The Effects Of Nutrients And Snails Grazing On Growth Of Submergerded Macrophytes (Vallisneria Spiralis)

In order to study the mechanisms involved in the decline of submerged macrophytes for eutrophication in Meiliang Bay of Taihu Lake,and to further provide theoretic bases for their restoration,this study assessed the effects of nutrient content,nutrient ratio and Bellamya sp.dense on the growth and morphological index of submerged macrophytes.Based on the nutrient level in Meiliang Bay of Lake Taihu,The effects of nutrients concentration on Growth of Vallisneria spiralis were sdudied in controllable outdoor experiment. Results show that:1)Vallisneria spiralis can grow in those water bodies if the light compensation point can be satisfied and without other macrophytes existing.2)The growth rate of Vallisneria spiralis showed a decreasing response to an increase in nutrient concentration,and the Vallisneria spiralis biomass began to decreased only when the nutrient level exceed 10mgTN/l and 0.4mgTP/l; there was little changes in Vallisneria spiralis leaves,otherwise,rhizome biomass decreased following the increasing of nutrients,and all morphological date,except leaf length,decreased significantly when nutrients concentration exceed 10mgTN/l and 0.4mgTP/l.In an outdoor controllable experiment,the effects of different TN:TP ratios(12.5:1,25:1,and 50:1)in eutrophic water on the growth of Vallisneria spiralis were investigated.When the TN:TP ratio was 25:1,Vallisneria spiralis had the best growth,with a biomass increment being 54%and 31%higher than that when the TN:TP ratio was 12.5:1 and 50:1,respectively,while the growth of periphyton,especially of epiphytic algae,was the worst,giving the least adverse effects on Vallisneria spiralis,indicating that a 25:1 of TN:TP ratio in eutrophic water was optimal for the growth of Vallisneria spiralis.However,such an optimal TN:TP ratio only worked when the nutrients concentration in eutrophic water was below the limited level,otherwise,it could be failed to be applicable.The effects of Bellamya sp.on Vallisneria spiralis was studied during Sept to Oct,2006. Results showed,there was no obvious change in plant biomass in control treatment,otherwise,the plant biomass increased significantly in other treatments,and showed positive response to an increase of density of snails,the growth ratio of plant biomass in 20,40 and 80 snails treatment are 4.5,5.3 and 6.2 times higher than in control treatment respectively.Besides,rhizome biomass of Vallisneria spiralis in treatments with snails notably higher than that in treatment without snail, while it was reverse for the fibre.Contrarily,periphyton biomass in treatment with 20,40 and 80 snails were significantly lower 33.3%,54.8%and 53.7%than in treatment without snail.