Propical Trofic

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TFopicalIakes: how latitude makesa difference
William M. Lervis.Jr. Depurtntent of Environntental, Population, and Organisntic Biology lJnit,ersiry of Coloratlo, Bouklet co 80309-0334

Keywords:tropical lakes,temperature, heatbudgets, phytoplankton, zooplankton, trophic dynamics

Abstract Tropical lakesarefar lessnumerous thantemperate lakesbecause lakesof glacialorigin are rare in thetropics. Also, the mixture of lake types differs from tropical to temperatelatitudes; Iakes of riverine origin are probablymorecommon thanany othertype in the tropics.For lakesof a given type and size,tatitudinaltrends can be tracedmainly to latitudinal gradients minimum monthly solar irradiance.Higher monthly minima in for irradiance in the tropics are responsible for higher minimum watertemperatures,higher mean temperatures the rvater column, and a smaller thermal differentiationbetrveenupper and lorver rvater for column' Tropical lakes of moderateto great depth are predominantlywamr monomictic, and shorv.great regularity in seasonalmixing, rvhich typically coincides rvith the hemispheric rvinter. Stratification is persistent, is lessstablethan at higher laritudes, seasonally but andthe amountof heatexchange requiredto causeimportant changesin stability is also smaller than at higher latitudes.For thesereasons, rvell as a as geostrophic factor that can magnify the variability of rvind-generated mixing, tropical lakesshorvmuch more intraseasonal variation in thickness of the mixed layer than morphometrically similar temperatelakes. Episodesof mixed layer thickeningaltematingrvith subsequent restorationof a thinner mixed layer affect nutrientcycling and planktondynamicsin tropical lakes.Efficiencyof nutrienrcycling is high at lorv latitudes because intraseasonal of deepmixing combinedrvith high mean temperatures, rvhich supporthigher nutrient regeneration rates.Primary productionis abouttrviceas high on a given nutrientbaseas it rvouldbe at higher latitudes, primarilybecause efficient nutrientcycling combinedrvith higher meantemperatures greater of and stability in solar irradiance.Nitrogen is more often a limiting elementfor autotrophsin the tropics than at temperatelatitudes.Phytoplankton,zooplankton,and benthic communities are no more complex at low latitudesthanat high latitudes, contrastto terrestrial in communities. Phytoplankton communitiesshorvbroadoverlap in speciescomposition rvith temperatecommunitiesand have fittle endemism.Zooplankton also show overlap, but have more endemism,contain more taxa that are characteristicallytropical, antl are composld'of smallerspecies than temperate communities. Efficiency of energytransferappeaisto be lorv in many-tropical lakes.Zooplanktonfishesare neitherabundant diversein many tropical lakes.Zooplanktonnor herbivbres often shorvsuppressed growth ratesindicatingnutritionalblockageof energytransfereven rvhen food is-abundant, and transferofenergy to fishesmay be intercepted suppressed Chaoborus,rvhich is or by abundant and rvidespread the tropics. in

Introduction The landsurface rheEarthholdsbetrveen and of 5 50 million lakes,as indicated extrapolation by of thefrequency distribution largelakes(Figurel) of plus scattered data on the numberof lakes in
Perspectircs TropicalLinnology,pp. 4344 in E Schiener K.T Boland(eds.) & @ 1996 SPB Acadenic Publishing Amsterdam, Netherlands bv, The

specific lake districts.The distributionfunction of lake areas shorvs that about 1500 of the largest Iakesaccountfor most ofthe global surfaceareaof lakes(Figure I, Schuiling 1977,Herdendorf1990). These1500 large lakes show a latitudinal distribution that reflects the abundanceof large

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Fig. /. Global lake surfaceareaas a function of lake size.The rarv data are from Schuiling (1977).

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