PJZ_57_6_2679-2689

INTRODUCTION

Zooplanktons are small sized animals that are weak swimmers and usually drift along the water currents. They act as primary consumers and carry out diurnal and nocturnal movements covering hundreds of meters. During the night, they feed on the water surface and graze the phytoplankton. They represent a link between the microbial portion and the large grazers (Lacerot et al., 2013). Zooplankton also plays a vital role in marine ecosystem by transferring energy from phytoplankton to higher tropic levels. They serve as an important food source to the marine organisms particularly to planktivorous fish. In plankton-based food webs, zooplankton is the major route for energy flux, making them an important element in functioning of marine ecosystems (Santos-Wisniewski et al., 2006; Chen et al., 2020).

The characteristics of zooplankton community structure are characterized by the intrinsic factors such as surface area, depth, trophic level, colour of water, and the biological community (Rahkola-Sorsa, 2008). Their abundance and biodiversity are linked to the health of marine ecosystems. The abundance and distribution of zooplankton are influenced by hydrographic condition and they have been suggested as good biological indicator species (Lan et al., 2004; Shin et al., 2022). Due to their small size, huge density, rapid metabolic activities, short life span, drifting nature, species diversity and high tolerance to the stress zooplankton are used as the indicator species for the physical, chemical and biological processes in marine environment (Gajbhiye, 2002). Potentiality of zooplankton as bioindicator is very high because their growth and distribution are dependent on some abiotic (e.g., temperature, salinity, stratification, pollutants) and biotic parameters (e.g., food limitation, predation, competition) (Beyst et al., 2001).

Marine coastal ecosystems are zooplankton rich and they are among the most productive environments in the world. Zooplankton is the main predator on phytoplankton and it is sensitively influenced by the fluctuations of environmental factors (Eisner et al., 2014; Zhao et al., 2022). The zooplankton plays an important role in the structure and functioning of coastal ecosystems. Moreover 75% of the marine fish feed on plankton and the availability of zooplankton as feed for larval fish is one factor that determines strength of commercial important fish species (Kane, 1993). Zooplanktons are involved in regulation of nutrients and phytoplankton population on which they feed. They consume large quantities of bacteria (Wroblewski, 1980), phytoplankton (Calbet et al., 2000) and organic detritus (Steinberg et al., 1998) and at the same time, they are preyed by higher trophic level such as fishes (Beaugrand et al., 2003).

Different studies have focused on the significance of the relationship between phytoplankton and zooplankton in marine ecosystems (Tan et al., 2004). An increase in nutrients can enhance phytoplankton productivity, leading to improved feeding conditions for zooplankton (Hansen et al., 2000; Sun et al., 2022). Elevated nutrient levels may also alter zooplankton species diversity and succession (Park and Marshall, 2000). Marine pollution due to industrialization and anthropogenic substances induces algal bloom which causes eutrophication affecting population of zooplankton present in the surrounding environment and they exhibit rapid changes in their populations when disturbance occurs. Therefore, they are used as indicator species for water pollution (Jakhar, 2013). Zooplankton species act as consumers of phytoplankton, particularly during algal blooms. At the early stage of algal bloom development, copepods, through their intensive grazing, are able to influence the food web structure and shift the phytoplankton species composition (Tan et al., 2004). Oceanographic processes such as temperature, salinity, wind forcing, current patterns, flooding and ebbing of the tides controlling the structure of plankton communities and generally reflected in patterns of abundance through space and time (Paffenhofer and Mazzocchi, 2003). Their community composition, richness and diversity also serve as good indicators of ecosystem health (Baliarsingh et al., 2014). In addition, climate change and human exploitation play a crucial role in zooplankton community and coastal ecosystems (Beaugrand et al., 2002; Bagheri et al., 2017). It is, therefore, important to assess species diversity and community structure of the zooplankton for evaluating potential fishery resources which have direct implications for commercial fisheries. The present study conducted monthly sampling from the coastal waters of Gadani ship breaking yard on the Balochistan coast and Sandspit on the Sind coast in Pakistan over a period of one-year and revealed variations in zooplankton populations across seasons and between sampling stations. The information obtained by the study may be utilized by different user groups, including government and non-government organizations working in the fishery industry.

