Nutrient and Organic Matter Dynamics in Tidally-Influenced Freshwater Sections of the Mission and Aransas Rivers, South Texas
Abstract
The Mission and Aransas rivers empty into Copano Bay, Texas, a part of the Mission-Aransas National Estuarine Research Reserve (MANERR). Both rivers have extensive agricultural and other developed land use in their watersheds. Export from the rivers is dominated by infrequent storm events. During normal, low- flow conditions, the lower sections of both rivers are pushed upstream during incoming tides, but a salinity gradient forms only relatively near the bay. Tidally-influenced freshwater reaches are poorly accounted for in riverine export modeling, but may significantly alter export due to processing during long residence times. Samples collected between June and August, 2008, were analyzed for nutrient and organic matter concentrations as well as particulate C and N stable isotope ratios. Although nutrient concentrations are high in upstream sections of the rivers, dissolved inorganic nitrogen concentrations are near limit of detection throughout the tidally-influenced freshwater reaches, and both systems are strongly N-limited. Autochthonous production dominates (POδ13C < -30‰), with 15N-enriched PON values indicating chiefly terrestrial-derived N from wastewater outfalls. Production (as measured by [PON], [POC], [Chl a], and surface %DO) is high throughout the tidal freshwater reaches and peaks near the beginning of the salinity gradient. The [DON] and [DOC] maximum lies just downstream of maximum productivity and corresponds with a peak in [POC]:[Chl a] ratios, indicating decomposition likely due to breakdown of freshwater phytoplankton encountering the increasing salinity. DON and DOC concentrations decrease downstream and into the bay, but remain elevated. Overall, the data indicate high production stimulated by inorganic nutrient inputs within the tidally-influenced freshwater reaches, resulting in an export of freshly produced organic material to the bay. The transformative effects of tidal freshwater reaches can be significant and should be considered in models and when designing management plans for such systems.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMOS41E1268K
- Keywords:
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- 4217 Coastal processes;
- 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460);
- 4845 Nutrients and nutrient cycling (0470;
- 1050)