A Study On Primary Productivity And Nutrients In Abijata Evaporative Concentration Ponds.

Changes in primary productivity and nutrients were studied ill- situ in ulree Abijata evaporative concentrationrnponds (B, C, D), intake channel (A) and ule main lake for seven months from Octoher, 1996 to April, 1997 (9rnsamplings). Data were also recorded for related physical and meteorological parameters including: temperature,rnwind speed, light, and water chemistry. Phytoplankton biomass was measured as chi-a. Photosynthesis wasrnestimated by oxygen evolution with the Winkler and Miller methods, and 14C was also used three times forrncomparison during the study period. The level of pond water fluctuated considerably in response to water influentrnfrom the main lake, precipitation and evaporative concentration. In all Ule studied water-bodies, there was nornappreciable stratification in temperature except pond D with greater temperature at the bottom. Intensive windrnmixing is a common feature in the ponds.rnIonic concentration in pond D exceed by more than 15 times over the channel and pond B. Similar patterns werernobserved in the composition of major nutrients, except phosphorus. There was strong positive relationshiprnhetween increasing salinity and alkalinity but inverse relationship with pH and phosphorus was observed. Thernconcentration of dissolved inorganic nitrogen is extremely high ( > 14mg/l ) in all water-bodies. No nutrientrnlimitation was found during the study period.rnA total of 11 phytoplankton genera were identified in the lake, channel and pond B, of which Allabaellopsisrnrepresent >50% of the commnnity in number. This persisted as an unialgal population in pond B during the dryrnmonths when ionic concentration was also high. Diatoms compose the greater part of the algal community,rnparticularly during the rainy months (March-April, 1997) and increased water level. Among diatoms, Navicula andrn- - / Nitzschia species were dominant at all times, except the dry months. Spirt/lilla platellSis was found in lowerrnfrequency when compared with reports from earlier studies.rnMean chI. a concentration in the main lake, channel and pond B were 8.16, 16.14, 4.23 mg m·', respectively,rnmuch lower than values reported by earlier investigators. Areal photosynthetic rates ranged from 0.2 to 1.05g Crnm·' h· l in the channel, and 0.042 to 0.414g C m·'h·l in pond B. It is about 5 g C m·' when extrapolated to dailyrnrate in pond B while in the lake and the channel it does not exceed beyond 11 g C m·' d·lrn. This is much lower thanrnthe usual rates for African alkaline-saline lakes. The major factors Ulat influence primary productivity during thernstudy period include the higher level of sulphate, declined alkalinity and higher salinity along the concentrationrngradient. It was found that Miller meulOd is the hest teclmique of estimation of photosynthetic rate in suchrnalkaline-saline waters.rnThese data for primary productivity and nutrients are discussed in relation to their implications for large scalernproduction and control purposes in the ponds.

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