Chesapeake Bay - Watershed Stream Health:

Introduction

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Bottom-dwellers, also known as benthic macroinvertebrates, are freshwater organisms including snails, mussels, and insects that live in and on the stream and river bottom. They are routinely monitored throughout the Chesapeake Bay watershed by the states and other organizations. The abundance and diversity of these organisms are good indicators of local stream health because they have more limited movement than fish and they respond quickly to pollutants such as nutrients and sediment and other environmental stressors. The health of bottom-dwellers is threatened by pollutants introduced into streams and rivers by sources such as mining, agriculture, stormwater, fossil fuel combustion, and household and industrial wastewater treatment facilities. These human activities can add nitrogen and phosphorus to the water, which lead to algal blooms and low dissolved oxygen in slow-moving streams. Mining, agriculture, and development also can add fine sediment to streams, which smothers benthic organisms and contributes to low dissolved oxygen. Mining adds toxic chemicals to the water that directly kill these bottom-dwellers.



 

Data Map

Water quality in Chesapeake Bay is linked to the health of the 64,000 square miles of land and associated streams and rivers that comprise its watershed. Land-based activities (e.g., development, agriculture) can add pollutants, such as nutrients and sediment, to local streams and rivers, which ultimately flow into Chesapeake Bay.

The new stream health indicator (Benthic Index of Biotic Integrity, or BIBI) illustrates this link between stream health and land-based activities. For example, stream health conditions tend to be very poor to fair in areas that have extreme land disturbance, such as new construction, which results in high levels of pollution, altered water flow, and poor quantity and quality of streamside vegetation. Such unhealthy streams tend to be clustered around large urban areas such as metropolitan Washington, D.C. in the lower Potomac River watershed, and in areas that have land-uses dominated by agriculture (e.g., Eastern Shore of Maryland) and mining (e.g., parts of Pennsylvania and West Virginia).

In contrast, stream health conditions tend to be good to excellent in areas with little land disturbance that offer low levels of pollution and natural in-stream and streamside habitat. Such healthy areas tend to be clustered around forested and prairie areas, such as the upper Potomac River watershed. The health of streams is variable throughout the Bay watershed and can vary even within a smaller subwatershed (e.g., the Potomac River watershed). Exceptions to these generalizations linking land-based activities to stream health are expected and are due to complexities within the ecosystem.

Overall, the analysis showed that over the time period of 2000-2008, 2,065 of the 4,248 random sampling sites in the watershed had very poor or poor conditions, 2,180 sites had fair, good or excellent conditions, and 3 sites were not able to be evaluated. Developing this indicator provides an important tool for managers and watershed groups who are focusing efforts to restore degraded streams and protect the quality of the healthiest ones.

 Interactive Google Map with data overlay

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Health Index Map

An analysis was conducted on a subset of 10,452 sites sampled during the time period of 2000-2008 to investigate regional variation in the BIBI scores. The subset of sites (4,248) were chosen where a random sampling design was used. By using only randomly selected sites, BIBI scores can be averaged across a large watershed area without introducing bias associated with sampling designs that target areas with known degraded or high quality waters.

The BIBI scores for those 4,248 sites were averaged across the 20 major watersheds within the Chesapeake Bay watershed, with results showing variation in stream health scores that can be linked to land-based activities within the individual watersheds. For example, the worst stream health occurs in highly urbanized watersheds such as the Patapsco, Back, and Lower Potomac River watersheds, which are within the Baltimore/Washington D.C. metro region. Stream health is compromised in these areas due to extreme land disturbance and an abundance of paved surfaces, which result in high levels of pollution, altered water flow, and poor quantity and quality of streamside vegetation. Lower scores are also present in areas with large amounts of agricultural activity such as the Lower Eastern Shore of Maryland, where excess nutrients and sediment compromise stream health. The best watershed average scores are often in watersheds with little land disturbance and that offer low levels of pollution and natural in-stream and streamside habitat. Such healthy areas tend to be clustered around forested areas such as in the upper reaches of the James and Potomac River watersheds and the West Branch of the Susquehanna River watershed.

These watershed average stream health scores tell only part of the story of overall stream health. Within watersheds, there is variation in the number of sites in each health category (very poor, poor, fair, good, and excellent) which is based on a combination of land-based and in-stream factors specific to each area. 

