Developing numeric chlorophyll a criteria through science, enhanced monitoring, and a collaborative stakeholder partnership

Chlorophyll a water quality criteria provide the means for regulating nutrient pollution in surface water systems without many of the technical challenges associated with nitrogen and phosphorus thresholds. However, the development of defensible chlorophyll a criteria is not without its own challenges, particularly in complex coastal ecosystems. We describe the evolution of water column chlorophyll a criteria specific to the Chesapeake Bay estuary, from a general narrative to numeric thresholds developed to meet multiple regulatory management goals. We then present a unique approach to numeric chlorophyll a criteria derivation, one that integrates continuous and underway water quality monitoring datasets. These datasets were used for the dual purposes of characterizing baseline conditions and determining chlorophyll a concentrations that confer unacceptable levels of risk to a specific water body in both space and time. This novel approach was used by the Commonwealth of Virginia to update the numeric chlorophyll a criteria it had originally established for the tidal James River, a subestuary of the Chesapeake Bay, in 2005. Tidal James River chlorophyll a concentrations were evaluated in terms of their relationships with harmful algal blooms, dissolved oxygen, pH, and water clarity. The importance of spatially and temporally intensive monitoring datasets to chlorophyll a criteria development is highlighted in this case study.