MATERIALS AND METHODS

The seawater samples were collected from Sandspit (Sindh coast) and Gadani ship breaking area (Baluchistan coast) for the analysis of zooplankton, distribution and abundance with reference to seasons and physico-chemical parameters (Fig. 1). Sandspit is located at the South west of the Karachi, Sind. It is 14.5 km long, the approximate distance from Karachi to Sandspit is 18 kilometers and Gadani is 50 kilometers. Gadani ship breaking yard is the world’s third largest ship breaking yard due to ship-breaking activities Gadani coastal area is considered most polluted (Saleem et al., 2016). Sandspit backwater is known as a nursery ground for fish and shellfish. Both Baluchistan coast and Sindh coast are important for economically important fisheries.

Zooplankton samples from these two sites were collected by vertical hauls from a depth of 10 m using a 55 μm mesh size net. Three, replicate samples (250 ml each) were taken every month for a period of one year for zooplankton abundance diversity. Water quality parameters including water temperature, salinity, pH, dissolved oxygen, nitrate, nitrite, ammonia, phosphate and chlorophyll a were determined according to Strickland and Parsons (1972). Mercury thermometer for temperature, refractometer (Atago, Japan) for salinity and pH meter (Hanna. Inc) for pH was used. Dissolved oxygen was determined by Winkler method (Strickland and Parsons, 1972). For enumeration of zooplankton, the samples were first preserved in 4% formalin and binocular microscope (Olympus CX-31) was used to identify the major taxonomic groups and presented in (no.ind./m³) number per cubic meter (Goswami, 2004).

Statistical analysis

Statistical analysis of data was carried out using PRIMER version 7 software and using PAST version 3 software. The biodiversity of zooplankton was calculated according to Shannon and Wiener (1949).

RESULTS

Table I shows water parameters which were variable throughout the year in Gadani and Sandspit. In order to check the seasonal abundance of zooplankton, seasons were divided into autumn inter-monsoon (October–November), northeast monsoon (December–February), spring intermonsoon (March–April), and south-west monsoon (May–September).

Table I. Environmental parameters at Gadani and Sandspit station.

Parameters	Mean	SD	SE	Range (Min-Max)
Gadani station
Temp. °C	25.667	4.053	2.866	20-31
Saliniy (ppt)	35.333	2.498	1.767	31-38
pH	8.075	0.469	0.332	7-8.7
Oxygen (ppm)	9.275	1.076	0.761	7.7-10
Nitrate (ppm)	9.967	0.049	0.035	9.8-10
Nitrite (ppm)	0.200	0.001	0.001	0.19-.20
Phosphate (ppm)	3.583	1.564	1.106	1.4-5
Ammonia (ppm)	1.750	0.544	0.384	0.09-2.5
Chlorophyll (µ/l)	0.946	1.088	0.769	0.5-2.4
Sandspit station
Temp. °C	25.333	3.916	2.769	19-30
Saliniy (ppt)	35.833	2.588	1.830	32-39
pH	8.108	0.355	0.251	7.7-8
Oxygen (ppm)	8.683	1.765	1.248	11-6.2
Nitrate (ppm)	9.983	0.039	0.028	9.9-10
Nitrite (ppm)	0.300	0.195	0.138	0.02-0.5
Phosphate (ppm)	2.667	1.614	1.142	0.42-4.9
Ammonia (ppm)	1.958	0.396	0.280	1.4-2.5
Chlorophyll (µ/l)	0.600	0.498	0.352	0.08-1.2

Variation in the zooplankton abundance was observed in all the three stations in four seasons in Gadani (Fig. 2). The peak abundance of zooplankton is in south west monsoon than in spring inter monsoon, autumn inter monsoon and north east monsoon at coastal waters of Gadani. Abundance of zooplankton varied from 176-12631 individuals/m³. The peak zooplankton abundance occurred in April and the lowest abundance was recorded in December. Copepod dominate the zooplankton group, highest percentage was recorded of Calanoid 18.80% than Cyclopoid 14.78% and Herpecticoid 11.44%. Percentage abundance of zooplankton species was highest in Station III as compared to Station I and Station II during 2016-2017 in Gadani ship breaking area (Fig. 3).

