Effects Of Fish On Tropical Shallow Lakes And The Implication For Lake Restoration

Human inputs of nutrients have accelerated the eutrophication of lakes.Eutrophication is a serious problem to many lake ecosystems.Though measures by reducing external nutrient loading and algae biomass removal etc.have been taken,the transparency of lakes is still low. Biomanipulation defined as re-structuring of the biological community to achieve a favourable response,usually leads to reduction in algae biomass,shifting to a clear water state and promotion of a diverse biological community.The term is typically applied to top-down manipulation of fish communities,i.e.enhancement of piscivores or reduction of zooplanktivores and/or benthivores.Until now,the successful cases have been reported in temperate lakes and no application has been done in tropical regions.Fish is an important component of aquatic ecosystems.Bethivorous fish have direct/indirect effect on lake ecolosystems via both predation,and nutrient excretion,and sediment resuspension.The aim of this study was to evaluate effects of fish on shallow lake ecosystems in tropical region.Enclosure experiments were carried out from August 29 to November 6,2007 in a shallow and eutrophic lake Huizhou West Lake in southern tropic region,Guangdong Province, China.Nanhu is one of the sub-lakes in West Lake,its mean depth is 1.8 m and 12 ha^-1, stocking in high density(2471.1 kg ha^-1) of fish including Tilapias niloticus,Cyprinus carpio,Cirrihia molitorella,Carassius carassius,Hypphthalmichthys molitri and Aristichthys nobilis. Six enclosures,three enclosure stocking 15 Cyprinus carpio and three enclosure without fish as control,were set up.During the fish-enclosure experiment period,total nitrogen(TN) and particulate nitrogen (PN) declined,from 1.67mg L^-1,0.91mg L^-1at beginning of the experiment August 8 to 0.92 mg L^-1,0.20 mg L^-1 at the ending of the experiment November 6,significantly different from fish-free enclosure(TN p=0.035,a=0.05,PN p=0.032,a=0.05).Total phosphorus(TP) and particulate phosphorus(PP) increased at the beginning of the experiment,then declined, from 0.10 mg L^-1,0.091 mg L^-1at beginning of the experiment August to 0.05 mg L^-1,0.04 mg L^-1,significantly different from fish-free enclosure(TPp=0.001,a=0.05,PP p=0.007;a= 0.05).Suspended solids(SS) in fish-enclosure increased from 11.00 mg L^-1 to 19.75 mg L^-1, and SS in fish-free enclosure was significantly lower than fish-free enclosure(p = 0.049,a = 0.05).Sedimentation rate was measured using 6 traps made of plastic tubes(diameter 6 cm;total height 30cm) placed in each enclosure on 29 August,2007.The traps were recovered on 10 and 17 November,2007.Sedimentation rate in fish-enclosure(269.21DWg m^-2 d^-1) is significantly higher than that in fish-free enclosure(9.41gDW m^-2 d^-1).Secchi depth(SD) was found to exhibit an opposite trend,SD in the fish-enclosure declined from 55.3 cm at the begging of the experiment to 40.3 cm at the end of the experiment, in contrast SD in fish- free enclosure increased from 60 cm to 82 cm,the difference between fish- and fish-free enclosure is significant(p=0.012,a=0.05).We also evaluated the response of a whole lake to fish removal.Fish were removed from Nanhu on 9 May to 31 July.The concentrations of TN,PN,TN,PP,SS and Chl-a of Nanhu increased:Chl-a rapidly ranged from 20.34 um L^-1 at the beginning of fish removal 9 May, 2007 to 93.11 um L^-1 on 21 June,2007.Meanwhile metazoan zooplankton spieces and abundance dramatic changed.The abundance of rotifers declined from 1022.5 ind.L^-1(9 May, 2007) to 216.6 ind L^-1(14 July,2007).The abundance of copepods increased from 1.1 ind·L^-1 to 60.5 ind L^-1,nauplii increased from 42.5 ind·L^-1 to 55.0 ind L^-1,copepods increased from 8.2 ind·L^-1 to 191.1 ind L^-1.These changes are likely due to decrease in predation after fish removal.This study suggests that stocking bethivorous fish such as carp may stir up sediment and lead to turbid water.Fish removal can improve transparency in a short term as increased algal biomass reduces water transparency and becomes turbid again.In order to establish a clear water state,it needs to combine fish removal with aquatic macrophyte restoration..