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Impacts of dredging on zooplankton communities of Warri River, Niger Delta, Nigeria
by Elijah I. OhimainEnvironmental Resources Managers Ltd, 107A Imam Abibu Adetoro Street, Victoria Island, Lagos, Nigeria. E-mail: eohimain@erml.net
Tunde O. ImoobeDepartment of Zoology, Faculty of Science, University of Benin, P. M. B. 1154, Benin City, Nigeria
Marian O. Benka-CokerDepartment of Microbiology, Faculty of Science, University of Benin, P. M. B. 1154, Benin City, Nigeria
Abstract The impact of dredging on zooplankton community in a tropical mangrove ecosystem was assessed. Prior to dredging, a total of 20 species of zooplankton at various stages of development made up of Copepoda (6), Cladocera (2), Cirripedia (1), Decapoda (3), Mollusca (2), Chaetognatha (1), Nematoda (1), Larvacean (1), Rotifera (1) and Fish larvae (1) were recorded. A checklist is given. Copepoda formed the dominant group in the river representing 81% of the population in the samples. This trend was similar at all other locations including the control stations 1000m upstream and downstream of the dredged section. Dredging resulted in a reduction in the population (by 91%) and taxa richness (by 72%). Factors responsible for the high mortality of zooplankton as a result of this dredging are examined and discussed in the light of available data. We conclude that dredging in the mangrove estuary resulted in heavy impacts on zooplankton population and diversity. Key words: Dredging, zooplankton, Niger Delta, population, diversity.
Introduction The Niger Delta of Southern Nigeria has continued to be the epicentre of hydrocarbon exploration and exploitation since the early 1960s. Access to these wetlands is one of the major challenges faced by oil companies. Typically, access is required during seismic operations; exploratory drilling; development drilling and well completion; construction of production facilities, base camps; and waste management (IUCN 1993). During dredging, vegetation, soil and sediments on the right of way (ROW) of the canal to be dredged are usually removed by dragline/ cutter/suction and deposited in mounds directly adjacent to the dredge channel as dredged spoils. Visible consequences of this practice are the loss of habitat, destruction of mangrove flora and fauna, and interrupted breeding cycles (Herbich 1981, IUCN 1993). The turbidity plumes created during dredging and disposal operations, although short-term often result in the reduction of phytoplankton and zooplankton population and diversity as well as estuarine productivity (Toumazis 1995). In addition to turbidity plumes, dredging also caused rapid depletion of dissolved oxygen in the water column through re-suspension of anoxic sediments containing organic matter and sulphide (Delaune & Smith 1995). The microbial oxidation of anoxic sediment containing sulphide often results in rapid drop in pH and re-mobilization of trace metals (Petersen et al. 1997). This increased acidity and trace metal mobilization into the environment may also have negative effect on plankton communities. This study is therefore commissioned to assess the impacts of dredging on the zooplankton communities of the Niger Delta.
This study was carried out in and around a dredged canal located in the mangrove swamp of the Niger Delta in Delta State, Southern Nigeria. The site is about 20km from Warri at latitude 50 31’N and longitude 50 31’E. The vegetation is typically mangrove swamp with a preponderance of Rhizophora racemosa. There are two seasons in the area. The rainy season lasts from March to October while November to February represents the dry season. Low humidity and high atmospheric temperature are generally recorded during the dry season, while the reverse is the case in the rainy season. Five sampling points were visited shortly before and after dredging in June and July 1998. These were at the dredged canal to the wellhead (STN 1), 500m upstream (STN 2), 1000m upstream (STN 3), 500m downstream (STN 4) and 1000m downstream (STN 5) of the canal along the main river course (FIG 1). Stations located 1000m upstream and downstream from the dredged canal are control stations.
