4.2 Plankton community composition, diversity and
abundance in relation to land and water use
4.2.1 Seasonal variation in plankton diversity and
abundance
This study found that the diversity and abundance of plankton
species varied seasonally. While this study failed to conclusively support this
variation with statistical significance, it is believed that rainfall patterns
were responsible for the noted seasonal variation. The two sampling periods
fell within the span of the normal rainy season that extends from November to
April. More conclusive evidence for seasonal variation in plankton diversity
and abundance may be reached if future sampling sessions took place both within
the normal rainy and dry seasons.
The slight differences in plankton species distribution
amongst the communal land sites (Fig 3.3) are probably due to documented
rainfall patterns. February sampling was conducted just after the short rains
of January and therefore could probably show a sign of seasonality in April
since March and April did not have rains. Differences in species composition
observed in the figures from communal lands and National Parks might also be
explained by the fact that the Matopos National Park received more rains from
November to January as compared to the communal lands that received rains only
in January. Thus, the abundance and diversity found in the National Park varied
more from February to April. It would be expected even greater seasonal
differences in plankton community structure should the samples be taken in the
period of November to April for the rainy season and the dry season that spans
from May to October.
The results of this study suggest that rainfall variability can
significantly impact the
diversity and abundance of plankton communities.
This is an important concept, as
rainfall patterns in this area have been
erratic in the past 10-15 years, and have the
potential to continue following non-normal trends due to
climate variability or climate change in the future. Rainfall patterns are
considered here in the sense that their intensity might induce the transport of
sediments from upstream of reservoirs to the reservoirs. These sediments, if
transported are likely to affect the reservoirs water quality, their biotic
composition and ecosystems health. This affirmation is supported by the
findings of a relation between the water quality parameters and the species
composition. These findings are presented in terms of factor analysis in Fig.
3.11 and Fig. 3.12.
Naz and Turkmen (2005) and Reynolds (1984) acknowledge the
fact that seasonal variations in plankton species composition and abundance are
believed to depend on interactions between physical and chemical factors that
are in turn influenced by climatic factors. The informal climatic sub-seasons
noticed during the sampling period were, therefore, likely to alter, though not
necessarily with significance, the plankton community structure.
Apart from this seasonal variation due probably to rainfall
patterns, no difference was noted between the communal lands and the National
Park related to land and water use. The reason, therefore, for a tendency to
acknowledge the good status of the health of the environment in the communal
lands. This shows similarities between reservoirs in communal lands to the
pristine-considered reservoirs in the National Park. However, care should be
taken to applaud this finding since a more detailed investigation including all
the seasons need to be done in order to be sure of the behaviour of these
ecosystems. As Cander-Lund and Lund (1995) confirm that like humans need a
regular check-up at the hospital, the health of aquatic ecosystems need to be
monitored through the observation of their plankton composition- a regular
monitoring might also be interesting to get on track with the evolution of the
status of these ecosystems.
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