"Biochemical aspects of phytoplankton communities in some khors of Lake Nasser"

Abstract

The sampling program took place seasonally during 2013 in Lake Nasser at four selected Khors covering each khor directions at surface and from 10 to 5 m depth: Kalabsha (7 sites), Wadi-Abyad (6 sites), Tushka (6 sites), and Korosko (7sites) focusing on the spatial and seasonal variation in biochemical composition of phytoplankton and the effect of changes in physico-chemical parameters quantitatively and qualitatively. Cyanophyceae was the most dominant group at the studied khors, where Microcystis aeruginosa, Microcystis flos-aquae, Lyngbya limnetica, Microcystis wesenbergii and Merismopedia glauca were the most dominant species. Protein represents the main biochemical content of phytoplankton, which generally reflects a physiologically healthy phytoplankton. Khor Korosko recorded the highest chlorophyll a and carbohydrates values (950.87 mg/l and 93.47 mg/l), while the maximum proteins and lipids investigated at Khor Kalabsha (79.25 g/l and 59.47 mg/l). Winter was the optimum season for the measured biochemical contents. Experiments were carried out under laboratory culture conditions to investigate the effect of amendment in the main nutrients nitrogen and phosphorus (N and P) of BG11 medium and different Zn concentrations not only on biological parameters M. aeruginosa, but also on the activity of catalase and alkaline phosphatase (AP) enzymes. Low metal dose (0.05mg/l) induced the algal growth, while higher doses suppressed it. Zn showed significant linear regression (P˂0.05) with the biochemical parameters. This reflected the fact that the toxicity of Zn is greatly dose-dependent. Catalase activity showed notable increase at low metal concentrations 0.05 and 0.1 mg/l to 0.02 and 0.028 U/L and decreased progressively to its lowest value 0.0032 U/L at higher dose 3 mg/l. Whereas, Zn caused a marked and gradual decrease in AP activity at which the lowest value (0.0041 U/L) was observed at higher Zn addition (3mg/l). There was a positive significant regression between Zn and both catalase and alkaline phosphatase (r2= 0.503 and 0.835). Chlorophyll a and the organic matter expressed in dry weight showed a preference for higher phosphorus levels at which their maximum values 892.38 and 0.0013 mg/l were obtained with the highest phosphorus concentration 175% (70 µg/l), whereas its lowest values recorded at the starvation state 25% (10 µg/l) of the nutrient. Similar tendency observed for protein, carbohydrate and lipid contents, where their values were gradually decreased with the deficiency of the phosphorous in the media. Data also elucidate how Chl a, dry weight and the determined biochemical contents of the tested alga increased in response to N supplementation. There was a significant regression (P˂0.05) under variable nitrogen concentration with chlorophyll a, dry weight, proteins and carbohydrates. A positive significant regression (r2= 0.878) of AP was detected under variable P concentrations. Key Words: Khors, Microcystis aeruginosa, chlorophyll a, biochemical constituents, nutrients, zinc, catalase, alkaline phosphatase.