Data collection and analyses Plankton samples were collected a week before and a week after dredging at each of the five stations, using Hydrobios plankton net of mesh size 55mm. Both qualitative and quantitative samples were collected. While the qualitative samples were collected by towing the net just below the water surface for five minutes at every station, the quantitative samples were collected by filtering fifty (50) litres of water through the same net. The concentrated samples collected were preserved separately in buffered 4% formaldehyde solution in plankton bottles and transported to the laboratory. In the laboratory, sorting and identification of specimens from the qualitative samples were done under a stereo zoom dissecting microscope and an Olympus universal Vanox Research Microscope. Representative specimens were mounted in 100% glycerin after dissecting relevant parts (where necessary). These were then identified using relevant literatures (Boxshall & Braide 1991, Crane 1973, Egborge & Chigbu 1988, Newell & Newell 1963, Smirnov 1974, Wickstead 1965). Abundance of the plankton was estimated by direct count. Each quantitative sample was concentrated to 25ml and from this a total count was made of species that were rare, while for those that were densely populated, 1ml of sample was taken and counted. The results for the various species were calculated for the sample using the formula,
Where N = the estimated number of species i per sample, a = volume of the sample (ml), b = volume of the sub-sample (ml) counted, and n = the number of species i in the sub-sample. Indices of species diversity were used to characterise the Zooplankton community structure. Taxa richness was computed using Margalef’s index (D) expressed as
Where: S = Number of taxa N = Total Number of all individuals. In= Natural logarithm General species diversity using Shannon-Wiener (H’) index was computed as
Where: N= total number of individuals, ni= number of individuals in species i k= total number of species.
This index take into account the total number of species present, as well as their respective abundance, thus providing a more convenient means of comparing differences within ecological communities. Since changes in the environment are reflected in the types and number of organisms present, diversity indices provide a tool for monitoring temporal changes. Evenness index, which expresses the degree of uniformity in the distribution of individuals among the taxa in the collections, was also calculated
Where H/=Shannon-Wiener index Hmax =maximum expected diversity expressed as Log S. S=Number of taxa. Besides the application of the diversity indices, interstation comparisons were carried out to test for significant differences in the faunal abundance using one-way analysis of variance (ANOVA) (Zar, 1984). Where significant values (p < 0.05) were obtained, least significant difference (LSD) test was subsequently applied to all pairs of means to detect the location of difference. To determine whether or not differences occurred between the faunal abundance before and after dredging, paired t-test was used to compare each of the two set of samples in the five sampling stations.
Results Composition, abundance and distributionA variety of animals were present in the plankton, these consisted of both holoplanktonic and meroplanktonic Zooplankton. A checklist is shown in Table 1. Figure 2 shows the relative abundance, and diversity of zooplankton in the study area before and after dredging. A total of 923 individuals were encountered in this study, about 57% of these (526) were recorded from all five stations before dredging, while the rest 43% (397) were found after dredging. The population per station before dredging ranged between 100 and 108, while that after dredging widened to between 9 and 102. Before dredging the highest number was recorded at the 1 Km upstream, however, it was not significantly different (p > 0.05) from the population at the other stations. On the contrary, after dredging, the population became drastically reduced at the dredged canal to 9% of what it was before dredging, with little change with distance from the dredged canal thus making the impact of dredging less obvious at the control stations (Fig. 2). A one way ANOVA, indicate that the dredged canal and 500m downstream were significantly different (p < 0.05) in abundance from all the other stations after dredging. At all other stations the observed variations were not statistically significant (p > 0.05).
Diversity Twenty different taxa of zooplankton at various stages of development were encountered from a total population of 923 individuals before and after dredging at all the stations. The highest number of taxa (20) was collected from 1km upstream of the dredged canal before and after dredging, while the least number (5) was recorded after dredging at the dredged canal. Taxa richness before dredging ranged between 18 and 20, whereas after dredging the range widened to between 5 and 20 being lowest at the dredged canal and highest at the 1Km upstream of the canal. The result of diversity analysis before dredging shows that diversity was equally high at all 5 sampling stations. However, after dredging there was a significant reduction in diversity at the dredged canal and 500m downstream (Fig. 2). A one-way ANOVA shows that there was no significant difference (p > 0.05) in taxa richness among the 5 stations before dredging. However, after dredging a significant difference (p < 0.05) in the taxa number of the dredged canal and 500m downstream from the rest 3 stations was observed.
