| dc.description.abstract | Previous studies in Tudor creek have suggested links between variations in general
environmental factors, primarily precipitation, and zooplankton distribution and
abundance, but have suffered from a lack of replication of samples. The aim of this study,
therefore, was to analyze, quantitatively and statistically, the relationship between nutrient
levels, phytoplankton biomass, and zooplankton composition, biomass and abundance by
means·ofreplicate sampling, at three stations.
All the environmental factors monitored showed a clear response to the monsoon
conditions and in addition local gradients from the mouth to the inner reaches of the
creek. Maximum nutrient levels, occurred during the intermonsoon (March), that is prior
to the peak of rainfall (May). This may be explained by a larger tidal prism during the
equinoctial spring tides and possibly more vigorous mixing of the water column. They
may also result from flushing in of litter, and consequent decomposition, at the start of the
rains in March. High levels of N03-N could also be due to nitrogen fixation by bluegreen
algae.
Plankton floral biomass and faunal composition, biomass and abundance were primarily
influenced by seasonal factors and to a lesser extent location in the creek. Their maxima
also occurred during the intermonsoon (March). The relationship between inorganic
nitrate concentration, phytoplankton biomass, as indicated by chlorophyll levels, and
zooplankton abundance was clear in station 1 only. This may be explained by the fact that
station 1 was dominated by holoplanktonic groups, particularly the copepods which are
mainly planktotrophic. In station 2 the relationship between inorganic nitrate
concentration and phytoplankton biomass as well as that between phytoplankton biomass,
as indicated by chlorophyll levels, and zooplankton abundance was obscure
XIII
Despite the relatively high levels of N03-N in station 2, both phytoplankton and
zooplankton biomass (F 0.05, 2, 59 = 3.475, P = 0.0038) were generally lower in this site
compared to the other two sites. Transparency depth also showed the least fluctuation in
this site and this was tentatively attributed to high turbidity as a result of anthropogenic
impacts such as sewage and waste disposal possibly from Coast General Hospital, the Old
Mombasa Port, and the adjacent human settlements. The low phytoplankton and
zooplankton biomass in this site may indicate that apart from discharges of nitrogenous
wastes, which would enrich the surface waters and therefore enhance planktonic floral and
faunal production, the wastes could include substantial amounts of toxic substances which
inhibit phytoplankton and directly or indirectly zooplankton production.
In station 3 total zooplankton densities were highest during the intermonsoon (March) up
to the early SE monsoon (May) despite the fact that peak levels of both forms of
chlorophyll a occurred in March and declined sharply thereafter. The lack of correlation
between zooplankton abundance and the levels of chlorophyll a and its derivatives in
station 3 may be explained by the fact that the peak from March to May is largely due to
the dominance ofbrachyuran larvae which are lecithotrophic.
Copepods and predatory groups such as hydromedusae, chaetognaths, siphonophores and
ctenophores showed some indication of a predator-prey relationship in station 1 only. No
such relationship was found in either station 2 or 3. Although the densities of the selected
predator groups and copepods, in station 1, are significantly correlated, these relations
may not be causal. The changing monsoon conditions seem to be the most important
factor affecting their abundances. This is indicated by the observation that the cycles of
copepods and the various predatory groups were almost in phase with one another | en |
