RESEARCH ARTICLE
The effect of intense hydrodynamic disturbance on chromophoric dissolved organic matter in a shallow eutrophic lake
Tingfeng Wu a,b *,Zhen Wang c ,Cheng Niu a ,Yunlin Zhang a ,Botong Li d and Peng Li d
a
State Key Laboratory of Lake Science and Environment,Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences,Nanjing,P.R.China;b State Key Laboratory of
Hydrology,Water Resources and Hydraulic Engineering,Nanjing Hydraulic Research Institute,Nanjing,P.R.China;c Department of Environmental Science and Engineering,School of Environment and Civil Engineering,Jiangnan University,Wuxi,P.R.China;d Department of Hydrology and Water Resources,College of Hydrology and Water Resources,Hohai University,Nanjing,P.R.China
(Received 8May 2014;accepted 25August 2014)
Chromophoric dissolved organic matter (CDOM)is an important component of aquatic ecosystems and its movement is a complicated process.To investigate the effect of strong wind-induced sediment suspension on CDOM concentration,we carried out an in situ observation in a large,shallow lake,which is seriously affected by eutrophication and cyanobacterial blooms,using a ?nely strati?ed sampling device and several automatic monitoring instruments.During the observation,a typhoon-induced strong wind with a mean wind speed of 6.6m/s swept the Lake Taihu basin.The strong wind-induced hydrodynamic disturbance caused explosive sediment suspension.During the strong winds,the maximum turbidity in the overlying waters reached 175.4NTU,which was 12.2times the background value recorded before the strong winds.Sediment suspension resulted in the release of CDOM from within the sediment.The CDOM absorption coef?cient a (355)in the overlying water showed that there was no signi?cant difference between any two water layers for all time points during the observation (13July to 16July).The CDOM concentration was fully mixed along the water depth.However,the mean a (355)values recorded in all time points during the wind were higher than that before the wind (p <0.001).The maximum value during the wind event was 2.524§0.097m ?1,which was 1.4times that of the value before the disturbance.CDOM concentrations were signi?cantly correlated with hourly mean wind speed (p <0.05)and turbidity (p <0.01).Because Lake Taihu is frequently affected by strong wind processes,CDOM concentration changes caused by intense hydrodynamic disturbance are important for quantitative remote sensing of water quality and primary productivity.
Keywords:Lake Taihu;hydrodynamics;sediment suspension;chromophoric dissolved organic matter;turbidity
Introduction
Chromophoric dissolved organic matter (CDOM)is the photoactive component of dis-solved organic matter (DOM)in water bodies and it signi?cantly affects light transmis-sion and related processes in aquatic habitats.Because it can absorb ultraviolet radiation,CDOM in water bodies can attenuate the damage of UV-B radiation on aquatic organisms (Williamson et al.1996;Laurion et al.2000;Zhang et al.2009).CDOM can absorb blue light in the visible spectrum,hence interfering with the quantitative remote sensing of
*Corresponding author.Email:tfwu@https://www.doczj.com/doc/922251365.html,
ó2014Taylor &Francis
Journal of Freshwater Ecology ,2015
Vol.30,No.1,143à156,
https://www.doczj.com/doc/922251365.html,/10.1080/02705060.2014.961043
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phytoplankton biomass,primary productivity and suspended solids (SS:Doxaran et al.2002;Arrigo et al.2011;Odermatt et al.2012).CDOM exposed to sunlight can also undergo self-photochemical degradation,thereby serving as an essential carbon and nitro-gen source for aquatic microorganisms (Stedmon et al.2007).
