Nutrient gradients in Panamanian estuaries: effects of watershed deforestation, rainfall, upwelling, and within-estuary transformations

Abstract

To test whether deforestation of tropical forests alters coupling of watersheds, estuaries, and coastal waters, we measured nutrients in 8 water- shed-estuarine systems on the Pacific coast of Panama where watershed forest cover ranged from 23 to 92%. Watersheds with greater forest cover discharged larger dissolved inorganic nitrogen concentrations and higher N/P into estuary headwaters. As freshwater mixed with seawater down-estuary, within-estuary biogeo- chemical processes erased the imprint of watershed deforestation, increased ammonium, lowered nitrate concentrations, and otherwise altered down-estuary water column composition. As estuarine water left man - grove estuaries, ammonium, nitrate, and phosphate, but not dissolved organic nitrogen, were exported to receiving near-shore waters. Mangrove estuaries in this region thus provide important ecological services, by uncoupling coastal waters from changes in terres- trial land covers, as well as by subsidizing adjoined receiving coastal waters by providing nutrients. The pattern of landsea coupling and exports was disrupted during La Nina-influenced conditions. In one instance when La Nina conditions led to upwelling of deeper layers, high concentrations of marine-derived ammo- nium were inserted into estuaries. In another instance, La Nina-associated high rainfall diluted nutrient con- centrations within estuaries and lowered salinity re- gionally, and the fresher upper layer impaired coastal upwelling. Regional rainfall has increased during the last decade. If La Nina rainfall continues to increase, disruptions of current landestuarysea couplings may become more frequent, with potentially signifi- cant changes in nutrient cycles and ecological services in these coupled ecosystems.