Basic HTML Version
International Journal of Marine Science 2013, Vol.3, No.14, 111-120
http://ijms.sophiapublisher.com
117
the circulation pattern that favored efficient water
renewal and higher energy because of seawater
intrusion and the absence of nearby sources of organic
matter (Uwadiae, 2009).
This study observed that mollusc densities showed
nearly the same trend related to the influence of
environmental gradients on dominant bivalve and
gastropod. Density of mollusc decreased towards the
upstream stations just as the occurrence of
P. aurita
and
M. Cumana.
The spp. were most numerous in
stations with relatively higher salinity values with
poor representations in upstream stations. Lowest
densities of the two spp. occurred in station three (for
M. cumana
) and station one (for
P. aurita
) whereas
highest densities were collected station eight (for
M.
cumana
) and station five (for
P. aurita
). The peak in
density (30 490 ind./m
2
) of gastropods in station six,
where a relatively higher water temperature was
recorded due to the discharge of heated water from the
Egbin Thermo-Electric power plant, agrees with the
observations of Hart and Fuller (1979), who reported
that, in the more tropical regions, many animals are
living near their upper thermal limits, and few degrees
increase in temperature may be fatal or result in the
interruption of the normal maturation and spawning
cycles. Molluscs are geared towards a cycle of growth
and build-up of reserves to be converted to gamete
and subsequent spawning. A continuous production of
gametes occurs, which would result in little or no
growth because all energy is used for producing the
gametes. Shell growth of hard clam (native to New
York) when transplanted to Florida waters, was
reported to have ceased when the temperature reached
about 27
℃
(Hart and Fuller, 1979). Warm water might
cause spawning when the needed kind of
phytoplankton food is not present in sufficient
abundance, or the larvae might be carried by the
currents to colder water, which would be detrimental
to them (Hart and Fuller, 1979). The specimens of
gastropods collected from this station were relatively
smaller than those of other stations (Uwadiae, 2009)
and among the bivalves, only two species were
recorded in station six, suggesting that the high
temperature recorded in this station may have affected
the molluscs (Teske and Wooldridge, 2001) of the
lagoon adversely.
The salinity gradient and sediment characteristics of
Epe Lagoon greatly influenced the species diversity
and density of molluscs. The downstream stations
provided a seemingly more favorable environmental
condition such as relatively higher proportion of sand
in sediment with low organic matter. The observed
pattern of diversity is consistent with the distributional
pattern proposed by Sanders (1958), which describes
the dominance of filter feeders or suspension feeders
on sandy sediments (Rhoads and Young, 1970), this
explains the dominance of the estuarine filter feeder,
P.
aurita
and suspension feeder,
M. cumana
in the study
stretch.
The present findings have important consequences for
studies of the distribution of benthic organisms in
estuarine ecosystems, including those concerned with
environmental monitoring. Although the diversity and
distributional pattern of molluscs in Epe Lagoon
seems to be related mainly to the natural estuarine
gradient and sediment characteristics, anthropogenic
factors indirectly may also influence molluscan
communities (Oyenekan, 1979; Clements, 1997;
Wiegner et al., 2003; Goto and Wallace, 2010),
especially in areas with multiple sources of human
stressors (Hewitt et al., 2005), where the dominance of
few species may indicate systems subjected to organic
enrichment and contaminated sediments (Gray and
Mirza, 1979; Dauer, 1993; Azrina et al., 2006). Epe
Lagoon is an estuarine system with a history of
pollution (Nwankwo, 1998; Nwankwo and Onitiri,
1992), therefore, monitoring programmers relating to
significant
long-term
changes
in
molluscs
communities (species disappearance, and shifts in
composition, densities and frequencies) will serve as
invaluable tools for the assessment of the impact of
human activities in the lagoon.
3 Material and Methods
3.1 Study area
Epe Lagoon (Figure 6) is located in Lagos State. It lies
between latitudes
3
°5
0
′~4°10′ N and longitudes
5°30′~5°40′ E. It has a surface area of about 243 km
2
(Kusemiju, 1988). The lagoon which has an average
depth of about 2.45 m is fed by the waters of
adjoining rivers and creeks. It is connected to the
Atlantic Ocean through the Lagos Harbour via the
Lagos Lagoon and tidal influence is relatively weak
especially in the upstream of the lagoon. The effects
of the tides is more in the dry season, and decreases