n late November re-oxygenated the water column of all the sites with about 10–12 mg/L of dissolved oxygen.
On the basis of pH and specific conductance measurements, Lake Tapps water is pH neutral and chemically dilute. Median pH values for water in the epilimnion and the hypolimnion ranged from 6.84 to 7.64 pH units and maximums did not exceed 7.8 pH units at any site. Median specific conductance was typically less than 70 microsiemens per centimeter at 25°C for the epilimnion and the hypolimnion.
Concentrations of nutrients and chlorophyll a in Lake Tapps were low. At most of the sites and in most of the samples from the epilimnion, total phosphorus concentrations were less than the Washington State Department of Ecology phosphorus criterion of 0.01 mg/L for maintaining oligotrophic conditions. Median concentrations of total nitrogen (unfiltered water) ranged from about 0.14 mg/L (Deep, Tapps Island, and Dike 2B sites) to about 0.18 mg/L (Allan Yorke and Lake Inlet sites). Chlorophyll a concentrations were low with median concentrations of 2.16 micrograms per liter (mg/L) or less. The majority of chlorophyll a concentrations were well below the Oregon Department of Environmental Quality action level of 10 mg/L.
Using the Carlson Trophic-Status Index and average measures of transparency, chlorophyll a, and total phosphorus data from this study, Lake Tapps generally fits within the oligotrophic classification, but with a few exceptions. At Allan Yorke, Lake Inlet, and Southeast Arm sites, the chlorophyll a and total phosphorus indexes of nearly 40 approach the upper limit of oligotrophic conditions. In addition, average concentrations of total phosphorus at Lake Inlet and Southeast Arm are at Nürnberg‘s (1996) threshold concentration of 0.01 mg/L, which suggests a slight tendency towards mesotrophic conditions at these two sites during summer July–September.
On the basis of epilimnetic nitrogen to phosphorus concentration ratios of greater than 17, Lake Tapps primary production is phosphorus limited at all but two study sites. At the Lake Inlet and Southeast Arm sites, ratios of 15 and 16, respectively, for the summer period suggest either nitrogen or phosphorus (or both) may limit algal growth.
Water samples collected at the Allan Yorke, Snag Island, and Lake Outlet study sites were screened for the presence of more than 250 organic chemicals. A total of 14 compounds were detected in trace amounts (or determined to be present) at one or more of the 3 sites. The Allan Yorke site had 9 detections, the Snag Island site had 10 detections, and the Lake Outlet site had 5 detections of compounds mostly belonging to the group of wastewater indicator chemicals. Compounds detected (or with verified presence) at all three sites included the herbicide 2,4-D, the insecticide and mosquito repellant DEET, the herbicide fluridone used for Eurasian watermilfoil eradication, and the herbicide prometon. The largest concentrations of these compounds were in samples from the Allan Yorke site; the lowest concentrations were from the Lake Outlet site.