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Original Article • Ocean Coast. Res. 69 • 2021 • https://doi.org/10.1590/2675-2824069.20-006akddss copy

Algal blooms and trophic state in a tropical estuary blocked by a dam (northeastern Brazil)

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

The Bacanga River Estuary is socioeconomically important due to artisanal fishing and aquaculture. It is blocked by a dam and is under human pressure along its drainage basin, intensifying the eutrophication process. This study reports on the occurrence of phytoplankton blooms and trophic state (TSI and TRIX) at six sampling sites during the annual cycle. The estuary was divided into downstream and upstream regions. Higher salinity, turbidity, depth, and lower dissolved oxygen levels were found downstream; whereas, high levels of chlorophyll a and nutrient concentrations were observed in both regions. There were blooms of Leptocylindrus danicus (1.45 × 10⁶ cells L^-1) and Skeletonema costatum (1.89 × 10⁶ cells L^-1) downstream; whereas phytoflagellate proliferation, such as those of Chlamydomonas sp. (13.17 × 10⁶ cells L^-1), Euglena gracilis (7.84 × 10⁶ cells L^-1), and Euglena proxima (1.03 × 10⁶ cells L^-1) were recorded upstream, with Chlamydomonas sp. as the discriminant species of this zone. Both trophic indices (TSI; TRIX) indicated elevated trophic conditions for the estuary, classifying it as hypereutrophic. Nevertheless, TSI only showed a significant relationship with some specific phytoplankton blooms. Thus, TSI seems to be the trophic index with a better response in the assessment of estuarine ecological functioning.

Descriptors:
Bacanga estuary; Chlamydomonas bloom; TSI; TRIX; Maranhão; macrotidal regime

TRIX	Conditions	Trophic State
< 2	Water very poorly productive and very low trophic status	Excellent (Ultra-Oligotrophic)
2-4	Water poorly productive and low trophic status	High (Oligotrophic)
4-5	Water moderately productive and medium trophic status	Good (Mesotrophic)
5-6	Water moderate to very productive and high trophic status	Moderate (Mesotrophic to Eutrophic)
6-8	Water very productive and high trophic status	Poor (Eutrophic)
>8	Water highly productive and highest trophic status	Very Poor (Hypereutrophic)

Variable	Unit.	Downstream	Median	Upstream	Median
DO	mg L^-1	5.44±2.39	4.95	7.32±4.28	5.08
pH	-	8.41±0.49	8.38	8.24±0.68	8.38
Temperature	ºC	29.77±1.24	29.98	30.34±1.19	29.77
Salinity	-	23.44±7.70	24.72	14.41±8.77	12.5
Turbidity	NTU	23.45±7.70	24.71	15.09±8.77	12.5
Secchi	m	0.78±0.24	0.70	0.79±0.28	0.79
Depth	m	3.05±2.52	2.71	0.82±1.13	0.55
NO₃^-	µmol L^-1	0.20±0.98	0.01	ND	ND
NO₂^-	µmol L^-1	0.49±0.39	0.43	0.42±0.30	0.28
NH₄⁺	µmol L^-1	111.57±113.49	80.68	31.80±23.25	30.22
DIN	µmol L^-1	112.26±113.39	83.53	32.23±23.51	30.55
DIP	µmol L^-1	26.53±47.06	0.32	26.53±48.13	0.32
DIS	µmol L^-1	60.46±82.38	17.98	77.81±96.58	33.29
TP	µmol L^-1	6.13±5.74	4.84	3.79±5.44	3.95
BOD₅	mg L^-1	13.03±8.33	10.79	10.86±6.98	7.00
DIN:DIP	-	211.38±257.20	162.26	57.71±78.40	27.02
DIN:DIS	-	3.05±2.52	2.71	1.09±1.38	0.83
DIS:DIP	-	158.66±272.15	52.69	224.48±320.83	79.03
Chlorophyll a	µg L^-1	34.26±32.70	26.54	33.20±46.33	14.15