Holoplankton Copepoda Ectocyclops phaleratus Koch, 1838 Microcyclops varicans Sars, 1863 Thermodiaptomus yabensis Wright & Tressler, 1928 Tropodiaptomus laurentii Gauthier, 1952 Eudiaptomus gracilis Sars, 1863 Bryocamptus minutus Claus, 1863 Nauplius Stage Cladocera Ilyocryptus spinifer Herrick, 1882 Diaphanosoma excisum Sars, 1885 Rotifera Filinia opoliensis Zacharias, 1898 Chaetognatha Sagitta enflata Larvacea Oikopleura sp.
MeroplanktonCirripedia Nauplius Stage Decapoda Anomuran larvae Brachyuran larvae Caridean larvae Mollusca Gastropod larvae Bivalve Larvae Nematoda Dorylaimus sp
Vertebrata Fish larvae.
DiscussionA total of 100 individuals belonging to 20 taxa were recorded prior to the commencement of dredging at the dredged canal. Copepods were the most dominant group accounting for 81% of the organisms counted here. Other groups of zooplankton encountered during the study include Cladocera, Cirripedia, Decapoda, Mollusca, Chaetognatha, Nematoda, Larvacean and Fish larvae. Detailed accounts of zooplankton of Warri River have been reported (Egborge & Okoi 1986, Gabriel 1986, Egborge 1994, Oronsaye & Egborge 1998). Taxa richness prior to dredging was high, but after dredging the number of species reduced drastically in the dredged canal (5 taxa) and at 500m downstream(9 taxa). This reduced species richness and diversity could be due to the increased turbidity, total suspended solids (TSS), declined oxygen and toxic effect of metals. According to Ohimain (2001), immediately after dredging in the study area, the surface water in the study site became acidic, turbid, dissolved oxygen declined while BOD5, COD, oil and grease, conductivity, TDS, heavy metals and sulphate increased. In particular, turbidity and total suspended solids (TSS) levels in all the stations were less than 20NTU and 20 mg/l respectively prior to dredging. Soon after dredging, turbidity plumes were observed with the turbidity and TSS values increasing drastically to 11,398NTU and 8,200mg/l at the dredged canal and 7,986NTU and 6,600mg/l at the 500m downstream location respectively. Other researchers have recorded similar impacts after dredging operations (USEPA/USACE 1992,1998, Turner 1997). The slight impacts recorded at 500m downstream locations may be due to the impacts associated with the heavy machinery used in the area and tidal fluctuations that may have carried turbidity plumes into these locations. Turbidity plumes have been reported to negatively impact estuarine organisms during dredging and disposal operations and to reduce primary production as a result of the restriction of photosynthetic processes in plants (IUCN 1993, Lannuzzi et al 1996). Reavell (1997) has also reported that dredging caused decrease in light penetration by 25% to 50% for a river distance of 12 km, and that this effect was still observed 18months after dredging. Reasons for this prolonged effect was ascribed to wash-in of silt and clay colloids from unvegetated spoil heaps. AcknowledgementBox shall GA & Braide EI (1991) The freshwater cyclopoid copepods of Nigeria with an illustrated key to all species. Bull. Br. Mus. Nat.Hist. (ZOOL). (2), 185-212. Crane JM (1973) Introduction to Marine Biology: A Laboratory Text. Charles Merrill Pub. Co., Ohio, USA. 179pp Delaune RD & Smith CJ (1985) Release of nutrients and heavy metals following oxidation of freshwater and saline sediments. J. Environ. 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Toumazis AD (1995) Environmental impact with dumping of dredged material at sea: A case study of the Limasol Port extension works. Wat. Sci. Technol. 3 : Total suspended solids 2: 9-10. Turner RE (1997) Wetland loss in the Northern Gulf of Mexico multiple working hypothesis. Estuaries: 20 1-3 USEPA/ USACE (1998) Evaluation of dredged material proposed for discharge in waters of the US. Inland testing manual report No .EPA –823-398-004. 391pp. USEPA/USACE (1992) Evaluating environmental effects of dredged material management alternatives: A technical framework. 96pp. Wickstead JH (1965) An introduction to the study of tropical plankton Hutchinson and co (publishers) Ltd. 160pp World Conservation Union (IUCN) (1993) Oil and gas exploration and production in mangrove areas. IUCN, Gland, Switzerland. 47pp. Zar JH (1984) Biostastistical analysis. 2nd end. Prentice Hall Inc. New Jersey, 718pp. |
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