International Journal of Marine Science 2015, Vol.5, No.13, 1-11
9
2.7 Relationship between nutrient concentrations,
Total organic content of formulated substrate and
fauna indices
Spearman's correlations indicated that there was
significant correlation between number of individuals
(rs =
-
0.932; p<0.5), number of species (rs = 0.952,
p<0.5) and nitrate concentration. Number of
colonizing individuals (rs=0.937; p<0.5) and species
(rs=0.876, p<0.5) also correlated significantly with
phosphate, while TOC in formulated substrate
correlated positively but not significantly with number
of individuals (rs=0.305; p<0.5) and species (rs=0.26,
p<0.5). In the overall relationship, nutrient
concentrations of formulated substrate had strong
effects on number of colonizing individuals and
species.
3 Discussion
The result of this experiment suggests that the lagoon
sediment (Cage 1) offered more suitable habitat
requirement for benthic macroinvertebrates than the
formulated substrates. Cage 1 recorded 133
individuals and 14 spp. However, we did not find
differences in taxon richness and assemblage
composition among the cages. These results indicate
that invertebrate assemblages showed an overall
response to the composition of formulated substrate in
cages without major shifts in composition. The lack of
clear-cut differences across assemblage in
experimental cages may be attributable to the
semblance in composition of formulated substrate.
Though quantitatively different, plant materials used
in the formulation of substrate in cages did not differ
in the state of decay and detrital quality
The lowest number of individuals and colonizing spp
was recorded in cage 3 which contained 2 kg of dried
plant material churned into 13 kg of oven dried sand.
This may be attributed to the low organic matter
content inform of plant detrital material which is an
important food source for benthic invertebrates. In the
same vain, benthic invertebrates do not thrive in
substrates of very high sand proportion owning to the
possible effects of erosion and burrowing difficult.
Forty-eight individuals and 10 spp were collected in
cage 2 with relatively higher mud and low plant
material. Hart and Fuller (1979) observed that very
few benthic organisms are able to survive in muddy
sediments, this is because mud smoothers benthic
organisms by blocking their respiratory organs. In
overall, numbers of individuals of macroinvertebrates
were higher in substrate with relatively equal
proportion of constituents. Number of individuals and
spp correlated positively with TOC in experimental
cages, indicating the importance of organic matter as
habitat requirement for benthic invertebrates. The
highest number of individuals occurred in the control
cage, this may be attributed to the seemingly
favourable substrate qualities in this cage. These
include a relatively high sand content with
proportionate admixture of mud and organic matter as
reported previous studies on Lagos lagoon sediment.
Owning to the response displayed by the community
recorded in this experiment, one may conclude that
formulated substrates were more of food source than
habitat for the colonizing spp. This is because, litter
from riparian vegetation is a primary source of coarse
particulate organic matter (CPOM) to aquatic systems
(Carvalho and Uieda, 2004). However, in low-gradient
Coastal Plain streams much of the CPOM becomes
buried within the sandy stream beds following floods
(Smith and Brumsickle, 1989). Leaf litter on the
surface of perennial stream beds function primarily as
a food source, whereas buried litter or wood additions
have had strong invertebrate response (Turner et al.,
1997), no response (William, 1977), or a variable
response over time (Tillman et al
.,
2003).
Large percentage of mud in sediment is known to be
detrimental to benthic invertebrates. This may have
accounted for the low number of individuals recorded
cage. Although muddy particles hold the largest
amounts of total TOC which represent a food source for
deposit feeding organisms (Butman, 1987), it limited
the abundance of the filter feeder. Induced
sedimentation resulting from high organic matter can
smother benthic molluscs both at their adult and
planktonic stages (Hart and Fuller, 1979). Increased
turbidities may increase the formation of pseudofeces
and decrease the amount of water that is pumped (Hart
and Fuller, 1979). The results of this study indicate that
overall environmental conditions appeared to be more
important in structuring the composition of invertebrate
assemblages than differences in substrate type.
Decaying leaf litter plays an important role in aquatic
food webs and has been reported to influence the
composition of FFGs in benthic macroinvertebrate