The importance of CDOM to aquatic ecosystems is widely recognized.However,the generation,transport and conversion of CDOM are very complicated processes and they are currently poorly understood (Zhang et al.2009;2013).Allochthonous CDOM originat-ing from human activities and decaying animal and plant remains can enter bodies of water through the transportation of precipitation,runoff and groundwater (Birdwell &Engel 2010;Zhang et al.2011).Waste breakdown and the metabolism of aquatic animals and plants,plankton and microorganisms can generate and consume CDOM (Lapierre &Fren-ette 2009;Romera-Castillo et al.2011).Some chemical processes (e.g.redox and photode-gradation reactions)can also in?uence CDOM content in water bodies (Zhang et al.2009).Another process that can affect CDOM in water bodies is sediment suspension.The research by Burdige et al.(2004)on the Chesapeake Bay and the mid-Atlantic shelf/slope break reported an increase in humic-like ?uorescence at the coastal sediment depth,and the ?uorescence was highly correlated to total organic carbon.Protein-like ?uorescence in interstitial water was notably higher than that in the overlying water,pointing to the sediment as one source of CDOM.Proof of the relative importance of this source requires further evidence.Based on surveys on the West Florida Shelf,Conmy et al.(2009)sug-gested hurricanes signi?cantly altered the typical CDOM distribution on the West Florida Shelf,which could be because of hurricane-induced sediment suspension,algal blooms,or allochthonous CDOM.Given the data set,it was not possible to determine which of the above was the major contributing factor.
To clarify the role of sediment suspension on the CDOM concentration,we employed a ?nely strati?ed sampling device and several automatic monitoring instruments,which simultaneously recorded typhoon-induced changes in meteorology,hydrology and aquatic chemistry.The study focused on the quantitative effect of strong wind-induced sediment suspension on changes in CDOM concentration in the overlying waters.
Methods Site description
Lake Taihu is a large,shallow and eutrophic lake located in the Yangtze Delta,in eastern China.The lake has an area of 2339km 2,is 68.5-km long (north àsouth),34-km wide (east àwest),and has a maximum depth of less than 3.0m (see Figure 1).Lake Taihu has an average depth of 1.9m with a ?at bottom and the average gradient of 19.7”.The hydrology of Lake Taihu is strongly affected by wind and the extensive lake surface and ?at underwater topography is favorable for the generation of wind waves and lake cur-rents.Hence,the lake is frequently subjected to hydrodynamic disturbances and there is no stable thermal strati?cation.The lake is connected to many in?ow and out?ow rivers.The complicated hydrology results in high spatial heterogeneity in the aquatic ecosystem of Lake Taihu (Qin et al.2007).The northwest region is predominantly an algal ecosys-tem,and cyanobacterial blooms occur frequently (Stone 2011).The higher a (350)values have been recorded in this region (Zhang et al.2011).The eastern region is predominantly macrophytic,with clear water and low a (350)values.The sediment of Lake Taihu also exhibits remarkable spatial heterogeneity.Silt is distributed mainly in the western region and Zhushan Bay and there are scattered silt areas in Meiliang Bay.The central and
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eastern regions are characterized by hard substratum (Shen et al.2011).These character-istics result in complexity surrounding the origin,composition,migration and conversion of CDOM in Lake Taihu.
Data collection
Investigating the effects of hydrodynamic disturbance on the vertical distribution of CDOM in the water column required a strong wind event suf?cient to cause explosive sediment suspension.From 12to 16July 2013,typhoon Soulik landed on the southeastern coast of China,which brought a strong wind pattern to the Taihu Basin and offered an excellent opportunity for in situ observation and investigation.The observation was car-ried out on an aquatic experimental platform of the Taihu Laboratory for Lake Ecology Research (TLLER),Chinese Academy of Sciences (see Figure 1).The platform ensured the safety of the observers during the typhoon.Water sampling was conducted by the device illustrated in Figure 2.Water samples were collected from 0.1,0.2,0.6,1.2and 2.0m above the sediment àwater interface,and three separate samples were collected
on
Figure 1.Map of Lake Taihu showing Taihu Laboratory for Lake Ecology Research (TLLER),Chinese Academy of Sciences,acoustic Doppler ?ow rate pro?ler (XR),in?ow and out?ow rivers,isobaths,YSI sonde and water sampling site.
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each level.From 12to 16July,sampling was done at least once daily,and the frequency of sampling was increased during the high wind event.