	Downstream				Upstream
Species Density (x 10⁶ cells L^-1)	B1	B2	B3	B4	B5	B6
Chlamydomonas sp.	0.00	0.00	0.00	0.00	0.00	13.17
Cylindrotheca closterium	0.00	0.00	0.01	0.00	0.06	0.11
Dinophysis sp.	0.18	0.02	0.02	0.05	0.04	0.00
Euglena gracilis	0.00	0.00	0.00	0.00	0.01	7.84
Euglena proxima	0.00	0.00	0.00	0.00	1.03	0.14
Leptocylindrus danicus	0.23	1.45	0.10	0.07	0.00	0.00
Protoperidinium sp1	0.18	0.07	0.03	0.04	0.02	0.00
Skeletonema costatum	1.89	1.49	0.05	0.00	0.01	0.01
Melosira varians	0.01	0.11	0.14	0.08	0.14	0.10
Trachelomonas sp.	0.00	0.00	0.00	0.00	0.76	0.00

	Characterizing species	Av.Density x10⁶ cells L^-1	Av.Sim	Contrib%	Cum.%
Downstream Average similarity: 30.57%	Skeletonema costatum	0.86	8.90	29.12	29.12
	Melosira varians	0.11	6.48	21.20	50.32
	Leptocylindrus danicus	0.46	6.29	20.57	70.89
	Protoperidinium sp₁	0.08	2.70	9.00	79.71
	Dinophysis sp.	0.07	1.60	5.00	84.94
	Chaetoceros lorenzianus	0.02	0.83	2.70	87.65
	Chaetoceros rostratus	0.01	0.73	2.38	90.03
	Coscinodiscus sp₁	0.01	0.59	1.94	91.96
	Chaetoceros teres	0.03	0.44	1.44	93.4
	Trachelomonas armata	0.01	0.31	1.03	94.43
	Chaetoceros simplex	0.00	0.20	0.67	95.00
Upstream Average similarity: 3.29%	Euglena proxima	0.59	1.11	33.59	33.59
	Melosira varians	0.12	0.80	24.45	58.05
	Cylindrotheca closterium	0.09	0.49	15.04	73.09
	Pleurosigma sp.	0.04	0.21	6.27	79.36
	Trachelomonas armata	0.05	0.14	4.31	83.67
	Phacus longicauda	0.04	0.12	3.53	87.19
	Chaetoceros subtilis	0.01	0.08	2.35	89.55
	Chaetoceros similis	0.33	0.07	2.06	91.60
	Skeletonema costatum	0.01	0.06	1.76	93.37
	Euglena gracilis	3.92	0.05	1.67	95.03
Discriminating species	Av.Density Downstream x10⁶ cells L^-1	Av.Density Upstream x10⁶ cells L^-1	Av.Diss.	Contrib%	Cum.%
Chlamydomonas sp.	0.00	6.59	27.18	29.41	29.41
Euglena gracilis	0.00	3.92	16.22	17.55	46.96
Euglena proxima	0.00	0.59	13.56	14.68	61.65
Trachelomonas sp.	0.00	0.38	9.78	10.59	72.23
Skeletonema costatum	0.86	0.01	9.43	10.2	82.43
Leptocylindrus danicus	0.46	0.00	5.26	5.69	88.13
Chaetoceros similis	0.03	0.33	1.61	1.75	89.87
Cylindrotheca closterium	0.01	0.09	0.93	1.01	90.88
Protoperidinium sp1	0.08	0.01	0.76	0.82	91.70
Dinophysis sp.	0.07	0.02	0.65	0.70	92.41

Instituto Oceanográfico da Universidade de São Paulo Praça do Oceanográfico 191, CEP: 05508-120, São Paulo, SP - Brasil, Tel.: (11) 3091-6501 - São Paulo - SP - Brazil
E-mail: diretoria.io@usp.br

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