Revisiting the concept of longitudinal gradients in reservoirs

Carneiro, Fernanda Melo; Bini, Luis Mauricio

doi:10.1590/S2179-975X1319

Brasil

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Original Article • Acta Limnol. Bras. 32 • 2020 • https://doi.org/10.1590/S2179-975X1319 linkcopy

Revisiting the concept of longitudinal gradients in reservoirs

Revisitando o conceito de gradientes longitudinais em reservatórios

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract:

Aim Reservoirs are an increasingly common type of aquatic ecosystem in countries where hydropower is a dominant energy source. Understanding the functioning of these systems is essential to establish monitoring strategies or priorities for their use. The main goal of this study was to test for the presence of longitudinal gradients in reservoirs.

Methods We gathered data along the main axes of 17 tropical reservoirs to test the relationship between different ecological indicators (e.g., turbidity, nutrient concentrations, and phytoplankton abundance) and distance to the dam. After, we tested whether the strengths of longitudinal gradients were related to a set of explanatory variables (occurrence in a reservoir cascade, presence of tributaries near the dam, land-use, morphology and reservoir age).

Results We did not find significant relationships between our set of ecological indicators and distance to the dam and the strengths of these relationships were not consistently predicted by the explanatory variables. However, different indicators suggested patterns consistent with expectations.

Conclusion The distance to the dam is not a consistent predictor of the spatial variation of different limnological variables in tropical reservoirs, highlighting a high degree of context-dependency.

Keywords: reservoir zonation; hydroelectric; phytoplankton; nutrients; monitoring

Factor	Predictions (and supporting references, when available)
Age	Reservoir recently created should exhibit less pronounced longitudinal gradients (Jorcin & Nogueira, 2008).
Land-Use	An increase in agriculture and cattle ranching in the watershed should decrease the strengths of the longitudinal gradients because these activities increase diffuse nutrient loading (in different regions of a reservoir), blurring the gradient (Espíndola et al., 2000; Karatayev et al., 2005).
Morphology	More dendritic reservoirs (e.g., high shoreline development) with complex hydrological dynamics should have less pronounced longitudinal gradients (Straškraba et al., 1993).
Tributaries	The presence of tributaries nearby the lacustrine zones of reservoirs should interfere with the formation of longitudinal gradients (Rice et al., 2001).
Cascades	Longitudinal gradients in isolated reservoirs should differ from those observed in reservoir cascade (Tundisi & Matsumura-Tundisi, 2003; Draštík et al., 2008).

	Turbidity (NTU)	Water Transparency (m)	Conductivity (µS/cm)	Total Phosphorus (µg.l^-1)	Total Nitrogen (µg.l^-1)
R1	2.03	3.63	45.35	2.83	1021.14
R2	1.8	4.88	3.19	1	355.35
R3	1.83	4.54	31.63	1.77	355.82
R4	1.53	5.12	38.25	4.3	504.05
R5	1.1	6.01	33.05	2.16	391.36
R6	1.9	4.21	32.7	2.46	450.43
R7	1.31	5.76	44.2	2.65	510.28
R8	10.09	1.23	38.2	12.22	384.82
R9	1.25	5.28	46	1.35	431.5
R10	2.41	3.3	18.57	2.03	387.71
R11	1	3.94	18.27	3.17	434.65
R12	1.8	3.68	41	7.99	325.54
R13	1.71	3.28	33.4	7.72	364.72
R14	1.25	5.88	3.99	3.6	280.68
R15	0.77	5.65	73.48	1.25	322.67
R16	0.75	6.63	92.2	2.22	365.47
R17	8.86	1.57	64	11.22	594.24