The sampling device was composed of a supporting steel pipe,Perspex tubes,silicone tubes,conical ?asks,a multi-pass box and a vacuum pump.The silicone tube on one end of the conical ?ask was connected to a Perspex tube ?xed on the supporting steel tube,while the silicone tube on the other end was connected to the vacuum pump through the multi-pass box.The vacuum pump suction sampled simultaneously from ?ve layers of water.The device avoided cross contamination between water samples,and enabled ?nely strati?ed,fast and simultaneous water sampling.
Automatic monitoring devices recorded wind speed,wind direction,precipitation,water depth,lake currents,wave height,turbidity,dissolved oxygen (DO)and phycocya-nin concentrations,during the in situ observations.Wind speed,wind direction and pre-cipitation were recorded by a weather station (WXT520,Vaisala Inc.,Finland),and the time interval between each recording was 10min.Water depth,lake currents and wave height were measured continuously every 30min using an integrated pressure sensor-acoustic Doppler pro?ler Argonaut XR (see Figure 1,Sontek Inc.,
http://www.sontek.
Figure 2.Diagram illustrating our ?nely strati?ed water sampling device.Water was simulta-neously collected from 0.1,0.2,0.6,1.2and 2.0m above the sediment àwater interface.
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com).The XR could monitor the lake current pro?le at a resolution of 0.001m/s,and simultaneously measure the wave height at 0.001-m resolution.At 1.5km north of the XR,a multiparametric water quality monitor was set up (YSI sonde,YSI Inc.,https://www.doczj.com/doc/922251365.html,).The YSI sonde recorded turbidity changes,DO and phycocyanin in the middle water layer every 10min.Sample processing and analysis
Water samples were ?ltered by GF/F membrane (0.7m m),and the post-?ltration mem-brane was used for the determination of suspended solids (SS).The water samples were subjected to a second ?ltration using a 0.22-m m Millipore membrane,which could then be used for determining the CDOM absorption coef?cient.The absorption was measured at 200à800nm by UV-2401spectrophotometer (Shimadzu,Japan),at 1-nm intervals.Ultrapure water was used as the blank standard.The following formula was used to calcu-late the CDOM light spectrum absorption coef?cient,after correction for the diffraction effect (Bricaud et al.1981):
a CDOM eλT?
2:303OD eλTr ?
2:303OD e700Tλ
700r
(1)
In the formula,a CDOM eλTis the absorption coef?cient at wavelength λ(m ?1),OD(λ)is the
optical density at wavelength λ,and r is optical path length (m).As the composition of CDOM is complex,the absorption at 355nm (a (355))was used to denote the concentra-tion of CDOM.The M value,the ratio of absorption coef?cients at 250and 365nm (a (250)/a (365)),was used to represent the size of CDOM molecules.M values decrease with an increase in CDOM molecular masses (Peuravuon &Pihlaja 1997).
A statistical software package was employed for statistical analysis (SPSS 18.0,SPSS,Inc.https://www.doczj.com/doc/922251365.html,).The parameters obtained from the spectrophotometer were used for the calculation of means and standard deviations.An independent-samples t -test was used for mean comparisons and three levels of signi?cance were reported:p <0.05,p <0.01and p <0.001.A partial correlation algorithm was used to calculate Pearson’s correlation coef?cient (r )and the level of signi?cance.Changes in the environmental var-iables with wind were analyzed using ordinary least squares regressions.Results
Weather and hydrodynamics
During the observation period,the Taihu Basin experienced a strong wind event caused by typhoon Soulik (see Figure 3).Before the in?uence of strong winds on Lake Taihu basin,from 00:00to 22:00on 12July,the wind direction was mainly south-southeast and mean wind speed was 3.8m/s.From 22:00,12July to 14:00,16July,large tropical storm fronts caused by typhoon Soulik passed across Taihu Basin and resulted in strong winds.Wind speed ?uctuated but had a mean value of 6.6m/s and a maximum of 13.8m/s (18:00,13July).The wind direction was mainly east-southeast at the beginning,gradually switching to south-southeast.After the strong winds ended,wind speed gradually decreased to 1m/s,and the wind direction changed to northeast.During the observation,the weather station did not record any effective precipitation.
