Effects of Tide on Zooplankton Community of a Tributary of Upper Bonny Estuary, Niger Delta, Nigeria

7/30/2019 Effects of Tide on Zooplankton Community of a Tributary of Upper Bonny Estuary, Niger Delta, Nigeria

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I nternational Journal of Scientifi c Research i n Knowledge (I JSRK), 1(9), pp. 325-342, 2013Available online at http://www.ijsrpub.com/ijsrk

ISSN: 2322-4541; 2013 IJSRPUB

http://dx.doi.org/10.12983/ijsrk-2013-p325-342

325

Full Length Research Paper

Effects of Tide on Zooplankton Community of a Tributary of Upper Bonny Estuary,Niger Delta, Nigeria

Onome Augustina Davies1*

, Obih Alex Ugwumba2

1Department of Fisheries and Aquatic Environment, Rivers State University of Science and Technology, Port Harcourt,

Nigeria2Hydrobiology and Fisheries Unit, Department of Zoology, University of Ibadan, Ibadan, Nigeria

*Corresponding Author: e-mail: [email protected]

Received 4 June 2013; Accepted 31 July 2013

Abstract. Okpoka Creek is a tidal tributary of the Upper Bonny Estuary in the Niger Delta receiving organic anthropogenic

effluents from Trans-Amadi Industrial Layout and the waterfront communities. The study investigated the influence of tide(low and high) on the species composition, diversity, abundance and distribution of zooplankton. The surface water and

zooplankton samples were collected monthly from May 2004 April 2006 at low and high tides from ten stations according to

APHA methods. Zooplankton was identified microscopically. Species diversity was calculated using standard indices. Data

analyses were done using analysis of variance, Duncan multiple range and descriptive statistics. Phosphate and ammonia

exceeded FEPA and USEPA acceptable levels of 0.10 mg/L and 0.10 mg/L respectively for natural water bodies. Phosphate

had significant tidal variations (P


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quality of aquatic environments (MBO, 2007a). Theyare useful indicators of future fisheries health becausethey are a food source for organisms at higher trophiclevels (Godhantaraman, 2001). Zooplankton biomass,abundance and species diversity are used to determine

the conditions of the aquatic environment (MBO,2007b). Generally, copepods dominate thezooplankton community in most aquatic ecosystems(Davies et al., 2008 and Ekwu and Sikoki, 2005).

Tides play a major role in the functioning of many

coastal systems. They are responsible for obviousmid-term (spring-neap cycles) and short-term (low-

high water cycles) variations in the abiotic and bioticcharacteristics of these systems (Villate, 1997). Tidalflushing is one of the main bottom-up factorscontrolling zooplankton biomass in estuaries besides

nutrients (Villate, 1997; Aliet al., 2011andTriguerosand Orive, 2000). These short/medium-term patternsof zooplankton abundance probably determine theavailability of food for their predators and may thushave significant implications for the foragingstrategies of juvenile fish preying upon zooplankton.On tidal coasts, the zooplankton community has tocope with continual changes in water level, and

current strength and direction (Krumme and Liang,2004). Quite a number of works had been conductedto evaluate tidal control of animal communities indifferent systems. These include: tidal influence on

bacteria, microphytoplankton and microzooplanktonabundance in a coastal lagoon of Baja California,Mexico (Morales-Zamorano et al., 1991), effects ofseasonality on zooplankton community dynamics inthe macrotidal coastal zone of the Amazon region (daCosta et al., 2001), tidal-induced changes in acopepod-dominated zooplankton community in amacrotidal mangrove channel in northern Brazil(Krumme and Liang, 2004) and tidal influence onzonation and occurrence of resident and temporaryzooplankton in Mundaka Estuary, Bay of Biscay(Villate, 1997).

There are many mans activities going on aroundand in the creek such as slaughtering of animals,

transportation (boating, navigation), fishing and wastedisposal. However, there has been no information on

the influence of tide on the zooplankton and nutrientsof the Okpoka Creek, a tributary of Upper BonnyEstuary. In order to bridge the existing gap inknowledge of the biotic and abiotic features of thisestuary, there is therefore the need to provide useful

information on the tidal variations of zooplankton andwater nutrients of this Creek. The study evaluated theinfluence of low and high tides on the species

composition, species diversity, abundance anddistribution of the zooplankton as well as nutrients.

