The influence of the coastal plain on the downstream material fluxes from a small coastal mountainous river basin

Abstract

Small mountainous coastal basins display steeper gradients, suggesting intensified river transport. However, there is little information about downstream trends in rivers with low discharge and material fluxes across coastal plains and its influence on land‐sea material transport. This study examined 2 years of river discharge, suspended sediment and nutrient concentrations, fluxes and yields from upstream to the upper estuary of a typical South‐east Atlantic Basin. This study provides data about the material retention capacity of the coastal plain. The results indicated that the coastal plain did not affect nutrient concentrations, but reduced turbidity, inducing chlorophyll a and consequently primary productivity in the lower river basin and upper estuary. The coastal plain attenuated the increasing downstream material fluxes, but also curbed suspended sediment and TN fluxes, across the lower river/upper estuary. In contrast, TP and PO43− fluxes increased sharply across the coastal plain. This was influenced by natural and anthropogenic P inputs from soils and run‐off to the river channel. These exceeded the retention capacity of the coastal plain and result in a high export efficiency of this nutrient to the sea.

The retention efficiency of a large dam and reservoir eutrophication in a tropical coastal watershed under a rainfall reduction scenario

“…The N and P emission loads estimated through the emission factor approach accounted for 1827 t N/year and 1139 t P/year, while the measured river fluxes averaged 373 t N/year (ranging from 19 to 2077 t/year) and 23 t P/year (from 3.4 up to 192 t/year). The differences were attributed to the low rainfall period and reduced river flow during the sampling period and the nutrient retention in the watershed that decrease downstream river transport (Molisani et al, 2021; Reddy et al, 1999). Concerning the input data for the Emission Factor Model, the reduced availability of some regional data for calculations contributed to the estimate's uncertainties.…”

Long‐term trophic state responses of a large tropical coastal lagoon to land use changes and nutrient transport

“…(2) sampling limitation to measure large river flows during episodic rainfall events which transport most of the nutrients; (3) unknow nutrient fluxes retained by two reservoirs located at both contributing rivers; (4) nutrient retention across the coastal plain and decrease of downstream export by rivers (Molisani et al, 2021;Reddy et al, 1999). Concerning the input data for the Emission Factor Model, the reduced availability of some regional data for calculations contributed to the estimate's uncertainties.…”

“…The N and P loads calculated by the Emission Factor Model (N: 4,471 and P: 1,404 t year −1 ) were higher than the river fluxes (N: 266 and P: 12 t year −1 ). The differences were attributed to (1) the upstream position of the gauging stations in the tributary rivers that underestimate the N and P fluxes; (2) sampling limitation to measure large river flows during episodic rainfall events which transport most of the nutrients; (3) unknow nutrient fluxes retained by two reservoirs located at both contributing rivers; (4) nutrient retention across the coastal plain and decrease of downstream export by rivers (Molisani et al, 2021; Reddy et al, 1999). Concerning the input data for the Emission Factor Model, the reduced availability of some regional data for calculations contributed to the estimate's uncertainties.…”