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Introduction

The transitional areas between terrestrial and aquatic ecosystems are called wetland where the water table is generally at or near the surface on the land which is covered by shallow water¹。It is estimated that, India contains about 757.06 thousand wetlands with a total wetland area of 15.3 m ha, which is nearly 4.7% of the total geographical area of the country²。In India, West Bengal holds a significant position for its large aquatic wealth in terms of floodplain lakes or natural wetlands³。Floodplain lakes are formed by cutting of river meanders from the main river due to erosion of river banks and siltation. Floodplain wetlands or ox-bow lakes supports a rich and profitable inland fishery system in eastern and north-eastern states of the country particularly in West Bengal⁴。More than 150 floodplain lakes are situated in West Bengal covering an area of 42,000 ha, which accounts almost 22% of the state’s total freshwater area⁵。These wetlands or floodplain lakes are locally named as beels or baurs. These beels are rich in finfish diversity which is economically beneficial. The productivity of fish can be increased by sustainable use of fish as well as water quality monitoring in regular basis. The physico-chemical parameters of water like pH, temperature, dissolved oxygen, free carbon di-oxide, hardness, alkalinity, salinity, biochemical oxygen demand etc. should be recorded regularly for keeping the aquatic habitat favourable to the fish⁶。

目前investigation was performed to study various physicochemical parameters of water in a floodplain lake for a period of three years from 2015 to 2017. River Jalangi and its branches form a large complex of floodplain lakes, locally known as “beels”. The study was based on the relationship between fish assemblage, seasonal fluctuation of water quality parameters and various biodiversity indices of such beel.

The water quality parameters have great influence on the fish assemblage of lakes⁷。In India, since the last few decades, the floodplain lakes have become victims of environmental deterioration. Most of the lakes are shrinking due to siltation, various anthropogenic activities like municipal discharge, agricultural run-off, eutrophication etc.⁸。

Therefore, seasonal changes in the physicochemical parameters of water and its relationship with various biodiversity indices of fish are very important for assessment of fish diversity in floodplain wetlands. So, the main objectives of the present study was to assess seasonal changes of various physicochemical parameters of water and their impact on various fish biodiversity indices of Hasadanga beel, a floodplain wetland of West Bengal to determine ecological health of the beel.

Materials and Methods

Study Area

The present studies were done on a floodplain lake namely Hasadanga beel. The Hasadanga beel is located besides NH34, near Bahadurpur rail station of the Sealdah-Lalgola Section of Eastern Railway, Under CD Block-Krishnagar-I, Sadar Sub-Division of Nadia District, West Bengal, India. The latitude and longitude of the area are 23⁰26日的41.56”N to 23⁰27’48.50”N and 88⁰27’26.54”E to 88⁰29’16.83”E respectively. The tropic of cancer passes beside the area. The nearest town is Krishnagar, District-Nadia. The primary Source of pollution of the beel is agricultural run-off. The nearest river of the beel is Jalangi. Controlling authority of the beel is district administration, Nadia. Average depth of the beel is ranges between 0.7-2.25 m.Surrounding temperature (⁰C) is ranges between Summer-40-42 and Winter-9-12. Average rainfall is ranged between 1165-1215mm. The beel is closed type and practices traditional type fisheries. Total length of the beel is 4.07 Km and water area is 64 Ha⁸。

Figure 1a,b,c,d: a. Map of West Bengal, b. Map of Nadia district, c. satellite view of Hasadanga beel, d. photograph of Hasadanga beel. (Ref. mapsofindia.com, Google earth, photograph was taken by author, * Sampling locations) Click here to view Figure

Sampling

The sampling was done at pre monsoon, monsoon and post monsoon season during the period from 2015 to 2017. Surface water samples were collected randomly from different sites of the beel (Fig. 1c) in 500 ml polypropylene bottles for determination of various physicochemical characters like temperature, pH, dissolved oxygen, free carbon di-oxide, alkalinity, hardness, biochemical oxygen demand etc. following the method of APHA (2012)⁹。Total 27 water samples were collected during the three-year study. A Celsius alcohol based thermometer (range 0^°C to 100^°C) was used to measure the surface water temperature. pH of water was measured directly by using pen pH meter made by Hanna Instruments, Italy. Dissolved Oxygen, Free Carbon di-oxide, Alkalinity, Hardness and Biochemical Oxygen Demand were measured by titrimetry. Random sampling for fish was done from three nettings in the Beel (Fig. 1c) to make ten kilogram (10 kg) of sample for determination of fish diversity. Total number of species, total number of individuals in a sample and total number of individuals in a species were recorded at pre monsoon, monsoon and post monsoon seasons during the period from 2015 to 2017. Shannon-Weaver species diversity index, Margalef’s species richness index, Pielou’s species evenness index and Simpson’s index of dominance^10-12were determined using the following equations:

