The present Virginia water quality standards for tidal James River and the District of Columbia’s 
water quality standards for its tidal waters, stated as seasonal chlorophyll a means, reflect the 
importance of the assessment in measuring central tendency compared to an acceptable upper 
bound for acceptable water quality conditions. Chlorophyll a is a parameter whose measures 
repeatedly show skewed distributions appropriate to log transformation to approximate a normal 
distribution for making inference with well developed, well tested statistical methods. It is 
therefore appropriate to use a statistic that addresses the central tendency respecting the 
appropriate statistical properties of such data, i.e., the geometric mean. 
The EPA Chesapeake Bay Program Office and its partners tested the recommended revised 
assessment methodology for Chesapeake Bay data (e.g., tidal James River) and compared the 
results with the application of the promulgated Virginia water quality standards’ chlorophyll a 
criteria. Results showed almost universally greater levels of chlorophyll a attainment using the 
recommended revised methodology compared with the previously EPA published criteria 
assessment method (and adopted into Virginia’s water quality standards). Acknowledging these 
findings, the revisions to the published criteria assessment method are recommended for 
ensuring consistency within the assessment procedures with acknowledged the statistical 
properties of the chlorophyll a data. 
LITERATURE CITED 
Barnes, H. 1952. The use of transformations in marine biological statistics. J. du Conseil 18:61- 
71. 
Bland, J.M and D.G. Altman. 1996. Transformations, means and confidence intervals. British 
Medical Journal 312:1079. 
Buchanan, C., R. V. Lacouture, H. G. Marshall, M. Olson, J. Johnson. 2005. Phytoplankton 
reference communities for Chesapeake Bay and its tidal tributaries. Estuaries 28(1): 138-159. 
Colorado lakes study. 
http://www.chatfieldwatershedauthoritv.org/pdf/Characterizimz%20Chlorophyll%20Distribution 
s%20in%20Colorado.pdf 
Gilbert, R.O. 1987. Statistical methods for environmental pollution monitoring. Van Nostrand 
Reinhold, New York, NY.320 pp. 
Harding, L. Jr. 1994. Long term trends in the distribution of phytoplankton in Chesapeake Bay: 
roles of light, nutrients and streamflow. Mar. Ecol. Prog. Ser. 104:267-291. 
Harding, L.W., and E.S. Perry. 1997. Long-term increase of phytoplankton biomass in 
Chesapeake Bay, 1950-1994. Mar. Ecol. Prog. Series. 157:39-52. 
38 
