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    Ross, Tori, undeclared, Mount Holyoke College, 50 College Street, South Hadley, MA, 01075,; Holt, Jack, Susquehanna University, 514 University Avenue, Selinsgrove, Pennsylvania, 17870,

    The impact of Walker Lake on the north branch of Middle Creek (Snyder County) was a focus of study during the summer months of June and July from 2021 to 2023. Walker Lake is large, about 46km3 and deep with a maximum depth >9m. Given its morphometry, the lake begins to stratify in early to mid-June and remains stratified through the rest of the summer. Throughout this period of stratification, the hypolimnion becomes hypoxic. Through the years of 2021 and 2022, the outflow of the hypolimnion from Walker Lake released hydrogen sulfide and generated a buildup of iron (III) oxide-hydroxide on downstream stones during 2021 and 2022. In mid-June of 2023, however, the outflow of Walker Lake was adjusted from hypolimnetic water to that of the lower epilimnion. As a result, the temperature and oxygen concentration rose to that of the epilimnion, and the hydrogen sulfide odor disappeared, as well as the iron (III) oxide-hydroxide that had begun to build up on the downstream stones. Each year of the three-year study, diatom biofilms were sampled three times through June and July as surrogates for the state of the streams above and below the impoundment, the first week of June, the last week of June, and the last week of July. In 2021 and 2022, the Walker system showed an average difference of 10°C between the epilimnion and hypolimnion and cooled the stream system by 3°C relative to the average upstream difference from above and below the lake. Measures of alkalinity and conductivity followed the same trends as temperature. Other metrics included Taxa Richness (TR), %Agricultural Guild (%AG), Sedimentation Index (Sed. Index), Total Phosphorus (TP) and Total Nitrogen (TN) Indices. How the reservoir impacted the stream was determined by the change in the metric from above to below the lake. The results in 2023 were mixed relative to the previous two years. Some of the metrics indicated increased impairment of the downstream site following the change in outflow. For example, the Shannon Diversity (SDI) metric, a measure of the complexity of the diatom community, fell indicating that the downstream site became simplified. Also, the %Agricultural Guild rose significantly relative to the first two samples taken in 2023 and relative to the samples taken in the earlier years. Total Nitrogen and Total Phosphorus indices showed no change. Metrics that respond to organic pollution are the Pollution Tolerance Index (PTI, range 1-4) and Generic Diatom Index (GDI, range 4-20). PTI and GDI values suggested that the Walker system improved downstream. However, Taxa Richness fell and the Trophic Diatom Index, a metric that estimates the trophic condition of a stream, rose. That outcome suggests that the downstream became even more eutrophic than during the previous years.

    diatom, reservoir, stratification, diatom-generated metrics