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It is widely recognized that nitrogen (N) inputs from watersheds to estuaries are modified during transport through river networks, but changes within tidal freshwater zones (TFZs) have been largely overlooked. This paper sheds new light on the role that TFZs play in modifying the timing and forms of N inputs to estuaries by (1) characterizing spatial and temporal variability of N concentrations and forms in the TFZs of the Mission and Aransas rivers, Texas, USA, and (2) examining seasonal fluxes of N into and out of the Aransas River TFZ. Median concentrations of dissolved inorganic N (DIN) were lower in the TFZs than in upstream non-tidal river reaches and exhibited spatial gradients linked to locations of major N inputs. These spatial patterns were stronger during winter than summer. The forms of N also changed substantially, with DIN changing to organic N (primarily phytoplankton) within the TFZs. Discharge and N flux comparisons for the Aransas River TFZ demonstrated that secular tidal patterns modulate the timing of N export during baseflow conditions: N export far exceeded input during winter, whereas export and input were relatively balanced during summer. While more data are needed to build an annual N budget, our results show that TFZ can change the timing and form of N export immediately upstream of estuaries. 

While organic and inorganic nutrient inputs from land are recognized as a major driver of primary production in estuaries, remarkably little is known about how processes within the tidal freshwater zones (TFZs) of riversmodify these inputs. This study quantifies organic matter (OM) decomposition rates in surface sediment layers in the lower reaches of two south Texas river channels and identifies key parameters that influence sediment decomposition rates. Sediment cores were collected from nontidal and tidal freshwater sites in theMission and Aransas rivers during two summers (June 2015 and June 2016) and two winters (February 2016, January 2017). We measured oxygen consumption rates, organic carbon and nitrogen content, stable isotope ratios (δ13C and δ15N of OM), and sediment porosity. O2 consumption rates in TFZ sediments were 385 ± 88 μmol O2 m−2 h−1 (summer) and 349 ± 87 μmol O2 m−2 h−1 (winter) in the Aransas River and 767 ± 153 μmol O2 m−2 h−1 (summer) and 691 ± 95 μmol O2 m−2 h−1 (winter) in the Mission River. These rates in TFZs were similar to rates in estuaries and higher than rates at non-tidal riverine sites. Rates of sediment O2 consumption were primarily controlled by OM content and temperature. Sediment OM was dominated by algal biomass from in situ production in both TFZs. We hypothesize that algal production and sinking within TFZs is a major pathway for translocation of watershed-derived nutrients from the water column to the sediments within TFZs. Further work is needed to quantify linkages between decomposition, nutrient remineralization, and potential removal through processes such as denitrification.