Chesapeake Bay - Watershed Stream Health: 2010
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.
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 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 the metropolitan Baltimore/Washington, D.C. region and in areas that have land-uses dominated by agriculture (e.g., Eastern Shore of Maryland and central Pennsylvania) 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 and James River watersheds. 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, 4,302 of the 7,886 random sampling sites in the watershed had very poor or poor conditions, 3,584 sites had fair, good or excellent conditions. 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.
Health Index Map
An analysis was conducted on a subset of 10,833 sites sampled during the time period of 2000-2008 to investigate regional variation in the BIBI scores. The subset of sites (7,886) 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 7,886 sites were averaged over the period of record for each location. Then the averages were aggregated by the smallest watershed size as possible while still remaining statistically valid. Results show 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 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 and central Pennsylvania, 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.
An analysis was conducted on a subset (7,886 of 10,833) 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 30 or greater on a scale of 0-100. 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 in order to reduce bias associated with sampling designs that target areas with known degraded or high quality waters. BIBI scores were analyzed for the percentage of the average scores that meet the criteria for a healthy stream for each of the subwatersheds within the Chesapeake Bay watershed.
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.