Modeling the species shift from Potamogeton malaianus Miq. to Potamogeton maackianus A. Bennett in the experiment pond

The outdoor pond experiment has been conducted from 2005 September till now, during which an interesting phenomenon of submerged macrophyte species shift was observed in 2007. A new macrophyte species, Potamogeton maackianus A. Bennett, appeared in 2007, and replaced the original species, Potamogeton malaianus Miq. gradually over 10months. In order to describe the submerged macrophyte species shift from Potamogeton malaianus to Potamogeton maackianus, a structural dynamic model had been developed by using eco-exergy index as the goal function, coupling the size functions of main biological components of the field experimental pond ecosystem. There were totally 5 size functions, including individual size of macrophyte, phytoplankton and zoobenthos, and leaf area index of macrophyte and photosynthetic area index of phytoplankton. The main physiological parameters (such as maximum growth rate, mortality rate and so on) of macrophyte, phytoplankton and zoobenthos had been characterized by the above concerned size functions. Eco-exergy index was optimized based on different combinations of these 5 size functions, which were changed ±5%, ±10%, ±15%, and ±20% every 10days during the simulation. The model yielded acceptable calibrations by comparing with the results from the pond experiments for the key state variables: total biomass of macrophyte, total dissolved nitrogen and phosphorous and organic nitrogen and phosphorous from September 22, 2005 to December 31, 2008. During the calibration period, the submerged macrophyte decreased in size gradually, while its leaf area index, maximum growth rate, biomass and the system eco-exergy index were all characterized with seasonal pattern, increasing from spring, reaching peak in summer and decreasing from autumn. The results showed that leaf area index was a meaningful parameter in the size functions and it determined the changes of eco-exergy index by translating into the biomass of the submerged macrophyte. The submerged macrophyte species shift happened in the experiment pond may due to the fitness of these two submerged macrophytes to the changes of the nutrients level. P. malaianus can be adaptable in higher nutrients water volume, while P. maackianus prefers lower nutrients environment. The nutrients content in the experimental pond was decreasing with time as to sustain the lives cycling. P. malaianus varied its size distributions to adapt the nutrients perturbations, afterwards when nutrients level decreased to some extent P. malaianus could not afford, P. maackianus took over rapidly enough to assure ecosystem stability and the ecosystem keep developing.