title: Controlling cyanobacterial blooms in hypertrophic Lake Taihu, China: will nitrogen reductions cause replacement of non-N2 fixing by N2 fixing taxa?
abstract: Excessive anthropogenic nitrogen (N) and phosphorus (P) inputs have caused an alarming increase in harmful cyanobacterial blooms, threatening sustainability of lakes and reservoirs worldwide. Hypertrophic Lake Taihu, China's third largest freshwater lake, typifies this predicament, with toxic blooms of the non-N2 fixing cyanobacteria Microcystis spp. dominating from spring through fall. Previous studies indicate N and P reductions are needed to reduce bloom magnitude and duration. However, N reductions may encourage replacement of non-N2 fixing with N2 fixing cyanobacteria. This potentially counterproductive scenario was evaluated using replicate, large (1000 L), in-lake mesocosms during summer bloom periods. N+P additions led to maximum phytoplankton production. Phosphorus enrichment, which promoted N limitation, resulted in increases in N2 fixing taxa (Anabaena spp.), but it did not lead to significant replacement of non-N2 fixing with N2 fixing cyanobacteria, and N2 fixation rates remained ecologically insignificant. Furthermore, P enrichment failed to increase phytoplankton production relative to controls, indicating that N was the most limiting nutrient throughout this period. We propose that Microcystis spp. and other non-N2 fixing genera can maintain dominance in this shallow, highly turbid, nutrient-enriched lake by outcompeting N2 fixing taxa for existing sources of N and P stored and cycled in the lake. To bring Taihu and other hypertrophic systems below the bloom threshold, both N and P reductions will be needed until the legacy of high N and P loading and sediment nutrient storage in these systems is depleted. At that point, a more exclusive focus on P reductions may be feasible.
authors.affiliation: Institute of Marine Sciences, The University of North Carolina at Chapel Hill, Morehead City, North Carolina, United States of America.
authors.forename: Hans W
authors.initials: HW
authors.lastname: Paerl
authors.affiliation: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, Nanjing, PR China.
authors.forename: Hai
authors.initials: H
authors.lastname: Xu
authors.affiliation: Institute of Marine Sciences, The University of North Carolina at Chapel Hill, Morehead City, North Carolina, United States of America.
authors.forename: Nathan S
authors.initials: NS
authors.lastname: Hall
authors.affiliation: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, Nanjing, PR China.
authors.forename: Guangwei
authors.initials: G
authors.lastname: Zhu
authors.affiliation: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, Nanjing, PR China.
authors.forename: Boqiang
authors.initials: B
authors.lastname: Qin
authors.affiliation: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, Nanjing, PR China.
authors.forename: Yali
authors.initials: Y
authors.lastname: Wu
authors.affiliation: Institute of Marine Sciences, The University of North Carolina at Chapel Hill, Morehead City, North Carolina, United States of America.
authors.forename: Karen L
authors.initials: KL
authors.lastname: Rossignol
authors.affiliation: Institute of Marine Sciences, The University of North Carolina at Chapel Hill, Morehead City, North Carolina, United States of America.
authors.forename: Linghan
authors.initials: L
authors.lastname: Dong
authors.affiliation: Marine Science Institute, The University of Texas at Austin, Port Aransas, Texas, United States of America.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: McCarthy
authors.affiliation: Institute of Marine Sciences, The University of North Carolina at Chapel Hill, Morehead City, North Carolina, United States of America.
authors.forename: Alan R
authors.initials: AR
authors.lastname: Joyner
mesh_terms: D018533:Biomass; D002681:China; D002734:Chlorophyll / Q000032:analysis; D000458:Cyanobacteria / Q000254:growth & development*; D004784:Environmental Monitoring / Q000379:methods*; D005068:Eutrophication / Q000502:physiology*; D060106:Lakes / Q000737:chemistry* / Q000382:microbiology*; D064307:Microbiota / Q000502:physiology*; D009584:Nitrogen / Q000032:analysis; D009586:Nitrogen Fixation / Q000502:physiology; D013045:Species Specificity
chemical_list: D002734:Chlorophyll; D009584:Nitrogen