Influence of freshwater inflow on source of particulate organic matter in a subtropical estuary, Texas

The Mission-Aransas National Estuarine Research Reserve (MANERR) in subtropical Texas is characterized by arid climate, drought, and sporadic precipitation events that affect the water quality of the system. Changes in the inflow regime in turn may alter the source and composition of the particulate organic matter (POM). The MANERR implements a system-wide monitoring program (SWMP) that continuously records water quality data and collects monthly nutrient samples at five stations across the estuarine salinity gradient. To evaluate the impact of freshwater inflow on the source of POM, monthly samples of surface water POM were collected at each station beginning in 2012 and analyzed for carbon and nitrogen concentrations and stable isotope content and pigment concentrations. These data were combined with a six-year time series (2010-2015) of SWMP salinity, stream flow and nutrient data. An increase in particulate organic carbon (POC) (54 to 136 µM) and particulate nitrogen (PN) (7 to 33 µM) concentrations was observed beginning in early 2015, as well as a depletion in the δ¹³C-POC (-24.56 to -26.82 permil); this pattern is consistent with a series of storm events during this time frame, which caused an increase in freshwater inflow and thus a precipitous drop in estuarine salinity. These results suggest a shift in the POM pool from dominance by marine algae towards terrestrial organic matter in the system as a result of increased freshwater inflow. This change of POC source likely occurred previously in our time series. In late 2009, the system experienced high rates of river discharge due to a series of recurring storm events which lasted until early 2011. This resulted in 2010 being a “wet” year with a much lower mean annual salinity (~8 ppt) than that of 2013, a “dry” year (~36 ppt). Data analysis of POC, PN, δ¹³C-POC, δ¹⁵N-PN, and pigments are still ongoing and will be presented.