Health Index scores for 2007 () compared to ()
|0 20 40 60 80 100|
|Upper Western Shore|
|Lower Eastern Shore (Tangier)|
|Upper Eastern Shore|
|Lower Western Shore (MD)|
|Patapsco and Back Rivers|
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.
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||Reference Community|
Thresholds (µg L-1)*
* Lacouture et al., Estuaries and Coasts (2006) & Buchanan et al., Estuaries (2005)