Hydrography, nutrients and abundance and distribution of zooplankton in Negombo Lagoon, Sri Lanka.
Negombo Lagoon is a productive fishing ground and also a sink for many anthropogenic effluents draining from its surrounding urban area. The seasonal abundance and distribution of zooplankton were investigated in relation to some physico-chemical parameters of the lagoon from March 2008 to September 2009. Zooplankton and surface water samples were collected from five sampling sites where effluents are discharged to the lagoon and analyzed using standard methods. Lagoon salinity ranged spatially from 0 - 25 ppt and mean monthly salinity varied from 1.08 ±1.75 to 15.23 ±5.33 ppt. Surface water temperature varied from 26.0 to 34.10C whereas mean pH ranged between 7.34 ± 0.32 and 8.02 ± 0.48. Variation of mean dissolved oxygen as between 5.73 ± 2.49 mg/l and 7.00 ± 1.87 mg/l. Micro-crustaceans represented the major component of zooplankton assemblage and ranged from 54.37 to 84.33% throughout the study period. A higher percentage of freshwater cladocerans was recorded during low salinity period (February-April, 2009) in Dundugam Oya and from April to June in Hamilton canal, the main freshwater inlets of the lagoon. Mean cladoceran density in Dandugam Oya was (1.96±3.50 individuals/l) higher compared to the cladoceran density in Hamilton canal (0.47±1.10 number/l). Nevertheless there was no significant relationship between salinity and the density of cladoceran. A positive significant correlation was found nitrate-N concentration and copepod density (r = 0.804; p<0.01) for the site in Dungalpitiya, where rotifer density did not show positive relationship but correlated positively with phosphate-P concentration although it was statistically insignificant (r = 0.489; p>0.05). Further, rotifer density in Madabokka area showed a positive and significant relationship with nitrite-N concentration (r = 0.731; p<0.01). The molluscan abundance showed a significantly positive correlation with salinity (r = 0.713; p<0.01) for the Dandugan Oya site. High concentration of phosphate and occurrence of pollution indicator species reveal organic pollution in the lagoon. Abundance of plankton was strongly influenced by the water circulation pattern and direct or indirect human impacts that occur at each site of the lagoon.