Mousing over the watersheds in the map provides a graph that illustrates the variation in scores within each major watershed.

Threshold Map

An analysis was conducted on a subset (4,248 of 10,452) of sites sampled during 2000-2008 to investigate how the BIBI scores relate to a benchmark for what would be considered a healthy stream. In this case, a healthy stream is defined as a stream that has a BIBI score of 3 or greater on a scale of 1 to 5. This is a benchmark for healthy bottom-dwelling communities deemed appropriate by scientists working in the estuary; however, this threshold might change in the future for this analysis as scientists continue to improve on the methodology and interpretation of the stream health indicator.

Sites were chosen where a random sampling design was used and BIBI scores were analyzed for the percentage of the average scores that meet the criteria for a healthy stream for each of the 20 major watersheds within the Chesapeake Bay watershed. By using only random sites, scores can be analyzed across a large watershed area without introducing bias associated with sampling designs that target areas with known degraded or high quality waters.  

Results from this analysis are similar to that of the analysis of average watershed health, illustrating the link between stream health and land-based activities in the watersheds. Watersheds with the lowest percentages of sites determined to be healthy are highly urbanized watersheds such as the Patapsco, Back, and Lower Potomac River watersheds, which are within the Baltimore/Washington D.C. metro region. Stream health is compromised in these areas by extreme land disturbance and an abundance of paved surfaces, which result in high levels of pollution, altered water flow, and poor quantity and quality of streamside vegetation. Lower percentages of healthy streams were also present in areas with large amounts of agricultural activity such as the Lower Eastern Shore of Maryland where excess nutrients and sediment compromise stream health. Watersheds with the highest percentage of sites meeting healthy stream criteria tend to be in watersheds with an abundance of natural forested and prairie land cover, such as the upper reaches of the James and the Potomac River, the West Branch of the Susquehanna River, and parts of the Rappahannock River watersheds. These areas have little land disturbance that offer low levels of pollution and natural in-stream and streamside habitat.

Mousing over the watersheds in the map provides a graph that illustrates the variation in scores within each major watershed.

Analysis

The health of the Chesapeake Bay is linked to the health of the 64,000 square miles of land and associated streams and rivers that comprise its watershed. To further examine the relationship between the Chesapeake Bay and its watershed, an analysis was conducted to investigate the connection between the health of the Bay and its watershed. A regression analysis was performed on data from 2000-2008 on 10 major river basins within the Chesapeake Bay watershed, which investigated the association between the Bay Health Index score (% of sites passing ecological thresholds) for the estuary portions of the rivers and the Stream Health Indicator (% of random sites defined as healthy streams) for the upper reaches of the rivers and streams draining to the estuary portions of the rivers.
A significant relationship was found between the Bay Health Index and the Stream Health Indicator, suggesting a link between the health of the watershed and the health of the Bay.


The graph at left shows the results for the Stream Health Indicator (x axis) and the Bay Health Indicator (y axis) for each watershed alongside a line showing where these points would plot if scores were equal.  Even though the sites do not lie on the line, there is significant agreement between the scores. Further analysis is needed to fully understand the underpinnings of this result and to test the relationship between other indices of watershed and Bay health. However, this analysis begins to investigate the linkage; such information could help inform decision-making to target resources for improving and protecting the Chesapeake Bay and its watershed.

Methods

Most monitoring programs in the Chesapeake Bay watershed collect samples of bottom-dwellers (benthic macroinvertebrates) with somewhat similar field methods and calculate a common suite of indicators from the data. However, the programs use state-specific protocols to score and evaluate these indicators in order to identify "impaired" waters for regulatory requirements. The purpose of this new stream health indicator is to evaluate benthic community health in a uniform manner and in the context of the entire Chesapeake Bay watershed. This approach incorporates the data into an overall watershed-wide Benthic IBI that is classified at the scientific family level. This method allows the results to be compared across state boundaries. This indicator is a first step toward a regional benthic community health assessment. Future work will continue to improve upon this indicator by standardizing methodologies, developing ways to combine results from different sampling designs (targeted vs. random samples), and incorporating data that were not available for analysis this year.

Further Information

Chesapeake Bay Program Stream Health Indicator website

Stream health indicator methods

State Assessments:

Delaware
Maryland
New York
Pennsylvania
Virginia
West Virginia