Abstract
Abstract This study aimed to estimate the nutrients concentration variability in the São Francisco estuary from observed data during the summer and winter seasons of 2014 and to assess the influence of outflow discharges, circulation and precipitation on nutrients fluxes exchange. The marginal difference in the streamflow (1,160 m3/s - summer; 1,260 m3/s - winter) reflected the small role of the river discharge on the variability of the nutrients load. The increase in the sediments load from February (13189.70 T/month) to July (36088.56 T/month) revealed that high precipitation (153.6 mm - winter; 37.6 mm - summer) and internal circulation might have contributed to enhancing the sediment budge towards the estuary mouth. The strong current velocity in July (~0.9 m/s) revealed more potential to export estuarine waters towards the coast when compared to ~0.7 m/s (February). The concentrations and nutrients load were higher during the winter season led by phosphate (3.70 μgL-1) and NID (83.64 μg L-1), against (1.38 μgL-1 and 30.70 μgL-1 - summer), except for silicates with 4.20 mgL-1 (summer) and 3.59 mgL-1 (winter). Despite the active control of outflows, the internal circulation, followed by local precipitation, are considered the main mechanisms behind the increased nutrients load within the estuary.
Highlights
The fluvial transport of matter from the continents to the oceans through the continental drainage is a fundamental process for the planet's structural and functional formation and the continent-ocean geology and biochemistry interface (Liu et al, 2010)
Much effort has been made for accurate detection, identification and estimation of nutrients fluxes in rivers and estuarine waters (Nowlin et al, 2005).As indicated previously, several studies have been conducted in the SFR estuary after the implementation of the last dam in late 1994 (Xingó dam), the majority of the studies were based on data before 2012, prior to the streamflow reduction caused by the extreme drought event initiated in late 2012 (Cavalcante et al, 2020)
The year 2014 was already under influence of the drought event that started in late 2012, the streamflow released at the Xingó's dam was within the minimum rate of 1,100-1,300 m3/s traditionally set by the ANA (ONS, 2018) to avoid ecological problems, which was reflected in the small role of the river discharge on the temporal variability of the nutrients load found in the SFR estuary
Summary
The fluvial transport of matter from the continents to the oceans through the continental drainage is a fundamental process for the planet's structural and functional formation and the continent-ocean geology and biochemistry interface (Liu et al, 2010). Numerous characteristics of the hydrological contribution of river flow are associated with the transport of matter, such as the total suspended solids and nutrients, and their final dispersion as coastal plumes on the continental shelf (Knoppers et al, 2018; Oliveira et al, 2012). Since several years of implementation of the dams in the SFR basin, a substantial number of studies have been reported on the impact of flow regularization in the lower stretch of the SFR on the water physical-chemical characteristics (Souza and Knoppers, 2003; Marques et al, 2004; Knoppers et al, 2006; Milliman et al, 2008), nutrient loads (Sabadini - Santos et al, 2009; 2013; Melo Magalhães et al, 2011; Oliveira et al, 2012; Medeiros et al, 2007; 2015; 2018; Holanda et al, 2017), and saline wedge migration (Cavalcante et al, 2017; 2020). Much effort has been made for accurate detection, identification and estimation of nutrients fluxes in rivers and estuarine waters (Nowlin et al, 2005).As indicated previously, several studies have been conducted in the SFR estuary after the implementation of the last dam in late 1994 (Xingó dam), the majority of the studies were based on data before 2012, prior to the streamflow reduction caused by the extreme drought event initiated in late 2012 (Cavalcante et al, 2020)
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