Reservoir	Chl-a	Richness	Density	GALD	Zygn	Baci	Cyan	Chlo	Cryp	Eugl
	(µg.l^-1)		(Ind.ml^-1)	(µm)	(Ind.ml^-1)	(Ind.ml^-1)	(Ind.ml^-1)	(Ind.ml^-1)	(Ind.ml^-1)	(Ind.ml^-1)
R1	0.43	53.75	1385	20.4	18	155	348	284	171	37
R2	0.13	19.67	519	10.2	16	42	119	104	95	19
R3	1.04	27.5	684	16.7	12	31	171	138	79	46
R4	1.22	42.75	2011	18.1	18	104	223	543	565	20
R5	0.5	23.75	419	19.8	1	4	73	94	56	0
R6	0.13	10.25	272	16.4	3	15	24	53	11	0
R7	0.75	19.5	555	15.1	28	74	89	49	122	0
R8	0.83	19.67	504	28.5	0	7	14	207	60	3
R9	0.43	20	951	15.3	120	15	260	168	106	0
R10	0.04	8.67	35	38.1	190	9	1	5	4	0
R11	0.52	24	387	18	84	3	38	126	9	4
R12	1.13	27.33	1844	31.5	20	7	1165	260	61	3
R13	1.84	24.33	1397	15.8	59	121	94	289	81	9
R14	0.08	10.67	62	23.3	325	209	4	31	5	0
R15	0.6	42.5	2934	39.8	132	95	1911	622	40	3
R16	0.93	51.17	2282	27.7	2	24	692	979	13	0
R17	0.23	20	235	25	0	0	42	82	60	0

Reservoir	Land-use (%)	SD_L	Age (years)	Tributary	Cascade
R1	84.31	6.67	4	1	1
R2	9.39	0.08	6	0	0
R3	45.03	6.31	27	1	1
R4	77.8	4.8	12	1	1
R5	56.13	6.31	29	0	1
R6	56.23	3.24	51	1	1
R7	60.64	4.58	31	1	1
R8	74	0.08	7	0	0
R9	11.85	4.91	5	1	0
R10	2.22	0.1	2	0	0
R11	0.4	0.15	3	1	1
R12	1.03	0.2	1	1	1
R13	0.5	0.14	3	1	1
R14	7.66	1.71	19	1	0
R15	47.67	4.61	7	1	1
R16	49.11	7.84	13	1	1
R17	99.73	0.2	54	1	0

Response variables	Intercept	Distance to the Dam	P
Turbidity	0.654	0.014	0.457
Water transparency	0.401	0.024	0.037
Conductivity	5.547	0.092	0.605
Total Phosphorus	0.977	0.698	0.290
Total Nitrogen	46.769	11.819	0.463
Chlorophyll-a	0.125	0.011	0.596
Phytoplankton richness	2.951	0.277	0.422
Total density	216.835	30.880	0.400
GALD	2.237	0.179	0.958
Zygnemaphyceae	21.613	1.124	0.142
Bacillariophyceae	14.124	2.209	0.082
Cyanophyceae	149.314	17.297	0.949
Chlorophyceae	52.168	6.391	0.834
Cryptophyceae	23.123	2.694	0.267
Euglenophyceae	5.311	0.763	0.916

	R²_adj	Age	Land-use	SL_D	Tributary	Cascade
Turbidity	-0.333	<0.001	<0.001	<0.001	<0.001	<0.001
Water Transparency	0.149	-0.133	-0.056	0.703	-0.125	-0.364
Conductivity	0.278	-0.204	0.354	0.568	0.043	-0.483
Total Phosphorus	0.738	-0.267	0.515	-1.02	0.421	-0.027
Total Nitrogen	-0.004	-0.501	0.481	0.109	0.039	-0.046
Chlorophyll-a	0.329	-0.435	0.006	-0.546	-0.003	0.449
Richness	0.596	-0.566	0.749	0.263	0.054	0.136
Density	-0.075	-0.347	0.297	0.29	0.108	-0.194
GALD	-0.231	0.283	-0.285	0.123	0.016	0.052
Zygnemaphyceae	-0.048	-0.075	-0.102	0.516	0.052	-0.08
Bacillariophyceae	0.438	-0.771	0.871	-0.053	0.092	-0.004
Cyanophyceae	-0.003	-0.064	0.205	0.417	-0.042	-0.397
Chlorophyceae	0.462	-0.304	0.422	0.613	0.003	-0.124
Cryptophyceae	0.221	-0.549	0.648	0.067	0.21	0.05
Euglenophyceae	0.08	-0.577	0.677	-0.225	0.153	-0.081

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