Phytoplankton Dynamics in A Lentic Water Body­-Bagha Dighi, Rajshahi, Bangladesh

Abstract

The dynamics of phytoplankton with the physico-chemical conditions of water, bottom soil nutrients, related flora and fauna of large lentic water body - Bagha dighi, have been studied during the period from July 2002 to June 2004. The physical conditions of the water body were as follows: Air temperature was found to vary from 18-36°C, surface water temperature from 12-35°C, bottom water temperature from 10-33.8°C, transparency 18-55 cm, average depth from 1.34-1.92 M, total suspended solid from 0.01-0.07 mg/I, total dissolved solid from 0.06-0.37 mg/I, total solid from 0.09-0.39 mg/I and electrolytic conductivity from 166-350 µ ohms/cm. Water colour was almost found blue, no remarkable taste and odour were found during the period of study. The chemical conditions of water were noted to vary from pH 6.5-7.6, free carbon dioxide 0-91 mg/I, carbonate 0- 30 mg/I, bicarbonate 30.3-118 mg/I, dissolved oxygen 2.6-7.7 mg/I, percentage of oxygen saturation 34.54-107.1 %, biochemical oxygen demand 1.1-6.21 mg/I, chemical oxygen demand 3.5-8 mg/I, Eh value 0.43-0.82, mv. rH7 value 29.32-40.87, calcium hardness 51.1-86.6 mg/I, magnesium hardness 2.68-18.01 mg/I, total hardness 44.67-148.7 mg/I, chloride 45-76 mg/I, ammonia nitrogen 0-0.08 mg/I, nitrate nitrogen 0.02-0.95 mg/I, total phosphorus 0.02-0.11 mg/I, siliccite 0.01-0.02 mg/I, soluble salt 0.01-0.21 mg/I and sediment activity 1.5-1.8 mg/I. The gross primary productivity was found to vary from 0.03-0.35 mg/C/1/h and net primary productivity from 0.02-0.21 mg/C/1/h, which showed the eutrophic nature of the dighi. The pH of bottom soil varied from 6.5-7.5, organic matter from 4.2-4.6%, potassium was always 0. 5 meq/100 gm soil, calcium from 23-33 meq/100 gm soil, magnesium from 2-4 meq/100 gm soil, total nitrogen was always 0.2%, phosphorus from 24-35 ppm, sulfur from 2.2-3.4 ppm, boron from 0.6-1.6 ppm, zinc from 8-10 ppm, iron from 60-128 ppm, manganese from 36-80 ppm, copper from 10-15 ppm. A total of 106 species of phytoplankton belonging to 4 classes have been recorded with their monthly quantitative variations of which 34 species belong to Chlorophyceae, 39 to Cyanophyceae, 7 to Euglenopllyceae and 26 to Bacillariophyceae. Among them Bacillariophyceae found to be dominant (39.65%) followed by Cyanophyceae (27.28%), Chlorophyceae (27.23%) and Euglenophyceae (5.84%). The seasonal abundance of phytoplankton was found to be in high peak in the month of June (summer) and minimum in September (post monsoon). Zooplankton consists of 27 species of which 10 belong to Rotifera, 9 to Copepoda, 7 to Cladocera, and 1 to Osterocoda. The high peak of zooplankton was observed in tl1e months of December and August and minimum in February. The phyla and zooplankton ratio was 2.4 : 1. A total number of 11 aquatic plant species, 50 fish and fisheries items including 42 fish species and 12 species of birds were observed in different seasons during the period of study. The water body is managed for fish cultivation and regularly cleaned and fertilized.