Spatiotemporal Variation of Trace Elements and Stable Isotopes in Subtropical Estuaries: II. Regional, Local, and Seasonal Salinity-Element Relationships

• Published in: Estuaries and coasts 2015-05, Vol.38 (3), p.769-781
• Main Authors: Mohan, John A, Walther, Benjamin D
• Format: Article
• Language: English
• Subjects: animals; Coastal Sciences; Earth and Environmental Science; Ecology; Environment; Environmental Management; Estuaries; Evaporation; Evaporation rate; Freshwater & Marine Ecology; Isotopes; Marine environment; rain; Salinity; Seasons; Stable isotopes; temperature; Trace elements; Water and Health
• ISSN: 1559-2723; 1559-2731
• DOI: 10.1007/s12237-014-9876-4

Trace elements and stable isotopes are commonly used in chronologically formed biominerals as proxies of temperature and/or salinity in estuarine and marine environments. To accurately use the chemistry of biominerals as salinity proxies, understanding the consistency of dissolved element-salinity relationships across spatiotemporal scales is essential. We examined relationships between dissolved Ba:Ca, Sr:Ca, Mg:Ca, Mn:Ca, δ¹⁸O, and salinity on regional, local, and seasonal scales in the lower portions of subtropical estuaries (salinities 15–42 ppt) of Texas, including locations where seasonal alternations between negative and positive estuaries can occur. Across all spatiotemporal scales, Ba:Ca displayed a negative linear relationship within the sampled salinity range and was elevated at sites furthest from the ocean and lowest at locations closest to the Gulf of Mexico. This pattern remained consistent as the neutral estuary switched from negative to positive after a rain event. On regional scales, δ¹⁸O displayed a positive linear relationship with salinity and was strongly related to evaporation rates. On local and seasonal scales, evaporation-enriched δ¹⁸O at upper enclosed estuarine sites and this pattern was consistent over time including periods of reverse estuary conditions. Dissolved Sr:Ca and Mg:Ca varied linearly with salinity on regional scales but displayed minimal variation across temporal scales within an estuary. High variability in dissolved Mn:Ca-salinity relations was found at all spatiotemporal scales, with localized episodic peaks of Mn:Ca possibly due to sediment disturbance. Although dissolved Ba:Ca and δ¹⁸O were not predictably related to salinity on local scales, consistent up-estuary enrichment and lower-estuary depletion make these two constituents reliable proxies for animal movements across the ocean–estuary gradient as recorded in chronologically formed biominerals.