The Fate Of Nutrients Released During The Decomposing Of Cyanobacterial Detritus In Submerged Macrophytes Zones

With the rapid development of the economy, human activities impose tremendous impacts on lake ecosystem. The most common and serious problems are eutrophication and cyanobacterial bloom. Cyanobacterial bloom commonly accumulated in the downwind littoral zones of eutrophic lake driven by the wind and in where they would die-off and become detritus, and then the nutrient which concluded nitrogen, phosphorus and organic carbon would be released during the decomposing of cyanobacterial detritus. The form and concentration of nutrient in water changed dramatically during this process.The littoral zones with submerged macrophytes of the lakes are ecotone between water and land. On one hand, the submerged macrophytes would reduce the impact of nutrient released by cyanobacterial detritus on aquatic systems. On the other hand, submerged macrophytes could utilize the nutrient originated from cyanobacterial detritus.In order to study the decomposition of cyanobacterial detritus, cyanobacterial blooms was harvested from Lake Taihu. The decomposition rate and the released forms of nitrogen and phosphorus from the decomposing cyanobacterial detritus were investigated experimentally at first. Each dried 0.1 g cyanobacterial detritus was added into one conical flask with 250ml filted water from Lake Taihu, and the concentration of Chl-a in each flask was 1600μg·L-1. And then we investigate the forms and concentration of nutrient which were released during the decomposing cyanobacterial detritus in water of ecosystems with submerged macrophytes Vallisneria spiralis or Elodea nuttallii. At last, the cyanobacteria which harvested from algae bloom of Lake Taihu was labeled by 15NH4+-N and dried to produce detritus, and then the cyanobacterial detritus were added to water of systems with submerged macrophytes Vallisneria spiralis or Elodea nuttallii. We studied whether the submerged macrophytes could uptook and assimilated the nitrogen released during the decomposing of cyanobacterial detritus through investigated the excess 15N in different tissues of submerged macrophytes.(1)The decomposition rate of cyanobacterial detritus was high and 41.9% dry mass was lost in two days. The release rate of total phosphorus of cyanobacterial detritus was faster than total nitrogen, but the increase of concentration of total dissolved nitrogen (TDN) in the water column lasted longer than total dissolved phosphorus (TDP) during the decomposing of cyanobacteria. After the first four days of increase, the concentration of dissolved inorganic nitrogen (DIN) was kept constant and the concentration of dissolved organic nitrogen (DON) continued to increase. The ammonia (NHV4+-N) was the main form of DIN in the water column, while the concentration of nitrate (NO3-N) and nitrite (NO2-N) was low during the decomposition. The Urea-N was not the main form of DON released by cyanobacterial detritus because the average percentage of Urea-N was low (3% of DON). The concentration of dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) increased in fist four days, and then DOP was transformed into DIP progressively. The present study demonstrated that the decomposition rate of cyanobacterial detritus is high, and decomposing cyanobacteria release organic and inorganic nitrogen and phosphorus.(2)The submerged macrophytes could reduce the concentration of total nitrogen in water through reduce the ammonia (NH4+-N). The concentration of DIP decreased in control group and submerged macrophytes groups, but the concentration of particle phosphorus (PP) decreased only in submerged macrophytes groups. The concentration of total nitrogen returned to the level before adding cyanobacterial detritus in submerged macrophytes groups but became higher in control group after the cyanobacterial detritus were added in the systems. But the concentration of phosphorus returned to the level before adding cyanobacterial detritus in both submerged macrophytes groups and control group. So the presence of submerged macrophytes could effectively reduce nutrient released from the decomposing cyanobacterial detritus.(3)We found that theδ15N in roots, leaves of submerged macrophytes Vallisneria spiralis or Elodea nuttallii increased with time. It suggested that the nitrogen released from the decomposing of cyanobacterial detritus rapid uptook and assimilated by the submerged macrophytes. The capacity to retain nitrogen derived from cyanobacterial detritus:the leaves of Elodea nuttallii>the leaves of Vallisneria spiralis>the roots Elodea nuttallii>the roots of Vallisneria spiralis. The submerged macrophytes could uptook the nutrient released duting the decomposing of cyanobacterial detritus, and that might be a reason why submerged macrophytes could reduce the nutrient level rapidly.This study showed that the submerged macrophytes could decrease the concentration of nutrient which released during the decomposing of cyanobacterial detritus and the nitrogen released during the decomposing of cyanobacterial detritus could uptook by submerged macrophytes.