Spatial and seasonal variation in N2-fixing cyanobacteria in Poyang Lake from 2012 to 2016: roles of nutrient ratios and hydrology

Large river floodplain systems provide a variety of societal, economic and biological benefits and are undergoing extensive and intensive environmental deterioration. Eutrophication coupled with undesired harmful cyanobacterial blooms is one of the most widespread and severe problems in floodplain ecosystems. However, our knowledge about cyanobacteria, particularly the biogeography of N-2-fixing (Nfix) cyanobacteria in shallow floodplain lakes, is very limited, and the relationships of cyanobacterial blooms with nutrient ratios and hydrological alterations remain unclear. We used a comprehensive database of field data compiled over several years (2012 to 2016) to compare the biomass and distribution of Nfix cyanobacteria between the northern (high water velocity) and southern (low water velocity) parts of Poyang Lake and to investigate the roles of hydrology and the critical nutrient mass ratios of total nitrogen:total phosphorus (TN:TP) and dissolved inorganic nitrogen:orthophosphate (DIN:PO4-P) as significant predictors of Nfix cyanobacterial biomass and distribution in eutrophic floodplain ecosystems. Dolichospermum spp. (Dolichospermum flos-aquae, D. azotica, D. circinalis, and D. spiroides) were the most widely distributed and frequent diazotrophic cyanobacteria in the regions considered in this study, followed by Aphanizomenon flos-aquae. The Nfix cyanobacterial biomass was generally low (mean=0.32mg/l) across Poyang Lake and was lower in the north than in the south. Using Spearman's rank correlations, we found that this pattern may be attributed to the high turbidity and washout from the high velocity of water flow in the north. The filament length and heterocyst frequency of Dolichospermum spp. in the lake were also strongly linked to variations in hydrological characteristics and water temperature. Our results imply that the nutrient mass ratios are more important determinants of Nfix cyanobacterial biomass than hydrology in the south part of the lake. The critical TN:TP mass ratio for the Nfix cyanobacterial communities in Poyang Lake is approximately 20, and the critical DIN:PO4-P mass ratio in the lake is approximately 40. Our analysis provides new information regarding the occurrence of bloom-forming Nfix cyanobacteria in Yangtze River floodplain lakes and thus fills an important knowledge gap in subtropical freshwater ecosystems.