TEMPORALAN DSPATIALVARIATIONSINBIOMASSAND PHOTOSYNTHETICPRODUCTIONOFPHYTOPLANKTONIN LAKECHAMO,ETHIOPIA

Abstract

Thetemporalandspatialvariationsinthehiomassandphotosyntheticproductionof phytoplanktoninrelationtosomephysico-chemicalenvironmentalfactorswere investigatedattwo(offshoreandnear-shore)stationsinLakeChamofromAugust, 2003toMay,2004.Inorganicnutrientsvariedtemporally,withtheirlowandhigh concentrationlevelsgenerallycorrespondingtothedryandrainyperiods respectively.Lake'stransparencywasalwayslow(Zsd<40cm).Because phytoplanktonhiomasswashighthroughoutthestudyperiod,itscontrihutiontothe extinctionofunderwaterlightisexpectedtoheconsiderahle.Phytoplanktonhiomass measuredaschlorophyllavaried43to95(mean=77.2)and47to108(mean=69.4) mgChiam'^atthenear-shoreandcentralstations,respectively.Thevertical distrihutlonofphotosyntheticactivitywasofatypicalpatternforphytoplanktonwith light-inhihitiononallsamplingdates.Thelight-saturatedrateofphotosynthesis(Amax) rangedfrom721to1789mgO2(~224.95to558.17mgC)m'^h'^atthecentral stationandtfrom543to1803mg02(»169.42mgC)m'^h'^atthenear-shore station.Lackofcorrespondencehetweenphytoplanktonhiomassand Amaxwas evident.Bipmass-specificrateofphotosyntheticproductionatlight-saturation, photosyntheticcapacity(<Pmax)rangedfrom9to34mgO2(mgChia)'''h'^atthenearshorestationandfrom10to34mgO2(mgChia)'''h'''atthecentralstation.An inverserelationshiphetweenphotosyntheticcapacityandphytoplanktonhiomasswas alsoohserved.Hourlyintegralphotosynthesisrangedfrom0.36to1.72gO2("0.11 to0.54gC)m'^h'^atthecentralstationandfrom0.47to1.8gO2(^0.15to0.56g C)m'^h'^atthenear-shorestation.Theproductionrateperunitareaseemstohave heeninfluencedhythelight-saturatedrateofphotosynthesisasthestrongand positivecorrelationhetweenthetwo(r=0.77)suggests.Thetemporalvariationsin phytoplanktonhiomassandprimaryproductioninrelationtoenvironmentalfactors arediscussed