Light Attenuation And Its Impacts On Photosynthetic Efficiency And Primary Productivity Of Phytoplankton In Lake Fuxian

Fuxian Lake is located in Yuxi City,Yunnan Province,one of "the nine large plateau lakes" in Yunnan Province.It is the second deepest freshwater lake in China.However,for a long time,Fuxian Lake has faced problems such as continuous decline in transparency,increased biomass of phytoplankton and decline in water quality.In lake ecosystems,light radiation is closely related to the physical,chemical,biological and ecological processes of lakes;phytoplankton is not only an important primary producer of lake ecosystems,but also a direct responder to changes in light radiation in water.From January 2017 to January 2019,the temporal and spatial distribution characteristics of the light penetration,the effects of light quality on the growth and photosynthetic efficiency of phytoplankton,and the phytoplankton primary productivity were sutdied in Fuxian Lake through field investigation and simulation experiments.The results of monthly(January-December,2017)field observation show that the average values of the diffuse attenuation coefficients of ultraviolet B(UV-B,305 nm),ultraviolet A(UV-A,340 nm)and photosynthetically active radiation(PAR,400-700nm)were 1.47±0.22 m-1,0.80±0.16 m-1 and 0.25±0.06 m-1,respectively;and,the depth at which 1%of surface light intensity occurs(Z1%)were 3.20±0.51 m,5.98±1.24 m,and 19.40±4.90 m,respectively.The results of statistical analysis indicate that phytoplankton biomass(characterized by chlorophyll a concentration)is the most important factor affecting the light attenuation of Fuxian Lake,followed by suspended matter and chromophoric dissolved organic matter.Spatially,the Z1%of PAR in the south region was significantly larger than that in the north region,and the similar results of the Z1%of UV-B and UV-A were also observed in several months.Seasonally,the Z1%of IUV-B,UV-A and PAR in autumn were significantly lower than those in the other three seasons,and the values in summer were significantly smaller than those in spring and winter,but there was no significant difference between spring and winter.To detect the effects of sunlight(with and without UV radiation)on the photosynthetic efficiency and growth of phytoplankton,a simulation experiment was carried out on the shore of Fuxian Lake from August 4 to September 3,2018.The phytoplankton fluorometer(Phyto-PAM)classification results showed that the algae in the raw water were mainly diatoms during the experiment,and the chlorophyll concentration(Chl.a(Bro,wn))of the control group(ultraviolet light+ visible light)was 3.677±13 ?g/L,the maximum quantum yield of phytoplankton(Fv/Fm(mean))was 0.57±0.16,the actual quantum yield(Y11(mean))was 0.13±0.04,and the non-photochemical quenching parameter(NPQ)was 0.11±0.07;meanwhile,the Chl.a(Brown),Fv/Fm(mean),YII(mean),and NPQ of the treatment group(UV removal)were 3.57±0.82?g/L,0.56±0.16,0.14±0.04,and 0.14±0.09,respectively.In the later stage of the experiment,the photoinhibition effect of UV radiation on phytoplankton was significant.That is,the existence of ultraviolet radiation would significantly reduce the actual quantum yield and light energy utilization efficiency of phytoplankton,and at the same time destroy the photoprotective ability of phytoplankton.This experiment also implied that UV radiation could enhance the tolerance of phytoplankton to glare.In situ experiments of phytoplankton primary productivity were carried out using light and dark(quartz)bottle technique in April 2018(spring),July 2018(summer),2018 October(autumn)and 2019 January(winter).The results showed that the maximum gross primary productivity and net primary productivity of phytoplankton in Fuxian Lake appeared at a water depth of 1 m due to the photoinhibition of the water column surface.The average values of gross primary productivity and net primary productivity were 0.499±0.18 mg O2/(L·d)and 0.3±0.20 mg O2/(L·d).The surface primary productivity in autumn is significantly larger than that in the other three seasons,which was probably due to the weak photoinhibition in autumn.The average gross primary productivity and net primary productivity of the water column in Fuxian Lake were 2.44±0.76 g O2/(m2·d)and 0.73±1.44 g O2/(m2·d).The average gross primary productivity of the water column in spring,summer,autumn and winter respectively were 2.62±1.15 g O2/(m2·d)?3.22±0.24 g O2/(m2·d)?1.83±0.23 g O2/(m2·d)and 2.09±0.10 g O2/(m2·d),and the net primary productivity were 0.16±1.15gOZ/(m2'd)?2.84±0.24gO2/(m2·d)?-0.61±0.179gO2/(m2·d)and 0.55±0.10 g O2/(m2·d).The gross primary productivity of water column in summer is significantly greater than that in autumn and winter.The net primary productivity in summer is significantly larger than that in the other three seasons.