Studies On Restoration Of Aquatic Vegetation And Engineering Practice In Polluted Urban Waterbodies

Eutrophication is a general character of the polluted waterbody (lake). To reestablish its aquatic vegetation is the key step of ecology rehabilitation. In an eutrophic lake, the difficulty in reestablishing aquatic vegetation is that those pioneer plants can settle down and live on the basin of lake. Three questions must be carefully dealt with during the process. First, selecting right pioneer plants. Second, improving the water quality. Third, using right tactics and methods to plant plants. Thus, Lake Yuehu and Lake Lianhuahu which locate in the Hanyang district, Wuhan were chosen as study objects, and the studies on restoration of aquatic vegetation and engineering practice were carried out in three levels---the lab, the lake enclosure and the lake itself. 1. Yuehu Lake and Lianhuahu Lake whose areas are respectively 0.66km2 and 0.085 km2, both consist of bigger and smaller parts with the average depth of less than 1.5m. They are hypereutrophic small shallow lakes. Accroding to our investigation, there were no other aquatic macrophytes, except some lotus in bigger Lake Lianhuahu, while there were many fishes in the two lakes before this project was carried out. From June 2003 to April 2005, the average concentrations of TN (total nitrogen) and TP (total phosphorus) were respectively 4.833 and 0.448 mg/L in the bigger Lake Yuehu, 9.920 and 0.490 mg/L in the smaller Lake Yuehu, 8.284 and 0.650 mg/L in the bigger Lake Lianhuahu, 2.771 and 0.198 mg/L in the smaller Lake Lianhuahu. The concentrations of heavy metal Cd,Cr,Pb,Cu and Zn were low in the water of the two lakes, but high in the sediment of the two lakes, especially Cd exceeded markedly the â…¢class standard of our country. 2. Under the stress of Hg²⁺, Cd²⁺ and (Hg²⁺+Cd²⁺), standing stock, dissolved protein concentration, chlorophyll content, chlorophyll a/b ratio, net productivity and respiration strength of Elodea nuttallii and Vallisneria spiralis depressed with the increase of the ion concentrations in 3d. The toxicity of Hg²⁺ on two plants is much bigger than Cd²⁺. The stress resistance of E. nuttallii to Hg²⁺, Cd²⁺ and (Hg²⁺+Cd²⁺) was very good, and clearly better than V. spiralis. V. spiralis would die in 3d under the Hstress of 10μmol/L Hg2+ or (Hg2++Cd2+). 3. Under the stress of NH4+-N or NO3--N, standing stock of E. nuttallii, V. spiralis and Potamogeton crispu could positively increase in 3d when [NH4+-N] =6.25, =1.56 and <1.56 mg/L, or [NO3--N] <100, ≤100 and ≤200mg/L, respectively. Under the stress of high concentration NH4+-N, V. spiralis and P. crispu would suffer serious injury in 3d, and the grade was V. spiralis>P. crispu>E. nuttallii. It means the stress resistance of the plants to NH4+-N is E. nuttallii >P. crispu >V. spiralis. While the stress resistance of them to NO3--N was P. crispu≈V. spiralis>E. nuttallii. There weren't obvious injury symptoms for the plants under the stress of high concentration NO3--N. The toxicity of NH4+-N on the plants was much bigger than NO3--N. 4. Under the stress of PO43--P, standing stock of E. nuttallii and V. spiralis could remain positive increase in 3d when [PO43--P] =0.8 mg/L, and =12.8mg/L for P. crispu. The stress resistance of the plants to PO43--P was P. crispu>E. nuttallii≈V. spiralis. There weren't serious injury symptoms in 3d for the plants under the stress of high concentration PO43--P. 5. Under the stress of (NH4+-N+PO43--P) (N/P=7.8125), standing stock of E. nuttallii, V. spiralis and P. crispu could remain positive increase in 3d when [NH4+-N] + [PO43--P] =7.05 mg/L. Under the stress of high concentration (NH4+-N+PO43--P), the plants will suffer injury in 3d. The stress resistance of the plants to (NH4+-N+PO43--P) was P. crispu≈E. nuttallii > V. spiralis. 6. Under the stress of (NO3--N+PO43--P) (N/P=7.8125), standing stock of E. nuttallii and V. spiralis could remain positive increase in 3d when [NO3--N] + [PO43--P] =28.2 mg/L, and =225.6 mg/L for P. crispu. There weren't serious injury symptoms in 3d for the plants under the stress of high concentration (NO3--N+PO43--P). The stress resistance of the plants to (NO3--N+PO43--P) was P. crispu > V. spiralis ≈E. nuttallii. The toxicity of (NH4+-N+PO43--P) on the plants was much bigger than (NO3--N+PO43--P). 