The Preliminary Analysis Of The Effects Of Nutrient, Grazer And Temperature Increase On Submerged Macrophytes And Their Epiphyton

Macrophytes are important structuring components in freshwater ecosystems as they contribute to establish and maintain clear water conditions. Epiphyton growing on the surface of leaves of macrophytes were closely connected with macrophytes. The role of macrophytes has been intensively studied while the effects of epiphyton on freshwater ecosystem are still vague. Our study aims to investigate the relationship between eutrophication and submerged macrophytes as well as epiphyton, and to explore how snail densities influence the complexity of submerged macrophytes and epiphyton. In addition, we also investigate the effects of global warming on snail-epiphyton-macrophyte relationship. Based on different scales of natural or artificial ecosystems, such as microcosm, mesocosm, small pond and lakes, we studied the effects of nutrient, grazer density and temperature increase on submerged macrophytes and their epiphyton:We investigate the direct effects of eutrophication on two submerged macrophytes Vallisneria spinulosa and Ceratophyllum demersum as well as their epiphyton. We found the biomass of epiphyton could reach the level of 2000μg chla g’1 (plant fresh weight) or above, which indicated that the shading of periphyton would be significant when lake revegtation was implemented under high nutrient concentration.When the phosphorus increased from 30μg L-1 to 800 μg·L-1, the epiphyton biomass of V. spinulosa grew as an unimodal curve, while that of C. demersum achieved the maximum in the relatively low nutrient concentration. Based on the meta-analysis, we found the relative growth rate of epiphyton through the process of eutrophication were dependent on leaf structure of submerged macrophyte, or more possibly, on the interaction between grazers and leaf structure.Snails, as the main and primary consumer, significantly affect the freshwater ecosystems, and snail densities were one of the most important factors which determined the growth of macrophytes and their epiphyton. In our study we set up four snail densities to investigate the grazing impact of snails on submerged macrophytes and epiphyton, and also the feedback of snails from the interaction. We found snails could strongly graze the epiphyton, and therefore promoted the biomass of the macrophytes, induced higher ramet number, and especially for higher densities. The direct grazing effects of snails on macrophytes were not significant, and submerged macrophyte alone could not support the high densities of snails. We also found that snails responded to the water nutrient level and food resource, and change the size structure of population.Warming may affect snail-periphyton-macrophyte relationships in lakes with implications also for water clarity. We explored the warming effects on snail-epiphyton-macrophyte relationship in two different systems:eutrophic microcosms and oligotrophic mesocosms. In the eutrophic system, when with snails, the biomass and length of V. spinulosa leaves increased more at the high temperature, and at both temperatures growth was higher than in absence of snails. The biomass of periphyton on V. spinulosa as well as on artificial plants was higher at the highest temperature in the absence but not in the presence of snails. The biomass of P. crispus (in a decaying state) declined in all treatments and was not affected by temperature or snails. While total snail biomass did not differ between temperatures, lower abundance of adults (size>1 cm) was observed at the high temperatures. In the oligotrophic system, we found higher periphyton on artificial strips in the higher temperatures, while epiphyton, macrophyte and snails biomass were not promoted by warmer conditions. The snail biomass was only correlated with epiphyton biomass, not with periphyton or macrophytes biomass. However, epiphyton biomass was not correlated with macrophytes biomass. Due to the insignificant effects of epiphyton on submerged macrophytes in low and clear water system, we also investigate the direct effects of temperature increase on macrophyte growth through a plant exchange experiment in a low nutrient system. After 10 years pre-treament under three different temperautres, plants are transferred to two heated scenarios, which are ambient+3℃ (T2) and ambient+5℃(T3). E. canadensis from T2 pretreatment had longer length and higher biomass than that from ambient pre-treatment. P. crispus from T2 and T3 pre-treatment had shorter length but more turions than those from ambient pretreatments. We did not discern any significance between the three pre-treatment, but P. crispus growing in T2 heated scenario had higher leaf biomass to total biomass ratio than that in T3 heated scenario.In the process of eutrophication, the growth of epiphyton increased significantly, and cascade effects from snails to macrophytes were gradually dominated in the chains of snail-epiphyton-macrophyte, and at a certain nutrient level, the role of snails would dominate in the chains, and via grazing on epiphyton, strongly simulated the macrophyte growth. In addition, the role of snails through the grazing chain would be more important in the scenario of the global warming.