1. Shannon-Weaver species diversity index (Hꞌ) =
   Where S is the total no. of species; N is the total no. of individual; Ni is the no. of specimens in each species.
2. Margalef’s Species richness index (D) =
   Where S is the total no. of species; N is the total no. of individuals.
3. Pielou’s Species evenness index (Jꞌ) = (Hꞌ)/log₂S
   Where Hꞌis the Shannon-Weaver species diversity index; S is the total no. of species.
4. Simpson’s index of dominance (ID) = Æ© (N_i/N)²
   Where N is the total no. of individual; Ni is the no. of individuals in each species.

Data Analysis

All the results were initially analysed by Shapiro-Wilk test for determination of normal distribution of a population and further statistically analysed by one-way ANOVA method described by R Development Core Team (2011)¹³followed by Duncan’s Multiple Range Test (DMRT)¹⁴。All data used here are the arithmetic mean of three observations. The collected fish were preserved in 4 % formalin solution and shifted to the laboratory for identification. Identification of fish species was done by following standard literatures like^15-18。Method of Ghosh and Biswas (2018)²²was used for determination of correlation.

Results

The seasonal fluctuation of physicochemical properties of Hasadanga beel during pre-monsoon, monsoon and post monsoon period are given in Table 1. Values are mean of three samples collected from three different sampling sites of the beel (Fig 1c). Hence Standard Deviation is mentioned after each value as ±. All values were initially analysed by Shapiro-Wilk test for determination of normal distribution of a population. Each value is super scribed by (a,b,c) and (m,n,o) which refers that values are significantly different (p<0.05) from each other following one way ANOVA and DMRT (Duncan’s Multiple Range Test) by the R software.

Table 1: Seasonal Variation of the Physicochemical Parameters of Hasadanga Beel at Pre Monsoon, Monsoon and Post Monsoon during the Period from 2015 to 2017.

Parameters	Seasons	2015	2016	2017	Mean
Water Temperature (°C)	Pre-Monsoon	31.3cmn±0.1	31.4cn±0.1	31.2cm±0.07	31.3
	Monsoon	28.5bm±0.07	29.1bo±0.07	28.7bn±0.07	28.77
	Post-Monsoon	20.2an±0.1	20.0am±0.04	20.9ao±0.04	20.37
pH	Pre-Monsoon	8.65cn±0.01	7.85bm±0.01	8.74bo±0.01	8.41
	Monsoon	8.43an±0.01	7.70am±0.01	8.75co±0	8.29
	Post-Monsoon	8.56bn±0.01	7.97cm±0	8.73ao±0	8.42
Free CO2(mg/l)	Pre-Monsoon	0am±0	0am±0	0am±0	0
	Monsoon	0am±0	11.5bn±0.1	0am±0	3.83
	Post-Monsoon	0am±0	15.0cn±0.07	0am±0	5
DO (mg/l)	Pre-Monsoon	4.5an±0.04	3.9am±0.07	4.4an±0.08	4.27
	Monsoon	4.7bn±0.04	4.3bm±0	4.7bn±0.07	4.57
	Post-Monsoon	5.0co±0.01	4.7cm±0.04	4.9cn±0	4.87
Alkalinity (mg/l)	Pre-Monsoon	190bn±0.71	189bn±0.7	187cm±0.71	189
	Monsoon	193co±0.71	175an±0.7	156am±0.71	175
	Post-Monsoon	184ao±0.71	175an±0.7	170bm±0.43	176
Hardness (mg/l)	Pre-Monsoon	145cn±0.71	140cm±0.71	141cm±0.71	142
	Monsoon	115an±0.43	117ao±0	113am±0.71	115
	Post-Monsoon	119bm±0	119bm±0.43	120bn±0.43	119
BOD (mg/l)	Pre-Monsoon	1.23bn±0	1.04am±0.01	1.85bo±0.01	1.37
	Monsoon	1.67cn±0.01	1.42cm±0.01	1.94co±0.01	1.67
	Post-Monsoon	1.03am±0.01	1.25bn±0.01	1.55ao±0.01	1.27

(Values within columns indicated by different superscript letter (a,b,c) and values within rows indicated by different superscript letter (m,n,o) are significantly different at 5% level determined by Duncan’s Multiple Range Test).

