A baseline taxonomic study of zooplankton in the lower Halda River, Bangladesh

Halda is a unique resourceful tidal river of Bangladesh, where Indian major carps (Catla catla, Labeo rohita, Cirhinnus mrigala, and Labeo calbasu) spawn from April to June. Ecological conditions are auspicious from time immemorial which serves as a natural breeding ground in the world. The present study was conducted over the period of January-June 2017 to identify the main species of Zooplankton fauna and also find out the dominancy of the highest plankton groups in the lower Halda River. A total of 3 major zooplankton groups-Copepoda, Rotifera and Cladocera were identified during the six months period. A total of 35 species under 4 family showing the dominancy for the class Rotifera consisting of 7 genera were recorded from five sampling sites. The lowest group was Cladocera consisting of 5 genera. The composition and dominance of the zooplankton community among five different sites in the lower Halda River showed some close inter-relationships. The present study suggests for conducting further research to know the overall conditions and to make a complete list of available zooplankton fauna that exert impact on the water quality of the Halda River.

Introduction

Bangladesh is blessed with small and large rivers which are mostly covered with water. The Halda River is one of the most unique resourceful rivers of Bangladesh where Indian major carps (Catla catla, Labeo rohita, Cirrhinus mrigala, and Labeo calbasu) spawn during April to June [1-3]. The river also provides navigations, supplies drinking water and generate employment opportunities for the local communities [1].

The biotic community of an ecosystem gives an insight into the conditions existing in the aquatic ecosystem. Changes in the structure and function of biological systems are induced by environmental disturbances in the composition of aquatic communities of rivers [4]. Zooplankton is one of the four selected bioindicators (benthic diatom, zooplankton, littoral macroinvertebrate, and benthic macroinvertebrate), uses for assessment in Ecological Health Monitoring (EHM) [5]. Zooplankton is considered the most important link between planktonic primary producers and large carnivores, amongst them fish species subject to human exploitation. They are minute aquatic animals that are non-motile or very weak swimmers. They serve as a good indicator of changes in water quality because it is strongly affected by the environment quality [6]. They play a crucial role in indicating the presence or absence of certain fishes. Zooplankton population is able to reflect the nature and potential of any aquatic systems [7,8].

Zooplankton constitutes an important food item of many omnivorous and carnivorous fishes. The larvae of white fish (Mullet) feed mostly on zooplankton [9], because zooplankton provides the necessary amount of protein for the rapid growth and especially that of the gonad. Zooplankton contributes about 82% of the food item of Anabas testudineus [10], 32% of Notopterus notopterus [11]. The main food item of Xenentodon cancila and the zooplankton contributes about 23% of the food item of Macrobrachium rosenbergii, 47% of the Catla catla and 6.37% of the Labeo rohita [12].

Resources of the Halda River are depleting day by day due to over-fishing, straightening of existing ox-bow bands, sedimentation on the river bed, changes in water quality, mismanaged sluice gate establishment, mechanical sand quarrying, water contamination with industrial wastes, denudation of various species of fishes towards gradual extinction including carps, unchecked riverbank erosion and above all global climate change [1]. Time may come when no more seed will be available from the Halda if the Halda River ecosystem destroyed. Therefore, the protection of the Halda River is an urgent need because most of the country’s pond carp culture reliant on the fish fry naturally produced in this river which is of insurmountable economic and nutritional value.

The taxonomic checklist of zooplankton along with its occurrence and distribution in the lower Halda River has become a prerequisite for fish production. Available information about the Halda River reveals that only a few works have so far been done in the country by different researchers. Thus, there is a research gap that needs to be bridged. Considering these facts, the present study was aimed to determine the main species of zooplankton fauna in the lower Halda River, Chittagong by making its list in terms of zooplankton species composition and group’s dominance. The present study will be a preliminary baseline for the zooplankton study in the lower Halda River.

Materials and methods

Study Area

The present study was carried out in the tidal Halda River (22°54′ N and 91°48′ E to 22°24′ N and 91° 53′ E), one of the tributaries of the River Karnaphuli originated from Haldachora at the area of 2 no (Table 1, Figure 1). Patachora Union in Ramgarh Upazila under Khagrachari districts (former Chittagong Hill Tracts), Bangladesh [13]. A number of canals such as Manikchori Khal, Khondokia Khal, Madari Khal, Khata Khal are connected with the Halda River and make Halda Khal a river [13,14].

Figure 1: Map showing the sampling sites of the study area.

Geological information

Five important sites named Karnaphuli (estuary), Krisno Khal, Khandokia Khal, Madari Khal and Garduara (Noyahat) were selected purposively for sample collection. There are a number of canals in the fish sanctuary of the Halda River. Khandokia Khal is one of the most polluted canals where untreated wastewater from the adjacent industries and Anannya residential area flow through the canal and finally ends up in the Halda River. Kalurghat is the confluence point of Halda and Karnaphuli River. Saline water from the Bay of Bengal enters the Halda River through the Karnaphuli River. During ebb and flow, the water of the Karnaphuli River mixes with the water of the Halda River through this confluence point. These indirectly bring the pollutants in the Halda River. The study of plankton of these sites gives an insight into the effects of environmental variables on the biodiversity and ecosystem of the Halda River and how the overall productivity fluctuates from site to site as well as gives the decision-makers a basis for coming up with its better management.

Sampling duration

The sampling was carried out from January to June 2018. Water samples containing zooplankton were collected from selected five points over the period of six months at the time of 6:30-10:00am but some time due to unfavorable condition time had to change. Samples were collected every month form every site.

