Carbon and nitrogen isotopes to distinguish sources of sedimentary organic matter in a Caribbean estuary

ABSTRACT

Carbon and nitrogen stable isotope compositions (δ13C and δ15N) of organic matter (OM) and total organic carbon to total nitrogen ratio (Corg/TN) in a sediment core collected in Sagua estuary (Cuba), were investigated to elucidate the origin of the Sedimentary OM (SOM) and changes in its main sources, over the last 100 years. Results showed almost constant values in the elemental and isotope composition of SOM from 1908 to 1970 with an abrupt change after 1970. From 1970 to 2005, δ13C increased from -21.2 up to -19.3 ‰, while δ15N declined from 1.5 to values close to 0 ‰. The output of the mass-balance model for the identification of OM sources indicated that δ13C and Corg/TN values are generally influenced by marine Particulate OM (POM) sources. Between 1900 and 1970, the main OM source in sediments was marine POM (>85 %), with freshwater POM contributing ca. 15%. Since 1970, the establishment of the Alacranes Dam determined drastic environmental changes influencing the OM sources in the area. Mixing models pointed to seagrasses (79 %) as the main contributors to SOM in the first period, while since 1973 onward, the contribution of human-derived sources such as fertilizers and urban discharges became greater. This information can provide baseline data for the environmental management of the Sagua watershed.

KEYWORDS:

• Carbon-13
• Caribbean Sea
• Cuba
• lead-210
• nitrogen-15
• organic matter
• radium-226
• sediments
• Sagua estuary

Acknowledgements

The authors would like to thank Miguel Gomez-Batista, Hector Cartas-Aguila, Joan Hernandez and Yoelvis Bolanos-Alvarez for their contribution to the collection and analysis of samples. Special thanks to the two anonymous reviewers who helped greatly to improve the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the International Atomic Energy Agency (IAEA) under TC Project CUB/7/008 ‘Strengthening the National System for Analysis of the Risks and Vulnerability of Cuba’s Coastal Zone through the Application of Nuclear and Isotopic Techniques and Cuban Nuclear Agency under Project PNUOLU No.3 /2014’.