Overall 34 fish species belonging to 8 Orders were recorded during the study period of three years (2015-2017) and are listed in table 2. Here IUCN refers to International Union for Conservation of Nature, PM refers to pre-monsoon, M refers to monsoon and PoM refers to post-monsoon. Here ‘+++’=highly abundant, ‘++’=moderately abundant, ‘+’=less abundant and ‘-’ =absent.

Table 2: Fish Species found in Hasadanga Beel during the Study Period of Three Years (2015-2017)

Order	Family	Species	Common name	IUCN status	Population trend	Occurrence
						PM	M	PoM
1.Cypriniformes	Cyprinidae	Labeo rohita	Rohu/Rui	Least Concern	Unknown	+++	+++	+++
		Labeo bata	Bata	Least Concern	Unknown	+++	+++	+++
		Labeo calbasu	Kalbose	Least Concern	Unknown	+++	+++	+++
		Gibelion catla	Katla	Not Evaluated		+++	+++	+++
		Cirrhinus mrigala	Mrigel	Least Concern	Stable	+++	+++	+++
		Puntius sarana	Sarpunti	Least Concern	Unknown	+++	+++	+++
		Puntius sophore	Punti	Least Concern	Unknown	+++	-	+
		Puntius ticto	Punti	Least Concern	Unknown	+++	-	+
		Hypophthalmichthys molitrix	Silver carp	Near Threatened	Decreasing	+++	+++	+++
		Cyprinus carpio	Common carp	Vulnerable	Unknown	+++	+++	+++
		Ctenopharyngodon idella	Grass carp	Not Evaluated		+++	+++	+++
		Amblypharyngodon mola	Mourola	Least Concern	Stable	-	+	+
		Mylopharyngodon piceus	Black carp	Data Deficient	Unknown	+	-	+
2.Siluriformes	Notopteridae	Notopterus notopterus	Pholui	Least Concern	Stable	+++	+++	+++
		Notopterus chitala	Chital	Not Evaluated		++	++	++
	Bagridae	Mystus vittatus	Tengra	Least Concern	Decreasing	+++	+++	+++
		Aorichthys (Sperata) aor	Aar tengra	Least Concern	Stable	+	-	-
	Siluridae	Wallago attu	Boal	Vulnerable	Decreasing	+++	+++	+++
	Schilbeidae	Eutropiichthys vacha	Vacha	Least Concern	Decreasing	+	-	+
	Clariidae	Clarias batrachus	Magur	Least Concern	Stable	+++	++	++
	Saccobranchidae	Heteropneustes fossilis	Singhi	Least Concern	Stable	+++	++	++
3.Perciformes	Gobiidae	Glossogibius giuris	Bele	Not Evaluated		-	++	++
	Anabantidae	攀鲈testudineus	Koi	Least Concern	Stable	+	++	++
	Nandidae	Nandus nandus	Bheda/Roina	Least Concern	Unknown	-	++	++
	Cichlidae	Oreochromis niloticus	Nilontica	Least Concern	Stable	+	+++	+++
4.Ophiocephaliformes	Ophiocephalidae	Channa marulius	Shal/Gajal	Least Concern	Unknown	+++	++	++
		Channa striata	Shol	Least Concern	Stable	+++	++	++
		Channa orientalis	Cheng	Vulnerable	Decreasing	+	+	+
		Channa punctata	Lata	Least Concern	Stable	+++	+++	+++
5.Mastacembeliformes	Mastacembelidae	Mastacembelus pancalus	Pankal	Not Evaluated		+	+	+
		Macrognathus aculeatus	Guchi	Not Evaluated		+++	++	++
6.Clupeiformes	Clupeidae	Gudusia chapra	Khoira	Least Concern	Decreasing	++	++	++
7.Beloniformes	Belonidae	Xenentodon cancila	Kankle	Least Concern	Unknown	++	++	++
8.Symbranchiformes	Symbranchidae	Monopterus cuchia	Ban/Cuche	Least Concern	Unknown	+++	+++	++

Number and Percent Composition of Families, Genera and Species under various Orders of fish fauna found in Hasadanga beel during the study period are listed in Table 3.

