Nutrients Load Estimation in a Regulated Streamflow Estuary: The São Francisco Estuary (NE/Brazil)

Melo, Edmara Ramos; Brandini, Nilva; Medeiros, Paulo Ricardo Petter; Silva, Rosiberto; Cavalcante, Geórgenes

doi:10.1590/0102-7786355000007

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Article • Rev. bras. meteorol. 35 (spe) • Oct-Dec 2020 • https://doi.org/10.1590/0102-7786355000007 copy

Nutrients Load Estimation in a Regulated Streamflow Estuary: The São Francisco Estuary (NE/Brazil)

Estimativa da Carga de Nutrientes em um Estuário com Vazão Controlada: O Estuário do Rio São Francisco (NE/Brasil)

Authorship SCIMAGO INSTITUTIONS RANKINGS

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.

Keywords:
river discharge; nutrients load; São Francisco estuary

Hidroelectric Station	Volume (hm³)	Physiographic regions
UHE Três Marias	19,528.00	Upper São Francisco
UHE Sobradinho	34,117.00	Sub - Middle São Francisco
UHE Luiz Gonzaga (Itaparica)	10,782.00	Sub - Middle São Francisco
UHE Apolônio Sales (Antiga Moxotó)	1,277.00	Lower São Francisco
UHE Paulo Afonso I/III	26.00	Lower São Francisco
UHE Paulo Afonso IV	1,277.00	Lower São Francisco
UHE Xingó	3,800.00	Lower São Francisco

Anchor station						Spatial stations
Months	Variables	Max	Min	$\bar{x}$	σ	cv	Max	Min	$\bar{x}$	σ	cv

February	Salinity (g/kg)	25.88	0.33	4.21	4.31	1.02	4.65	0.01	2.12	1.39	0.65
	Temperature (°C)	29.33	27.75	28.81	0.36	0.01	28.64	27.67	28.29	0.35	0.01
	pH (íons gL^-1)	8.17	7.45	7.71	0.18	0.02	7.90	7.63	7.73	0.08	0.01
	Turbidity (NTU)	4.90	2.70	3.57	0.51	0.14	4.80	2.70	3.31	0.65	0.20
	TSS (mgL^-1)	10.93	1.80	4.32	2.03	0.47	7.60	1.77	3.80	1.75	0.46
	Cl-a (μgL^-1)	1.79	0.13	1.03	0.42	0.40	1.44	0.77	1.16	0.24	0.21
	Si-SiO₂ (mgL^-1)	4.99	2.67	4.20	0.69	0.16	4.61	2.97	3.83	0.53	0.14
	P- ${PO}_{4}^{-3}$ (μgL^-1)	7.99	0.31	1.38	1.69	1.22	1.30	0.31	0.64	0.49	0.77
	P- ${HPO}_{4}^{-2}$ (μgL^-1)	55.80	4.22	13.29	10.15	0.76	13.96	7.47	9.99	1.83	0.18
	N-NH₄ (μgL^-1)	29.40	0.14	13.48	8.69	0.64	33.60	4.22	12.02	9.55	0.79
	N-NO₂ (μgL^-1)	0.57	0.14	0.19	0.12	0.65	0.29	0.14	0.19	0.07	0.39
	N-NO₃ (μgL^-1)	153.99	0.14	17.03	30.80	1.81	3.92	0.14	1.82	1.48	0.81
	DIN (μgL^-1)	168.05	1.12	30.70	30.92	1.01	36.96	5.88	14.03	9.98	0.71
	N:P	167.60	3.62	45.65	45.41	0.99	119.23	9.74	31.95	33.76	1.06
July	Salinity (g/kg)	29.39	0.42	4.92	6.26	1.27	6.56	0.79	3.46	1.82	0.52
	Temperature (°C)	26.24	25.28	25.85	0.24	0.01	26.33	25.79	26.05	0.17	0.01
	pH (íons gL^-1)	8.29	7.10	7.61	0.30	0.04	7.95	7.68	7.80	0.10	0.01
	Turbidity (NTU)	13.70	3.60	5.32	2.09	0.39	8.00	4.30	5.24	1.10	0.21
	TSS (mgL^-1)	57.00	4.75	11.57	10.71	0.93	13.25	6.50	9.18	2.53	0.28
	Cl-a (μgL^-1)	2.35	0.38	1.51	0.51	0.34	6.49	1.20	2.35	1.67	0.71
	Si-SiO₂ (mgL^-1)	4.37	3.00	3.59	0.38	0.11	4.15	3.66	3.90	0.17	0.04
	P- ${PO}_{4}^{-3}$ (μgL^-1)	7.79	1.30	3.70	1.63	0.44	6.17	2.92	4.36	0.98	0.22
	P- ${HPO}_{4}^{-2}$ (μgL^-1)	33.43	7.47	14.72	6.19	0.42	17.20	9.09	12.52	2.62	0.21
	N-NH₄ (μgL^-1)	55.66	5.06	17.38	11.76	0.68	21.00	4.22	9.35	4.90	0.52
	N-NO₂ (μgL^-1)	2.00	2.00	2.00	0.00	0.00	2.00	2.00	2.00	0.00	0.00
	N-NO₃ (μgL-1)	87.60	36.64	64.26	14.47	0.23	75.28	59.04	67.88	5.63	0.08
	DIN (μgL^-1)	110.60	53.82	83.64	13.79	0.16	97.72	65.26	79.22	9.11	0.11
	N:P	78.24	8.64	29.60	19.45	0.66	26.26	12.76	18.97	4.60	0.24

Months	Parameter	Average concentration	Average monthly flow (m³/s)	Daily loads (T/day)	Monthly loads (T/Month)
February	TSS	4.32 mg.L^-1	1262.06	471.06	13189.70
	SiO₄	4.20 mg.L^-1		458.26	12831.37
	${PO}_{4}^{-3}$	1.38 μg L^-1		0.15	4.22
	${HPO}_{4}^{-2}$	13.29 μg L^-1		1.45	40.57
	NID	30.70 μg L^-1		3.35	93.73
July	TSS	11.57 mg.L^-1	1164.00	1164.15	36088.56
	SiO4	3.59 mg.L^-1		361.28	11199.71
	${PO}_{4}^{-3}$	3.70 μg L^-1		0.37	11.54
	${HPO}_{4}^{-2}$	14.72 μg L^-1		1.48	45.90
	NID	83.64 μg L^-1		8.41	260.84

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E-mail: sbmet@sbmet.org.br

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[1] Correspondeing authors: Edmara Ramos Melo, edmara-melo@hotmail.com; Geórgenes Cavalvante, georgenes.cavalcante@icat.ufal.br.