The Role Of Zooplankton In The Pelagic Food Webs Of Tropical Lakes
thesisposted on 13.05.2021, 11:21 by Ahmed S. M. AL-Budeiri,
There is a general paucity of studies concerning trophic interactions between zooplankton and Cyanobacteria, and about the general role which zooplankton play in pelagic food webs in tropical saline lakes. Although the relative importance of allochthonous and autochthonous carbon resources to the diet of zooplankton in temperate lakes is well understood, significant knowledge gaps remain in tropical systems.
This thesis addresses three principal questions: (1) What are the principal trophic interactions between zooplankton and Cyanobacteria in tropical lakes, with a specific focus on the lakes of the East African Rift Valley?; (2) Is there potential for competition between zooplankton and lesser flamingos in the Rift Valley lakes?; (3) What is the relative importance of allochthonous versus autochthonous carbon sources for aquatic consumers in tropical lakes? These questions were answered by analysing the compositions and trophic interactions in the pelagic food webs of four contrasting East African lakes (Naivasha, Baringo, Bogoria and Sonachi). The lakes were sampled over two campaigns in different seasons. The planktonic compositions and size distributions were assessed by microscopic identification of individual planktonic taxa. Fish and flamingos were also sampled. The trophic levels and potential dietary interactions of each identified taxa were then evaluated using natural abundance stable isotope analyses (δ13C and δ15N). The potential contribution of other carbon sources, such as terrestrial particulate organic carbon, was also assessed.
In Chapter 3, observations from Lake Sonachi suggest that the pico-alga Synechococcus sp. was the dominant food item for the principal zooplankton taxon (the large calanoid Lovenula sp.). This finding differs from reports in other in tropical lakes which had suggested that large calanoids mainly consume colonies of Microcystis sp. The findings from Lake Bogoria, presented in Chapter 4, suggest a pronounced seasonality in the occurrence of Moina sp. and Cyclotella sp. This was predominantly a consequence of lake level rise and associated freshening during the wet season. These organisms do not appear to be utilised as a significant food source by flamingos in this lake. This suggests that seasonal shifts in the planktonic food web structure are not beneficial for flamingos in this lake. The results also suggest that rotifers may compete with flamingos for their main food item, the Cyanobacterium Arthrospira sp. In Chapter 5, stable isotope abundance and C/N ratio data from Lakes Baringo and Naivasha suggest that pelagic zooplankton in both lakes were largely dependent on autochthonous carbon in both sampling seasons, despite potentially large catchment sediment fluxes. This challenges previous suggestions that allochthonous carbon is an important basal resource for pelagic food webs in many lakes. Such assumptions, as derived from temperate lake systems may not always apply to tropical lake systems, as sampled in this study.