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Location Dynamics of the Major Phytoplankton and Zooplankton Communities and Their Role in the Food-Web of Lake Tana, Ethiopia

  • Student: Ayalew Wondie Melese
  • Institution: Addis Ababa University
  • Degree: Ph.D.
  • Posted on: August 02, 2012

Thesis Abstract

Spatial and seasonal dynamics of the major planktonic communities were studied from June 2003 to July 2005 in relation to environmental factors in Lake Tana, a large lake in the highlands of Ethiopia. Abundance, biomass and production rates were measured at monthly intervals in six stations evenly spaced around the river mouths. 85 species of phytoplankton were identified over the study period. Primary production and chlorophyll a were substantially higher in the inshore zone. Most probably wind-induced mixing has caused resuspension of bottom substrates, which have improved the availability of nutrients in the shallow inshore zones. Average gross primary production and Chlorophyll a concentration were 1.8 g O2 m-2 d-1 and 3.34 g l-1, respectively. Higher algal biomass and production rates were observed in the post-rainy (Oct–Nov) and pre-rainy seasons which coincided with larger phytoplankton bio-volume and higher chlorophyll levels. These high production rates were the combined results of increased availability of nitrates and high wind-induced turbulence which increase nutrient concentrations in the water column and bring photosynthetic organisms to the euphotic zone. The literature data show that primary productivity (i.e. gross primary production per unit chlorophyll) in L. Tana is in the same range as in 30 other tropical lakes and reservoirs, of which 27 were from Africa. The average daily gross primary production is however the lowest of studied lakes and confirms the oligotrophic nature of the lake. The zooplankton community is represented by 3 copepod, 8 cladoceran and 2 rotifer species. Copepods were numerically higher than cladocerans and rotifers and accounted for 52% of the total zooplankton abundance while cladocerans and rotifers accounted for 11% and 37%, respectively. Copepods comprised 76% of the total zooplankton biomass while cladoceran and rotifers contributed 17% and 7%, respectively. The percentage contribution of the major zooplankton taxa to total production in Lake Tana were 47%, 30% and 23% for Copepoda, Cladocera and Rotifera, respectively. Numerical density, biomass and production of the zooplankton community in Lake Tana were significantly higher in the inshore zone as compared with the open water zone. The contribution of cladoceran egg production in Lake Tana was very low when compared with other tropical lakes and total cladoceran production is low when compared with results from other tropical lakes or the mean of temperate lakes. The overall annual integrated production of zooplankton in Lake Tana was estimated as 12.3 g DW m-2 yr-1 . With an average biomass of 362.4 mg DW m-2, zooplankton have a P/B ratio of 33.94 year-1. In Lake Tana, all the taxa showed peak abundance during the post-rainy season and relatively increased during the pre-rainy season. The overlap of these seasonal peaks (coexistence of zooplankton communities) indicated that they were not influenced by competition and predation pressure from carnivores but rather by food limitation and to a lesser degree by turbidity. The high production rates of zooplankton in the post and pre-rainy seasons correspond to the highest concentrations of chlorophyll a and primary productivity in the lake, in addition to the imported organic matter and wind-driven mixing effect. Maximum secondary production had short time lag with maximum primary production, which was mainly due to blue greens and diatoms. Herbivores utilized decomposing Microcystis sp.and Melosira sp. cells and bacteria after their sedimentation during the dry season. Trophic analysis of the 17 functional (ecological) groups in Lake Tana was done using Ecopath model (EwE version 5.0). Trophic flows and mixed trophic impact (MTI) were constructed. Although zooplankton show similar pattern of seasonality with phytoplankton, Ecopath results indicated that phytoplankton were utilized moderately by zooplankton. This inefficient consumption on phytoplankton can be explained by large filamentous and colonial algal cells. There is also alternative /complementary feeding of zooplankton on suspended organic material (probably imported) and bacteria in the sediment. It appears that bottom-up effects are more important than top-down interactions in structuring communities and determining productivity in the food web structure of Lake Tana. Further research gaps and problems that may help to elucidate the Lake Tana ecosystem in detail are pointed out.