2. MATERIALS AND METHODS

2.1. Study Area

The Trans-Amadi Creek is located between longitudes7

o00E and 7

o15N and latitudes 4

o28E and 4

o40N.

It is a tributary of the Upper Bonny Estuary in the

Niger Delta, South -South of Nigeria (Fig.1). Thevegetation is dominated by nypa palm (Nypa

fructican) and mangroves, red mangrove (Rhizophoraracemosa) and white mangrove (Avecennia nitida). Itpasses through many communities namely: Oginigba,Woji and Azubiae, Many mans activities going on

within and around this creek include dredging,fishing, boating, navigation, washing, disposal ofexcreta, bathing and swimming, to mention but a few.This aquatic body receives effluent discharges fromthe many industries (Snig, Far East paints, RIVOC,General-agro, Michelin tyres, Cocacola, Hallibuton,Schlumberger, Acorn, etc) and main abattoir housesited close to it.

2.2. Sampling stations

A total of ten stations were chosen at least 500 metres

apart along the main creek course. These stationswere Station 1 (Oginigba), Station 2 (Trans-Amadi bySchlumberger), Station 3 (by main abbatoir house),Station 4 (Azubiae), Station 5 (Woji), Station 6(Okujagu), Station 7 (Okuru-ama), Station 8(Ojimba), Station 9 (Oba-ama) and Station 10 (Kalio-ama). There are many industries sited close to theriver shore discharging effluents into the creek. Thedominant vegetation is nypa palm (Nypa fructican)followed by drying up red mangrove. Patches ofwater hyacinth were seen during the rainy season.Manual dredging of sand is constantly going on.

2.3. Collection of phytoplankton samples and

analyses

Samplings were done at low tides and high tides.

Plankton net of 55m mesh was used to collectzooplankton sample in each station. It is the mostefficient device for concentrating zooplankton (Boyd,1981). The net was towed on a slow-moving engineboat for five minutes and the filtrate was kept in aone-litre wide mouth plastic container and fixed with

few drops of 10% formalin.


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Fig. 1: Study Area Map

In the laboratory, the filtrate was made to form auniform volume of 100ml. The sample was properlyagitated and 1ml of sub sample was collected from it

and transferred into a SedgwickRafter countingchamber using a stampel pipette. Identification and

enumeration (standing crop estimation) was carriedout under a binocular compound microscope withmagnification 40 x 400.Three replicates of thesubsamples were analysed. For each sample, eachsolitary cell or group of cells were counted as one unit

except for the diatoms which were counted in a cell bycell base. Results were expressed in a number of

organisms per ml of sample. The Sedgwick-Raftercounting chamber contains exactly 1ml (50mm long x20mm wide x 1mm deep) and has a surface area of1000mm

2. The exact area viewed within the ocular

micrometer grid is also known. The followingformula was used for the calculation of planktondensity:

Density of plankton (Number of plankters per ml)

= (T) 1000 x Volume of concentrate ml (Eq. 1)

AN Volume of sample (ml)

Where: T = Total number of plankters counted A =area of grid in mm

2; N = number of grids employed

1,000 = area of counting chamber in mm2 (Boyd,1981).

Identification and characteristics of planktonicspecies were made by the descriptive keys byNeedham and Needham (1962); Newell and Newell

(1963); Patrick and Reimer (1966), Han (1978),


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Durans and Leveque (1980), Prescott (19982); Kadiri(1988) amongst others. Margalef species diversityindex was estimated by this formula:H = S1

InN (Eq. 2)

Where S = the number of species (or othertaxonomic group); N = total number of plankers(Boyd, 1981).

Index of dominance (C) was determined by the

formula: = (Eq. 3)

Where ni= importance value for each species(number of individual, biomass, production, etc)

N = total of importance values.Importance values were assigned to the

zooplankton species based on the contribution of eachspecies to total net primary production (Odum, 1971)and also to pollution.

Table 1: Important values assigned to ith speciesabundance

Range Importance value

1-10 1

11-20 2

21-30 331-40 441-50 5

51-60 6

61-70 771-80 881-90 9

91-100 10100 and above 11

Other indices of species diversity calculated are asfollows:(1) Shannon index of general diversity (H) wascalculated thus.

