Temporal And Spatial Characteristics Of Eutrophication And Diatom Communities In Erhai Lake In Recent 100 Years

Erhai is a large plateau lake in Northwest Yunnan and is characterized bycomplicated lake typology with environmental pressure from multiple stressorsincluding eutrophication and intensive human disturbances. All these can lead tospatial heterogeneity in lake environment and ecosystem structure. This study appliedhigh-resolution multi-proxy analyses of sediments from the three basins of Erhai (i.e.the north, south and central basins). Based on210Pb-inferred sediment dating andpigments records, we firstly reconstructed the eutrophication history for the differentparts of Erhai during the past100years. With the sedimentary diatom records, weidentified the historical variation of phytoplankton community structure for the threecores and the spatio-temporal patterns among the cores from the perspective ofbiodiversity. Finally we discussed the timing and spatial synchrony of regime shiftamong the three basins of Erhai in terms of ecosystem stability.(1) Based on the sedimentary pigment data, we found that there existed temporalasynchrony in the primary production for different regions of the lake because of theirdifference in lake typology and history of human disturbance. The south part (EH-1)is characterized of the shallow waters and being nearest to the urban area. The timingof the pigment increase is earlier than the other two parts of the lake with two spikingperiods. During ca.1920AD-1971AD, there is a rise of the pigment content for southregion lake, which indicates the beginning of nutrient enrichment. During1971AD-1990AD, the pigments content in the core generally decreased indicating aslow-down of lake eutrophication. From1990AD to now, the pigment content risedrapidly and exceed anytime in the history covered in this study, inferring anaccelerated eutrophication process. The pigment content for north and middle parts ofErhai obviously increased during1986AD-2000AD, suggesting that both regionsexperienced a rapid process of eutrophication at the time period. This can be due tothe massive input of sewage wastes and agricultural fertilizers and pesticides leadingto the enrichment of nutrient elements (i.e. N and P). In addition, average depth of thesouth basin is much shallower than the other two northern regions and the shallow basin is characterized by abundant benthic habitats that are suitable for macrophytegrowth and the algal bloom.(2) There existed strong spatial heterogeneity in the patterns of diatom communityresponses among the three lake basins. The occurence of eutrophic plankton diatomswas much earlier and experienced strong fluctuation in the south part of the lake. Theoligotrophic epiphytic and benthic diatoms were not found in the bottom of the corefrom the south but occurred earlier from the other two cores after the first peak ofplankton diatoms that were counted around1925AD. The eutrophication process inthe south basin speeded up after ca.1955AD. Diatom records from the north basinonly showed one shift from oligotrophication to eutrophication and were dominantedby oligotrophicbenthic diatoms. After around1979AD the eutrophic plantonicdiatoms became dominant suggesting the process of nutrient enrichment. The diatomcommunity response from the middle part of the lake was relatively straightforward:dominant taxa shifted from oligotrophic benthic diatom to eutrophic planktonicdiatoms from around1938AD onward implying a process of eutrophication.(3) There also existed temporal coherency for the primary producers reflected by thepigments content and diatom community structure, while the PCA and null model testsuggested that the underlying environmental gradients that controlled the diatomcommunity structure differed sigficiantly among the cores. In particular, the changeof diatom community structure in the south part was influenced by both trophic leveland coverage of submerged vegetation, but only the nutrient level was significant inthe both north and middle parts of Erhai Lake.(4) The diatom species diversity patterns among the cores are different according tothe analysis of diatom alpha diversity. The diatom species diversity in south regionwas the highest possibly due to its shallow depth and more available habitats fordiatoms, while the diatom species richness in the north basin is lower than that fromsouth part and middle region, the deepest part of the lake, displayed the lowest diatomdiversity probably as a result of its deep basin and relatively lower nutrient level.(5) The timing and frequency of regime shift also showed differences among thebasins. The south shallow region may have experienced the first regime shift as earlyas around1925AD and may have occurred shift at least3times since the past100 years. While north and middle parts only experienced the regime shift once and thetiming of each was found at around1986AD and2000AD respectively mainly basedon the diatom community records.(6) Over all, our sedimentary records showed that different parts of the lake havebeen driven by different environmental gradients and to a different degree, implyingthat spatial heteogeneity in water quality, habitat availability and the intensity andtypology of human acitvities. These are cleally evidenced in our analyses andcomaprision of diatom community structure and species diversity among the differentbasins of the lake. One important implication is that spatial heterogeity in lakeenvironment and ecological response in large plateau lakes can be significant and thispattern need to be taken into account in future research on limnology and lakesediments. Our results also provide insight into the history of eutrophication andenvironmental changes at Erhai Lake and can serve as important basis foridentification of the reference conditions for lake restoration and biodiversityconservation.