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International Journal of Marine Science 2013, Vol.3, No.14, 111-120
http://ijms.sophiapublisher.com
116
Table 4 Spearman's correlations between biotic and environ-
mental parameters in the study area
Salinity
TOC Sand
Mud
Overall Diversity
+*
-ns
-
ns
+ns
Gastropod
Density
+**
-
**
-
ns
+ns
Species richness
+ns
-
**
Bivalvia
Density
+ns
-
*
-
*
-
ns
Species richness
+ns
+ns
+ns
+ns
Note: +: positive correlation;
-
: negative correlation; ns: no
significant correlation; p > 0.01/0.05; *: significant correlation;
p < 0.01; **: significant correlation; p < 0.05
2 Discussion
The result of this study suggest that the observed
spatial pattern in diversity and distribution of benthic
molluscs were majorly controlled by the salinity
gradient and sediment characteristics of the lagoon,
although the localized effect of elevated temperature
in station six is also worthy of note. The importance of
salinity and sediment characteristic in the distribution
of benthic organisms is widely recognized. This
assertion agrees with results of studies carried out in
Nigeria (Wbb, 1958; Webb and Hill, 1958; Sandison,
1966) and in other parts of the world, such as
Guanabara Bay (Van Der Ven et al., 2006; Mendes et
al., 2007; Santi and Tavares, 2009); and South
American estuaries (Paranagua´ Bay: Boehs et al.,
2004; Patos Lagoon: Bemvenuti et al., 1992; Todosos
Santos Bay: Venturini et al., 2004; Samborombon Bay:
Ieno and Bastida, 1998; Rio de la Plata Estuary:
Gilberto et al., 2004), where salinity gradient and
sediment type strongly influenced the spatial
distribution and diversity of benthic fauna.
The molluscan community recorded in this study was
majorly euryhaline species, occasioned by the low and
fluctuating salinity of the study area resulting from
weak tidal salt water incursion experienced in the
lagoon. This observation fulfills the predictions of
Olaniyan (1975) who noted that, although the bottom
fauna of Epe Lagoon has not been studied, it is very
likely to consist of animals which can tolerate low
salinity, and low fluctuations in salinity. Epe Lagoon
is sandwiched between two lagoons, the brackish
water Lagos Lagoon and the freshwater Lekki Lagoon
(Figure 6), and is weakly influenced by tidal water
input through the Lagos Lagoon. Higher salinities
values occurred in study stations close to the Lagos
Lagoon than those located towards Lekki Lagoon and
the discharge points of Oshun and Oni Rivers.
Mollusc diversity followed the salinity gradient, with
the highest diversity in the downstream stations which
were mostly influenced by marine intrusion, and
decreasing towards upstream. Community composition
gradually changes according to the salinity gradient of
the lagoon, a trend similar to those reported in Gaston
et al (1998) and Giberto et al (2004). Species (
M.
edulis, M. perna
and
C. gazar
) known to be intolerant
of wide fluctuations in salinity were restricted to
station seven where a higher and relatively stable
salinity was recorded, this observation has also been
reported in other estuarine systems (Oyenekan, 1988).
Sediment type is a limiting factor for the spatial
occurrence and abundance of benthic organisms
(Ysebaert and Herman, 2002; Bemvenuti et al., 1992),
and can be important in the structure of benthic
communities (Teske and Wooldridge, 2001; Ysebaert
and Herman, 2002). Sediment characteristics
(Percentage of sand particles and TOC) strongly
affected the mollusc community in the study area
(Table 4). Organic matter in sediment strongly
influenced the density and species composition of
molluscs. The lowest density (810 ind./m
2
) of mollusc
was recorded in station 3 which was characterized by
high mud and TOC. Although, organic matter is
important as food for benthic species (Newell et al.,
2002; Arrighetti and Penchaszadeh, 2010), induced
sedimentation resulting from high organic matter can
smother benthic molluscs, both at their adult and
planktonic stages. Increased turbidities arising from
the presence of large amount of organic matter may
heighten the formation of pseudofeces and decrease
the amount of water that is pumped by molluscs
during feeding and respiratory activities (Hart and
Fuller, 1979). Higher percentage of organic matter
recorded in station 3 compared to other stations was
caused by the large quantity of organic debris from a
farm located at the bank of the lagoon, this may have
accounted for the low density and diversity of benthic
molluscs recorded in the station. In general, values
(2.1%~10.45%) of organic matter in the upstream
stations were higher than those (1.01%~7.5%)
recorded for downstream stations, this may have
contributed to the variation in density and diversity of
mollusc observed. The lower deposition rate of
organic matter in the downstream stations results from