THE CATION CONUNDRUM: BENIGN INDICATORS OR IMPORTANT MECHANISMS IN AQUATIC ECOSYSTEMS?

Urbanization alters the cation (Ca, Mg, Na, K) concentration and composition of surface water in cities. Often these changes accompany other sources of pollution which degrade water quality and have deleterious effects on aquatic ecosystems. Despite this common occurrence, our understanding of the sources and how cations interact with the aquatic ecosystems remains poorly developed. Here I explore two questions, 1) can cations help serve as indicators of surface water pollution and changing watershed processes, and 2) do cations act as mechanisms in biogeochemical cycling and aquatic ecosystems in cities? To explore these questions, I compare the water chemistries from two urban watershed gradient case studies located in diverse geological and climate regions: Carters Creek in College Station, TX and Stroubles Creek in Blacksburg, VA. The results from these and other watersheds indicate the concentration and composition of cations in urban watersheds vary spatially and temporally by climate, geology, and urban gradient. In Stroubles Creek, by examining a broad suite of elements in combination with multiple microbiological water quality indicators, we find the relationships within and between water chemistry and water quality indicators to be diverse and complex. Surprisingly, traditional biogeochemical elements (N, P, C) were less strongly related to water quality indicators than were Ca, Mg, Na in watersheds without wastewater treatment plants. The results suggest that wastewater via subsurface flowpaths may more broadly impact surface water chemistry and quality than expected and that cation chemistry may serve as a background mediator of watershed biogeochemical cycling in cities.