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 watershed-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 biogeochemical processes erased the imprint of watershed deforestation, increased ammonium, lowered nitrate concentrations, and otherwise altered down-estuary water column composition. As estuarine water left mangrove 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 terrestrial land covers, as well as by subsidizing adjoined receiving coastal waters by providing nutrients. The pattern of land−sea coupling and exports was disrupted during La Niña-influenced conditions. In one instance when La Niña conditions led to upwelling of deeper layers, high concentrations of marine-derived ammonium were inserted into estuaries. In another instance, La Niña-associated high rainfall diluted nutrient concentrations within estuaries and lowered salinity regionally, and the fresher upper layer impaired coastal upwelling. Regional rainfall has increased during the last decade. If La Niña rainfall continues to increase, disruptions of current land−estuary−sea couplings may become more frequent, with potentially significant changes in nutrient cycles and ecological services in these coupled ecosystems.The impacts of anthropogenic changes in land cover on the export of nutrients to tropical estuaries are attenuated by mangroves (inset).Photos: I. Valiela FREE REE ACCESS CCESS

Dynamics of nutrient and phytoplankton in Epe Lagoon, Nigeria: possible causes and consequences of reoccurring cyanobacterial blooms

“…Seasonality and distribution of terrestrial and aquatic organisms in the tropics are determined by rainfall pattern (Webb 1960). Rainfall is a driving force in aquatic ecosystems of south-west, Nigeria (Olaniyan 1969;Oyenekan 1988;Nwankwo 1996) because it influences flow rate, mixing, dilution and nutrient recycling (Valiela et al 2013). The observed lowest temperature (≤ 27 °C) in the wet season (late wet I) might be attributed to the influx of cooler flood waters from wetlands, adjoining creeks and rivers.…”

Effects of Extreme Climatic Events on the Hydrological Parameters of the Estuarine Waters of the Amazon Coast

“…the input of freshwater, which in turn may modify saline stratification patterns, seawater intrusion, renewal time, and the processes inherent to the biogeochemical cycles (Grimm et al 2000;Garcia et al 2004). During drier conditions, estuarine waters become more saline, and dissolved inorganic nutrient concentrations decline (Wilkerson et al 2002;Wetz et al 2011;Valiela et al 2013), with the opposite pattern being observed during events of increased rainfall (Valiela et al 2012;Thompson et al 2015). These alterations affect biological productivity, and impact fishery production, which may have serious socio-economic implications (Garcia et al 2004;Rossi and Soares 2017).…”

Satellite observations estimating the effects of river discharge and wind‐driven upwelling on phytoplankton dynamics in the Chesapeake Bay

“…On continental shelves, the effect of upwelling was associated with the intrusion of deep ocean water, characterized by N/P balance close to 16:1, in contrast to river discharge where this ratio was typically much higher. This effect was reported at the American Pacific coast (Brown & Ozretich, 2009;Colbert & McManus, 2003;Valiela et al, 2013), Iberian Peninsula (Alvarez-Salgado et al, 1996;Bode et al, 2017;Pérez et al, 2000), the East China Sea (Pei et al, 2009;Tseng et al, 2014), and included complex interaction between turbid river plumes and low-turbidity, low-nutrient ocean waters. Given that Chesapeake Bay is sheltered from the ocean by a shallow entrance and that its circulation and nutrient dynamics are largely driven by riverine influence, it contrasts with environments typical of large continental shelves and is less affected by upwelling.…”