Introduction

The Iraqi marine water is located at the north-western part of the Arabian Gulf. This water body consists of Shatt Al-Arab Estuary (shallow, average depth of 5 meter at low water) and several open lagoons such as Khour Al-Khafka and Khour Al-Amaya (deep and narrow, average depth of 25 m at low water) besides Khour Abdullah. Shatt Al-Arab River is the main source of freshwater to the northern zone of the Arabian Gulf (Al-Mahdi et al., 2009).

Zooplankton are generally very small animals, usually weak swimmers and therefore just drift along with the currents. They live all or part of their life as plankton and living virtually everywhere in the ponds, swamps, rivers and seas. They are found near the surface in aquatic environments where there is enough sunlight to support phytoplankton. They are considered the key components of marine ecosystems forming the base of most marine food webs which comprise the first link in the food chain and food for many zooplankton. These include a wide range of animals, from primitive protozoan’s to the larvae of more complex animals and range in size from microscopic organisms to some of the longest animals in the sea.

However, research on the zooplankton in Iraqi marine water was few, the first study by Gurney (1921) who surveyed Crustaceans of the lower Mesopotamia for the first time. Mohammad (1965) then identified Cladocera from the middle and south of Iraq in different areas including Al-Musab in North West Arabian Gulf to Al-Qurna region. Khalaf (1988) studied the quality of zooplankton in Khour Al-Zubair and Khour Abdulla. Other studies included Salman et al. (1990) study in Khour Abdullah, Ajeel (1990), Khalaf (1991; 1992), Khalaf and Ajeel (1994), Khalaf (1994), Ajeel and Khalaf (1995), Ajeel and Khalaf (1997) and Ajeel (1997) in Khour Al-Zubair and Khour Abdullah. Al-Zubaidi (1998), Al-Zubaidi and Salman (2001) studied the zooplankton of the southern Shatt Al-Arab River estuary and Northwest Arabian Gulf. Ajeel (2004) investigated the zooplankton of the Umm Qaser and Khour Al-Zubair canal followed by Khalaf (2008) and Al-Shawi (2010) study in Khour Al-Zubair. Then Morad (2011) investigated the seasonal changes of zooplankton in Iraqi Marine coastal and estuarine brackish water. Whereas, Ajeel (2012) investigated the zooplankton of the Khour Al-Zubair and Shatt Al-Basrah Canal. Ajeel et al. (2015) studied the chemical elements in zooplankton at Iraqi marine water. Ajeel (2016) studied the zooplankton of the south Shatt Al-Arab River.

Due to the environmental importance of the North-West Arabian Gulf, and the significant role played by Khour Al-Zubair, Shatt Al-Basrah channel and Shatt Al-Arab River for being good places for spawning, breeding and feeding of many fish, and good sources for fishing and important outlets for the Iraqi territorial waters. Therefore, the present study aims to estimate the zooplankton production in North West Arabian Gulf, identify the environmental characteristics and to study some characteristics of biological organisms by means of the biomass of zooplankton. In addition to that the study aims to give basic information about the diversity and occurrence of the zooplankton from 7 stations in Iraqi marine water at the period of the peak time.

1 Materials and Methods

1.1 Sample collection

Seven stations were selected in various localities in North West Arabian Gulf. Shatt Al-Arab estuary represented in St. 1 Al-Faw located in the Shatt Al-Arab River, before the flow in the Arabian Gulf (29^°59^׳20.06^״N 48^°28^׳02.22^״E), St. 2 Ras Al-Bisha located in the end of Shatt Al-Arab River (29^°56^׳3.20^״N 48^°35^׳45.98^״E), St. 3 Al-Musab, located in the beginning of the Arabian Gulf (29^°50^׳54.8^״N 48^°4933.1^״E), St. 4, 5 and 6 at Khour Abdulla located between Al-Faw peninsula and Warbah and Bubiyan Islands }St. 4 Buoy 5 located (29^°46^׳57.6^״N 48^°34^׳15.4^״E), St. 5 Buoy 13 located (29^°50^׳26.4^״N 48^°25^׳49.7^״E), St. 6 Buoy 29 located (29^°59^׳54.8^״N 48^°12^׳33.1^״E){ and St. 7 Umm Qaser Port located (29^°59^׳54.00^״N 47^°59^׳30.37^״E) (Figure 1). The nature of these stations is quite different from each other, plankton net of 120 micron mish-size and 40 cm mouth diameter was towed behind a boat for 15 min. A flow-meter was mounted at the mouth of the net to determine the volume of water filtered by net (DeBernardi, 1984). Water temperatures were recorded to the nearest 0.1°C. Salinity determinations were carried out immediately with a digital salinometer M.C.5 type Salinity Temperature Bridge. Samples were preserved in 4% formalin immediately after towing, while those used for biomass measurements, were deep-freezed without fixation.

