SEASONAL AND LONG-TERM (1996-2012) TRENDS IN THE CONCENTRATIONS AND RATIOS OF DISSOLVED SILICA AND DISSOLVED INORGANIC NITROGEN IN THE GREAT MIAMI RIVER AT MIAMISBURG, OHIO

Nutrient balance controls the health and survival of ecosystems. A low ratio (<1) of dissolved silica (DSi) to dissolved inorganic nitrogen (DIN) may limit diatom growth. Here we explore, DSI, DIN, and DSi:DIN long-term and seasonal behavior from April 1996 to September 2012 for the Great Miami River below Miamisburg, Ohio. We examined a high-resolution dataset from the National Center for Water Quality Research at Heidelberg University with over 7000 samples collected (8-hour to daily resolution). The Great Miami River below Miamisburg drains an area of 6954 km² and is located within the Till Plains, underlain by limestone, dolomite, and shale. Land use is primarily agricultural (>80%), but includes several major urban areas. The ratio of DSi to DIN shows a significant decrease through time associated with a corresponding significant decrease in DIN concentrations. There was no significant trend in Si concentrations through the long-term record. Seasonally, the lowest mean monthly DSi:DIN (<0.70) occurred between March-May, corresponding with low mean DSi concentrations (<0.20 mM), and high mean monthly flow (>150 m³/s). The highest mean monthly DSi:DIN occurred in July (1.47) during summer low flow (<70 m³) when mean DIN concentrations drop by more than 30% from the previous month. Of note, is that only during summer months were DSi:DIN ratios above 1, suggesting that DIN loading has impaired the nutrient balance within the river. To improve our understanding of the controlling processes of the DSi:DIN ratio during its low period, we plan to compare with a late winter-early spring diel study in Buck Creek (363 km²), a dominantly agricultural sub-watershed of the Great Miami River also influenced by urbanization. Our goal is to understand the controls of DSi:DIN for distinct scales of observation within the Ohio River Basin.