7. Under 4800lx light intensity and day/night =1/1, the growth speed and the optimum temperature of E. nuttallii and Ceratophyllum demersum depressed with the time prolongation, E. nuttallii and C. demersum could keep positive growth respectively 15d and 5d under 39℃, E. nuttallii grew faster and the heat-resistance ability was stronger than C. demersum in 1/5 Hoaglands cultivate solution. While, under the same light, and in tap water with sediment from Lake Yuehu, E. nuttallii and C. demersum could keep positive growth respectively 25d and>25d under 39℃,and the average growth speed of C. demersum was 4.54 times as E. nuttallii. The heat-resistance abilities of them were highly increased. 8. A week after the treatment with simulation wave, adsorbability of eight species submerged macrophytes on SS (suspended substance) could be divided into two groups. The better group consisted of six species---E. nuttallii, P. crispu, Hydrilla verticillata, Myriophyllum spicatum, P. malaianus and Najas graminea. The worse group consisted of two species---V. spiralis and C. demersum. While sedimentation effects of the plants on SS could be divided into three groups. The best group consisted of P. crispu and H. verticillata. The worst group consisted of E. nuttallii, M. spicatum, N. graminea, C. demersum and P. malaianus. V. spiralis were in the middle of two groups. 9. The establishment of small enclosures (5×5m2) in Lake Yuehu could result in the difference distinctively of [TN] and [TP] both inside and outside. Algal bloom appeared more frequently and worse in the enclosures. Aeration on the bottom of waters, placement floating beds with Alternanthera philoxeroides or planting Spirodela polyrhiza could control algal bloom, and the best way was to plant S. polyrhiza. By aerating on the bottom of waters for 13 months (2³ h/d), [TN] depressed markedly, while [TP] and [CODCr] didn't change markedly. Aeration could depress Chl a (chlorophyll a) content, and increase transparency because it could restrain algal bloom during the time of algal bloom, but it had no effects on [chl a] and transparency when there was no algal bloom. Through 11 months experiment, [Chl a] and [CODCr] could be depressed, and transparency could be increased, while [TN] and [TP] couldn't be changed markedly by placing 2m2 AquaMats which a kind of ecology purification mat into the enclosures. From July to November 2004, water quality wasn't improved in the enclosures, [TP], [CODCr] and transparency didn't change markedly too in a big enclosure which was built in Lake Yuehu and the area is 5000m2, but [TN] and [Chl a] depressed in the big enclosure by dousing LLMO (liquid live micro organisms, from USA) into the small and the big enclosures nine times. In spring, after all steps (except planting S. polyrhiza and dousing LLMO) what introduced above were done for 2⁶ weeks, the seeds of V. spiralis and the shoots of E. nuttallii were planted into the enclosures, but only a few plants were alive. Up to winter, E. nuttallii were planted and could live in every enclosure with some steps (such as aeration, aeration+AquaMats, dousing LLMO and planting S. polyrhiza) done. E. nuttallii grew very well in the big enclosure and the enclosures which had planted S. polyrhiza, the bad effect was found in small enclosures by dousing LLMO.10. Pollution-cutting and water level control were two important qualifications to restore aquatic vegetation in eutrophic lake. In general, water level is low and transparency is high correspondingly in winter. So, planting right aquatic plants in winter is a good strategy. To clean out fishes was effective on improving transparency in winter. Through biomanipulation, controling water level, purifying water by artificial wetland, using right planting strategies and techniques,et al, the aquatic vegetation could be restored in the hypereutrophic shallow lakes. The coverage and total biomass were respectively 95% and 66.64 ton in small Lake Lianhuahu, April 2005; 80% and 118.5 ton in small Lake Yuehu, May 2005; 45% and 810.4 ton in big Lake Yuehu, May 2005. It won primary success in restoration of aquatic vegetation in these lakes. During the time, [Chl a] depressed and transparency increased markedly, but [TN] and [TP] didn't change markedly in Lake Yuehu because pollution-cutting didn't achieve thoroughly.