Emission load, flux and estuarine modulation of ammonium and phosphate from a small subtropical river basin to the coast

Abstract

Small river basins represent an important source of dissolved nutrients to the coast, which are fundamental to coastal primary production. However, such transport is a resultant of nutrient fluxes from small rivers, estuarine modulation and, currently, anthropic emission loads. In this context, this study assessed ammonium (NH 4 + ) and phosphate (PO 4 3− ) dynamics in a small subtropical river-estuary system by addressing seasonal fluxes from the river to the estuary, as well as the estuarine spring-neap tidal modulation, the resultant fluxes to the coast, and the influence of natural processes and anthropogenic activities on the land-sea nutrient dynamics. The results indicated that NH 4 + and PO 4 3− fluxes in the small river-estuary system were influenced by anthropic emission loads and by estuarine modulation through amplification of river discharge and NH 4 + and PO 4 3− concentrations and fluxes within the estuary. During most of the time, the small river-estuary system exported such dissolved nutrients to the coast, likely contributing to coastal primary production. • NH 4 + and PO 4 3− budgets were assessed for a small subtropical river-estuarine system. • Small estuary amplified the river NH 4 + and PO 4 3− fluxes to the coast. • NH 4 + in the estuary was influenced by raw sewage discharge. • Small river exported nutrients contributing to coastal primary production.