Relationship between nutrient levels, phytoplankton biomass and zooplankton composition, biomass and abundance in tudor creek-Mombasa Kenya.

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