This dataset contains taxonomic and biovolume data for phytoplankton sampled from Detroit Lake, OR, in August, 2019, and August, 2020. Taxonomy is reported to the lowest possible taxonomic level along with total and percent biovolume. Surface grab samples for phytoplankton enumeration were collected with a van Dorn type sampler and then transferred into a clearn polycarbonate churn. Whole water samples were dispensed for total nutrients and phytoplankton identification and enumeration, the latter preserved with 1 percent Lugol's iodine solution. Samples were analyzed at BSA Environmental Services in Beachwood, Ohio. Cell numbers of all identified phytoplankton taxa were quantified on a per milliliter basis using the Utermohl method (Lund et al. 1958). Samples were thoroughly mixed prior to settling an aliquot to ensure that the organisms were evenly distributed. Various aliquots were measured and delivered into custom Utermohl chambers and the organisms allowed to settle in darkness undisturbed for at least 20 hours within an enclosure protected from vibration and temperature alteration (Burkholder and Wetzel 1989). Counts were performed on a LEICA DMiL at 800X or 1260X. Scans at multiple magnifications were conducted and the abundance of common taxa estimated by random field counts. At least 300 natural units (colonies, filaments, unicells) or 50 random fields were enumerated to the lowest possible taxonomic level from each sample. Cell biovolumes were estimated using formulae for solid geometric shapes that most closely match the cell shape (Hillebrand et al., 1999), based on measurements of 10 organisms per taxon for each sample where possible. These monitoring data were collected as part of the City of Salem's source water protection program that includes several sites within Detroit Lake and sites downstream in the North Santiam River down to Salem's Geren Island Drinking Water Treatment Plant. These data can be used to understand which phytoplankton taxa are dominant and a measure of their biomass (biovolume). These data also may be used to trigger sampling for cyanotoxins. REFERENCES: Burkholder, J.M. and R.G. Wetzel. 1989. Epiphytic microalgae on natural substrata in REFERENCES a hardwater lake: seasonal dynamics of community structure, biomass and ATP content. Arch. Hydrobiol., Suppl. 83:1-56. Hillebrand, H., C.D. D?rselen, D. Kirschtel, U. Pollingher, and T. Zohary. 1999. Biovolume calculation for pelagic and benthic microalgae. Journal of Phycology. 35: 403-424. Lund, J.W.G., C. Kipling and E.D. LeCren. 1958. The inverted microscope method of estimating algal numbers and the statistical basis of estimates by counting. Hydrobiologia 11:143-170.