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International Journal of Marine Science 2013, Vol.3, No.14, 111-120
http://ijms.sophiapublisher.com
112
Bolufawi, 1986; Oyenekan and Adediran, 1987, Ajao
and Fagade, 1990, 1991; Brown and Oyenekan, 1998)
have been carried out on the benthic communities of
some of these estuarine systems, none has investigated
the spatial patterns displayed by benthic communities
with a view to identifying the factors responsible for
the observed patterns. This study therefore, is aimed at
evaluating the spatial patterns of mollusc community
in a weak tidal estuarine system along the Nigerian
coast, with the intent of contributing to the
understanding of the distribution of benthic
macroinvertebrates in relation with environmental
variables.
1 Results
1.1 Environmental characteristics
The values of selected environmental variables
recorded in this study are summarized in Table 1.
Water temperature ranged from 23
℃
to 43
℃
.
Relatively higher values of temperature were observed
at station 6 where a range of 29
℃
to 43
℃
was
recorded. There was significant difference (ANOVA,
P < 0.05) in water temperature at the study stations
with station 6 having significantly higher values.
Values (2.4~10.5 mg/L) of DO show no particular
trend. Salinity values varied between 0.0
0
‰ and
19.72‰. Values were low during the wet season
ranging between 0.00‰ and 3.4‰ and relatively
higher during the dry season with values ranging
between 0.01‰ and 19.30 ‰(Table 1). Salinity at the
study stations were significantly different (ANOVA, P
< 0.05), with downstream stations (4, 5, 6, 7, 8) closer
to the Lagos Lagoon having relatively higher salinity
values than upstream stations (1, 2, 3).
The study area was predominantly sandy intermixed
with varying proportions of mud in the stations
sampled. The highest value of sand fraction (93.6%)
occurred in station five, while the least (54.4%) was
recorded in station three. The highest amount (44.6%)
of mud in sediment was recorded at station three.
Total organic content of sediment was highest
(10.45%) in station three, while the lowest value
(1.01%) occurred in stations 4, 6, 7 and 8. The trend
displayed by TOC values shows higher values in
upstream stations than downstream stations.
1.2 Mollusc assemblage
A total mollusc density of 174 680 ind./m
2
comprising
12 species belonging to seven families from two
molluscan groups (Gastropoda and Bivalvia) were
recorded. Twelve molluscan species made up of six
gastropod species belonging to three families
(Neritidae, Melaniidae, Potamididae) and six bivalve
species belonging to four families (Tellinidae, Avcidae,
Osteridae and Aloididae)) were identified. Gastropod
dominated in terms of population of individuals
collected with a density of 119 200 ind./m
2
constituting 68.2% of the total population of mollusc
in the study area, while bivalvia constituted 32.8%
with a density of 55 480 ind./m
2
. Most of the
gastropods were widely distributed occurring in all
study stations, except
Pachymelania fusca
var
quadrisiata
which was not found in stations one and
three (Figure 1). The dominant species observed
among the gastropod was
P. aurita
, which recorded
total density of 78 520 ind./m
2
and constituted 45.05%
of the molluscan population, the species ranked
highest in abundance among the species of molluscs
encountered and recorded 982 ind./m
2
as mean spatial
density. Other gastropod species recorded in the study area
include;
Neritina kuramoensis
(Density=13 290 ind./m
2
;
mean spatial density = 1661 ind./m
2
; % contribution =
7.62%),
N. glabarata
(Density = 11 110 ind./m
2
; mean
spatial density = 1389 ind./m
2
; % contribution =
6.37%),
Tympanotonus fuscatus
(Density=8 560 ind./m
2
;
mean spatial density = 1 070 ind./m
2
; % contribution
=4.91%),
T. fuscatus
var
radula
(Density=6912 ind./m
2
;
mean spatial density = 864 ind./m
2
; % contribution =
3.96%) and
P. fusca
var
quadrisiata
(Density=810 ind./m
2
;
mean spatial density=102 ind./m
2
; % contribution=0.5%).
Among the bivalves collected from the study stretch,
Macoma cumana
dominated in terms of density and
distribution. Of the total bivalve density of 55 480 ind./m
2
recorded,
M. cuman a
contributed 51 170 ind./m
2
,
thereby constituting 29.35% of the total molluscan
population. With mean spatial density of 6 396 ind./m
2
,
the animal ranked second in terms of abundance and
constitutes another keystone species after the
gastropod,
P. aurita
. Another major representative of
the bivalve group was
Aloidis trigona
which recorded
a total density of 2 440 ind./m
2
, mean spatial density
of 305 ind./m
2
and constituted 1.19% of the total
molluscan population. Other bivalve species collected
were;
Tellina nymphalis
(Density=1 240 ind./m
2
, mean
spatial density = 155 ind./m
2
; % contribution = 7%),
Crassostrea gazar
(Density=270 ind./m
2
; % contribution