Seasonality of water chemistry in blackwater streams in South Carolina
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Abstract for presentation at 2025 National Water Quality Monitoring Conference.
Blackwater streams are an important part of the landscape in South Carolina, occurring commonly in the Southeastern Plains (SEP) and Middle Atlantic Coastal Plain (MACP) ecoregions of the state. Using data collected as part of South Carolina’s Ambient Surface Water Quality Monitoring Program, we compared monthly trends in water chemistry between streams identified by South Carolina Department of Environmental Services as blackwater and non-blackwater in these two ecoregions. This very large dataset was summarized to median values for each site-month-analyte combination, then plotted as means by analyte, site type, and ecoregion to examine monthly patterns. Most analytes showed a seasonal pattern of values, but some (pH, total alkalinity, nitrate-nitrate (NOx), total phosphorus, and turbidity) also exhibited differences between blackwater and non-blackwater sites during at least part of the year. A few analytes, including total Kjeldahl nitrogen (higher in MACP) and dissolved oxygen (higher in SEP), showed strong differences between ecoregions but not between blackwater and non-blackwater sites within a region. Within each ecoregion, we also ran Principal Components Analysis (PCA) using monthly data during the growing season of April to October to explore relationships among analytes. In the Southeastern Plains, pH, turbidity, and total phosphorus were associated with one another and most important in separating site-month data points in ordination space. In contrast, the Middle Atlantic Coastal Plain, dissolved oxygen, NOx, and pH were positively correlated and most important in separating site-month data points. Using boxplots of PCA axis 1 for each ecoregion plotted by site type and month, we could see that Southeastern Plains blackwater sites were consistently different from non-blackwater sites over the months included in the analysis. However, this pattern was weak in the Middle Atlantic Coastal Plain sites, showing little separation between site types on a month-to-month basis. This analysis indicates that the ability to differentiate and manage blackwater streams may vary by ecoregion and potentially by month or season and will inform future research on this resource.