Pearson correlation coefficient was applied to observe the relationships between zooplankton communities with Hydrographical parameters and nutrients. Zooplankton abundance was positively correlated with temperature and chlorophyll a whereas inverse correlation was observed with salinity, pH, dissolve oxygen, nitrate, nitrite, phosphate and ammonia in station 1. Zooplankton abundance was positively correlated with temperature whereas inverse correlation was observed with salinity, pH, dissolve oxygen, chlorophyll a, nitrate, nitrite, phosphate and ammonia in stations 2 and 3 (Table II).

Table II. Correlation (Pearson) of total abundance of zooplankton with water parameters at Gadani in station I, II and III.

	Abundance St. 1	Temp	Salinity	pH	Oxygen	Nitrate	Nitrite	Phosphate	Ammonia
Gadani station I
Temperature	0.145
Salinity	-0.294	-0.170
pH	-0.436	0.101	0.791*
Oxygen	-0.441	0.061	0.477	0.843*
Nitrate	-0.429	0.078	0.825*	0.988*	0.810*
Nitrite	-0.408	0.083	0.849*	0.951*	0.763*	0.985*
Phosphate	-0.740	0.378	0.304	0.435	0.309	0.475	0.509
Ammonia	-0.080	0.272	0.708*	0.876*	0.689*	0.875*	0.854*	0.300
Chlorophyll	0.010	0.406	-0.206	0.021	0.359	0.027	0.070	0.149	0.157
Gadani station II
Temperature	0.189
Salinity	-0.290	-0.170
pH	-0.447	0.101	0.791*
Oxygen	-0.490	0.061	0.477	0.843*
Nitrate	-0.445	0.078	0.825*	0.988*	0.810*
Nitrite	-0.436	0.083	0.849*	0.951*	0.763*	0.985*
Phosphate	-0.557	0.378	0.304	0.435	0.309	0.475	0.509
Ammonia	-0.028	0.272	0.708*	0.876*	0.689*	0.875*	0.854*	0.300
Chlorophyll	-0.008	0.406	-0.206	0.021	0.359	0.027	0.070	0.149	0.157
Gadani station III
Temperature	0.061
Salinity	-0.239	-0.170
pH	-0.398	0.101	0.791*
Oxygen	-0.497	0.061	0.477	0.843*
Nitrate	-0.410	0.078	0.825*	0.988*	0.810*
Nitrite	-0.411	0.083	0.849*	0.951*	0.763*	0.985*
Phosphate	-0.055	0.378	0.304	0.435	0.309	0.475	0.509
Ammonia	-0.294	0.272	0.708*	0.876*	0.689*	0.875*	0.854*	0.300
Chlorophyll	-0.360	0.406	-0.206	0.021	0.359	0.027	0.070	0.149	0.157

In Gadani, Shannon diversity index and Margalef diversity index in station I was highest in North east monsoon as compare to other seasons. Pielou’s evenness index was highest in station III in North east monsoon. Maximum number of species was recorded in station I in South west monsoon as compared to other seasons (Table III).

In Sandspit, lowest abundance of zooplankton was recorded in October in Station 1 i.e., 530.75 individuals/m³ and high abundance was recorded in December in station 2 i.e., 441323 individuals/m³. Variation in the zooplankton abundance was observed in the two stations in four seasons (Fig. 4). Annual percentage abundance of zooplankton species shows variations at two stations in Sandspit (Fig. 3).

Pearson correlation coefficient was applied to observe the relationships between zooplankton communities with Hydrographical parameters and nutrients. Zooplankton abundance was positively correlated with temperature, Salinity, pH and Ammonia and inverse correlation was observed with dissolve oxygen, chlorophyll a, nitrate, nitrite and phosphate at station 1 (Table IV). However, zooplankton abundance was positively correlated with salinity, dissolve oxygen, nitrate and ammonia and inverse correlation was observed with temperature, pH, chlorophyll a, nitrite and phosphate at station 2 (Table IV).

Table III. Seasonal abundance of zooplankton (cells/L) recorded from Gadani and Sandspit.

