4.2.2 Plankton composition
There significant difference found in phytoplankton abundance
samples using the chi- square method might be due to natural differences
between the National Park and the communal lands. The most abundant
phytoplankton taxon found during this investigation was Hydrodictyon
spp. in April samples. Its abundance might be due to the fact
that the species is known to break into pieces (Cander-Lund and Lund, 1995) and
the counting of those small pieces might mislead to an acknowledgement of
abundance. It was found, however, in April samples that the taxa was widely
distributed. Hydrodictyon was rarely observed in February samples.
Anabaena sp., a blue-green algae (Cyanophyta), was the second
most abundant species observed in April though it was rare in the February
samples. Though Anabaena sp. is always associated with algal
blooms, its abundance was not high enough to create an algal bloom. Literature
shows, in fact, that Anabaena sp. can be found in non-polluted waters
(Cander-Lund and Lund, 1995). However, the presence of this species, and others
that prefer similar ecological conditions, in areas where they are not expected
to normally occur might be a sign of the enrichment of waters, a term referred
to as eutrophication. The current aquatic community structure would likely
change with the onset of eutrophication, perhaps altering water quality and
rendering the reservoirs unsuitable habitat for a variety of plankton species
and unsuitable for human uses as they currently stand. One particular risk of
the cyanophytes group is the fact that most of the species (including
Anabaena sp.) contain toxic substances that can lead to fish
kills wherever their blooms occurs, especially in hyper-eutrophic ecosystems.
They have Nitrogen-fixing sites (heterocysts) on their organisms and are
therefore able to fix nitrogen; which means that they can proliferate rapidly.
Anabaena is, particularly, known to produce neurotoxins that affect
the human central nervous system and hepatotoxins that affect human liver
(Chipfunde, person.comm.).
Ceratium (fig. M, annex 3), a dinophytes that is
likewise known to produce toxic substances and red water blooms, was found in
both samples of February and April. This taxon belongs to the major group of
dinophytes and is common in the plankton of lakes. Some species are rich in
plants nutrients such as phosphates and nitrates (Cander-Lund
and Lund, 1995). Cander-Lund and Lund (1995) states that even
in such lakes it is often accompanied by cyanophytes. The results obtained in
this work have shown the presence of Ceratium as well as cyanophytes,
though the water bodies were nutrient-poor (oligotrophic). Ceratium
and Peridinium (another dinophytes) increased in abundance in April
samples due probably to favourable conditions to their proliferations such as
an increase in total nitrogen levels. This cannot be confirmed since total
nitrogen was not analysed for February samples. An increase in nutrient levels
in the study area would enhance a high productivity level of dinophytes and
cyanophytes, leading to algal blooms, which would compromise health of the
ecosystems as they currently stand. It is therefore crucial to keep the water
bodies under observation.
The species that could cause algal blooms like
Anabaena were mostly present in the communal lands (Table 3.4).
Ceratium and Peridinium have also been found in high
abundance in the communal lands as compared to the National Park. These taxa
are known to be proliferating in nutrient rich waters (Cander-Lund and Lund,
1995); Ceratium being able to exploit organic and inorganic nutrients
and gain competitive advantage over purely photosynthetic species (Smalley and
Coat, 2002). Because these nutrient enrichment indicative species are abundant
in the communal lands, an argument would be made that communal land sites
should be monitored for an influx of nutrients that could spur them into an
algal bloom.
The zooplankton community was less diverse and less abundant
as compared to ponds and reservoirs (from other studies) though the most common
zooplankton groups were represented. The lower diversity and abundance found in
this study might be explained by the presence of planktivorous fishes and most
probably low light penetration (low transparency, especially in the communal
lands). Though fish abundance was not part of this study, it was noted that
fishes were present in all of the reservoirs. Humans were observed actively
fishing on the reservoirs. Planktivorous organisms have preferences for
specific food items (Wetzel, 1983). Large planktons are the preferred food
item, as they contain the most energetic reward to balance the energy loss the
fish has most incurred when hunting. In the case of the studied reservoirs,
large planktons were composed of big cladocerans like some species of
Daphnia and Calanoids. The idea of
active hunting on large zooplankton can easily be depicted
from Table 3.9 where large Calanoids are scarce while their juveniles are
abundant. Large Cyclopoids and cladocerans were also rarely found in the
samples and most of the time when they were found they were only carcasses. So
there seems to be a good zooplankton productivity, which is very well regulated
by high predation by fish. This is in accord with Hrbàéek et
al. (1958) in Wetzel (1983) who shows that the size of the zooplankton
community is regulated by the presence of fish predators. The zooplankton
community structure found is also in agreement with Arcifa et al.
(1986) who concluded that plankton proliferation is greatly affected by the
predator-prey relationships in reservoirs.
The correlation found in zooplankton and phytoplankton species
might be explained by the availability and preference of food. The availability
of a certain phytoplankton species that constitute a preferred source of food
to a zooplankton counterpart will allow it to grow easily, following the same
abundance curve. This tends to confirm that the availability of nutrients and
necessary conditions for phytoplankton growth has a pulling effect on
zooplankton species.
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