Unraveling The Optical Characteristics,Sources And Bioavailability Of Organic Matter In Lake Taihu,China

Lake Taihu,the third largest fresh water lake in China,is located in the Yangtze River Delta.Due to its location in the most developed region in China,Lake Taihu receives excessive nutrient input from the upstream watershed,and algal blooms have appeared persistently in the summer for the last three decades.The distributions,sources and compositions of OM in Lake Taihu are extremely complex,which is related to that Lake Taihu has the complex river networks,the algal-dominated lake and the macrophyte-dominated lake,algal blooms in summer and sediment resuspension caused by strong wind and wave disturbance.In this paper,the sources,compositions,and bioavailability of OM in Lake Taihu,a large eutrophic,shallow lake in China are discussed.The main findings are as follows:1.Based on the Geostatistics analysis,we studied the sources and occurrence form of nitrogen and phosphorus in different area of Lake Taihu,a large shallow and eutrophic lake in China,and the spatial distribution of the type of nutrients-restricted and its reason by monitoring from August 2014 to May 2015 in seasons.The results showed that:?1?the concentration of soluble nitrogen and phosphorus was higher in winter than other seasons,and decreased gradually from northwest to southeast in spatial.The concentration of particulate nitrogen,phosphorus and chlorophyll a was highest in summer,and the high-concentration regions in winter and other seasons were located in south and northwest part of lake,respectively.?2?The composition of nitrogen and phosphorus in algae-and macrophyte-dominated regions changed greatly with the seasonal change.The algae-dominated regions were dominated by nitrate nitrogen and organic phosphorus in winter and particle nitrogen and phosphorus in other seasons.However,the macrophyte-dominated region was dominated by particle nitrogen and phosphorus in winter and ammonia and organic nitrogen and phosphorus in other seasons.?3?The ratios of total nitrogen to total phosphorus in the algae-dominated region were larger than 16 in autumn and winter and decreased to less than 16 in summer and spring.On the contrary,the ratios of total nitrogen to total phosphorus in the macrophyte-dominated region increased from less than 16 in autumn and winter to more than 16:1 in summer and spring.In the algae-dominated region,the character of spatial variation in dissolved N/P ratio was consistent with the ratio of total nitrogen to total phosphorus.In the macrophyte-dominated region,the dissolved N/P ratio increased from less than 16 in autumn to more than 16 in summer,winter and spring.The temporal-and spatial-variation of the ratio of particulate nitrogen to phosphorus was not significant?P>0.05?,and this ratio was less and larger than 16 in algae-and macrophyte-dominated regions,respectively.2.The formations,transformations,and fates of particulate?POM?and dissolved organic matter?DOM?,and their accompanying nutrient regeneration processes are key links in the biogeochemical cycling of nutrients in aquatic ecosystems.Furthermore POM and DOM,as the providers of nutrients,also play very significant roles during the outbreak of algal blooms in inland lakes.To characterize the similarities and differences of the spatial-temporal variations of the optical components and sources between POM and DOM,we applied the combined POM-DOM PARAFAC model and geostatistics method to analyze the outbreaks of problematic cyanobacterial blooms in Lake Taihu between August2014 and February 2015.Two extensive sampling campaigns were conducted at the same sites?n=34?in Lake Taihu in August of 2014 and February of 2015,respectively,and a total of 68?34×2?samples were collected.In summer and winter,four kinds of fluorescence peaks were found in DOM.However,there were obvious seasonal differences in the types of POM fluorescence peaks.The POM had 6 kinds of fluorescence peaks?peak B,D,M,C,A and U?in summer and 5 kinds of fluorescence peaks?peak T,D,M,C and A?in winter.The excitation wavelength of peak D of POM in summer had an obvious“blue shift”phenomenon,indicating that the molecular weight,condensation degree and aromaticity of the tyrosine fluorescence of POM in summer were lower than those of other seasonal organic matter.There were five kinds of fluorophores of POM and DOM,which were tyrosine-like fluorophores?C1 and C2?,humic-like fluorophores?C3 and C4?and tryptophan-like fluorophores?C5?.There was strong spatial-temporal variation in fluorescent characteristics of OM in Lake Taihu.The POM pool was dominated by C5p and C3p,with peaks in summer and winter,respectively.Concerning spatial variation of the relative amount of C1-C5 of POM,maximum values normally occurred in the Zhushan Bay in summer,and maximum values were normally confined to the central lake.In contrast,the DOM was dominated by C1,C2 and C5 during all two seasons.For DOM,the fluorescence intensity in Zhushan Bay was significantly higher than that in other areas.The algae-dominated region had a higher relative contribution of tryptophan-like components of POM than the macrophyte-dominated region in summer and winter?t test,P<0.01;t test,P<0.01?,respectively.The DOM in winter approached the characteristics of autochthonous material.The DOM in summer and the POM in all two seasons simultaneously had both autochthonous and exogenous characteristics,among which,the POM in summer showed strong autochthonous characteristics.3.The migration and transformation of organic matter and the accompanying regeneration of nutrients should be the main"fresh forces"of rapid supply of nutrients during the outbreak of algal blooms.The phytoplankton-derived particulate organic matter was taken in Lake Taihu as the breakthrough point.We studied the changing process of C,N and P elements in the degradation process of phytoplankton-derived particulate organic matter under the different conditions?light and dark?.The new algal grew along with the degradation of the phytoplankton-derived particulate organic matter under the light condition,absorbing C,N and P elements in the water and converted to biomass.The light had a significant impact on the degradation of C,N and P,for the degradation rates of C,N and P in the dark were twice than in the light,and the proportion of degradable in the dark was 2.5 times of that in the light.The net temporal decreases in total fluorescence intensity of colored dissolved organic matter?CDOM?were 37.25%and 56.62%in the light and dark treatments,respectively.Principal component analysis showed that the CDOM in dark controls was more resistant to further degradation than that in the light.The tryptophan-like C1 to the tyrosine-like C2 ratio decreased with C1:C4 ratio increasing during the 14 d light exposure,indicating that the tryptophan-like C1 was more photo resistant than the tryptophan-like C4,but less than the tyrosine-like C2.We found a significant correlation of NH₄⁺photoproduction with DOC concentration in the light treatment,indicating that higher photoammonification may be expected from more ambient DOC.