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Strong winds caused intense hydrodynamic disturbance.As shown in Figure 4,the mean ?ow velocity recorded by the Doppler was 3.6cm/s,and the maximum value was 10.9cm/s (17:00,15July).The curve of ?ow velocity generally exhibited a single peak and was signi?cantly correlated with wind speed (r D 0.18,p <0.01).However,the ?ow velocity ?uctuated wildly so the Pearson’s coef?cient was small.Similarly,wind speed and wave height exhibited a signi?cant correlation during the observation period (r D 0.55,p <0.01)despite the change of wind fetch.The two wave height peaks both hap-pened during strong winds.The ?rst peak lasted from 19:20,14July to 16:50,15July and the maximum value was 5.1cm.The later peak lasted from 21:20,15July to 14:00,16July and the maximum value was 5.4cm.For the rest of the time the wave height in Lake Taihu was less than 1cm.The water depth ?uctuation was relatively mild,ranging from 2.14to 2.36m.
Water characteristics
Strong winds induced hydrodynamic disturbance,which acted on the sediment àwater interface,resulting in erosion and subsequent suspension of sediment.During the event,turbidity and wind speed exhibited a signi?cant positive correlation (r D 0.56,p <0.01).As shown in Figure 5,a maximum turbidity of 175.4NTU was recorded at 04:00,15July,which was 12.2times the minimum value before strong winds.During strong winds,the mean turbidity was 102.6NTU,which was 2.4times the mean turbidity value before and after strong winds.Figure 6showed the results of synchronous monitoring of suspended sediment (SS).Before strong winds,the mean SS value along the depth pro?le was 19.2mg/L,and there was no signi?cant difference in SS concentrations between any two water layers.Water body SS concentrations rose with time and reached a maximum
W i n d s p e e d (m s -1
)
0.0
2.04.0
6.08.010.012.014.016.0
00:00Jul-1212:00Jul-1200:00Jul-1312:00Jul-1300:00Jul-1412:00Jul-1400:00Jul-1512:00Jul-1500:00Jul-1612:00Jul-1600:00Jul-17
4590135180225270315
360W i n d d i r e c t i o n (o
)
Figure 3.Changes in wind speed and wind direction in Lake Taihu basin during the observation.
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value at 00:00on 15July.From Figure 6,at 00:00on 15July there were no signi?cant differences in SS concentrations above 0.6m but the SS concentrations at 0.1and 0.2m were signi?cantly greater than those at 0.6,1.2,or 2m (p <0.05).SS concentrations in the water body demonstrated strati?cation.Speci?cally,the mean SS concentration at
W a v e h e i g h t (c m )
0.0
1.0
2.03.04.05.0
6.0
00:00Jul-1212:00Jul-1200:00Jul-1312:00Jul-1300:00Jul-1412:00Jul-1400:00Jul-1512:00Jul-1500:00Jul-1612:00Jul-1600:00Jul-17F l o w v e l o c i t y (c m s -1
)
0.02.0
4.06.08.010.0
12.0Figure 4.Changes in wave height and ?ow velocity during the observation.
T u r b i d i t y (N T U )
0.0
20.040.060.080.0100.0120.0140.0160.0180.0200.0
00:00Jul-1212:00Jul-1200:00Jul-1312:00Jul-1300:00Jul-1412:00Jul-1400:00Jul-1512:00Jul-1500:00Jul-1612:00Jul-1600:00Jul-17
D i s s o l v e d o x y g e n (m g /L )
0.0
2.0
4.0
6.0
8.010.0C y a n o b a c t e r i a b i o m a s s (106 i n d /L )
0.0
2.04.06.08.010.012.014.016.018.0
20.0
Figure 5.Changes in turbidity,dissolved oxygen and cyanobacterial biomass during the observation.
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0.1m was 288.8mg/L,which was 2.4times that of the surface water layer and 11.8times of the corresponding value before strong winds.
