Abstract
Monsoon-bound seasonal abundance of two species of rotifers (Brachionus calyciflorus and Keratella tropica) was examined in relation to the population densities of Cladoceran zooplankton and phytoplankton and some limnlogical characteristics in Kandy Lake, a tropical urban water body in Sri Lanka, using standard techniques. Kandy Lake did not show marked seasonal variation influenced by the monsoons with respect to limnological parameters except for bicarbonate alkalinity and chlorophyll-a content. The population densities of Brachionus calyciflorus declined and Keratella tropica became dominant during the first inter-monsoon (March-April) when the abundance of total cladocerans and Microcystis aeruginosa, a cyanobacterial species with toxigenic strains, were high under relatively turbid conditions. Brachionus calyciflorus was re-established progressively during the subsequent season with decreasing populations of Microcystis aeruginosa and increasing densities of Aulacoseira granulata, a filamentous centric diatom, and Pediastrum simplex, a colony forming green algae. The likely explanation for the marked suppression of Brachionus calyciflorus during the first inter-monsoon could be the abundance of toxin forming cyanobacteria strains of Microcystis aeruginosa or else a combined effect of Microcystis aeruginosa and interference competition resulting from high densities of total cladocerans under unfavourable environmental factors such as high turbidity.
Highlights
Rotife$ are of fundamentalimportancein aquaticecosystemsdue to their high efficiency in trarsfoming primary production of phytoplanktonandenergystoredin bacteriainto energyaccessibleby other organisms.They showquick rcsponsesto envirctrmentalchangesand arc thereforeconsideredas good indicato$ of water quality and trophic states(Slddecek1983; Matveeva1991)
The relative abundanceof Bftuhionus cal)ciJlorus andKeralelkr tropican rclationto lhe concentrationof chlorophyll-a. It experieDcews eatherandclimatet)?ical to the wet zone of Sri Lanka,KandyLake did not showamarkedmonsoon-boundseasonal variation in environmentalparameters(i.e. Temperatue, pH, EC and tmnsparency)exceptfor bicarbonatealkalinity and chlorophyll'a content drring the studyperiod,perhapsdue to the uniform distributionof rainfall throughoutthe year 2000
Rotifers dominated by the genus Brachionus,arc cofiirJtonin pelagiczooplanktoncornmunitiesin Sd Lankan reservoirs(Femando1980a;Rajapaksaand Femando1982;Jayalunge 1986).Fultler, it hasbeenemphasizetdhatrotiferpopulationdensitiesin hy&olo8ically dynamic_reseryohecosystemsareregulatedeither by rapid tl[oughput or predatorpressureof zooplanktivorousfish @uncan 1984; Duncan and Gulati 1983; Jayarunge1986)
Summary
Zooplankton and phytoplankton were collected biweekly ftom Januaryto December2000 at four samplingsitesof Kandy Lake (Fig. 1) usinga SchindlerPatalassampler(15 l). Watersampleswerccollectedfrom 1 m depthsat eachsampling site aIId fanspofied immediatelyto the laboratory,which is located km southwestof theLake.In thelabomtory,zooplankonsampleswerefixed in 5Vo fotmalln and phytoplankton samples were preserved in Lugol's solution. R€sults The magdtude and dishibution of rainfall in the watershedof lhe Lake was more or less similar dwing four monsoon-definedseasonsi,e., Dec-Feb,northeastmonsoon(NEM); Mar-AF, first inter-monsoon(FIM); May-Seps, outlwestmonsoon(SWM)andOct-Nov,secondinter-moDsoon (SM) in 2000 (Table l). Daytime surface warer rempefttue ranged between26.0'Cand28.ffC andelectical conductivityvariedbetween212 /S crrr and 239 pS cr.tt dudng the srudy period wirh no significanr seasonavlariation(p>0.05)(Table 1). The seasonaclhangein pH showeda similarpattembutit variedbetween6.96and8.61andthewidest range was recordedduring the soutlwestmonsoon(Table). RotiJets in KandY Inke monsooD.One-wayANOVA showedno significant seasonavl ariatioDin Secchidisk visibility (p>005) (Tablel).
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