In the laboratory, the samples were poured into a graduated vessel, and diluted if densely populated. Then a 10 ml subsample was taken. The sample was placed in a Bogorov chamber, examined and counted under a stereomicroscope. This procedure was repeated for 3 times, and then the whole sample was examined for the rare species.

1.2 Biomass of zooplankton

1.2.1 Displacement volume

The volume of water displacement of zooplankton was measured for all samples. The samples were put in the volumetric flask 500 ml and completed the volume to the final mark by the addition of water, then the sample was filtered through a net of a mish-size less than that used for sample collection, in another volumetric flask (500 ml), the volume then was completed to the mark.

The added volume of water is equal to the displacement volume of the zooplankton. The volume of zooplankton (ml/m³) was then obtained by dividing the volume of zooplankton by the volume of the samples filtered by the net.

The standing crop of the zooplankton (mg C/m³) was calculated using the conversion factor of 65 mg C/ml of displacement volume. (Jacob et al., 1979).

1.2.2 Wet weight and Dry weight

Fresh weight and dry weight of the zooplankton were estimated by filtering the sample through a wet filter paper of a known weight using a vacuum pump and the wet weight was recorded by subtracting the weight of the wet filter paper from the paper with the zooplankton. Then the paper was oven–dried at 60ºC for 24 hours and the dry weight was recorded. The dry weight of the filter paper was subtracted from that of the paper with the sample and the dry weight of the sample was obtained. Then the wet weight and dry weight were converted into mg/m³ by dividing the weight of the sample by the volume of the sample filtered.

2 Results

2.1 Environmental factors

Water quality parameters varied according to seasonal norms. Temperature ranged from 20.4°C in March 2010 at St. 7 (Umm Qaser Port) to 29.5°C in July 2009 at St. 5 (Khour Abdulla (Buoy 13), salinity from 5.87 psu in March 2010 at St. 1 (Al-Faw) to 47 psu in July 2009 at St. 5 (Khour Abdulla (Buoy 13)) (Figure 2).

2.2 Density of zooplankton

The density of zooplankton ranged between 185 ind./m³ during March 2010 to 32856 ind./m³ during July 2009 at St. 3 (Al-Musab) (Figure 3). The average density of zooplankton was 12940 ind./m³ in the study area during July 2009 and 5216 ind./m³ during March 2010 (Table 1 and Table 2). Crustaceans were the dominant of the total zooplankton in seven stations respectively (82.8%, 95.3%, 69.7%, 85.3%, 94.5%, 19.3% and 88.0%). Copepoda constitute (76%) of the total zooplankton. The second important group was Bivalve (9%), then Rotifera (6%), whereas Cirripede larvae and Appendicularia (2%) followed by the other zooplankton (5%) (Figure 4). Among Copepoda, Calanoid was the dominant group in all stations (67%). Then Cyclopoida (15%), nauplii (12%), Harpacticoida (4%), and Poecilostomatida (2%) of total Copepoda (Figure 5).

2.2.1 Station 1 (Al-Faw)

The density of zooplankton reached 16696 ind./m³ in July 2009. It was found that the Copepoda (11827 ind./m³) constituted about 70.8% of the total zooplankton, and the important species were Acartia pacifica (1119 ind./m³) (6.7%) and Paracalanidae (4137 ind./m³) (24.8%). The second important group was Bivalve (3796 ind./m³) (22.7%), then the Cirripede larvae (584 ind./m³) (3.5%) of the total zooplankton (Table 1).

Whereas in March 2010 The density of zooplankton reached 9895 ind./m³. The Copepoda (9413 ind./m³) constituted 95.1% of the total zooplankton, It was found the density of Cyclopoida reached 9215 ind./m³ (93.1%), Rotifers 276 ind./m³ (2.8%) of the total zooplankton.

2.2.2 Station 2 (Ras Al-Bisha)

The density of zooplankton reached 32856 ind./m³ in July 2009. It was found that the Copepoda (30368 ind./m³) constituted about 92.4% of the total zooplankton, and the important species were Acartia pacifica (9557 ind./m³) (29.1%) and Paracalanidae (8254 ind./m³) (25.1%). The second important group was Cirripede larvae (790 ind./m³) (2.4%), then Polychaets and Appendicularia (513 ind./m³) (1.6%) of the total zooplankton.

Whereas in March 2010 the density of zooplankton reached 185 ind./m³. The Copepoda (51 ind./m³) constituted 27.6% of the total zooplankton, It was found the density of Megaloba reached 109 ind./m³ (58.9%) of the total zooplankton (Table 1).