N

ni

N

ni (Eq. 4)

Where ni = importance value for each species: N =total of importance values; Pi = importance

probability for each species =N

ni

(2) Evenness index (e)

S

H

elog

(Eq. 5)

Where H = Shannon index; S =number of species

2.4. Physico-chemical parameters of the water

The following parameters were measured in-situ andin laboratory following standard methods (APHA,

1985). One litre clean containers were used to collect

water samples for physico-chemical parameters ateach station. All the kegs containers were kept in ice-chest box for laboratory analyses. Nitrate wasdetermined by the Brucine method (APHA, 1985).Spectrophotometer (Spectronic 21D) was used to

measure the nitrate at 410nm wavelength. Sulphatedetermination was carried out by turbidimetricprocedure (APHA, 1985). This involved the use ofspectrophotometer (Spectronic 21D). Theconcentration of sulphate (mg/l) was measured thus:

Sulphate = mgSO4 x 1000 (Eq. 6)

Volume of sample used (ml)

Phosphate-in-water levels were determined by

standard test (APHA, 1985). It involved the use of

spectrophotometer (Spectronic 21D). Theconcentration of phosphate (mg/l) was measured thus:

Phosphate = mgPO4 x 1000 (Eq. 7)

Volume of sample used (ml)

Ammonia concentrations in water samples were

determined by the indophenol or phenate (Phenol-hypochlorite) method. It was spectrophotometrically

measured at 630nm wavelength with Spectronic 21D.The concentration of total ammonia in the sampleswas computed from the equation.

C1 = A1C2 = A2Where A1 = the absorbance of the total ammonia-

nitrogen standard: A2 = the absorbance of the sample;C1 = the concentration of the total ammonia-nitrogenstandard; C2 = the concentration of total ammonia-nitrogen in the sample (Boyd, 1981).

2.5. Data analyses

SAS (2003) was used to analyse data for analysis ofvariance (ANOVA), Duncan multiple range (DMR)

and descriptive statistics.

3. RESULTS

3.1. Zooplankton

The study recorded 30,742 zooplankton made up of 7taxa, 66 genera and 85 species. They includedRotifera (29 species), Copepoda (26 species),Cladocera (12 species), Protozoa (11 species),

Ostracoda (5 species), Euphausiacea (1 species) andBranchiura (1 species).


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3.2. Rotifera (Rotifers)

Rotifers represented 9.05% of the zooplankton

abundance with a mean of value of 789.11 124.12

org/ml in the study area (Fig. 2). Tide had nosignificant effects on parameters (P>0.05). At hightide, lower values were observed except Shannon andEvenness indices (Figs. 3 to 7). High and low tides

density ranged from 460.0261.10 org/ml and

919.70170.49 org/ml, Margalef 0.400.06 and

0.410.04; Shannon 0.220.03 and 0.230.02;

Evenness 0.190.02 and 0.240.04 and Dominance

0.210.02 and 0.230.03. Ten genera and 29 speciesof rotifers recorded (Fig. 8). The prominent species

were: Cryptochrysis commersalis (56.7833.70%)

Vorticella sp (60.2411.14%), Condonella uncinata

(53.9712.03%), Rotaria rotatoria (55.117.74%),

Cephalodella catellina (56.256.25%), Branchionus

angularis (46.314.91%), Notonunata aurita(32.566.55%), Dicranophonus forcipatus

(33.223.95%), Linda torulosa (26.0510.51%) and

Diurella porellus (24.775.14%). Cryptochrysiscommersalis and Rotaria rotatoria were only presentat low tide (Fig. 9). C. catellina was higher at hightide than low tide. Other rotifer species had higherdistribution percentage at low tide.

Fig. 2: Overall mean values of zooplankton density in Okpoka Creek

Fig. 3: Variation of zooplankton density in relation to tide in Okpoka Creek


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Fig. 4: Variation of zooplankton Margalef index in relation to tide in Okpoka Creek

Fig. 5: Variation of zooplankton Shannon index in relation to tide in Okpoka Creek

Fig. 6: Variation of zooplankton Evenness index in relation to tide in Okpoka Creek

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Branchiura

Clado

cera

Copepoda

Eupha

usia

Ostra

coda

Prot

ozoa

Rotifera

Zooplankton class

Zooplanktonevennessindex

Low

High


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Fig. 7: Variation of zooplankton Dominance index in relation to tide in Okpoka Creek

Fig. 8: Overall mean values of rotifer species of Okpoka Creek(1=Mesocyclops leukarti; 2=Temora longicornis; 3=Anomalocera patersoni; 4=Centropages typicus; 5=Metridia lucens; 6=Oithona similis;

