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International Journal of Marine Science 2014, Vol.4, No.42, 1-11
http://ijms.biopublisher.ca
9
be due to a change in the balance between grazing and
the growth of suspended microalgae (Cloern, 1987).
The dominant frequencies of change in chl -a
concentration are therefore the same as those of
changes in the clarity of the water column. Both vary
on shorter time-scales than the diel variability
associated with the daylight cycle.
Overall monthly values of chl-a in sediment varied
from 1.99 to 3.44 mg/g, with the highest concentration
recorded in the dry season (March), highest values of
chl-a in water was also recorded in the same month.
This result is expected since in the dry season
turbidity is lower and transparency higher owing to
the water clarity due to reduced flooding associated
with the wet season which brings a lot of debris into
the lagoon. The overall values of chl-a in water was
higher than that recorded for sediment. This is also
expected since the higher amount of radiation
reaching the surface water is likely to engender more
photosynthetic activities than in sediment that receives
lower amount of radiation. The effect of this
variability on productivity depends on the covariance
of changes in, turbidity and microalgal biomass and
on any changes in the photosynthetic responses of the
microalgae (Baillie and Welsh, 1980). Resuspension
of sediment alone would cause a drop in system level
productivity because of the decrease in light
availability. This depression can be offset where
photosynthetically-competent benthic algae are
resuspended in association with the sediment because
an increase in algal biomass compensates for the
decrease in light availability. Covariance between
changes in chl-a and turbidity has been shown in
several studies including Murphy et al. (2008).
Analysis of the data recorded for macroinvertebrate
shows that the amount of chl-a may affect the
abundance and diversity of benthic macroinvertebrates
in an estuarine ecosystem. Most of the correlation
analyses carried out between concentration of chl-a
and parameters of benthic macroinvertebrates
indicated different levels of relationship. Concentrations
of chl-a in water and sediment affected the density and
diversity of benthic macroinvertebrates differently.
While sediment chlorophyll correlated negatively with
density and diversity of benthic macroinvertebrates,
both parameters were positively and significantly
correlated with chl-a in water (See Table 3). Dudley et
al. (1986) reported that effects of measures of
productivity such as algae production as represented
by chl-a concentration on benthic maccroinvertebrates
may be “positive” resulting in increasing measures of
macroinvertebrate communities or populations with
increasing algae or “negative” decreasing macroinvertebrate
community/population measures with increasing algae.
Maasri et al. (2008) showed that the concentration of
chl-a is important in explaining variation in
chironomid community composition and the
abundance of certain chironomid populations in
Mediterranean streams.
Effects of benthic chl-a on macroinvertebrates may be
summarized as follows. Positive effects may include:
Increased habitat structure (attachment space,
predation refuges, etc.); increased food resources
(epiphytes or macroalgae; filamentous algal mats trap
additional fine particulate organic matter). Negative
effects may include: Disruption of habitat
(smothering); competition for space with invertebrates;
interference with feeding or other behavior (direct
physical effects; alteration of current flow patterns);
water quality changes (reduced dissolved oxygen
[DO]; increased variation in water quality) as well as
production of toxins (Cyanobacteria).
Steigerwalt (2005) found that macroinvertebrate
community diversity (Shannon-Wiener Index, H’) and
community evenness (Pielou’s evenness, E) declined
significantly with increasing algal biomass on
Ichetucknee River snag habitat. Her data indicate that
taxa richness increased with algal biomass, the decline
in diversity was observed to be due to the reduction in
evenness. This indicates that increased algal biomass
results in a less diverse benthic macroinvertebrate
community due to increased dominance by a few
invertebrate taxa better adapted to exploit the algal
habitat. These observations corroborate the findings of
this present study. The significant positive relationship
between density and diversity of macroinvertebrates
and chl-a in water shows that higher phytoplankton
production is likely to result in larger amount of
materials available for the benthic filter feeders, while
the negative correlation displayed by the two
parameters with chl-a in sediment suggests that large
algal biomass may result in detrimental effects on
benthic macroinvertebrates.