Seasons	Stations	No. of species	Total no. of Ind.	Margalef diversity index	Pielou's evenness index	Shannon diversity index
Autumn inter monsoon	Station 1	16	6546	1.707	0.8162	2.263
	Station 2	14	6511	1.48	0.8037	2.121
	Station 3	16	10155	1.626	0.7767	2.153
North east monsoon	Station 1	18	7678	1.9	0.8465	2.447
	Station 2	17	7041	1.806	0.7242	2.052
	Station 3	15	8457	1.548	0.8499	2.302
Spring inter monsoon	Station 1	13	4529	1.425	0.7652	1.963
	Station 2	15	4954	1.646	0.8085	2.19
	Station 3	14	18399	1.324	0.8366	2.208
South west monsoon	Station 1	19	21867	1.801	0.8047	2.369
	Station 2	18	18293	1.732	0.7737	2.236
	Station 3	15	14932	1.457	0.8214	2.224
Autumn inter monsoon	Station 1	10	8209	0.9986	0.7599	1.75
Autumn inter monsoon	Station 2	11	14649	1.043	0.5328	1.278
North east monsoon	Station 1	26	8560	2.761	0.5882	1.917
North east monsoon	Station 2	26	116315	2.143	0.5676	1. 849
Spring inter monsoon	Station 1	26	16769	2.57	0.5662	1.845
Spring inter monsoon	Station 2	26	130964	2.122	0.5574	1.816
South west monsoon	Station 1	26	33889	2.397	0.5777	1.882
South west monsoon	Station 2	26	363593	1.953	0.5641	1.838

Table IV. Correlation (Pearson) of total abundance of zooplankton with water parameters in Sandspit at station I and II.

	Abundance St. 1	Temp	Salinity	pH	Oxygen	Nitrate	Nitrite	Phosphate	Ammonia
Sandspit station I
Temperature	0.103
Salinity	0.144	0.158
pH	0.396	-0.081	-0.107
Oxygen	-0.127	-0.105	-0.109	0.096
Nitrate	-0.643	-0.020	0.421	-0.383	0.135
Nitrite	-0.308	-0.036	-0.054	0.380	0.089	0.478
Phosphate	-0.439	-0.168	-0.297	0.449	-0.171	-0.096	0.288
Ammonia	0.057	0.244	0.436	-0.126	0.007	0.540	0.059	-0.450
Chlorophyll	-0.341	0.470	0.118	-0.611	0.238	0.441	-0.076	-0.336	0.301
Sandspit station II
Temperature	-0.195
Salinity	0.513	0.158
pH	-0.102	-0.081	-0.107
Oxygen	0.120	-0.105	-0.109	0.096
Nitrate	0.129	-0.020	0.421	-0.383	0.135
Nitrite	-0.175	-0.036	-0.054	0.380	0.089	0.478
Phosphate	-0.142	-0.168	-0.297	0.449	-0.171	-0.096	0.288
Ammonia	0.033	0.244	0.436	-0.126	0.007	0.540	0.059	-0.450
Chlorophyll	-0.008	0.470	0.118	-0.611	0.238	0.441	-0.076	-0.336	0.301

Table V. Percentage abundance of zooplankton species in Sandspit and Gadani.

S.	Species	Sandspit %	Gadani %
1	Calanoid	9.66	16.39
2	Cyclopoid	2.53	13.44
3	Herpecticoid	7.65	11.86
4	Crab larva Zoea	0.03	0.29
5	Megalopa larva	0.04	0.06
6	Shrimp larva	0.11	5.51
7	Lobster larva	0.06	0.29
8	Lucifer	0.83	0.95
9	Nauplius	6.45	8.64
10	Squilla larva	0.08	5.69
11	Cyprus Balanus	0.09	1.92
12	Acorn Barnacle nauplii	0.10	1.26
13	Isopoda	0.11	0.03
14	Oikopluera	0.88	1.87
15	Doliolum	0.17	0.58
16	Bristle worm	35.31	5.03
	Table continued on next column.............
S.	Species	Sandspit %	Gadani %
17	Evadne	1.36	2.17
18	Penilia	0.14	2.45
19	Egg mass	0.19	1.00
20	Fish larva	0.15	0.11
21	Jellyfish larva	0.18	0.18
22	Veliger larva	6.23	7.26
23	Brittle star larva	0.21	0.00
24	Sagitta	0.19	1.90
25	Foraminifera	27.01	8.14
26	Ciliates	0.24	2.98

In Sandspit Shannon diversity index and Margalef diversity index in station I was highest in North east monsoon as compare to other seasons. Pielou’s evenness index was highest in station I in autumn inter monsoon. Minimum number of species was recorded in station I in autumn inter monsoon as compared to other seasons (Table III).

