PHYSICAL AND CLIMATIC CONTROLS ON NITROGEN EXPORT ACROSS THE PANAMA CANAL WATERSHED

Previous studies have shown that a disproportionate amount of nitrogen is delivered to the ocean from tropical rivers compared to those in temperate regions. It is difficult to draw proper relationships between the timing of nutrient export with physical and climatic controls because many of these studies generally lack long-term data and there are multiple methods to calculate long term fluxes and yields. In this study, we used long-term discharge records (1998–2019) and nitrate (NO₃) data (2003­­­­–2019) records of high temporal frequency from 8 drainage basins of the Greater Panama Canal watershed collected and reported by the Autoridad del Canal de Panamá (ACP). Long-term (1998–2019) NO₃ fluxes and yields were calculated using two regression models: (1) a linear regression that uses discharge as an independent variable, and (2) a USGS LOADEST model that uses discharge and time as independent variables. The LOADEST regression was a better estimate of NO₃ flux than the linear regressions in all but two basins. Long-term NO₃ yields ranged from 0.109-3.482 kg/km²/yr and 0.109-0.339 kg/km²/yr for the LOADEST and linear models, respectively. We also compared 2015 nitrate yields to potential controlling variables including annual rainfall, suspended sediment, and land use/cover. Pearson correlations show nitrate yields have a positive significant relationship with forest cover (R=0.41, R=0.36) and with suspended sediment yields (R=0.86, R=0.90) for the LOADEST and linear model, respectively. Our results demonstrate variability in predicted NO₃ export based on the regression model employed. Regardless of model choice, correlations between NO₃–land cover and NO₃–suspended sediment were similar. This study demonstrates that land cover is an important control on nitrate export in the tropics.