7=Pseudocanus elongatus; 8=Candacia armata; 9=Acartia loniremis; 10=Acartia sp; 11=Onchocamptus sp; 12=Paracyclops affinis; 13=Paracyclopsfimbriatus; 14=Calanus sp; 15=Macrocyclops distinctus; 16=Schmackeria inopinus; 17=Onchocamptus mohammed; 18=Eurytemora hirundoides;

19=Acanthocyclops bicuspidatus; 20=Pareachaeta norvegica; 21=Candacia sp; 22=Microcalanus sp; 23=Paracalanus sp; 24=Parapontella sp; 25=Temora

sp; and 26=Cyclops strenus)

3.3. Copepoda (Copepods)

Copepods density ranged between 3249.63286.92

org/ml (low tide) and 5582.42438.45 org/ml (high

tide) with a mean of 3783.80248.33 org/ml (Figs. 2and 3). This accounted for 43.40% of zooplanktonpopulation and the highest among the zooplankton.Copepods had the highest species diversity indices:

Margalef (0.980.03), Shannon (0.500.02), Evenness

(0.350.01) and Dominance (0.250.01). The effect oftide on copepods diversity indices were significant

except on Dominance index (P


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Fig. 9: Percentage distribution of rotifer species in relation to tide in Okpoka Creek1=Mesocyclops leukarti;2=Temora longicornis;3=Anomalocera patersoni;4=Centropages typicus;5=Metridia lucens;6=Oithona similis;7=Pseudocanus

elongatus;8=Candacia armata;9=Acartia loniremis;10=Acartia sp;11=Onchocamptus sp;12=Paracyclops affinis;13=Paracyclops fimbriatus;14=Calanus

sp;15=Macrocyclops distinctus;16=Schmackeria inopinus;17=Onchocamptus mohammed;18=Eurytemora hirundoides;19=Acanthocyclopsbicuspidatus;20=Pareachaeta norvegica;21=Candacia sp;22=Microcalanus sp;23=Paracalanus sp;24=Parapontella sp;25=Temora sp;and 26=Cyclops

strenus

Fig. 10: Overall mean values of copepod species in Okpoka Creek(1=Mesocyclops leukarti; 2=Temora longicornis; 3=Anomalocera patersoni; 4=Centropages typicus; 5=Metridia lucens; 6=Oithona similis;7=Pseudocanus elongatus; 8=Candacia armata; 9=Acartia loniremis; 10=Acartia sp; 11=Onchocamptus sp; 12=Paracyclops affinis; 13=Paracyclops

fimbriatus; 14=Calanus sp;1 5=Macrocyclops distinctus; 16=Schmackeria inopinus;17=Onchocamptus mohammed;18=Eurytemora

hirundoides;19=Acanthocyclops bicuspidatus;20=Pareachaeta norvegica;21=Candacia sp;22=Microcalanus sp;23=Paracalanus sp;24=Parapontellasp;25=Temora sp;and 26=Cyclops strenus)

(13.550.65%), Onchocamptus sp (12.111.89%),

Metridia lucens (12.041.30%) and Mesocyclops

leukarti (12.800.22%). Out of the 26 speciesidentified, 15 species were calanoid copepods andothers were cyclopoid copepods. M. leukarti and A.bicuspidatus were absent at high tide. T. longicornis

and C. typicus had higher percentage distribution at

high tide (30.419.99% and 17.482.92%) than at low

tide (21.633.19% and 15.920.94%). Others weremore at low tide than high tide (Fig. 11). Tidal effect

was higher at low tide than at high tide.

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Rotifera zooplankton Species

Percentagedistribution

(%)

Low

High

0

5

10

15

20

25

30

35

40

45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Species

Percentagedistribution(%)


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Fig. 11: Percentage distribution of copepod species in relation to tide in Okpoka Creek(1=Mesocyclops leukarti; 2=Temora longicornis; 3=Anomalocera patersoni; 4=Centropages typicus; 5=Metridia lucens; 6=Oithona similis;

7=Pseudocanus elongatus; 8=Candacia armata; 9=Acartia loniremis; 10=Acartia sp; 11=Onchocamptus sp; 12=Paracyclops affinis; 13=Paracyclops

fimbriatus; 14=Calanus sp; 15=Macrocyclops distinctus; 16=Schmackeria inopinus; 17=Onchocamptus mohammed; 18=Eurytemora hirundoides;