Sample collection and preservation

Zooplankton samples were collected from the subsurface water using a zooplankton net of 70μm mesh size. A flow meter was attached at the mouth of the net. The net was thrown to the desired distance (7m) and allowed to reach the desired depth of the sampling sites from the boat. Then the throne rope with the net was pulled quickly towards the boat for the collection of samples. The water was passed down through the net and the plankton condensed at the end of the plankton net then it was collected into a glass test tube and fixed firmly. The net was pulled 3 times from several places of the sites. Then the samples were transferred to a plastic jar, labeled detective of time, date and sampling sites. The collected samples were preserved in 5% formalin in the plastic jars immediately after collection and were taken to the laboratory of the Department of Zoology, University of Chittagong. For further study, samples were preserved in 70% alcohol.

Identification

The sorted organisms were brought under microscope and identified following Ahlstrom [15], Edmondsom [16], Davis [17], Pennak [18], Needham and Needham [19], Williamson [20], Brooks [21], Wilson [22], Wilson and Yeatman [23], Smith and Fernando [24], Battish [25], Bhouiyan and Asmat [26], Macan [27], Das and Bhouiyan (1974), Mellanby [28], Delorme [29], Wickstead [30], Mizuno [31], Nayar and Nair [32], Ward and Whipple (1963), Balcer, et al., [33], Victor and Fornando [34], Ahmed [35]; Elias [36]; Zafar [37]; Mohi [38] etc.

Results and discussion

Zooplankton species composition

A total of 35 species of zooplankton was identified. 15 species of Rotifera belonging to 4 family and 7 genera; 9 species of Copepoda belonging to 2 family and 5 genera; 9 species of Cladocera belonging to 4 family and 5 genera also pelecypod larva, Nauplius larva, etc. were recorded from the lower Halda River during the study period. Major taxa of zooplankton which was found during the investigation were presented in Table 2.

Table 2: Major taxa of zooplankton along with their characteristics.

The dominance of Rotifera was due to its preference for warm waters as highlighted by Dumont [39]. The dominance of Brachionus is an indication that the Halda River is eutrophic and their abundance due to the presence of high levels of organic matter in the river basically at Khondokia Khal (Figure 2). The relatively low species diversity of Copepoda and Cladocera was a result of the hydrodynamics of the river such as low water volume, short residence time and morphometric.

Figure 2: Percentage of various groups of Zooplankton in five different sites.

The low genera diversity of Copepoda and Cladocera has also been documented in the other water bodies [40], the Ogun and Ona rivers [41] and the Niger-sokoto River [42]. According to Rocha, et al., [43], an increase a primary production (phytoplankton) tends to be followed by an increase in zooplankton number and biomass. Lampert [44], have emphasized these factors as being responsible for zooplankton number and biomass reduction. Food resource [45], the ability to adapt to food conditions and less predation [46], maybe the reasons for the significant abundance of Rotefers, Cladocera, and Copepoda in the aquatic ecosystem. Generally, the zooplankton community of the Halda River was dominated by Rotifera, which due to their short generation time and their high reproductive rate, dominate in rivers. Among the species identified, rotifer regarded as the indicators of eutrophication in the river, the rotifer Brachionus stands in its great tolerance to extremely eutrophic environments [47], Regarded it as a good indicator of eutrophication.

The abundance of the genera Brachionus and Keratella showed that rotifer fauna was made up of atypical tropical assemblage. The predominance of Brachionidae could, however, be attributed to their omnivorous nutrition and the widespread geographical distribution of most of the members [48]. In the present investigation, members of Brachionidae were dominant both in species numbers.

Members of the rotifer were dominant at Khondokia Khal and Krisno Khal of the Halda River during the investigation period comparatively more than that of the other three sites. Due to industrial and municipal wastes and also other factors the abundance of Brachionus is a common fact at Khondokia Khal and Krisno Khal of the Halda River. Because Brachionus, Keratella, and Lecane sp. are considered as euryhaline and suggested as mesohaline indicators and tolerance high ranges of salinity [49-53].

Conclusion

The findings of the present study indicate that zooplankton occupies a significant position in the river ecosystem. The Halda River is mostly known for its natural spawning instance. Zooplankton diversity in this river is an important criterion for natural spawning instance. Most of the taxa of zooplankton was highly variable over time. As they constitute the most important link in the energy transfer between phytoplankton and higher aquatic fauna. They are essential for estimation of secondary production and fishery potential of a water body. Almost every fish depends on Zooplankton in their entire life cycle. Since the Zooplankton community is an integral part of tropic level, conservation is very essential for healthy and sustainable ecosystem management. It can be concluded that the occurrence of zooplankton required for maintaining sustainable healthy conditions in the Halda River. The presence of zooplankton communities was higher in Garduara (Noyahat) than the other four sites because it is called the main breeding point of the Halda River where flow of industrial wastewater is very low. On the other hand, the presence of zooplankton communities was very low in Khondokia Khal because different industries are connected to this Khal. As a result, water becomes polluted and ultimately these polluted water are getting mixed with the Halda River through Khondokia Khal. The presence of zooplankton communities is decreasing at an alarming rate due to various types of natural and anthropogenic disturbance, withdraw of water from the upstream, global climate change, etc. Besides, highly fluctuation of various Zooplankton communities reveals a sign of serious threat to destroy the natural ecosystem in the Halda River.

Limitations

As it was a study of six months period it cant be concluded the overall problems of the Halda River ecosystem. Long time-intensive research will give a better result which will be more effective to conserve the biodiversity of the Halda River.

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