Table 3: Number and Percent Composition of Families, Genera and Species Under Various Orders.

Sl no	Order	Families	Genera	Species	% of Families in an Order	% of Genera in an Order	% of Species in an Order
1	Cypriniformes	01	09	13	6.25	34.62	38.25
2	Siluriformes	06	07	08	37.5	29.92	23.53
3	Perciformes	04	04	04	25	15.38	11.76
4	Ophiocephaliformes	01	01	04	6.25	3.85	11.76
5	Mastacembeliformes	01	02	02	6.25	7.69	5.88
6	Clupeiformes	01	01	01	6.25	3.85	2.94
7	Beloniformes	01	01	01	6.25	3.85	2.94
8	Symbranchiformes	01	01	01	6.25	3.85	2.94
	Total	16	26	34

Percentage occurrence of fishes of Hasadanga beel under the various conservation categories of IUCN are listed in Table 4.

Table 4: Percentage Occurrence of fishes of Hasadanga Beel under the Conservation Status IUCN (2020) (Ref: https://www.iucnredlist.org/)

	EN	VU	NT	LC	LR	DD	NE	Total
Number of species	00	03	01	23	00	01	06	34
Percent contribution	00%	8.82%	2.94%	67.65%	00%	2.94%	17.65%	100%

EN=Endangered

VU=Vulnerable

NT=Near Threatened

LC=Least Concerned

LR=Lower Risk

DD=Data Deficient

NE=Not Evaluated

Number of Families, Genera and Species under various Orders of fish fauna found in Hasadanga beel during the study period are graphically presented at Fig. 2.

Figure 2: Number of Families, Genera and Species Under Various Orders. Click here to view Figure

Percentage occurrence of fishes of Hasadanga beel under the various conservation categories of IUCN are graphically presented in Fig. 3.

Figure 3: Pi Diagram showing the no. and Percentage of Species Under Various Threat Categories as per IUCN Status. Click here to view Figure

The number of individuals belonging to 8 different orders of fish found per ten kilogram sample is represented graphically in figure 4.

图4:Seasonal Variation in Number of Individuals Between 8 Orders of Fishes found /10 Kg Sample. Click here to view Figure

Shannon-Weaver species diversity index, Margalef’s species richness index, Pielou’s species evenness index and Simpson’s index of dominance for fish were determined at pre-monsoon, monsoon and post-monsoon period for 2015, 2016 and 2017 are listed in table 5. Each value is super scribed by (a,b,c) and (m,n,o) which refers that values are significantly different (p<0.05) from each other following one way ANOVA and DMRT (Duncan’s Multiple Range Test) by the R software.

Table 5: Various species diversity indices for fish of Hasadanga Beel at at pre-monsoon, monsoon and post-monsoon period for 2015, 2016 and 2017.

Sl no	Diversity indices	Season	2015	2016	2017	Mean
1	Shannon-Weaver species diversity index (Hꞌ)	Pre-monsoon	1.35cn	1.30bm	1.40co	1.35±0.04
		Monsoon	1.29an	1.25am	1.32ao	1.28±0.03
		Post-monsoon	1.34bn	1.31cm	1.37bo	1.34±0.02
2	Margalef’s Species richness index (D)	Pre-monsoon	13.24bn	13.10bm	13.44bo	13.26±0.14
		Monsoon	12.72an	12.55am	12.82ao	12.69±0.11
		Post-monsoon	14.15cn	13.98cm	14.25co	14.12±0.11
3	Pielou’s Species evenness index (Jꞌ)	Pre-monsoon	0.91cn	0.90cm	0.92co	0.91±0.01
		Monsoon	0.88an	0.87am	0.89ao	0.88±0.01
		Post-monsoon	0.89bn	0.88bm	0.91bo	0.89±0.01
4	Simpson’s index of dominance (ID)	Pre-monsoon	0.053an	0.054ao	0.052am	0.053±0.001
		Monsoon	0.071cn	0.073co	0.070cm	0.071±0.001
		Post-monsoon	0.057bo	0.056bn	0.055bm	0.056±0.001

(Values within columns indicated by different superscript letter (a,b,c) and values within rows indicated by different superscript letter (m,n,o) are significantly different at 5% level determined by Duncan’s Multiple Range Test).