19=Acanthocyclops bicuspidatus; 20=Pareachaeta norvegica; 21=Candacia sp; 22=Microcalanus sp; 23=Paracalanus sp; 24=Parapontella sp; 25=Temora

sp; and 26=Cyclops strenus)

3.4. Cladocera

Mean density of cladocera ranged from 696.4667.96

no/ml (low tide) to 1059.57104.89 org/ml (high

tide) with a mean value of 810.96 57.93 org/ml

(9.30% abundance). Mean value of Margalef,Shannon, Evenness, and Dominance species diversity

indices were: 1.040.07, 0.220.02, 0.250.02 and

0.250.02 respectively. The ANOVA of theseparameters were significant (P


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Fig. 13: Percentage distribution of cladocera species in relation to tide in Okpoka Creek

(1=Podon evadne;2=P. polyphemoides;3=Alatonopsis australissar;4=Daphnia carinata;5=D. cristata;6=Bosmina sp;7=B.

longirostris;8=Alona affinis;9=A. quadragularia;10=Evadne nordmanni;11=Simocephalus serrulatus ;and 12=Penilia

avirostris;

3.5. Protozoa

Tide had significant effect on density (P


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Fig. 14: Overall Percentage distribution of protozoa species in Okpoka Creek

Fig. 15: Percentage distribution of protozoa species in relation to tide in Okpoka Creek(1=Mesocyclops leukarti;2=Temora longicornis;3=Anomalocera patersoni;4=Centropages typicus;5=Metridia lucens;6=Oithona similis;7=Pseudocanus

elongatus;8=Candacia armata;9=Acartia loniremis;10=Acartia sp;and 11=Onchocamptus sp)

0

10

20

30

40

50

60

70

80

90

100

Percentagedistribution

(%)

Strobilidium

gyra

ns

Tintinnopsiswangi

Tsinen

sis

Tconic

us

Tstrigo

sa

Ts

pp

Leprotintinnis

sp

Pempholyxsalcata

Trinema

sp

Rhabdonella

sp

Halteria

sp

Protozoan species

65.35

42.23

23.19

100

9.3

73.95

14.88

17.03

16.67

1.86

46.62

35.29

38.06

20.59

0

0

0

0

0

0

0

68

0 20 40 60 80 100 120 140

1

2

3

4

5

6

7

8

9

10

11

Protozoanspecies

Percentage distribution (%)

Low

High


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Fig. 16: Overall means values of percentage distribution of ostracoda species in Okpoka Creek

Fig. 17: Percentage distribution of ostracoda species in relation to tide in Okpoka Creek

3.9. Nutrients of surface water from Okpoka Creek

High tide ammonia level (0.190.02 mg/L) washigher than low tide ammonia (0.170.01 mg/L) witha mean of 0.170.01 mg/L (Table 2). Tidal variations

were significant (P>0.05). Nitrate concentration(0.680.02 mg/L) was high at low tide and low(0.480.04 mg/L) at high tide with a mean of0.640.02 mg/L. Tidal effect was highly significant

(P


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environment. Olaleye and Akinyemiju (1999) reporteda total of six (6) zooplankton species (rotifers, 3species; copepods, 3 species) from Abiala Creek,Niger Delta; da Costa et al. (2011) recorded fortyeight (48) taxa from the macrotidal coastal zone of the

Amazon region and copepods were 48%. This presentobservation of higher zooplankton density and speciespossibly indicates high nutrients status of OkpokaCreek. The copepoda was the dominant zooplanktonclass. This observation is consistent with other studies

on the aquatic ecosystem (Oronsaye and Okaka, 2000;Kolo et al., 2001; Aminu and Ahmed, 2000;

Kemdirim, 2000; Krumme and T. Liang, 2004; Ekwuand Sikoki, 2005; Davies et al., 2008 and da Costa etal., 2011). The report of Okayi et al. (2001) ofdominant cladocera disagrees with the present

observation. That study recorded 20 species ofzooplankton which is far lower than the observedzooplankton in this study. The dominance of copepodsin terms of abundance indicates pollution according toRuivo (1972). Falomo (1998) reported the dominanceof copepods in the Oil Refinery-Axis of the OkrikaCreek and their absence in the NAFCON-ONNE axisof the creek. Edoghotu (1998) recorded copepods as

the dominant zooplankton in Okpoka Creek. Theobserved tidal variations of the zooplanktonabundance might be traced to the varied nutrientsconcentrations. The low species diversity values

might be associated with environment under stress. Itis agreed by pollution biologists that species diversitydeclines as pollution effects are more severe. Lowerdominance denotes higher species diversity of thezooplankton.

