Comparative Study on the Physico-Chemical and Biological Diversity of two Urban Lakes of Dhaka City

Abstract

The present study was conducted from April 2010 to March 2013 in two urban lakes, Ghulshan lake and Dhanmondi lake to determine the limnological quality, heavy metals of water and sediments and diversity of plankton and benthos. Water quality results showed that the Gulshan lake water were higher values of ammonia-nitrogen, alkalinity, hardness, conductivity, total dissolved solids, biological oxygen demand and chemical oxygen demand. Dissolved oxygen and transparency showed higher values in the Dhanmondi lake. Water depth, air temperature, water temperature, carbon dioxide and pH values recorded normal values. In the Gulshan lake water depth, dissolved oxygen, pH, carbon dioxide, alkalinity and hardness were not statistically significant differences in three years data. Air temperature, water temperature, transparency, ammonia-nitrogen, conductivity, total dissolved solids, biological oxygen demand and chemical oxygen demand values of the Gulshan lake shows statistically significant differences. In the Dhanmondi lake water depth, air temperature, water temperature, dissolved oxygen, pH, hardness, conductivity, total dissolved solids, biological oxygen demand and chemical oxygen demand values were not statistically significant differences in three years data. Transparency, ammonia- nitrogen and alkalinity values showed statistically significant differences in the Dhanmondi lake water. In comparison of the two lakes in first year air temperature, water temperature, transparency, pH, carbon dioxide and hardness values were showed no statistically significant differences between Gulshan and Dhanmondi lakes. Water depth, dissolved oxygen, ammonia-nitrogen, alkalinity, conductivity, total dissolved solids, biological oxygen demand and chemical oxygen demand values showed statistically significant differences between the Gulshan and Dhanmondi lakes. In second year water depth, air temperature, water temperature, dissolved oxygen, pH, carbon dioxide and hardness values were not statistically differences among two lakes. Others parameters values showed statistically differences. In third year water depth, transparency, dissolved oxygen, ammonia-nitrogen, alkalinity, conductivity, total dissolved solids, biological oxygen demand and chemical oxygen demand values showed statistically significant differences. Others parameters were not statistically different among two lakes. Among heavy metals of lake water Chromium and Nickel were found not detection level in both the lakes. Other heavy metals Cadmium, Zinc, Copper, Lead and Manganese were found higher level in the Gulshan lake water. Among them Manganese was detected higher values while Cadmium showed minimum values. Heavy metals of water like zinc, lead, cadmium, copper and manganese were recorded higher concentrations in the Gulshan lake water compare to the Dhanmondi lake.

Sediments heavy metals Lead showed higher concentration and Cadmium were found in minimum during the study period. Zinc, Chromium, Copper, Nickel and Manganese were present in both the lake sediment. All heavy metals were recorded higher concentrations in the Gulshan lake sediments than the Dhanmondi lake sediments. Average total plankton density was higher in the Gulshan lake. The phytoplankton was about 80% and their density ranged between 14532 to 42200 ind/L in the Gulshan lake. In the Dhanmondi lake phytoplankton contributed 55% out of total plankton and density ranged between 821 and 2386 ind/L. The major groups of phytoplankton were Cyanophyceae, Chlorophyceae and Bacillariophyceae. Zooplankton density ranged between 4720 to 10515 ind/L in the Gulshan lake. In Dhanmondi lake density ranged between 710 and 1580 ind/L. Zooplankton fauna comprised of Protozoans, Copepods, Cladocera and Rotifers. Statistically highly significant differences were recorded in both phytoplankton and zooplankton among two lakes. Benthos population in the Gulshan lake ranged from 950 to 2237 ind/m2. In the Dhanmondi lake it ranged from 862 to 1930 ind/m2. Benthic population was higher in the Gulshan lake where bivalve molluscs were very few in number. The following groups and species were identified from benthic organism. Chironomids: Chironomus sp larvae (Red blood), Chironomus larvae (others); Oligochaetes: Lumbriculus sp, Nais sp, Tubifex sp, Chaetogaster sp, Branchiodrillus semperi, Branchiura sowerbyi, Aelosoma sp; Molluscs : Lamellidens sp, Bellamya bengalensis, Brotia costula, Terabia sp. Benthic organism shows statistically highly significant differences among two lakes. The diversity of fishes in the lake was limited. The Dhanmondi lake is partially used for sport fishing and thus major carps and some exotic fishes were introduced. In the Gulshan lake some exotic species like, Thai pangus (Pangasiodon hypopthalmycthyes) and Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio) were found. Both the lakes were infested with an exotic aquarium fish named Sucker fish (Catostomus commersonii). The Gulshan lake water was moderately polluted throughout the year. The Dhanmondi lake water was found pollution free. Contamination sources were the surroundings polluted materials that drained into the lake water.