Correlation between various physicochemical parameters with different species diversity indices of Hasadanga beel during the study period (2015-17) are listed in table 6.

Table 6: Correlations between Physicochemical Parameters and Species Diversity Indices

Parameters	Shannon-Weaver species diversity index (Hꞌ)	Margalef’sSpecies richness index (D)	Pielou’s Species evenness index (Jꞌ)	Simpson’s index of dominance (ID)
Temperature	-0.16742	-0.80804	0.40141	0.09948
pH	0.97974	0.84137	0.70887	-0.96372
Free CO2	-0.41729	0.33551	-0.84775	0.47857
DO	-0.13207	0.59732	-0.65465	0.19967
Alkalinity	0.65947	-0.05247	0.96393	-0.70942
Hardness	0.71298	0.02113	0.98090	-0.75934
BOD	-0.93050	-0.92200	-0.57656	0.90322

Discussion

In the current study, temperature ranges between 20.2 to 31.3, 20.0 to 31.4 and 20.9 to 31.2⁰C during 2015, 2016 and 2017 respectively (table 1). The temperatures are significantly varies (p<0.05) at all the seasons in every year. The highest and lowest temperature was recorded at pre-monsoon and post-monsoon season of 2016. High solar radiation and low level of water may cause the comparative higher temperature during summer season. Oppositely, high water level and low solar radiation may cause the lower temperature of winter season at every year^{26, 27}。The values of pH range between 8.43 to 8.56, 7.70 to 7.85 and 8.73 to 8.75 during 2015, 2016 and 2017 respectively (table 1). Comparatively higher pH value may be caused by higher amount of macrophytes with algae and phytoplankton in the beel which consume Carbon dioxide from the water for photosynthesis thereby increase of pH level²⁷。The free CO₂remains nil at all seasons of 2015 and 2017 (table 1). But it ranges from 0.00 to 15.0 mg/l at 2016. The highest level of free CO₂recorded at post monsoon season which correspond the pH value of the water. The dissolved oxygen ranges between 4.5 to 5.0, 3.9 to 4.7 and 4.4 to 4.9 mg/l during 2015, 2016 and 2017 respectively (table 1). Comparatively higher DO value recorded at post-monsoon season and lower DO value was recorded at pre-monsoon season. High water temperature during summer reduces the holding capacity for oxygen molecule and thereby decreases the solubility of oxygen which results low DO value during summer season²⁸。The total alkalinity ranges between 184 to 193, 175 to 189 and 156 to 187 mg/l during 2015, 2016 and 2017 respectively (table 1). The higher alkalinity value during summer season may be the result of organic decomposition, which releases CO₂which form bicarbonate ions causing increase in total alkalinity in water. Dilution of water due to rainfall causes reduction of total alkalinity at monsoon²⁷。115年到145年之间的硬度值范围,117 to 140 and 113 to 141 mg/l during 2015, 2016 and 2017 respectively (table 1). The maximum value of hardness was observed during summer season whereas minimum at monsoon season. Reduction in water level at pre-monsoon may cause concentration of water with Ca and Mg salts which are responsible to increase hardness of water. Oppositely, at monsoon and post-monsoon seasons, the hardness has been decreased due to the dilution of the salts in water²⁹。是生化需氧量值范围en 1.03 to 1.67, 1.04 to 1.42 and 1.55 to 1.94 mg/l during 2015, 2016 and 2017 respectively (table 1). Maximum BOD value was observed during monsoon season of every year. It may be due to huge inflow of rain-wash which contains detergents, domestic sewage and agricultural effluents. Higher BOD value may causes decline in aquatic biodiversity. These results corresponds the trends of earlier workers²⁷。

Overall 34 fish species belonging to 8 Orders and 16 families were recorded during the study period of three years (2015-2017) (Table 2). Out of 34 species, three vulnerable and one near threatened species were found. Besides that, 23 least concerned species were found which constitutes almost 68% of total species. Almost 18% species found which were not evaluated by IUCN. Order Cypriniformes contains most numbers of Genera and Species followed by Siluriformes. Earlier workers²³records 46 fish species in total at the beels of Nadia district. Ghosh and Biswas²⁴recorded 33 species of finfish belonging to 8 orders and 17 families in a study on Chhariganga Oxbow Lake located in Nakashipara development block of Nadia district.

