| Submerged aquatic vegetations(SAV) often disappeared in eutrophication lakes afternutrient loading. Then lead to the changes in the lake ecosystem. Reconstruction of SAV inthese lakes has become a hotspot. Restore the SAV in water bodies have been researched alot, but most of them were in relatively closed lakes, such as Wuli Lake. Few restoration ofSAV in an open water has been reported. This paper is focused on a shallow lake, GehuLake, a headwater of Taihu Lake. Because of its water quality was below to the ClassⅤofwater quality of China in2009, so it was named as “heavily polluted lake”. In order toconstruct the basic condition for SAV restoration, a flurry of multi-treatments was inprocess from2008to2011in north area of Gehu Lake. The multi-treatments were included:control the nutrition flowed into the northwest of Lake Gehu by pre-dam system, dredgingthe sediment from lake, purification the water by aquatic plants, nonclassicalbiomanipulation by Silver Carp and Bighead Crap, salvage of cyanobacteria bylarge-biomimetic machines, etc. Whether or not the basic conditions for SAV growth hadimproved hasn’t been analyzed. This work attempts to analyse the water quality andunderwater light climate as they were significant to rebulit a steady-state which isdominant by SAV, and then put forward some suggestions to restore the SAV in area I inthe following years.Compared with2009-2010, TP has decreased about63%in area I, but it was the limitingfactor in area I to reconstruction a steady-state in summer and autumn; however TN hasdecreased about24%in area I, it was also a limiting factor to re-establishment of SAV inautumn, winter and spring seasons.That underwater light climate monitoring showed that the optical properties of the wholelake had spatio-temporal variations before the engineering，so did after it. Underwater lightclimate in summer was also a limit factor to restore SAV in area I. A significantimprovement of underwater light climate in summer and winter in area I was found whencompared to the same period in July and Jan.2009, which was benefit for the growth ofSAV.Factors which affecting the underwater light climate in summer, autumn and winterpresented temporal and spatial changes.Underwater light climate in area I was better thanthe neighboring areas where were without multi-treatments(area II). The differencebetween area I and area II were varied with time caused by different factors. ISS, OSS, ISS, ISS and Chl.a, ISS were the reasons for which caused the difference between July, October,November, January, February in2013, respectively. Although CDOM had significantspatial and temporal variations, but it had little effect on the underwater light climate.Underwater light climate between two adjacent months also had some differences inarea I, which caused by diverse factors. The difference of Chl.a, ISS and Chl.a, ISS andCDOM, OSS in area I was the reason for caused the notable difference between July andAugust, September and October, December and January, January and February,respectively.Improving the water quality can also improve the underwater light climate, which willfavor the growth of SAV. At the same time, the underwater light climate in area I in fall andwinter seasons were significantly better than that in summer, so reconstruction of SAV inautumn and winter is recommend, and it can also prove a precursor light environment forSAV grow in spring. Myriophyllum spicatum L. is recommended to plant in area I for it cantolerance suspended matters. When SAV growth in virtuous circle throughout the lakeannually, it should have a reasonable coverage to comprise the steady-state need and otherdemands. |
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