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International Journal of Marine Science 2013, Vol.3, No.14, 111-120
http://ijms.sophiapublisher.com
115
Figure 4 Variations in density of Gastropoda and Bivalvia in the study stations
Table 2
Summary of biotic metrics at the Study Stations
Study stations
1
2
3
4
5
6
7
8
Total density (No. of ind/m
2
)
of molluscs
1090i
9940i
810i
33070a
35850a
31130a
24170a
38260a
Number of species
8
9
8
9
9
8
12
9
Shannon-Wiener index
0.33
0.37
0.17
0.52
0.52
0.43
0.58
0.53
Evenness
0.33
0.10
0.31
0.24
0.19
0.25
0.20
0.22
Note: Lower case alphabets indicate no significant difference (P > 0 .05), * Turkey’s test (P < 0.05).
1.3 Spatial patterns of ecological indices
Species representation of the two groups (Gastropod
and Bivalvia) is depicted in Figure 3. Analysis of the
variation in the density of molluscan species in the
study stretch at the spatial scale indicates that there
were great variations among the study stations (Table
2). Generally, stations downstream and close to the
brackish water Lagos Lagoon recorded higher
densities of molluscs than upstream stations. Station
eight recorded the highest density (38 620 ind./m
2
),
followed by station 5 (35 850 ind./m
2
). Densities of
mollusc observed in other stations were; 33 070 ind./m
2
for station four, 31 130 ind./m
2
for station six, 24 170
ind./m
2
for station seven, 1 090 ind./m
2
for station one,
9 940 ind./m
2
for station two and 810 ind./m
2
for
station three. Highest gastropod density (30 490 ind./m
2
)
was recorded in station 6, and the lowest (190 ind./m
2
)
was observed in station 1, whereas highest density of
bivalvia occurred in station 8 (14 330 ind./m
2
) and the
lowest (580 ind./m
2
) recorded in station 3 (Figure 4).
Turkey‘s test indicated that the densities of mollusc
collected in the downstream stations were similar,
higher than and significantly different from those in
upstream stations (Table 2).
Number of species of molluscs recorded at the study
stations ranged between 8 and 12 (Table 2). Highest
number of molluscan species occurred in station 7,
while the least number (8) was observed in stations 1,
3 and 6. Stations 2, 4, 5 and 8 recorded 9 species of
molluscs each. The diversity of benthic mollusc as
illustrated in Table 2, was highest (0.58) in station 7
and lowest (0.17) in station 3, while values of
evenness varied between 0.1 (Station 2) and 0.33
(Station 1). There was a remarkable increase in
density and diversity toward the downstream stations.
The cluster analysis presented in Figure 5 shows
clearer separation of the study stations into groups
with similar ranges of density of mollusc. Two main
groups can be recognized from the dendogram, group
I comprised stations 1, 2 and 3, while stations 4 to 8
occurred in the same group. At higher level of
separation the analysis revealed that the density of
mollusc in stations 4, 5, 6 and 7 were similar, while
that of station 8 was relatively lower.