In contrast to the turbidity variation,DO exhibited greater ?uctuation before and after strong winds but was relatively stable during the strong wind event.As a result,DO exhibited a signi?cant negative correlation with wind speed (r D ?0.35,p <0.01).Before strong winds,DO showed a clear daily cycle.The minimum value of 7mg/L was seen at 08:00on 12July,while the maximum value of 7.8mg/L was recorded at 16:00on 12July.During strong winds,the DO only varied between 6.7and 7.3mg/L,which was relatively minor.
The strong wind event also resulted in variation in the cyanobacterial biomass in the water body.Cyanobacterial biomass was signi?cantly positively correlated with wind speed (r D 0.28,p <0.01).With the increase in wind speed,the maximum cyanobacterial biomass reached 18.7￡106ind/L (16:50,14July).The biomass then decreased as the winds calmed.During strong winds,the mean value was 6.3￡106ind/L,which was 1.7times the mean values before and after strong winds.
There was no signi?cant difference in CDOM between any two water layers at any time points,showing that CDOM was evenly distributed throughout the water column.However,CDOM concentration showed signi?cant changes with time.Based on correla-tion analysis,the CDOM absorption coef?cient a (355)was signi?cantly correlated with the wave height and hourly mean wind speed of the corresponding time point (r D 0.78,p <0.05;r D 0.79,p <0.05,respectively).Figure 7shows the regression analysis of mean a (355)along water depth with (a)hourly mean wind speed or (b)turbidity.With an increase in wind speed or turbidity,there was a signi?cant rise in CDOM concentrations in the water body.The rate gradually decreased as wind speed or turbidity increased.CDOM concentration in the water body before strong winds (12:00,12July)was signi?-cantly lower than that during the wind event (p <0.001,see Figure 8).Speci?cally,a (355)in the water body at 12:00on 15July was the maximum (a (355)D 2.524§
Suspended soild (mg L -1
)
50.0
150.0250.0
350.0
T h e d i s t a n c e t o s e d i m e n t -w a t e r i n t e r f a c e (m )0.0
.5
1.0
1.5
2.0
Figure 6.Suspended solid concentrations at 0.1,0.2,0.6,1.2and 2.0m above the sediment àwater interface.Water samples collected at 12:00on 12July 2013,12:00on 14July,and 00:00on 15July.
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0.097m ?1),which was 1.4times the corresponding value before strong winds.After 12:00on 15July,the CDOM absorption coef?cient decreased to 2.332§0.295m ?1(12:00,16July,p <0.05)at a rate of 0.008(m h)?1.
Similarly,there was no signi?cant difference between M values of any two water layers at any time points during the observation.However,M values showed signi?cant changes with time.Although M values were not signi?cantly correlated with wind speed,an increase of M values in the water body was observed.Except for 12:00on 13July and 18:00on 16July,the M values before strong winds were signi?cantly lower than those
Hourly mean wind speed (m s -1)3.0
4.0
5.0
6.0
7.0
8.0
9.0
C D O M a b s o r p t i o n c o e f f i c i e n t a (355) (m -1
)
0.0.51.01.52.02.5
3.0
Turbidity (NTU)
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
C D O M a b s o r p t i o n c o e f f i c i e n t a (355) (m -1
)
0.0
.51.01.52.02.5
3.0Figure 7.Regression analysis of the mean CDOM absorption coef?cient a (355)along water depth and (a)hourly wind speed and (b)turbidity.
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during strong winds (p <0.001).The maximum value of 12.594§1.016appeared at 12:00on 14July,which was 1.3times that observed before strong winds.However,after that time,M values quickly decreased.The M values at 12:00,16July were higher than those observed before the wind (p <0.05),but the M values at 18:00,15July did not show a signi?cant difference from the value determined before strong winds.
