Evolutionary Process And Identification Of Dominant Environmental Factors Of Algae In Erhai Lake

Eutrophication of freshwater resources due to nutrient enrichment has become a global phenomenon,of which algal blooms are a prominent sign.Algal plants play a very important role in aquatic ecosystems as primary producers of aquatic environments.chlorophyll a(Chla)is a valuable and feasible indicator of algal bloom in lakes and is directly associated with phytoplankton growth.Past evidence show that variation in climate cycles may increase Chla and promote algal bools in lakes.However,individual or cumulative response of climate variation and nutrient concentration on growth of phytoplankton is still questionable.In lake ecosystems,regime shifts can induce changes in physical and biological components,particularly phytoplankton community structure,as well as across multiple trophic levels.Phytoplankton community succession may be driven by different environmental factors before and after regime shift;however,owing to the abruptness of the regime shift,long-term observations encompassing both periods are scarce,limiting our ability to identify the shift of key drivers of phytoplankton community succession.For the above questions,this study analyzed the phytoplankton data(including phytoplankton data from 1997 to 2008)based on 30 years(1988 ～ 2018)of long time series environmental monitoring data from the Erhai Lake.Trends in environmental monitoring data were first analyzed,while a parametric data-driven regression model(GAM)was used to investigate the relationship between chlorophyll a and environmental factors.To verify whether a regime shift occurred between 2001 and 2003,an abrupt change analysis combined with biodiversity analysis was used.To investigate the key environmental factors driving phytoplankton community succession in Erhai Lake before and after the regime shift,phytoplankton functional groups were used and used for correlation analysis.The results of the study show that there are cyclical fluctuations in the climate data,with an overall increase of 1.35 °C in temperature and a 30% decrease in wind speed.In the early 20 th century,the levels of TN and TP showed a significant increasing trend.The dynamics of climatic factors and the increase of nutrients further promote the growth and reproduction of algal plants,which can also be shown by the trend of chlorophyll a.GAM analysis showed a very strong relationship of Chla with TN(P<0.001)during all seasons,TP(P<0.05)in winter only,while mean temperature(P<0.001)in winter season.Under the interaction of climatic factors and nutrients,algal plants proliferate and algal blooms occur frequently.The dynamics of hydrochemical parameters and phytoplankton community composition indicated a distinct regime shift between 2001 and 2003.The phytoplankton community evolved over a decadal period toward community structure simplification,dominance of individual species,and homogenization of species composition.Prior to the regime shift(1997–2000),the eutrophic status of Erhai Lake was oligotrophic,and the phytoplankton community was characterized by high biodiversity and low biomass,with Cyanophyta,Bacillariophyta,and Cryptophyta being the dominant species.Nutrient concentrations had a significant limiting effect on phytoplankton,with total nitrogen concentration being the primary limiting factor.Following the regime shift(2005–2008),the phytoplankton community exhibited low biodiversity and high biomass,with the dominant taxa shifting to Cyanophyta,Chlorophyta,and Bacillariophyta.Light became the primary driver of phytoplankton community succession.The study was based on long-term observations covering regime shifts in Erhai Lake,providing strong evidence that the phytoplankton community is influenced by nutrients in oligotrophic status.However,as nutrient concentrations increase,especially once the threshold is exceeded,species interactions may shift from competition for nutrients to competition for light.This implies that the resource competition theory,which integrates nutrient-and light-based approaches,provides an effective approach to predicting phytoplankton community succession,and that measures to improve underwater light environments should be considered as an additional option for nutrient load reduction in eutrophic lakes.