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Browse History: Phytoplankton Index of Biotic Integrity (2011)
Indicator Icon Phytoplankton Index of Biotic Integrity

Phytoplankton (microalgae) is an important component of the Bay's food web. The Phytoplankton Index of Biotic Integrity (PIBI) is a measure of the microscopic algal community condition.

Chesapeake Bay - Indicator Details:

  Indicator Selection Diagram

Threshold Map

This map illustrates the overall Phytoplankton Index of Biotic Integrity (Phytoplankton IBI) scores for each of the 15 reporting regions. The Lower Bay region had the highest (best) phytoplankton community score in 2011, and increased from moderately good in 2010 to good this year. This was a return to phytoplankton diversity scores that were seen in the early 2000's. Seven regions of the Bay (the Upper Eastern Shore, Choptank, Elizabeth, York, Potomac, Patuxent, and James Rivers) had very poor scores in 2011. No phytoplankton monitoring is conducted in the Lower Eastern Shore (Tangier) or Upper Western Shore regions.

Creating this map
The frequency that the Phytoplankton IBI scores exceeded the threshold level of 3 was determined during the spring-summer sampling periods. In 2011, samples were collected up to 6 times at each station. The overall reporting region scores were calculated by accounting for the surface area of the region represented by each sampling station.

Data Map

This map shows the average spring-summer Phytoplankton Index of Biotic Integrity (Phytoplankton IBI) score for 2011. In 2011, all sampling stations except two (~90% of stations) within the Bay's tributaries had average Phytoplankton IBI scores between 1 and 2.9 out of a possible 5, indicating that the phytoplankton communities were in degraded conditions relative to reference communities. In contrast, ~55% of monitoring stations within the Bay's mainstem had moderately good phytoplankton communities (scoring between 3-4). Some regions improved (Upper and Lower Bay, Patapsco and Back Rivers and the Lower Western Shore (MD), and others declined (Mid Bay, Patuxent, Choptank and James Rivers).

Understanding this map
In 2011, phytoplankton samples were collected at 31 stations during the spring (March-May) and summer (July-September). Samples were collected up to 6 times at each station. Data from each sampling event were used to calculate a Phytoplankton IBI score. Phytoplankton IBI scores for each sampling station were then averaged and mapped.


Comparison of Bay Health Index scores for 2011 () compared to

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

This figure ranks each region from best to worst for the Phytoplankton Index of Biotic Integrity (Phytoplankton IBI). The Lower Bay overtook the Mid Bay as the highest scored Phytoplankton IBI region. The Mid Bay had the highest Phytoplankton IBI score for six years in a row but decreased by 33% in 2011, though it still remains the second best score. The next highest-ranked region was the Rappahannock River. The overall Bay score is moderate because the Mid Bay and Lower Bay scores are good and these regions represent a large area of the Bay. Regions with the lowest Phytoplankton IBI scores were the Elizabeth (which scored a zero), Potomac, Choptank, and Patuxent Rivers. No phytoplankton monitoring is conducted in the Lower Eastern Shore (Tangier) and Upper Western Shore regions.

Excel Spreadsheet