Discussion
The strong winds caused by typhoon Soulik resulted in intense hydrodynamic disturbance in Lake Taihu and caused explosive sediment suspension.One major feature of shallow lakes is the lack of stable thermal strati?cation,so the mixed layer is in direct contact with the sediment.Once the shear stress exceeds the critical value,the sediment is subjected to erosion,and subsequently affects the whole lake ecosystem (Bachmann et al.20:00;Ding et al.2012).James et al.(2008)on Lake Okeechobee reported that a hurricane could cause large-scale suspension of the lake sediment that has a large impact on the lake ecosystem.Based on observations on the erosion-suspension of the sediment in Lake Taihu,Qin et al.(2006)found that wind speeds exceeding 6m/s resulted in large-scale sediment suspension.Our observations also show that a strong wind event with a mean wind speed of 6.6m/s causes substantial sediment suspension in Lake Taihu making the turbidity in overlying water notably higher.According to the formula for calculating the suspension equivalent of Lake Taihu,the sediment thickness needed to generate the SS concentration pro?le at 00:00on 15July (see Figure 6)would be 1mm (Hu et al.2006).
The erosion-suspension of sediment caused by hydrodynamic disturbance not only resulted in an increase in SS concentration in the overlying waters,it also led to the release of CDOM stored in the sediment.However,CDOM concentrations in the overlying water
C D O M a b s o r p t i o n c o e f f i c i e n t a (355) (m -1
)
1.0
1.5
2.02.5
3.03.5
4.0
00:00Jul-1212:00Jul-1212:00Jul-1312:00Jul-1400:00Jul-1400:00Jul-1512:00Jul-1500:00Jul-1612:00Jul-1600:00Jul-1300:00Jul-17
M v a l u e
0.0
2.04.06.08.010.012.0
14.016.0Figure 8.Changes in mean CDOM absorption coef?cients a (355)and M values along water depth over time.
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returned to pre-wind values with the re-deposition of SS.The in situ study by Conmy et al.(2009)on the West Florida Shelf suggested that hurricanes substantially alter the typical distribution of CDOM.The authors believed that sediment suspension induced by the hurri-cane might be a contributing factor.However there was not suf?cient evidence to support their claim.In our in situ observation,we showed that the CDOM absorption coef?cient a (355)was signi?cantly correlated with wind speed and water turbidity.The organic matter content is far higher in the sediment compared to the overlying water in Lake Taihu (Trolle et al.2009;Wang et al.2013),and when there is erosion-suspension of the sediment,the organic matter stored in the sediment is also released.
The re-suspension of sediment results in the enrichment of CDOM concentrations in the overlying water in two ways.The ?rst is the direct release of highly concentrated CDOM from the interstitial water into the overlying water body.The second is the sub-stantial adsorption of CDOM by suspended particulates.The study by Binding et al.(2008)on Lake Erie discovered that suspended particulates could adsorb large quantities of CDOM,resulting in an increase in the absorption coef?cient of suspended particulates in the short wavelength region.In the current study,we did not consider the adsorption of CDOM on particulates.Thus,the release of CDOM into the water body by hydrodynamic disturbance might be higher than the data suggested in the current report.With the decline in wind speed,the water body CDOM returned to pre-wind values so the effect of hydro-dynamic disturbance on CDOM concentrations was short-term.The decrease in CDOM concentration might be caused by the re-deposition of SS,transferring CDOM in the water back to the sediment layer.
CDOM in the sediment could also enter the overlying water body through molecular diffusion.However,according to our current study,the contribution of molecular diffu-sion to the CDOM concentration over 0.1m above the sediment àwater interface was negligible.Burdige et al.(2004)reported the existence of a CDOM concentration gradient near the sediment àwater interface of the Chesapeake Bay,supporting the idea that CDOM in the sediment could diffuse into the water body through molecular diffusion.Despite the fact that in Lake Taihu organic matter content in the sediment was much higher than that in the overlying water (Trolle et al.2009;Wang et al.2013),the observa-tion in the current study found that the CDOM absorption coef?cient a (355)more than 0.1m above the interface did not show any signi?cant differences.CDOM was fully mixed along the water depth.As the average depth of Lake Taihu was just 1.9m,the hydrodynamic mixing usually can act directly on the sediment àwater interface.Thus,there is no macroscopic concentration gradient of dissolved substances in Lake Taihu (Ding et al.2012).Further research at a detailed level is needed to understand the effects of molecular diffusion on CDOM concentrations at 0à0.1m above the sediment àwater interface.The strati?cation sampling technique used in the current study limits the inves-tigation to <0.1m above the interface.Future research on CDOM distribution on a micro-scopic scale necessitates the use of more delicate microelectrodes.
