The Role Of Periphyton In Shallow Lakes Regime Shifts

Shallow lakes are the most abundant lentic ecosystems. They have high environmental and economical values. They provide resources and maintain ecological environment. However, in recent years most of the urban and suburban shallow lakes have undergone accelerated eutrophication owing to economic developmemt. As the representative urban lake, Donghu Lake was a eutrophic, subtropical shallow lake in Central China. High periphyton biomass was apparent on stones in the littoral zones of Donghu Lake. Periphyton communities had received relatively little attention in lake ecosystems. However, evidence was increasing that they played a key role in primary productivity, nutrient cycling, and food web interactions. The lake-macrocosm and Laboratory-scale experiments were conducted from March 2012 to October 2013. The lake-macrocosm experiment was to investigate: 1) the environmental factors and biomass of periphyton. 2) The contents of total phosphorus and forms of periphyton. 3) The patterns of benthic diatoms and their response to the environmental factors. Laboratory-scale experiments were conducted to asses the water quality parameters, the chlorophyll a of phytoplankton, V. natan, periphyton and the physiological activity of V. natan.The macrocosm experiments were conducted from March 2012 to February 2013. The main results are as follows: The Chl a and Ca-P contents of benthic algae mat in the macrocosm were higher than those in control, but Fe/Al-P content of benthic algae mat in macrocosms was lower. We concluded that the benthic algae mats in macrocosms could uptake more nutrients and the retained phosphorus forms of benthic algae mat in the macrocosms were more beneficial to the restoration of submerged macrophytes in shallow eutrophic lakes. Water quality was remarkably improved in macrocosms. The chemical factors in macrocosms were significant lower than those in control. The Chl a values of were lower in macrocosms. Lake-Macrocosms related to lake water characteristic changes were having a positive effect on the biodiversity and functioning of ecosystems. Lake-macrocosms revealed that Donghu Lake could switch from a turbid state dominated by phytoplankton toa clear-water macrophyte dominated state when the TP contents in the water were ranged from 0.02-0.05 mg/L.A total of 148 diatom taxa were recorded across one year in all sites, and the dominant benthic diatom communities in the macrocosms were higher than those in control. They were changed with different seasons in all sites. The average diatom densities in macrocosms and control were 8.4 × 109 and 2.1× 1010 cells/m2, respectively. Achnanthes minutissima was the dominant diatom in all sites with a maximum abundance of 65%. Diatom taxa, which were abundant and associated with good quality environmental variables in macrocosms, were Cymbella gracilis, Cymbella affinis, Gomphonema truncatum, Gomphonema olivaceum. Besides, the dominant species observed in macrocosms, such as Cocconeis placentula and Rhopalodia gibba indicated the restoration of submerged macrophytes. Nitzschia palea, Gomphonema parvulum, Cymbella ventricosa and Navicula cryptocehala only found in control were especially tolerant of the poor water quality conditions. CCA analyses indicated that the variation in benthic diatom communities was strongly related to phosphorus, nitrogen in macrocosms and pH, TN in control. This study indicated that Lake-Macrocosms related to lake water characteristic changes were having a positive effect on the biodiversity and functioning of ecosystems.Laboratory-scale experiments were performed to develop a procedure of piggery wastewater using the periphyton and the submerged macrophyte-V. natan. The results showed that the maximum Chemical Oxygen Demand(COD) removal rates were more than 86% in high level groups. In the mixed systems, the average phosphorus(P) and nitrogen(N) removal rates were over 62 and 70% in high and medium nutrient groups, respectively. The average P and N removal rates were less than 59 and 65% in the separate treating groups, respectively. Periphyton and V. natan illustrated significant phosphorus retention capacity. Total phosphorus of periphyton and V. natan was retained mainly in inorganic forms. In addition, the variations of physiological activity indicated that V. natan suffered a short-term adversity stress in high level of piggery wastewater. However, the Chlorophyll a content of V. natan was low, and the malondialdehyde content was high in treatment groups without periphyton. In conclusion, the treatment systems with high nutrients removed COD, N and P effectively. The medium nutrient with periphyton was the optimum condition for the restoration of V. natan.