Impacts of Colombo-Katunayake Expressway Bridge Opening on Water Quality at Madabokka in Negambo Lagoon, West Coast of Sri Lanka

Abstract

The proposed 5 year Colombo- Katunayake expressway construction project which is targetto reduce travelling time between Colombo and Katunayake international airport in SriLanka. This project would build 42 bridges and 88 culverts. One of the bridges opened atMadabokka inside the Negambo Lagoon is identified as highly productive ecosystem whichprovides breeding, feeding and security grounds for variety of aquatic lives. Thus, currentStudy the seasonal impact on Madabokka is carried out using water quality analysis.

Water samples from nine locations of three main sites (southern side, Northern side andoutside of the Madabokka) are analysed for physical, (temperature, salinity, pH andtransparency or sechchi depth) chemical, [NO3, NO2, PO4, SiO2, Total suspended solids(TSS), and Dissolved Oxygen (DO)], and biological parameters (chlorophyll a,phytoplankton and zooplankton) once in two months during the year 2012.

Results indicate that average salinity and temperature are varied from 14.24-24.54 PSU and310C (September) – 340C (March) respectively, while the warmest water is existed insouthern part of Maddabokka which has no free exchange with Negambo lagoon. However,the observed spatial and temporal variation would be related to climatic conditions ratherthan bridge constructions. Both the lowest (4.6) and the highest values (7.9) pH values andwere recorded outside Maddabokka during June and March respectively. Low pH values inmonth of June would be associated with South-West monsoon and particular low valuescould be a result of fresh water runoff canals from Katunayake airport area. Sechchi depth isalways ≤ 1.00m and higher depth remained as turbid water due to rapid constructions. DOvalues are varied from 5.9 - 8.3 mg l-1 with no impacts from the bridge constructions. TSSvalues are varied from 13.9-84.3 mg l-1 and the highest value is recorded in southern side ofthe bridge during November. Level of nutrients, (Nitrite; 0.05-5.34 mg l-1, Nitrate; 0.06-0.17mgl-1, Phosphate; 0.00-0.84 mg l-1, Silicates; 2.68-14.88 mg l-1) are varied temporally andthere is no construction interferences. Chlorophyll a (1.10-20.51 mg l-1) is distinctive intemporally and no spatial variations. The highest abundance (355000 Ind/L) of Phytoplanktonare recorded in Southern side of the bridge in March. All guilds were dominated by centricdiatoms. The lowest and the highest Zooplankton abundance (27 ind/l and 530 ind/L) arerecorded at outside of Madabokka during March and September respectively. Speciesdiversity (7 groups) is high in southern part of the Madabokka and dominated by Calanoids inJune and November.