Effects Of Eutrofication And Changing Water Level On Biomass Of Submerged Macrophytes

Three microcosm experiments were conducted to study the effects of the shading on water surface and shortening of the inundation period on biomass and production of Vallisneria species. 1) Above- and below-ground biomass was significantly reduced at treatments above 50% shading. There were significant delays in first pistillate and staminate florescence dates above 75% and 50% shading, respectively. There were no significant differences in the ratio of mature to unripe fruits produced (both in number or dry weight) between different shading treatments, but there were significant reductions in dry weight fruit production at 90% shading. 2) Individual below-ground tuber biomass increased significantly with length of growing season, but their number did not. Significantly more above-ground tubers were initiated by the ramets in the later harvest, but these showed no difference in dry weight biomass. Fewer ramets initiated sexual reproduction by the time of the September harvest compared to those of the October harvest, but differences in this and dry weight production of fruits failed to reach statistical significance. However, almost all October fruits were mature, when the ratio of mature to unripe fruits was significantly higher than in September. 3) Above and below ground biomass generally increased with length of growing season. There were no significant differences between early and late harvest measures for each start date, except for the earliest planting date, where biomass was significantly less in November compared to October. There were no significant differences in the dry weight production of fruits between any of the treatments, nor in the ratio of unripe to mature fruits produced in each treatment. There was however a significant increase in the dry weight of mature fruits with length of growing season.Based on these results, we contend that in Yangtze River ephemeral wetlands 1) the recent expansion in the extent of floating water chestnut Trapa spp. at could (by shading) have contributed to the reduction in annual biomass production and seed production of Vallisneriaand contributed to declines in its distribution and abundance; 2) Shortening of the inundation periods experienced by Vallisneria spinulosa plants will reduce the dry weight biomass of below-ground tubers and the production of above-ground tubers; 3) Shortening of the inundation periods experienced by Vallisnaria natans plants will reduce annual biomass production, and potentially reduce investment in seed production in this important submerged macrophyte.