Macrofauna as Bioindicators for Monitoring Pollution Status in Gulshan lake of Dhaka City Bangladesh

Abstract

Gulshan lake of Dhaka is a polluted lake as it receives discharge of domestic sewage, drains, gullies and untreated effluents from different sources. To prevent the depletion and preserve the water quality a regular monitoring is needed. Biomonitoring based on macrofauna combined with traditional physico-chemical methods have been widely used as an integral part of water quality monitoring in many countries. As macrofauna are constantly exposed to their ambient environment, they provide more accurate conditions of their habitat. In Bangladesh monitoring of water quality is limited to physico-chemical analysis only. In the present study, a potential use of macrofauna as bioindicators for monitoring pollution status of Gulshan lake of Dhaka, Bangladesh was explored. Water samples and macrofauna were collected from eleven selected stations of Gulshan lake and were examined for a period of two years from April 2012 to March 2014. To achieve the goal findings estimation of selected water quality parameters, identification and analyzing the abundance and distribution of macrofauna, exploration of relationships between selected water quality parameters with macrofaunal community were analyzed. Physico-chemical parameters, including water and air temperature, Dissolved Oxygen (DO), pH, total alkalinity (TA), total hardness (TH), ammonia-nitrogen (Ammonia-N), nitrite-nitrogen (Nitrite-N), total dissolved solid (TDS), electrical conductivity (EC) were recorded. The results revealed that the average DO (3.7 ± 1.3) mg/L was very low throughout the investigation period, which is an indication of pollution (domestic and industrial effluents). Value of DO was negatively correlated with air and water temperature. pH (7.7 ± 0.18) remained mostly alkaline during present study. Total hardness ranged 73 - 124 mg/L and was found relatively higher in winter (110.5) mg/L and lower in summer (83.5) mg/L. The result of high concentration of Ammonia-N (14 ± 2.41) mg/L indicates the water of Gulshan lake is polluted. The high value of Ammonia- N was perhaps due to sewage contamination from surrounding areas. Water quality classification (WQC) was performed for each sampling stations based on mean ranks of physico-chemical parameters. The stations were categorized as class1 (station W4) lest polluted, class2 (stations E1, E4 and W3) slightly polluted, class3 (stations E3 and W5) moderately polluted and class4 (stations E2, E5, E6, W1 and W2) grossly polluted. Abundance, community composition, diversity and distribution of macrofauna are important topics which are frequently used as bioindicator to evaluate the status of aquatic ecosystem. It is interesting to note that the identified macrofaunal biodiversity was considerably high during the present study with a total of 46 genera belonging to 3 phyla, 5 orders and 30 families. Most dominant was Arthropoda with a total number of 1,94,239 (85%) individuals followed by Annelida 23,151 (10%) and Mollusca 11,760 (5%). Macrofaunal diversity mainly represented by Oligochaeta, Hemiptera, Diptera and Gastropoda groups in the present study. Seasonally, from the combined result the highest average value (No/m²) for Oligochaeta (126 ± 77) and Hemiptera (184 ± 112) were observed in summer. But highest value of Diptera (837 ± 675) was recorded in winter and Gastropoda (67 ± 30) in monsoon. Seasonally, from the combined result the lowest average value (No/m²) of Oligochaeta (44 ± 22) and Hemiptera (45 ± 16) were observed in winter. But lowest abundance (no/m²) of Diptera (185 ± 157) was found in monsoon and Gastropoda (47 ± 24) in summer. During the present investigation Diptera was the dominant group, due to the presence of large number of Chironomous spp. with a total of 5609 recorded from the combined result. Richness of organic matter in the water body might have favored the abundance of large number of pollution tolerant macrofauna. Presence of pollution tolerant macrofauna (Chironomous sp., Limnodrilus sp., Tubifex sp. and Lumbriculus sp.) indicated poor water quality of the lake and characterized by low dissolved oxygen and high nutrient concentrations. The findings for both physicochemical parameters and macrofaunal abundance indicate that the water quality of Gulshan lake is polluted. In the present study biological assessment of water quality were explored. Monthly taxa richness of macrofauna ranged from 15 to 21 and almost evenly distributed throughout the year. The total taxa (Family) counted were 30 of which, the number of Biological Monitoring Working Party (BMWP) taxa were 17. The BMWP score ranged from 23 to 39. The Average Score Per Taxon (ASPT) score ranged from 2.88 to 3.3 which followed same pattern as BMWP score. From the combined result the values of biological indices were, Shannon-Wiener diversity index (0.51 - 2.45), Simpson’s index (0.18 -0.89), Margalef index (1.81 - 2.58) and Pielou Evenness index (0.09 - 0.55). Shannon-Wiener index (1-3) value indicates the lake is moderately polluted. Simpson’s index value indicates a fairly diversified community structure. Biotic and diversity index emphasize the environmental degradation. During present study notable relationship between the biological parameters with physico-chemical parameters was observed. Taxa richness exhibited decreased response from lest polluted site Water Quality Class 1 (WQC1) to grossly polluted Water Quality Class 4 (WQC4) with the values of 19 to 15. Both BMWP score and BMWP-ASPT displayed a relationship with pollution status by a gradual decline with the scores from lest polluted station to grossly polluted stations. Taxa richness exhibited negative correlation with water temperature (r= -0.619) and pH (r= -0.544). BMWP score negatively correlated with water temperature (r= -0.637) and pH (r= -0.587). BMWP-ASPT also exhibited significant negative correlation with water temperature (°C) (r = -0.708), pH (r = -0.744) and DO (r = -0.470) but positively correlated with TDS (0.612). The relationship between some pollution tolerant macrofauna with Water Quality Class (WQC) was examined. Remarkable response was observed. The result revealed that Tubifex sp., Lumbriculus sp., Lethocerus sp., Corixa sp. and Planorbis sp. increased in number from least polluted WQC1 to grossly polluted WQC4. The abundance of Tubifex sp. 12 No/m² for WQC2 and 50 No/m² for WQC4 were found. Lumbriculus sp. 122, 299, 223, 895 No/m², Lethocerus sp. 194, 478, 211, 649 No/m² was found for WQC1, WQC2, WQC3 and WQC4, respectively. Corixa sp. also increased from 190 No/m² for WQC1 to 1424 No/m² for WQC4. The abundance of Planorbis sp. was 7 No/m² for WQC1, 57 No/m² for WQC3 and 200 No/m² for WQC4. Thus, under the existing environmental condition the observed macrofauna; Tubifex sp., Lumbriculus sp., Lethocerus sp., Corixa sp. and Planorbis sp. could be considered as bioindicators for monitoring Gulshan lake ecosystem.