Tidal cycles had significant effects on thezooplankton density, species diversity indices,composition (species richness) and distribution. Therecorded higher densities of rotifera, protozoa andbranchiura at low tide might indicate that thesezooplankters were true residents of Okpoka Creek.Tides induce flow and transport of sediments,

animals, plants, other organisms and suspendedparticles at high tides and leave the sediments and

suspended particles behind but the biota go with theebbing tide back to the sea (Blondeaux and Vittori,

2005; SlideShare Inc., 2009). At high tide, it isexpected to have increased abundances of thesezooplankters but the opposite was observed. It couldbe that these zooplanktons had vertically migrateddownward into the sea before the high tide thus they

were not drifted into Okpoka Creek. In addition, itcould be that they did not respond as otherzooplankton taxa to the high tide. On the other hand,they might have vertically migrated further down intothe creek in response to high salinity induced by the

influence of the sea at high tide hence they could notleave with the ebbing tide to the sea. This might

signify that these taxa were low-salinity, low-sediment load and low-turbidity friendly. Verticalmigration is an aspect of zooplankton behaviour. Itmay be affected by abundance of both food items andpredators and other environmental cues such as light,

tide, salinity and temperature (Avent et al., 1998). Thevertical migration of these taxa could be traced toabundant of food items (autotrophs) and absence ofpredators. High light intensity or transparency at lowtide leads to high primary productivity and this might

indicate that these zooplankters feed onmicrophytoplankton.

Zooplanktons respond selectively to variousphysical and chemical factors such as tide, watercurrent, salinity to mention but a few and can exertsome influence on the ultimate outcome of their

transport (Knott, 2013). Some planktonic species maybe distributed only within restricted zones in coastalwaters, such as the low-salinity regions of estuaries,while others may reside only in the coastal waters andthe high-salinity reaches near the estuary mouth.Retention and settlement of the true resident taxa intothe creek might be the possible reasons for their lowdensity at low tide. Zooplankton samples were

collected within few centimeters of the water column(surface water). Retention mechanism washypothesized by Krumme and Liang (2004) forcopepods resident in the Furo do Meio (a macrotidal

mangrove channel in northern Brazil) so this could beused to give possible reason for the observed higherdensities of rotifera, protozoa and branchiura speciesand densities at low tide. Little settlement will occurduring periods of stronger flow (Villate, 1997) thusstrong settlement at low tide might be a factor for thepresent observation.

Migration of non-resident species of cladocera,copepoda, euphausia and ostracoda from the sea intothe Okpoka Creek at high tide and moving out withthe ebbing tide into the sea at low tide could be thepossible explanation for the higher densities of these

taxa at high tide. This present observation correspondsto Roberson et al. (1988) (in mangrove and other

nearshore habitats in tropical Australia) and Wang etal. (1995) (in a megatidal estuary (Seine, eastern

English Channel) who found higher densities and totalabundances of copepods at high tide than low tide. Incontrast, Krumme and Liang (2004) reported greatestabundance and biomass of copepods at low waterwhen the number of species was lowest and lowest at

high water when the number of species was greatest.However, this study recorded higher number ofcopepod species at low tide which disagrees with thereport of Krumme and Liang (2004). Furthermore,these taxa might be high-salinity friendly thus their

high density at high tide. Villate (1997) reported thatestuarine zooplankton populations move towards


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more saline waters from spring to neap tidesespecially the copepods. Higher abundances ofheterotrophic protists such as ciliates anddinoflagellates than autotrophs at high tide indicatethat these zooplankters are omnivores. At high tide,

the sediment load is high hence high turbidity and thisfavours the growth of heterotrophic protists and notautotrophs. Salinity affects the distribution pattern andrelative abundance of organisms (Sharipova, 2005;Frankovich et al., 2006).

The observed higher densities of copepoda,cladocera, ostracoda and euphasiacea at high tide

could be attributed to the high inflow of water fromthe sea containing estuarine zooplankton. Theseorganisms moved out from the Okpoka Creek at ebbtide hence the observed low densities at low tide. At

high tide, current velocities are high exceeding 1.5m/sand at low tide, these zooplankton species wereapparently not carried into the creek because of lowtide current speeds that might not be sufficient tosuspend and transport them into it. These zooplanktonassemblages at low tide demonstrated a rather uniformstructure due to low tidal turbulence.

