Understanding the ?Lag Times? Affecting the Improvement of Water Quality in Chesapeake Bay

Better quantifying the ‘lag time’ between changes in nutrient and sediment sources in the Chesapeake Bay watershed and improvement in the Bay?s water quality and submerged aquatic vegetation (SAV) is critical to help resource managers to implement the most effective nutrient and sediment reduction strategies and for scientists to improve monitoring and modeling. Tributary strategy plans for basins within the Bay watershed have been developed to implement appropriate best management practices (BMP?s) to reduce nutrient and sediment loads to the Bay. These practices are designed so water-quality criteria (for dissolved oxygen, water clarity, and chlorophyll) can be met in the Bay by 2010. However, there is a large degree of uncertainty about the ‘lag time’ between implementing the nutrient and sediment practices and detecting an actual improvement of water quality and SAV in the Bay. The objectives of the workshop were to provide the CBP with a better understanding of the factors affecting the ‘lag time’ associated with improving water quality and SAV in the Bay and provide recommendations for improved monitoring and modeling of these factors. Results from the workshop suggest that ?lag times? associated with implementation of management practices, impacts of watershed properties, and response of the Bay water quality will make it very difficult to meet water-quality criteria in the Bay by 2010. Additionally, there are lag times associated with the movement of nutrients and sediment in the watershed. These include the influence of ground water which may cause a lag time from months to decades for improvement in nitrogen concentrations. Watershed properties affecting the storage and transport of phosphorus and sediment may cause lag times of years to decades in water-quality improvements. Lag times in the tidal waters appear to be much shorter. The findings suggest that water-quality conditions in tidal waters may improve within a season as nutrient and sediment loadings are reduced to the Bay.