Fig. 10.

Relationships between Na⁺, Ca²⁺, and Mg²⁺ (total sum of these three base cations), and K⁺ and dissolved organic carbon concentrations in streams and rivers across the U.S. In addition, we present examples of relationships between dissolved organic carbon (DOC) and individual ions at specific sites. Concentrations of DOC appear to increase initially with elevated concentrations of Na⁺, Ca²⁺, and Mg²⁺ (sum of these base cations) across sites in the Eastern U.S., but then decrease likely as ionic strength and coagulation increases. However, there are typically declining relationships between DOC concentrations and concentrations of Na⁺, Ca²⁺, and Mg²⁺ at individual local sites. Typically, K⁺ concentrations show a strong positive relationship with DOC concentrations, which is likely because of the importance of biological controls on cycling of potassium; potassium is a limiting nutrient in terrestrial systems (Tripler et al. 2006). Information on U.S. Geological Survey (USGS) sites can be found in Supporting Information Table S1