Species diversity indices provide important

information about rarity and commonness of speciesin a community(http://www.tiem.utk.edu/~gross/bioed/bealsmodules/shannonDI.html). Zooplankton species diversity

indices of this present study were generally low. Lowspecies diversity suggests relatively few successfulspecies in the habitat, the environment is quitestressful with relatively few ecological niches andonly a few organisms are really well adapted to thatenvironment, food webs which are relatively simpleand change in the environment would probably havequite serious effects. Branchiura and euphausia wererepresented by only one species thus their speciesdiversity indices were zero. This could mean that thespecies diversities of these taxa were very low. Therecorded Margalef index for rotifera, cladocera and

protozoa at low tide than high tide could signifyhigher species diversity due to retention and high

settlement of these species before the period of strongflow at high tide. Also, high river discharge at low

tide is pronounced and might account for thisobservation as these taxa species might be low-salinity lovers. High solar radiation and lightpenetration denoting high temperature are high at lowtide thus the high abundance of microphytoplnakton

or autotrophs. The observed higher Margalef index forcopepoda and ostracoda at high tide than low tidecould possibly be explained by the strong flow fromthe sea at high tide carrying estuarine species and highamount of sediments. Also, it might be these taxa can

withstand the high tidal turbulence. These taxa are

suspended by tidal turbulence and omnivorous feedingon heterotrophic protists and detritus.

Shannon Index is a measurement ofboth species richness or abundance and evenness; itgives an informative description of a

site's biodiversity. It is particularly useful whencomparing similar ecosystems or habitats, as it canhighlight one habitat as more species rich or moreeven than another. Evenness is the relative abundancewith which each species is represented in an area

(https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-

diversity/species-diversity/). Generally, the presentstudy Shannon and evenness indices were low for allthe taxa. They were below 1. This signified that thesetaxa had low species richness and evenness.

Typically, the value of the Shannon index ranges from1.5 (low species richness and evenness) to 3.5(high species evenness and richness), though valuesbeyond these limits may be encountered(https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/). An ecosystem in whichall species are represented by the same number of

individuals has high species evenness.An ecosystem in which some species are representedby many individuals and otherspecies are representedby very few individuals has low species evenness. In

this study, all species were not represented by thesame number of individuals or some species wererepresented by many individuals and other species byvery few individuals. The higher Shannon andevenness indices for cladocera, copepoda, ostracodaand rotifera at high tide than at low tide might betraced to high sea discharge, bringing in temporaryspecies of these taxa therefore increased the equitabledistribution (evenness) of these individuals. Protozoacould have retained and settled before the strong flowof the sea thus their low Shannon and evennessindices at high tide.

Dominance index was generally low and it denotedhigher species diversity. Dominance index was higher

at low tide than high tide for copepoda, protozoa androtifera. These taxa could be the dominant and true

resident zooplankton in Okpoka Creek. Thisobservation might further be attributed to retentionand settlement at low tide. Villate (1997) stated themodels that predict that little settlement will occurduring periods of stronger flow. These zooplanktons

sink to the ground level at low tide and could notmove out with the receding tide at low tide. This isretention and settlement at weaken flow. Dominanceindex was higher for cladocera and ostracoda at hightide than low tide. It might be that these zooplanktons

could withstand turbidity caused by the high tideturbulence. The recorded high species distributions at
http://www.tiem.utk.edu/~gross/bioed/bealsmodules/shannonDI.htmlhttp://www.tiem.utk.edu/~gross/bioed/bealsmodules/shannonDI.htmlhttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_biodiversityhttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_habitathttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_ecosystemhttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_ecosystemhttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_ecosystemhttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_ecosystemhttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/https://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_habitathttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_biodiversityhttps://www.boundless.com/biology/community-dynamics/trophic-structure-and-biological-diversity/species-diversity/#key_term_glossary_specieshttp://www.tiem.utk.edu/~gross/bioed/bealsmodules/shannonDI.htmlhttp://www.tiem.utk.edu/~gross/bioed/bealsmodules/shannonDI.html


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low tide might be linked to high nutrients (nitrate,phosphate and sulphate) status. Low tidal turbulence,turbidity, sediment suspension or load, decreasedsalinity and current velocities could be linked to thisobservation.