There is variation in the percentage abundance of zooplankton species in Sandspit and Gadani (Table V). The highest percent abundance was of Bristle worm 35.31 no.Ind/m3 in Sandspit than Foraminifera 27.01 no.Ind/m3 and third abundance was of Calanoid Copepod 9.66 no.Ind/m3. However, the highest percent abundance was of Calanoid Copepod 16.39 no.Ind/m3 in Gadani than Cyclopoid 13.44 no.Ind/m3 and third abundance was of Herpecticoid 11.86 no.Ind/m3.

DISCUSSION

The present study was conducted in Gadani, situated along the Baluchistan coast and Sandspit, located on the Sind coast within the Northern Arabian Sea. This region experiences significant influences from both the south west monsoon (summer monsoon) and north-east monsoon (winter monsoon). During summer, the south west monsoon leads to upwelling, bringing nutrient rich cooler waters parallel to the coast into the euphotic zone, creating a strong connection between biological and physical processes. This upwelling phenomenon brings nutrient rich water to the surface that influence phytoplankton growth thus increasing chlorophyll a content and high gross primary productivity of the region (Banse, 1987; Kumar et al., 2001).

In the current study, data on the seasonal abundance and distribution of zooplankton with physico-chemical parameters are studied. In the south west monsoon, the peak abundance of zooplankton was noticed due to high chlorophyll a content in Gadani and Sandspit. Our result corresponds with Soriede et al. (2010), who reported high abundance of zooplankton can be related to the high content of chlorophyll a in the area. In the south west monsoon, the temperature was recorded high in both sites (Gadani and Sandspit) as compared to other seasons. Temperature is an important factor that regulates biochemical attributes of marine environment. The disparity in temperature may be due to variations of seasonal impact. In winter, when the temperature was low, the dissolved oxygen value was high, whereas dissolve oxygen was low in summer when temperature was high. In high temperature the organisms demand for oxygen increases results in low dissolved oxygen retaining capacity of water (Hussain et al., 2013).

Salinity has a great effect on the ecological processes of marine environment and is a factor affecting the biodiversity of zooplanktons (Zadereev et al., 2022), an increase in salinity results in loss of biodiversity (Schallenberg et al., 2003). In Sandspit salinity is positively correlated with zooplankton abundance but in Gadani it is negatively correlated. The pH is also an important variable in assessment of water quality because it has a great impact on chemical and biological processes. The pH value was 7.4-8.6 in the Gadani ship breaking area whereas in Sandspit sites it was 7.7-8.6. The pH value showed negative correlation with zooplankton density in Gadani and Sandspit however, pH value showed positive correlation with zooplankton density in Sandspit in station 1. High pH value is directly proportional with water productivity. The pH value greater than 7 but less than 8.5 is best for productivity while pH value less than 4 is harmful for marine life (Abowei, 2010).

