Pesticide Concentrations Measured in Zooplankton Samples Collected from the Sacramento River, Yolo Bypass, and Cache Slough Region, California, 2022 and 2023

Zooplankton samples were collected in the summer/fall of 2022 and 2023 at multiple sites in the Sacramento River, Yolo Bypass, and Cache Slough Region of California and analyzed for a suite of 160 current-use pesticides and pesticide degradates by the U.S. Geological Survey Organic Chemistry Research Laboratory. In 2022 samples were collected at two sites in the Bypass and at a comparison site on the Sacramento River from early August to mid-October. In 2023 zooplankton samples were collected at one site upstream of the Yolo Bypass in Northern California, four sites within the Bypass, one site downstream of the Bypass in the Cache Slough region, and at a comparison site on the Sacramento River from late July through early October. Samples were collected every two weeks by towing a 150-micron conical plankton net for 5 minutes by boat at each site. In the laboratory, zooplankton samples were filtered through a 63-micrometer sieve to separate the zooplankton and vegetation/detritus from the water. Large sticks, twigs, rocks, and leaves were rinsed with organic free water into the sieve and then discarded. The remaining mass in the sieve was transferred into 50 milliliter plastic centrifuge tubes, frozen overnight at -20 degrees Celsius, dehydrated completely using a freeze dryer, and then stored at -20 degrees Celsius. Immediately prior to analysis, each freeze-dried sample was homogenized and sub-sampled to a targeted final weight of 0.5 grams. Sub-samples were further homogenized with magnesium sulfate and extracted with acetonitrile at 100 degrees Celsius using an EDGE Automated Extraction System following procedures outlined in Black and others, (2023). Extracts were cleaned up using activated carbon and evaporated to 200 microliters in acetonitrile. Each sample had internal standards added and was analyzed by gas chromatography-tandem mass spectrometry for 27 pesticides and by liquid chromatography tandem mass spectrometry for 133 pesticides following methods described in Gross and others, (2024). A total of 15 pesticides were detected in the 18 zooplankton samples collected in 2022, and 16 of the samples contained multiple pesticides (up to 9 per sample). The most frequently detected compounds in 2022 were azoxystrobin, bifenthrin, fluridone, and p,p’-DDE. A total of 25 pesticides were detected in the 55 zooplankton samples collected in 2023, and 50 of the samples contained multiple pesticides (up to 16 per sample). The most frequently detected compounds in 2023 were azoxystrobin, bifenthrin, and p,p’-DDE.