The Shannon-Weaver species diversity index (Hꞌ) ranges between (1.2911-1.3502) which corresponds earlier workers²⁴where Shannon-Weaver species diversity index ranges between 1.19-2.02, Margalef’s species richness index (D) ranges between (12.72-14.15), Pielou’s species evenness index (Jꞌ) and Simpson’s index of dominance (ID) ranges between (0.8829-0.9140) and (0.05346-0.07139) respectively which does not correspond with earlier workers²⁴where Pielou’s species evenness index ranges between 0.36-0.64 and Simpson’s index of dominance ranges between 0.21-0.51. These may be the result of jute retting and other environmental and anthropogenic factors in those beels studied by earlier workers^{24, 25}。

Shannon-Weaver species diversity index (Hꞌ) is highest in post-monsoon period of 2017 whereas lowest in monsoon season of 2016. Hꞌ has positive correlation with pH, alkalinity and hardness whereas negative correlation with temperature, free CO₂, DO and BOD. Margalef’s Species richness index (D) is highest in post monsoon period of 2016 and lowest in monsoon season of both 2015 and 2017. D has positive correlation with pH, free CO₂, DO and hardness whereas negative correlation with temperature, alkalinity and BOD. Pielou’s Species evenness index (Jꞌ) is highest in pre monsoon period of 2017 and lowest in pre monsoon season of 2015. Jꞌ has positive correlation with temperature, pH, alkalinity and hardness and whereas negative correlation with free CO₂, DO and BOD. Simpson’s index of dominance (ID) is highest in monsoon period of 2016 and lowest in pre monsoon season of 2017. ID has positive correlation with temperature, free CO₂, DO and BOD whereas negative correlation with pH, alkalinity and hardness.

Margal Shannon-Weaver物种多样性指数ef’s species richness index and Pielou’s species evenness index values are highest in 2017 and lowest in 2016 at all seasons. But Simpson’s index of dominance value is highest at 2016 and lowest at 2017 for all seasons. This may be due to low pH value, low DO value and low BOD value in 2016.

The Shannon-Weaver species diversity index is highest in pre monsoon season and lowest in monsoon season for the year 2015 and 2017. But in 2016, it is highest in post monsoon. The Margalef’s species richness index and Pielou’s species evenness index are lowest in monsoon and highest in post monsoon for all the seasons (except J’ for 2017). The Simpson’s index of dominance shows highest value in monsoon and lowest in pre monsoon period for all the years. These are probably due to fall of agricultural runoff from adjacent agricultural land in monsoon season.

So, the impact of seasonal variations of physicochemical parameters like temperature, dissolved oxygen and pH are found to influence in fish diversity of the Hasadanga beel. The physicochemical parameters and anthropogenic activities can be considered as the key factors for reduction of biodiversity of the beels. Similar results were also observed by earlier workers^{19-21, 27, 28}。

Conclusion

From the present study, it is evident that, Hasadanga beel, a floodplain wetland of Nadia district, harbours a large number of freshwater fish fauna as compared to similar works carried out in West Bengal^{23, 24}。但diversit Shannon-Weaver物种的范围y index (Hꞌ) (1.2911-1.3502) suggests that the diversity of fish is moderate here. The low level of SW in monsoon season also indicates that the water is moderately polluted during monsoon season, which may be due to the inflow of agricultural run-off and domestic sewage. Comparatively high SW value in pre-monsoon season can suggest that the water temperature can also support positively the fish diversity in the wetland. The seasonal variation of fish diversity indices should be taken into account for making long term policies for sustainability of wetlands in the country.

Acknowledgement

The authors are thankful to the Head, P.G. Dept. of Zoology, Krishnagar Govt. College and Head, Dept. of Zoology, University of Burdwan for providing infrastructural facilities to carry out the work.

Conflict of Interest

Authors have not any conflict of interest including any financial, personal or other relationships with other people or organizations that can influence the work.

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