| dc.description.abstract | Two creeks along the Kenyan coast, Mtwapa and Shirazi were investigated in a bid to
establish the ecological status as a result of various human impacts on the marine waters.
Nutrient distribution patterns in the two creeks were investigated from January-August 2001in order to cover the dry and wet season. Mean concentrations recorded for ammonium, . nitrate and phospha.te, during the entire sam.pling period, in Mtwapa creek were 2.4 ± 0.2
ugl', 4.1 ± 0.5 ugl' and 1.2 ± 0.1 ugl' respectively. In Shirazi, mean nutrient concentrations
were 1.4 ± 0.1 ugl', 1.5 ± 0.1 ugl" and 0.9 ± 0.1 ugl' respectively for ammonium, nitrate and
phosphate. Significant differences in nutrient levels between the two seasons in both creeks
were also observed (p<0.05).
Inorganic nutrient levels in Mtwapa were higher than Shirazi creek, during the rainy season as
a result of nutrient loading. Spearman correlations showed that there were positive
correlations between total rainfall.and all three nutrient concentrations in Mtwapa creek (r =
0.6, 0.5 and 0.5 respectively for ammonium, nitrate and phosphate). This demonstrates that
the main contributor of the. three nutrients in Mtwapa creek was as a result of surface runoff
and riverine inputs. However, in Shirazi creek total rainfall correlated positively with nitrate
concentrations only throughout the e~lt1re study period (r = 0~5). There was no correlation
between total rainfall and ammonium or with phosphate concentrations (r = 0.2 and -0.2
respectively).
Though rainfall is considered to "be the main source of nutrients in Mtwapa creek, the creek
also receives raw sewage from various point and non-point sources such as the Shimo-la-
Tewa prison. The highest nutrient concentrations in Mtwapa creek was in station MB which
was the area below the Mtwapa bridge whereby sewage outfalls were physically observed
coming from the Shimo-Ia tewa Government Prison. This station recorded the highest
nutrient levels for ammonium, nitrate and phosphate respectively (18.2 ± 0.55 ugl", 28.83
ug.l' ± 0.75 ugl' and 6.33 ± 0.19 ugl' ).
Biological parameters were also analysed to determine nutrient-plankton interactions in both
creeks. Phytoplankton biomass (Chi a) and zooplankton compositions, abundance and
diversity patterns were established for both creeks.
Mtwapa creek had higher mean chlorophyll a concentrations than Shirazi (t (339) = 2.69, P <
0.05). An increasing gradient in chlorophyll a concentrations was observed from the mouths
of the creeks to stations within the creeks. The tukey HSD test showed that stations MB, MJ
and MK were significantly different from station ME (p < 0.05), while in Shirazi creeks there
were no significant differences between stations (p> 0.05). Spearman correlations showed that
in Mtwapa creek there was a slight positive correlation between chlorophyll a and ammonium
(r = 0.61), and also a slight positive correlation between chlorophyll a and nitrate (r = 0.50).
In Shirazi creek Spearman's correlation showed that there was only a slight positive
correlation between chlorophyll a and nitrates (r = 0.53). Generally, the poor correlations
between nutrients and chlorophyll a, co~lc!. be an indication of patchiness of phytoplankton
communities in the creeks.
In both creeks, zooplankton standing stocks displayed wide seasonal variations. The two
creeks differed in total zooplankton ~bundan~e, with higher mean zooplankton abundances in
Mtwapa than Shirazi (t (42) = 2.54~ p<0.05). Spatial variability in total zooplankton
abundances in stations in both creeks was notsignificant (p>0.05). The most abundant
zooplankton group that was observed in both creeks were the calonoida copepods.
An inverse relationship between zooplankton abundances and diversities was observed in
both creeks. Although Mtwapa creek had higher zooplankton abundances, diversities were
relatively lower in comparison. In Shirazi creek, higher zooplankton diversities were
observed in comparison to their observed abundances. The highest zooplankton diversity
was at the mouth of Shirazi creek, while the lowest was below the Mtwapa bridge. Lowered
zooplankton diversities in Mtwapa creek can be attributed to the to the presence of sewage
outfalls as was observed physically during the study period
The interactions between the physico-chemical variables and the zooplankton communities in
Mtwapa and Shirazi creek were examined using Canonical Correspondence Analysis (CCA),
a Multivariate ordination analysis technique. The main physico-chemical parameters that
affect zooplankon distribution patterns, differed in both creeks. The 1110stimportant physicochemical
parameters that affected zooplankton distribution patterns in both creeks however,
were the nutrient concentrations. In Shirazi creek, the concentration of phosphate seems to
affect the distribution of zooplankton group's more than the other two nutrients. While in
Mtwapa creek, the distribution of zooplankton groups is affected by the interactions of all the
three main inorganic nutrients, which are more tightly coupled.
In Mtwapa creek, tw,o main Clusters were formed. On the upper left quadrant, oxygen was an
important environmental variable that affected the distribution of caJanoids, cycJopoids and
mysids. This is because they had the shortest distance in right angletto oxygen, which had the
greatest length in that cluster. However, on the lower right quadrant, the most important
environmental parameters were the inorganic nutrients and chlorophyll a, which seemed to
have affected the distribution ofa great number of most of the meroplanktonic zooplankton
groups such as the brachyuran larvae and megalopas, decapods, gastropods and ostracods. A
contrast was observed in Shirazi creek, where the most important physico-chemical
parameters that affected the distrbution of zooplankton were transparency and pH, which
affected the distribution of the cylopoids, fish eggs, harpacticoids and the sergestids. Another
important cluster was observed between inorganic nutrients, cholorophyll a, brachyuran
larvae, decapods and the gastropods.
Predation impacts, on copepod populations is crucial to understanding the complex trophic
dynamics of marine ecosystems. Arnphipods and Mysids were the most important predatory
groups of copepods in Mtwapa creek ( r = -0.70 ; r = -0.54 respectively), While in Shirazi
creek, the most important predatory groups of the copepods were the Ctenophores and the
Chaetognanths. (r = -0.60 ; r = -0.50 respectively). Spearman correlations showed that the
combined densities of predator groups in Mtwapa and Shirazi creeks were negatively
correlated to the total cope pod abundances ( r = 0.50 & r = 0.51; p<0.05, for Mtwapa and
Shirazi respectively). The high grazing effects of these predatory groups plays a role III
plankton dynamics in these creeks.
In conclusion, the hydrodynamics of Mtwapa 'creek has managed to prevent eutrophic
conditions from occuring, although th.e creek undergoes nutrient loading during the rainy season, while in comparision, Shirazi creek can be referred to as a pristine ecosystem due to
the low nutrient level concentrations recorded in this study.
Keywords: Nutrient loading, zooplankton diversity and abundance., | en |
