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
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.
- Balesdent, J., and A. Mariotti. 1996. Measurement of soil organic matter turnover using 13C natural abundance. In Mass spectrometry of soils, ed. T.W. Boutton and S. Yamasaki, 83–11. New York: Marcel Dekker, Inc.Google Scholar
- Barros, V.G., J. Mas-Plas, T.M. Oliviera Novais, E. Sacchi, and G.M. Zuppi. 2008. Hydrological mixing and geochemical processes characterization in an estuarine/mangrove system using environmental tracers in Babitonga Bay (Santa Catarina, Brazil). Continental Shelf Research 28 (4-5): 682–695.CrossRefGoogle Scholar
- Cernusak, L.A., K. Winter, J. Aranda, and B.L. Turner. 2008. Conifers, angiosperm trees, and lianas: Growth, whole-plant water and nitrogen use efficiency, and stable isotope composition (δ13C and δ18O) of seedlings grown in a tropical environment. Plant Physiology 148 (1): 642–659.CrossRefGoogle Scholar
- Mariotti, A., F. Gadel, P. Giresse, and Kinga-Mouzeo. 1991. Carbon isotope composition and geochemistry of particulate organic matter in the Congo River (Central Africa): Application to the study of Quaternary sediments off the mouth of the river. Chemical Geology 86 (4): 345–357.Google Scholar
- Martinelli, L.A., M.V. Ballester, A.V. Krusche, R.L. Victoria, P.B. de Camargo, M. Bernandes, and J.P.H.B. Ometto. 1999a. Landcover changes and the δ13C composition of riverine particulate organic matter in the Piracicaba River Basin (southeast region of Brazil). Limnology and Oceanography 44 (7): 1826–1833.CrossRefGoogle Scholar
- Ometto, J.P.H.B., J.R. Ehleringer, T.F. Domingues, J.A. Berry, F.Y. Ishida, E. Mazzi, N. Higuchi, L.B. Flanagan, G.B. Nardoto, and L.A. Martinelli. 2006. The stable carbon and nitrogen isotopic composition of vegetation in tropical forests of the Amazon Basin, Brazil. Biogeochemistry 79 (1-2): 251–274.CrossRefGoogle Scholar
- Parnell, A.C., R. Inger, S. Bearhop, and A.L. Jackson. 2010. Source partitioning using stable isotopes: Coping with too much variation. PLoS One 5: 5.Google Scholar
- Parnell, A.C., D.L. Phillips, S. Bearhop, B.X. Semmens, E.J. Ward, J.W. Moore, A.L. Jackson, J. Grey, D.J. Kelly, and R. Inger. 2013. Bayesian stable isotope mixing models. Environmetrics 24: 387–399.Google Scholar
- Terwilliger, V.J. 1997. Changes in the δ13C values of trees during a tropical rainy season: Some effects in addition to diffusion and carboxylation by Rubisco? American Journal of Botany 84: 1693–1700.Google Scholar
- Valiela, I., L. Camilli, T. Stone, A. Giblin, J. Crusius, S. Fox, C. Barth-Jensen, R. Oliveira Monteiro, J. Tucker, P. Martinetto, and C. Harris. 2012. Increased rainfall remarkably freshens estuarine and coastal waters on the Pacific coast of Panama: Magnitude and likely effects on upwelling and nutrient supply. Global and Planetary Change 92-93: 130–137.CrossRefGoogle Scholar
- Valiela, I., M. Bartholomew, A. Giblin, J. Tucker, C. Harris, P. Martinetto, M. Otter, L. Camilli, and T. Stone. 2014. Watershed deforestation and down-estuary transformations alter sources, transport, and export of suspended particles in Panamanian mangrove estuaries. Ecosystems 17 (1): 96–111.CrossRefGoogle Scholar