This case study describes the procedures used to develop proposed chlorophyll a numerical criteria. The regulations for Virginia’s state water quality standards require state waters to support balanced indigenous populations of aquatic life. The tidal James River—a portion of the river that stretches from Richmond, Virginia, to its confluence in the Chesapeake Bay—was listed as impaired under the Clean Water Act because of excessive nutrients that nourished undesirable levels of algae and created poor water clarity that negatively affected the health and distribution of underwater bay grasses. In response, Virginia proposed adopting river segment- and season-specific numerical chlorophyll a criteria.
An approach based on reference phytoplankton community conditions was used to help derive the chlorophyll a criteria for the tidal James River. While phytoplankton communities themselves do not demonstrate any direct relationship between chlorophyll a concentrations and designated use impairments, they do provide solid insights into the chlorophyll a concentrations that will likely be observed in nutrient-limited and light-saturated conditions.
The water quality and phytoplankton data used to determine reference phytoplankton community conditions were collected at Chesapeake Bay Program (CBP) biomonitoring stations between 1984 and 2001. Collected data were characterized spatially and temporally using box-and-whisker plots and other exploratory techniques.
The tidal James River exhibits a salinity gradient as well as changes in depth, hydrology, and biological conditions as it flows from the upland to the Chesapeake Bay. Those changes form the basis for spatial classification of the estuary to reduce data variability. Five segments were identified for deriving and applying the chlorophyll a criteria: (1) upper and lower segments in the tidal fresh section; (2) a transitional segment where freshwater mixes with the higher salinity bay and ocean waters; and (3) upper and lower segments in the higher salinity region. Because of the large contrast between seasonal conditions, separate spring and summer criteria were developed for each of the segments.
To derive the reference communities, least impaired water quality conditions were defined as the co-occurrence of high light penetration, low dissolved inorganic nitrogen (DIN), and low dissolved inorganic phosphorus (PO4) concentrations. Low DIN and PO4 concentrations are the threshold concentrations shown in the literature to limit phytoplankton growth in Chesapeake Bay waters whereas high light penetrations are the Secchi depth values identified by EPA and other sources as necessary for restoring underwater bay grasses. Thresholds for DIN, PO4, and Secchi depth for spring and summer across the salinity zones were applied to the 1984–2001 CBP monitoring database to bin the data records into water quality categories. Reference phytoplankton communities and associated chlorophyll a thresholds were derived from the least impaired water quality categories found in each season-salinity regime.
The final recommended segment-specific numerical chlorophyll a criteria for spring and summer seasons were based on best professional judgement using the reference phytoplankton community approach described above and other lines of evidence, including historical concentrations, model simulations using the Chesapeake Bay water quality model, and reviews of scientific literature and other studies (VDEQ 2004).
Reference:
VDEQ. 2004. Virginia Department of Environmental Quality Technical Report – Chlorophyll a Numerical Criteria for the Tidal James River. Virginia Department of Environmental Quality. Accessed October 2016. http://www.deq.virginia.gov/Portals/0/DEQ/Water/WaterQualityStandards/JamesRiverChlorophyllStudy/James_chl_a_criteria_tech_doc_2004.pdf EXIT.