2.2.3 Station 3 (Al-Musab)

The density of zooplankton reached 27547 ind./m³ in July 2009. It was found that the Copepoda (18386 ind./m³) constituted about 66.7% of the total zooplankton, and the important species were Acartia pacifica (1797 ind./m³), Paracalanidae (2549 ind./m³), Cyclopoida (1373 ind./m³), Nauplii (2614 ind./m³), Acrocalanus sp. (1275 ind./m³) and Euterpina sp. (1209 ind./m³). The second important group was Bivalve (6765 ind./m³) (24.5%) then Cirripede larvae (588 ind./m³) (2.1%) and Appendicularia (556 ind./m³) (2.0%) of the total zooplankton (Table 1).

2.2.4 Station 4 (Khour Abdullah, Buoy 5)

The density of zooplankton reached 4499 ind./m³ in July 2009. It was found that the Copepoda (3381 ind./m³) constituted about 75.1% of the total zooplankton, and the important species were Corycaeus sp. (642 ind./m³), Cyclopoida (654 ind./m³), Nauplii (618 ind./m³) and Copepodite stages (557 ind./m³). The second important group was Bivalve (412 ind./m³) (9.1%) then Rotifers (303 ind./m³) (6.7%) of the total zooplankton.

Whereas in March 2010 the density of zooplankton reached 6418 ind./m³. The Copepoda (5774 ind./m³) constituted 89.9% of the total zooplankton. The imported groups of copepods was Copepodite stages (2489 ind./m³), Paracalanidae (1102 ind./m³), Cyclopoida (938 ind./m³) and Nauplii (388 ind./m³). The second important group was Polychaetes (163 ind./m³) (2.5%) then Foraminifera (122 ind./m³) (1.9%) of the total zooplankton (Table 2).

2.2.5 Station 5 (Khour Abdullah, Buoy 13)

The density of zooplankton reached 5114 ind./m³ in July 2009. It was found that the Copepoda (4861 ind./m³) constituted about 95.0% of the total zooplankton, and the important groups were Copepodite stages (1649 ind./m³) and Paracalanidae (1093 ind./m³) Nauplii (914 ind./m³). The second important group was Appendicularia (72 ind./m³) (1.4%) of the total zooplankton.

Whereas in March 2010 the density of zooplankton reached 2881 ind./m³. The Copepoda (2658 ind./m³) constituted 92.2% of the total zooplankton. The important groups of copepods were Copepodite stages (1674 ind./m³), Nauplii (230 ind./m³). The second important group was Appendicularia (77 ind./m³) (2.7%) then Bivalve and Fish eggs (55 ind./m³) (1.9%) of the total zooplankton (Table 2).

2.2.6 Station 6 (Khour Abdullah, Buoy 29)

The density of zooplankton reached 1223 ind./m³ in July 2009. It was found that the Copepoda (1094 ind./m³) constituted about 89.4 % of the total zooplankton, and the important species were Copepodite stages (439 ind./m³), Nauplii (268 ind./m³), Paracalanidae (225 ind./m³) and Acartia pacifica (96 ind./m³). The second important group was Megaloba (43 ind./m³) (3.5%) then Polychaetes (32 ind./m³) (2.6%) of the total zooplankton. While at March 2010 the density of zooplankton was 7029 ind./m³. The important group Rotifers (6566 ind./m³) (93.4%) then Copepoda 416 ind./m³ (5.9%) (Table 2).

2.2.7 Station 7 (Umm Qaser Port)

The density of zooplankton reached 2642 ind./m³ in June 2009. It was found that the Copepoda (1267 ind./m³) constituted about 47.9% of the total zooplankton, and the important species were Copepodite stages (469 ind./m³), Paracalanidae (297 ind./m³) and Nauplii (172 ind./m³). The second important group was Megaloba (906 ind./m³) (34.3%) then Polychaetes (141 ind./m³) (5.3%) of the total zooplankton. Whereas at March 2010 the density of zooplankton was 7662 ind./m³. The prevailing group was Copepoda (3722 ind./m³) (45.9%) and the second prevailing group was shrimps larvae (Megaloba) (2522 ind./m³) (32.9%) then Bivalve (525 ind./m³) (6.8%) of the total zooplankton (Table 2).

2.3 Biomass of the zooplankton

The biomass of zooplankton, in terms of wet weight varied from 30.256 mg/m³ at St. 6 (Buoy 29) during March 2010 to 1792.6 mg/m³ at St. 2 (Ras Al-Bisha) during July 2009 and the average was 387.762 mg/m³ (Figure 6). Whereas in terms of dry weight ranged between 1.120 mg/m³ at St. 5 (Buoy 13) to 84.216 mg/m³ at St. 2 and the average was 18.738 mg/m³ (Figure 7). While the biomass of zooplankton in terms of displacement volume and standing crop ranged from 0. 065 ml/m³ and 4.233 mg C/m³ to 2.005 ml/m³ and 130.335 mg C/m³ in July 2009 at St. 6 and St. 2 respectively and the average 0.508 ml/m³ and 33.015 mg C/m³ respectively (Figure 8 and Figure 9).