Higher concentration of ammonia at high tidefavoured the observed zooplankton (cladocera,copepoda, euphausia and ostracoda) especially thedominant copepod at high tide. Dominance ofcopepods in terms of abundance and species

composition indicates pollution (Krumme and Liang,2004). This shows that copepods are resilent to the

increased anthropogenic input and high turbidity. Thelower concentration of ammonia at low tide favouredthe other zooplankton groups (rotifera, protozoa andbranchiura) and this further shows that these

zooplankters are autotrophs. High turbidity is notlimiting to reduced visual ranges in predators but alsoto high concentrations of detritus and associatedmicrobes that provide an important source of energyfor copepods (Krumme and Liang, 2004). Ammonia isone of the nutrients required by phytoplankton forprimary productivity. Others are nitrate, phosphorusand sulphate (Yamamaro et al., 1993). Furthermore,

ammonia is a source of nitrogen and contributes to thefertility of water since nitrogen is an essential plantnutrient. Nitrate-nitrogen and ammonia-nitrogendetermine community productive levels. The lower

concentrations of sulphate, nitrate and phosphate athigh tide might signify low primary productivity andsupport the possible reason that cladocera, copepoda,euphausia and ostracoda are omnivores and detritusfeeders.

The higher concentration of nitrate, phosphate andsulphate at low tide might be attributed to the higherabundance of rotifera, protozoa and branchiura at lowtide. These nutrients signify high primary productivityand that these zooplankters are microphytoplanktonfeeders. Nutrients availability especially phosphorusstructures the algae community (Frankovick et al.,

2006). In addition, primary productivity(phytoplankton productivity) is expected to be high at

low tide as turbidity is low and solar and lightpenetrations are high. High primary productivity will

enhance the high abundances of these zooplankters atlow tide.

The observed assemblage ammonia possiblyindicated increased anthropogenic inputs at high tide.The highest nitrate recorded at low tide might be

indicative of high human excrement and industrialdischarges. Stabilization period might be the possiblereason for this observation. It occurs during slacktides or slow currents and results in increased rates ofnutrient uptake (Zimmerman, 2013). The higher

phosphate at low tide could be due to highdecomposition of organic matter and stabilization

period. The recorded sulphate values at low and hightides could be attributed to the higher biologicaloxygen demand and stabilization period at low tidethan at high tide. Oxidation of the organic materialsand burning of fossil fuel used up oxygen thereby

exerting higher biological oxygen demand in theOkpoka Creek.

5. CONCLUSION

Tide affects the zooplankton community of OkpokaCreek in terms of species composition, diversity,

abundance and distribution. Copepods are thedominant taxa and euphasiacea being the seconddominant zooplankton fauna. They are sensitive toboth low and high tides. Rotifera, protozoa and

branchiura densities are higher at low tide whilecopepoda, cladocera, ostracoda and euphasiacearecorded higher densities at high tide. Branchiura andeuphausia are represented by only one species thustheir species diversity indices are zero. The presenceof dominant copepods as well as euphasiacea andbranchiura being represented by one species indicateorganic pollution or stress in Okpoka Creek. The two

alternate tidal cycles of six (6) hours of low and hightides within twenty-four (24) hours contribute to thestress condition of Okpoka Creek. Phosphate andammonia (nutrients) exceeded FEPA and USEPA

acceptable levels of 0.10 mg/L and 0.10 mg/Lrespectively for natural water bodies. These nutrientsindicate also organic pollution in Okpoka Creek. Theflushing action of the tidal flows contributes tomoving these potential pollutants from the BonnyEstuary down into this creek.

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Dr. Davies, Onome Augustina is a visible Senior Lecturer in the Department of Fisheries and Aquatic

Environment, Rivers State University of Science and Technology (RSUST), Port Harcourt, Nigeria. She

obtained B.Sc., M.Sc. and Ph.D degrees in Zoology from University of Ibadan, Ibadan, Nigeria in 1991,

1994 and 2008 respectively. She is a hydrobiology and fisheries consultant.

Prof Obih Alex Ugwumba has a B.Sc Zoology, 1979; Ph. D Zoology 1984 and was made Professor of

Zoology in 1999 in the Department of Zoology, University of Ibadan, Ibadan, Oyo State, Nigeria. He is a

Marine Biologist and an Environmental Pollution Consultant.

Effects of Tide on Zooplankton Community of a Tributary of Upper Bonny Estuary, Niger Delta, Nigeria

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