U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigations Report 99-4018B

Do Herbicides Impair Phytoplankton Primary Production in the Sacramento-San Joaquin River Delta?

By Jody L. Edmunds, Kathy M. Kuivila, Brian E. Cole, and James E. Cloern

This paper is available in pdf format: Edmunds.pdf

ABSTRACT

The effect of herbicide concentration on the maximum rate of phytoplankton primary production (P_max) was examined for 53 water samples collected at 9 sites in the Sacramento-San Joaquin River Delta between May and November, 1997. Samples were analyzed for P_max and the concentrations of diuron, atrazine, cyanazine, simazine, thiobencarb, and hexazinone. The herbicide concentrations ranged between 0 µg/L and 2.1 µg/L, with 50% of the values (n=318) between 0 and 0.018 µg/L. Herbicide concentrations in 52 of the water samples were well below the lowest observable effect concentrations (LOECs) that have been reported in laboratory experiments to inhibit primary production. P_max ranged between 2 and 11 milligrams of carbon per milligrams of chlorophyll a per hour (mg C (mg chla-h))^-1 for the 52 samples where the herbicide concentrations were less than any reported LOEC. However, for the one sample where the diuron concentration (2.1 µg/L) exceeded the reported LOEC of 2.0 µg/L, P_max was the lowest observed during the study, 0.9 mg C (mg chla-h)^-1. Herbicide concentrations we observed throughout the system do not appear to limit production; however, localized occurrences of elevated herbicide concentrations exist and may affect primary production.