The average value of zooplankton biomass at Shatt Al-Arab estuary (St. 1, 2 and 3) 726.665 and 33.305 mg/m³ in terms of wet weight and dry weight respectively, and 0.905 ml/m³ and 58.825 mg C/m³ in terms of displacement volume and standing crop respectively. Whereas at Khour Abdulla (St. 4, 5 and 6) 131.018 and 8.435 mg/m³ in terms of wet weight and dry weight respectively and 0.219 ml/m³ and 14.235 mg C/m³ in term of displacement volume and standing crop respectively, while in Umm Qaser Port 141.285 and 5.915 mg/m³ in terms of wet weight and dry weight respectively and 0.183 ml/m³ and 11.927 mg C/m³ in terms of displacement volume and standing crop respectively (Figure 10 and Figure 11).

3 Discussion

Zooplankton plays vital roles in energy and matter transfer through the system. Despite their importance, understanding of zooplankton biodiversity is limited because of their fragile nature, small body size, and the large number of species from various taxonomic phyla. The zooplankton distribution varies both spatially and temporally according to the environmental conditions prevailing in the region. Differences may also arise due to the nature of distribution of the plankton, namely patchiness which may be the cause of the great variations in the catches of the nets (Raymont, 1983). Moreover, the mesh-size of the net is an important factor controlling the quality and quantity of the catch; in general the crop of a small apertures nets more than larger aperture nets (Ajeel, 1990).

The results showed that the Crustacean constituted a large proportion of zooplankton in the study area, which comprised (82.8%, 95.3%, 69.7%, 85.3%, 94.5%, 19.3% and 88.0%) of the total zooplankton in seven stations respectively. This is consistent with the study of Salman et al. (1990) in Khour Abdullah, who found that the crustaceans comprised 88 % of the total zooplankton, and the study of Ajeel (1990) in Khour Al-Zubair port and Umm Qasr port and Khour Abdullah, who found that the crustaceans constituted about 97.9%, 90.7% and 94.1%, respectively. However Al-Zubaidi (1998) found that the crustaceans constituted about 88.4% in the Northwest Arabian Gulf, while Ajeel (2004) stated that the proportion of crustaceans amounted to 85.4% in Shatt Al-Basrah and 92.1% and 99.5% in the Khour Al-Zubair ports and Umm Qasr ports respectively. Whereas 62.9% in Shatt Al-Basrah and 83.7% in Khour Al-Zubair (Ajeel, 2012).

The present results indicate that there were great differences in the abundance of zooplankton among the seven stations sampled. Moreover, at first three stations (Shatt Al-Arab estuary) are the richest among all in zooplankton abundance. That’s agreement with Al-Zubaidy and Salman (2001). Similarly the highest zooplankton biomass estimations were occurred at the second station (Ras-Al-Bisha) and first station (Al-Faw) more than other stations during July 2009 and March 2010 respectively, this may be due to changing environmental conditions especially salinity concentration. According to Day et al. (1989) the temperature and salinity have been identified as important factors regulating zooplankton abundance in estuaries. As well as the effect of nutrients from Shatt Al-Arab to the Arabian Gulf.

Also it is obvious that the average highest density of zooplankton (16518 ind./m³) is recorded at station 2 (Ras Al-Bisha) and the lowest density (3997 ind./m³) is recorded at station 5 (Bouy 13). Several researchers in other estuaries elsewhere exhibited a similar pattern of zooplankton abundance (Pace et al., 1992; Laprise and Dodson, 1994). Moreover the Shatt Al-Arab estuary represents a calm environment (Albadran et al., 1995). Therefore, higher salinities which have reduced current velocities and increase transparencies at this part of the estuary may have resulted in increasing retention time, phytoplankton and high zooplankton abundance observed (Bakker, 1994; Bakker and Rijswijk, 1994).

A comparison of the results of the present study with those of previous studies in different regions may be meaningful because of the different mesh sizes of nets used in the collection of samples (Table 3). While Table 4 shows a comparison of the biomass of zooplankton in the Garmat Ali River, Shatt Al-Arab River, Shatt Al-Arab estuary, Khour Al-Zubair, Khour Abdullah and Arabian Gulf. The highest value of biomass was 3.461 ml/m³ and recorded in Khour Al-Zubair in February 1990, while the lowest value was 0.001 ml/m³ and reported in the Shatt Al-Arab in June 1996. The results show the average biomass of the zooplankton was high in the Shatt Al-Arab estuary at (St. 1, 2 and 3) and lower in the St. 7 (Umm Qaser), these differences are likely locality variations.

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