Abstract
The species composition and production of the phytoplankton community of the Shesher and Welala floodplain Wetlands, on the eastern side of Lake Tana, were studied during four seasons from July 2009 to May 2010. We investigated the spatial and temporal dynamics of phytoplankton, densities, biomass, in relation to physico-chemical conditions. Gross and net primary production was studied at one site in each Wetland. Temperature, dissolved oxygen, conductivity, Secchi-disc depth, nitrate, phosphate and silicate concentrations showed significant temporal variation (p 0.05). Thirty six phytoplankton genera/species, belonging to 7 higher taxa were identified. The Chlorophyta dominated the phytoplankton community and contributed 42% - 53 % of the total observed phytoplankton numbers. Average phytoplankton biomass (chlorophyll a content) over four seasons ranged from 9 - 121 μg.l-1 in Shesher and from 27 - 206 μg.l-1 in Welala, whereas the average gross primary production over three seasons was 10.5 in Shesher and 7.7 mg.O2l-1d-1 in Welala. The peak concentration of chlorophyll a was observed in the pre-rainy season, which coincided with a bloom of Microcystis spp. Both Shesher and Welala Wetlands are very productive and have a good water quality but they are threatened by low water inputs since in the summer of 2009 the construction of a dam by local people and facilitated by local officials prevented overflow from Ribb River into the Wetlands. We conclude that the good water quality, the relative high water temperature and high primary productivity make the two Wetlands suitable for culture based fisheries and/or aquaculture, but a wise water resource management is crucially important.
Highlights
Temporal variability in the structure and function of phytoplankton communities is of fundamental importance to the metabolism of an aquatic system [1]
We investigated the spatial and temporal dynamics of phytoplankton, densities, biomass, in relation to physico-chemical conditions
We conclude that the good water quality, the relative high water temperature and high primary productivity make the two Wetlands suitable for culture based fisheries and/or aquaculture, but a wise water resource management is crucially important
Summary
Temporal variability in the structure and function of phytoplankton communities is of fundamental importance to the metabolism of an aquatic system [1]. Aquatic environments are subject to high temporal variability, with frequent reorganization of the relative abundance and species composition of phytoplankton as a result of the interactions between physical, chemical and biological variables [2]. In floodplain Wetlands, there are additional factors owing to the hydrodynamic differences arising from the location, morphometry and the main function of a given system. The study of changes in phytoplankton densities, biomass, species composition and primary production is, fundamental to the understanding of water quality [3]. Fish yields depend on the primary production of phytoplankton, in water bodies in which fish fauna is primarily composed of planktivorous species such as tilapia [4]
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