Contributions of external nutrient loading and internal cycling to cyanobacterial bloom dynamics in Lake Taihu, China: Implications for nutrient management

Abstract

Harmful cyanobacterial blooms (CyanoHABs) are linked to increasing anthropogenic nitrogen (N) and phosphorus (P) inputs. However, CyanoHABs in many large lakes continue despite extensive abatement efforts, mostly focused on external P loading. Internal nutrient cycling can modify nutrient availability and limitation; thus, understanding the relative importance of external vs. internal nutrient loading is essential for developing effective mitigation strategies for CyanoHABs. We estimated long‐term nutrient budgets for Lake Taihu, China, from mass balance models using extensive monitoring of input and output nutrient data from 2005 to 2018 to quantify contributions from internal nutrient loading. The nutrient mass balance showed that 9% and 63% of annual external N and P inputs, respectively, were retained in the lake. Denitrification removed 54% of external N loading and can thus help explain rapid decreases in lake N concentrations and summer N limitation. Water column NH4+ regeneration can help sustain CyanoHABs over the short term and contributed 38–58% of potential NH4+ demand for summer‐fall, Microcystis‐dominated blooms. Internal P release contributed 23–90% of CyanoHABs P demand, although Taihu was a net P sink on an annual scale. Our results show that internal nutrient cycling helps sustain CyanoHABs in Taihu, despite reductions in external nutrient inputs. Furthermore, N is leaving the lake faster than P, thereby creating persistent N limitation. Therefore, parallel reductions in external N loading, along with P, will be most effective in reducing CyanoHABs and accelerate the recovery process in this and other large, shallow lakes.

Increased Nitrogen Loading Boosts Summer Phytoplankton Growth by Alterations in Resource and Zooplankton Control: A Mesocosm Study

“…In shallow eutrophic lakes, in-lake TN:TP ratios are often low in summer due to the high denitrification and strong P release at high temperatures (Xu et al, 2010;Xu et al, 2021;Søndergaard et al, 2013;Søndergaard et al, 2017;Shatwell and Köhler, 2019). From a resource availability perspective, excess N loading could directly increase the summer phytoplankton biomass via alleviation of temporal N deficiency, and this has been confirmed by numerous bioassays and mesocosm experiments (Xu et al, 2010;He et al, 2015;Kolzau et al, 2018).…”

“…blooms (Guo et al, 2007;Qin et al, 2019). Previous bioassay experiments and field monitoring studies have revealed that the phytoplankton in Lake Taihu is generally N limited or N, P co-limited during summer and fall (Xu et al, 2010;Xu et al, 2021;Paerl et al, 2011). Thus, our hypothesis was that with increasing N loading in summer, the phytoplankton biomass would increase due to the alleviation of N deficiency and higher sediment P release.…”

High ammonium recycling in an anthropogenically altered Yeongsan River Estuary, South Korea

“…The regeneration of ammonium through active ammonium recycling is likely to supports high phytoplankton production during the springsummer season in the YRE. Previous studies have suggested that water column N recycling contributes significantly to phytoplankton N demand in aquatic environments (Bronk et al, 2014;Hampel et al, 2018;Jiang et al, 2019;Xue et al, 2021a;Xue et al, 2021b;Xu et al, 2021;Hoffman et al, 2022). In the West Florida Shelf, regenerated ammonium was sufficient to supply the ammonium demand, and supported 85% of the ammonium uptake even during blooms of Karenia brevis (Bronk et al, 2014).…”

Phosphorus Accumulation in Extracellular Polymeric Substances (EPS) of Colony-Forming Cyanobacteria Challenges Imbalanced Nutrient Reduction Strategies in Eutrophic Lakes

“…In world’s large bloom-impacted lakes, such as Lake Taihu (China) and Lake Champlain (Canada/USA), , nitrogen (N) concentration often tended to be scarce with the development of cyanobacterial blooms, presumably a result of the high rate of denitrification . Consequently, cyanobacterial aggregates generally experienced strict N limitation when the blooms reached their peak biomass. , The intensity of N limitation is expected to be stronger because of the imbalanced nutrient reduction efforts/efficiencies, for example, along with the substantial and constant reductions of N concentration and nutrient stoichiometry (i.e., TN:TP ratios) in Lake Taihu during the past decade. , Nitrogen can play an important role in the regulation of EPS production in cyanobacteria. − Specifically, N limitation can often elevate EPS production, alter EPS structure and composition, and then shape the morphology and colony size of cyanobacterial aggregates . The imbalanced nutrient reduction is thus likely to promote the cyanobacterial capacity for P storage in their EPS and then increase the total cyanobacterial particulate P quota (including both P stored in EPS and cells).…”