BIOLOGICAL, CHEMICAL AND PHYSICAL FACTORS INFLUENCING THE SEASONAL CHANGE OF POLYSACCHARIDE CONCENTRATION IN IRRIGATION-CHANNEL SEDIMENT

Variation of polysaccharide concentration in irrigation-channel sedime nt was determined concurrently with biological, chemical and physical factors influencing the benthic algal community. Phenol-sulphuric acid method was used to measure polysaccharide concentration. Polysacchari de concentration, biomass of benthic algae, and species composition ch anged spatially and temporally. Fluctuations of total suspended solid (TSS) concentration and exposure of channel bed to direct sunlight had major effects on algal growth and polysaccharide production. Polysacc haride concentration was correlated to chlorophyll a concentration (r= 0.73, P<0.001) and algal biomass (r=0.57, P<0.001). Fragilaria constru ens and Aulacoseira (Melosira) italica were the most common diatoms in the benthic flora. Chlorophyll a concentration in the sediment showed a strong negative correlation (r=-0.99, P<0.001) with the seasonal va riation of TSS concentration in channel water. The polysaccharides pro duced by benthic microorganisms play a major role in clogging channel bed and thereby reducing seepage from earthen irrigation channels. Cor relations between polysaccharide concentration and chlorophyll a (and algal biomass) further indicate the importance of benthic algae for po lysaccharide production. Since availability of light to the algal flor a is critical for the production of polysaccharides, the effect of clo gging can be maximized by exposing the channel bed to direct sunlight during non-irrigation period (winter).