Studies On The Community Structure And Ecological Function Of Macrozoobenthos In Lake Nanhu, Wuhan City, China

Macrozoobenthos is an important part of the lake ecosystem which has many ecology functions, such as accelerating the decomposition of the organics detritus, adjusting the substantial exchange of mud-water microcosms and promoting the self-cleaning of the water bodies. Meanwhile, macrozoobenthos itself is a significant link in the food chain in the lake ecosystem. The density of Chironomid larvae in the eutrophic lake is high. They can consume a large amout of organics detritus before the adult emergence and fly away to land. As the cleaner of nitrogen and phosphorus contained in sediments, macrozoobenthos broke a new path to clean nurition accumulated at the bottom of the lake. On the other hand, macrozoobenthos can be ingested by fish through food chain and then fished out of the water bodies, which makes another way to clean nitrogen and phosphorus in lakes.Since 1980s, as the development of industry and urbanization and the booming popultion of human being as well as the inappropriate fishing, the eutrophic degree of lakes has become higher and higher. Consequently, the envrionment changed dramatically. What is the effect on macrozoobenthos brought by the changed envrionment and what will macrozoobenthos do as a response? Research on these problems is lacking. In order to find the answers to these questions, we have studied the community structure, temporal and spatial distribution of macrozoobenthos in Nanhu Lake, a shallow eutrophic lake. We have also studied the relation between the release, as well as reproducing of nitrogen and phosphorus and fish. The results of our study are as follow:1. We have collected 34 kinds of macrobenthos from the Nanhu Lake in total, which includes 7 kinds of Oligochaeta,12 kinds of Mollusca,12 kinds of aquatic insects and 3 kinds of other animals. Only Oligochaeta and aquatic insects larval have been found in the epilimnion while no living mollusk has been found. Oligochaeta was mainly composed by tolerant species, with an annually average density of 3802 ind/m2 and the maximum reached 16576 ind/m2. The preponderant Chironomid larva of aquatic insects was Tanypus punctipennis with an annually average density of 730 ind/m2. Next to T. punctipennis was Propsilocerus akamusi, with an annually average density of 50 ind/m2. From the aspect of seasonal distribution, the largest density of Limnodrilus hoffmeisteri was in May while the least in August and the largest density of both T. punctipennis and P. akamusi were in November.2. The largest depth of macrozoobenthos was 25 cm and the depth range of major distribution was 0～15cm. In 30cm sediment core, the content of Oligochaeta within the depth of 0-20cm took up 99.28% of the total and the percentage of Chironomid larvae was 99.31%. in 20cm sediment core, the amount of macrozoobenthos was above 99% of the total. The horizontal motion distance in 24h of B. purificata and B. aeruginosa are different from each other. The maximum distance of the former is 7 m and the average distance is 3.4 m, while the figures of the latter are 3.6 m and 2.4 m.3. The biodiversity of macrozoobenthos in the Nanhu Lake is comparatively low. The dominant species are Oligochaeta and Chironomid larvae and the biomass of the two are 176759.35kg and 46810.30kg, respectively. Next to Oligochaeta and Chironomid larvae are freshwater snails, the biomass of which is 899.34 kg. The total nitrogen and total phosphorus transferred or removed by Oligochaeta as well as Chironomid larvae and freshwater snails are 3463.61kg and 350.92 kg respectively. As a result, macrozoobenthos is an important approach to remove nitrogen and phosphorus in the Nanhu Lake. However, the total nitrogen of macrozoobenthos takes up only 2.49% of the whole water body, so the nitrogen and phosphorous removed by macrozoobenthos is very limit before sewage interception is completely realized in the Nanhu Lake.4. The average absolute fecundity, the maximum and the minimum number of embryo and the relative fecundity of B. aeruginosa are 63.97 eggs,169 eggs,4 eggs and 26.85 eggs/g. While the figures of B. purificata are 38.57 eggs,115 eggs,3 eggs and 23.28 eggs/g. B. aeruginosa begins to spawn from March while B. purificata begins from April. Radix auricularia begins to spawn from the beginning of March (water temperature 8～12℃), and the peak appears in March or April. After May, both the fecundity and the frequency of spawning decline. The most suitable water temperature for spawning is 16～24℃. R. auricularia can spawn several times at the peak of fecundity and the interval between two spawning is 1～10 days, but normally 1-3 days. Mature individuals may spawn 4-5 oocysts during the whole life and the amount of eggs of each oocyst vary from dozens to hundreds. The fertilization rate of R. auricularia normally is 95% to 100%. The average hatching rate is 95.28%, while the maximum is 100% and the minimum is 85.7%. Incubation time is highly related to water temperature. The higher the water temperature is, the shorter the incubation time will be. 5. The centralized adult emergence of Chironomid larvae appears twice a year and normally appears in April to May and November to December. There are some differences between both the individual fecundity and the structure of oocyst. The average individual absolute fecundity of T. punctipennis is 665.7 while the figure of P. akamusil is more than 1000.6. In the Nanhu Lake, within the depth range between 0cm and 10cm, the density of Oligochaeta and Chironomid larvae is highly related to the amount of aquatic plant residues. As the amount of aquatic plant residues declines, the density of Oligochaeta and Chironomid larvae declines, too. The relation between total nitrogen and sediment depth is not obvious. In terms of vertical distribution, there is a highly negative relation between total phosphorous of aquatic plant residues and sediment depth. That is to say, the earlier the sedimentary age is, the lower the total phosphorous will be. This relationship can illustrate the change of nutritional salts in the Nanhu Lake very well. Aquatic plant residues can serve as an evidence for sedimentology of lake.7. Fish can cause a top-down effect on food organisms (including benthic animal). Cyprinus carpio haematopterus and Carassius auratus auratus are compelled to change their reproductive behavior, changing their spawning place from the grass to the rocks along the shore of the lake. The rocks along the shore are not only the habitat and spawn place of snails, but also the spawn place of Chironomid and C. carpio haematopterus and C. auratus auratus. Meanwhile, they are also the hatching place of Chironomid larvae, fry of C. carpio haematopterus and C. auratus auratus. So there is overlap in terms of geographical distribution. Complicated assemblage is formed on the same rocks. Bellamy a occupies the top of the food chain, so it can affect the hatching rate of fishes's adhesive eggs and Chironomid larvae through predation, so as to affect the increase of both demersal fishes's and aquatic insects'population. Therefore, the effect on the breeding and resource enhancement of demersal fishes brought by freshwater snail cannot be ignored.