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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.
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Defining a Riverine Tidal Freshwater Zone and Its Spatiotemporal Dynamics
Abstract Tidal freshwater zones (TFZs) are transitional environments between terrestrial and coastal waters. TFZs have freshwater chemistry and tidal physics, and yet are neither river nor estuary based on classic definitions. Such zones have been occasionally discussed in the literature but lack a consistent nomenclature and framework for study. This work proposes a measurable definition for TFZs based on three longitudinal points of interest: (1) the upstream limit of brackish water, (2) the upstream limit of bidirectional tidal velocities, and (3) the upstream limit of tidal stage fluctuations. The resulting size and position of a TFZ is transient and depends on the balance of tidal and riverine forces that evolves over event, tidal, seasonal, and annual (or longer) timescales. The concept, definition, and transient analysis of TFZ position are illustrated using field observations from the Aransas River (Texas, USA) from July 2015 to July 2016. The median Aransas TFZ length was 59.9 km, with a late summer maximum of 66.0 km and a winter minimum of 53.6 km. The TFZ typically (annual median) began 11.8 km upstream from the river mouth (15.4 km winter/11.2 km summer medians) and ended 71.7 km upstream (69.0 km/77.2 km). Seasonally low baseflow in the Aransas River promoted gradual coastal salt encroachment upstream, which shortened the TFZ. However, sporadic large rainfall/runoff events rapidly elongated the TFZ. The TFZ definition establishes a quantifiable framework for analyzing these critical freshwater systems that reside at the nexus of natural and human‐influenced hydrology, tides, and climate.
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- Award ID(s):
- 1417433
- PAR ID:
- 10446488
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 56
- Issue:
- 4
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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