My name is Tricia Light, and I’m a junior majoring in chemistry at Scripps College in Claremont, California. This summer I’m working with Dr. Amber Hardison and Dr. Jim McClelland as part of a 3-year study on the “oscillic freshwater zone” (OFZ) of rivers. They are proposing a definition of the OFZ as the region of a river that contains only freshwater but is tidally influenced, so it is not quite an estuary but also not quite a river. Little research has been done on this region before, but there is some evidence suggesting that important nutrient transformations occur in the OFZ. Rivers bring both natural and anthropogenic nutrients from land into estuaries and, ultimately, the ocean, so the manner in which the OFZ transforms these nutrients can have a large effect on coastal ecosystems.
My project is investigating the transformation of nitrogen in OFZ sediments. While nitrogen makes up most of the atmosphere, nitrogen is often a limiting nutrient for biological processes because most organisms can only use certain forms of bioavailable nitrogen. Sediments are important sites for the complex nitrogen transformations that occur in aquatic environments. For example, nitrogen fixing bacteria within sediments turn unavailable N2 into bioavailable NH4+, nitrifying bacteria turn NH4+ into NO2– and NO3–, and denitrifying bacteria turn NO3– and NO2– back into N2. I will take sediment cores from 5 stations in the OFZ of the local Mission and Aransas Rivers and determine where and to what extent nitrification and denitrification are occurring within the sediment. First, I will record the oxygen profiles of each core because different nitrogen cycling processes require conditions with or without oxygen. Next, I will measure the concentration of NO3–, NO2– and NH4+ in the water trapped in the sediment at various depths within the core. This information, along with data collected by my labmates, will help us determine how the OFZ might be transforming nitrogen from the river before it enters the estuary.