The zooplankton abundance and diversity show variations with different season and varied from station to station. The peak abundance is in south west monsoon than in spring inter monsoon, autumn inter monsoon and north east monsoon at coastal waters of Gadani ship breaking area. Whereas in Sandspit the peak abundance of zooplankton is in south west monsoon than in spring inter monsoon, north east monsoon and autumn inter monsoon. In Gadani copepod dominate the zooplankton group, highest percentage was recorded of Calanoid 16.39% than Cyclopoid 13.44% and Herpecticoid 11.86%. Copepods were the most abundant group in Gadani 41.6% than 19.8% in Sandspit of total zooplankton group. Copepod abundance may be due to high temperature and high rate of productivity. In Gadani in all zooplankton groups, Calanoid copepod shows highest in number. The Calanoid copepod shows the peak abundance in autumn inter monsoon and spring inter monsoon whereas, Foraminifera shows abundance in south west monsoon and Veliger larva in north east monsoon. Copepods are the most adaptable and abundant group in marine waters, can tolerate wide range of fluctuations in different physico-chemical parameters and are dominant group. They are the most important secondary producers in coastal and marine ecosystems, forming an important link between phytoplankton, microzooplankton and higher tropic levels such as fish (Beyst et al., 2001). Copepods are herbivorous, some are omnivorous and carnivorous. They are widely distributed in world Ocean and contribute 80% of the biomass (Sampey et al., 2007). Copepods have sturdy exoskeleton and the longest and toughest appendages which assist them to swim faster than other zooplankton. The three orders of copepod differ in their feeding habitats. The cyclopoid copepods are generally carnivorous and feeds on other zooplankton and fish larvae, they also feed on detritus, bacteria and algae. The calanoid copepods are omnivorous feed on rotifers, ciliates, algae, detritus and bacteria. Their food is dependent on age, sex, season and food availability. The herpacticoid are primarily benthic and third group of copepod. Herpacticoid physical shape and diverse feeding habits help them to tolerate severe environment as compared to Cladocera (Kalff, 2016). Cladocerans and copepods serve as the bioindicator to determine the health of the aquatic ecosystem (Jha and Barat, 2003).

Studies on the effect of pollution on zooplankton is scarce (Zaitsev, 1992). However, efforts have been made to initiate categories of zooplankton on the basis of their tolerance to pollution (Soetaert and Van Rijswijk, 1993). Changes in zooplankton diversity, abundance and populations seem to be correlated with factors related with pollution. Anoxic conditions may be the factor for decrease of zooplankton population and diversity in polluted environment (Park and Marshall, 2000) and particulate matter concentrations may affect the death rate of copepods (Castel and Feurtet, 1992). The present study reveals that the environmental conditions of coastal waters of Gadani ship breaking area is polluted and there is less zooplankton but in offshore waters high abundance and diversity of copepod is present.

Increase in nutrient may alter zooplankton species diversity and marine pollution due to anthropogenic activities induces algal bloom affecting population of zooplankton present in the surrounding environment and they exhibit rapid changes in their populations (Park and Marshall, 2000; Jakhar, 2013). Coastal environments are highly productive areas owing to food supplies Cebrian and Valiela (1999). The zooplankton abundance and composition generally varied due to the seasonal variations and their habitat like coastal and mangrove ecosystem. In the present study the abundance of zooplankton was greater in open-ocean as compare to near shore waters. In Sandspit total abundance per year of zooplankton in Station II was 727186.8 individuals/m³ as compare to Station I was 67777 individuals/m³. Whereas in Gadani total abundance per year of zooplankton in station III was maximum 51942 individuals/m³, in station II was 36798 individuals/m³ and in station I was 40620 individuals/m³. In ship breaking area (station I) there is less abundance of zooplankton as compared to offshore (station III). This may be due to the organic and inorganic materials dissolved in the water that support zooplankton growth. Similar results also reported by Bhuiyan et al. (1982), Ali et al. (1985), Zafar (2000) and Khan et al. (2015).

CONCLUSION

This study provides recent and up-to-date data on diversity and abundance of zooplankton in two distinct sites: The Gadani ecosystem on the Balochistan coast (a polluted site) and Sandspit on the Sind coast (a non-polluted) within the Northern Arabian Sea. It shows the significance of physico-chemical parameters and nutrient levels as pivotal factors that influence the production of phytoplankton and zooplankton essential for sustainable fisheries. Moreover, the study highlights the impact of seasons on the physiological and chemical condition of marine waters that determine composition and distribution of zooplankton species. Further studies of the seasonal zooplankton abundance and diversity in the inshore and offshore waters of the Pakistan coast are required to understand the dynamics of the zooplankton to sustain fisheries in this area.

Declarations

Acknowledgement

The authors would like to thank Higher Education Commission (HEC) for providing financial support of the project No. 20-2768, National Research Program for Universities (NRPU) to carry out the present research and the facility provided by Centre of Excellence in Marine Biology is well appreciated.

Funding

Statement of conflict of interest

The authors have declared no conflict of interest.

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