Allochthonous sources,horizontal transfer and conversion from other forms of organic matter also were not major contributors to the increase in CDOM during the observation.Import from allochthonous sources was not signi?cant because no precipita-tion was recorded,and the gates guarding the estuaries near the sampling site were closed.The large spatial variation in the CDOM of Lake Taihu (Zhang et al.2011)might affect the CDOM concentrations at the sampling site through horizontal migration.Based on on-site observation and model building,Hu et al.(2006)and Wu et al.(2013b )believed that the concentration in overlying waters was mainly in?uenced by local vertical hydro-dynamic disturbances rather than horizontal migration.
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Algae degradation is one of the major sources of CDOM in Lake Taihu (Zhang et al.2009)and a signi?cant increase in phycocyanin was observed because of spatial distribu-tion differences.However,because the CDOM concentration rose from a minimum to a maximum in only 24h and during this interval the largest range of variation in DO was just 0.2mg/L,algal metabolism probably contributed to the rise in water body CDOM,but was not a major factor.
Although the rise in CDOM concentration in the overlying water caused by sediment suspension was ephemeral and small in magnitude,it is important to the ecological study of shallow and eutrophic lakes.From April to October,when cyanobacterial blooms fre-quently occur,Lake Taihu was subjected to winds stronger than 6m/s for a total of 322h (Wu et al.2013a ).This shows that Lake Taihu is frequently subjected to strong winds,which result in persistent erosion and suspension of sediment and lead to continuous vari-ation in the CDOM concentrations in the overlying water.As CDOM can absorb visible light,it can interfere with quantitative remote sensing of phytoplankton biomass,primary productivity,and SS (Doxaran et al.2002;Arrigo et al.2011;Odermatt et al.2012).The persistent ?uctuation of CDOM concentrations also results in uncertainty in the remote sensing of those parameters.Thus accurate retrieval in the remote sensing of shallow lakes necessitates further investigation on the effect exerted by the instantaneous ?uctua-tion in CDOM concentrations caused by hydrodynamic disturbance.Moreover,when there is an outbreak of cyanobacterial blooms in Lake Taihu,nutrient limitation is likely to occur (Xu et al.2010).Short term,strong hydrodynamic disturbance can not only offer nutrients for algal growth,but can also indirectly affect algal growth through altering the underwater light regime.
Conclusion
This study conducted in situ observations on Lake Taihu which has frequent cyanobacte-rial blooms.We observed that the intense hydrodynamic disturbance induced by a typhoon led to erosion and suspension of lake sediment.The water turbidity during the strong wind event was 12.2times of the background value recorded before strong winds.Throughout the observation process,CDOM concentrations were evenly distributed along the water depth.During the strong wind event,CDOM concentrations increased,and were signi?cantly correlated with hourly mean wind speed and turbidity.Sediment suspension related to the strong winds was the major reason for the increase in CDOM concentrations in the overlying water,while contributions of in?ow/out?ow rivers,precipitation and organic matter oxidation were https://www.doczj.com/doc/922251365.html,ke Taihu is frequently subjected to strong winds,which cause short-term ?uctuation in CDOM concentrations in the overlying water,and in turn affect quantitative remote sensing of water quality parameters and primary productivity.Acknowledgements
The authors thank the Taihu Laboratory for Lake Ecosystem Research.
Funding
This work was supported by the National Natural Science Foundation of China [grant number 41230744],[grant number 41471021],[grant number 41101458];the Key Program of Nanjing Insti-tute of Geography and Limnology,Chinese Academy of Sciences [grant number
154T.Wu et al.
D o w n l o a d e d b y [G r a d u a t e S c h o o l ] a t 17:37 20 A p r i l 2015
NIGLAS2012135003];the External Cooperation Program of the Chinese Academy of Sciences [grant number GJHZ1214];the Open Foundation of State Key Laboratory [grant number 2012491511];the Major Projects for National Science and Technology Development [grant number 2012ZX07101-007-005].
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