Watershed-Estuary Coupling in Pacific Panama: Isotopic Evidence of Forest and Pasture Land Covers on Watersheds and Marine Contributions to Suspended Particulate Matter in Mangrove Estuaries
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Tropical estuaries are increasingly altered by inputs from watersheds subject to widespread deforestation, as well as by globally driven hydrodynamic changes in adjoining seas. To assess contributions of C4 and C3 plants (from pasture and forest vegetation cover, respectively) to particulates exported from Pacific Panama watersheds, we measured δ13C and δ15N in suspended particulate matter (SPM) within eight mangrove estuaries whose watersheds differed in degree of conversion from forest to pasture land cover. These measurements also allowed evaluation of down-estuary transformations and the relative marine influence on transport and exchanges of particles between land, estuary, and sea. Imprint of watershed mosaic was detectable in δ13C of SPM within upper reaches of estuaries but disappeared down-estuary. Detectably heavier δ13C suggested that C4 plants contributed to SPM in upper reaches of estuaries. δ13C signatures were sufficiently sensitive to reveal presence of a small, but still detectable, contribution by C4 grasses to SPM. Influence of heavier marine-derived sources increased down-estuary, erasing terrestrial imprints. δ13C and δ15N in SPM, and in mangrove species present, became enriched down-estuary, likely from increased inputs of particulates bearing heavier signatures from upwelled waters. In this tropical Pacific region, estuarine particulates are subject to increasing shifts in land cover as deforestation increases, and to global-scale changes in hydrodynamic forcing of upwelled waters.
KeywordsLand-sea coupling Mangrove forests Suspended particulate matter Stable isotopes Carbon Nitrogen
This work was funded by NSG Grant BIO-0842413; we thank Henry L. Gholz and Timothy K. Kratz of NSF for their support of this work. We would not have been able to carry out the fieldwork without the excellent resources of the Liquid Jungle Laboratory (LJL) built and operated by Jean Pigozzi and the Canales de Tierra Foundation, and we are much indebted to the LJL staff for providing excellent support and facilities to this work. We thank L. Madin, L. Camilli, and the Ocean Life Institute at the Woods Hole Oceanographic Institution for initial support and throughout the work. The support of the Woods Hole Consortium was instrumental to facilitating work by our multi-institutional research team. S. Wilkins, S. Baldwin, R. M. Oliveira, J. Tucker, R. McHorney, S. Kelsey, J. Brennan, K. Hernández, N. Mueller, and J. Bissonette helped carry out the demanding field work involved in the project.
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