ABSTRACT

Lakes dominated by cyanobacterial harmful algal blooms (CyanoHABs) are geographically widespread. These blooms are associated with fish kills and toxin production and have negative impacts on human health. There is compelling evidence that, in addition to nitrogen and phosphorus, iron (Fe) can also regulate CyanoHABs. Low Fe concentration (tens of ng L⁻¹) has been documented to limit CyanoHABs in freshwaters, whereas high Fe concentration (hundreds of μg L⁻¹) can promote CyanoHABs. However, the extent to which Fe influences CyanoHABs in lakes that lack an extensive anoxic zone and where the Fe concentration ranges from tens to <400 μg L⁻¹ is unclear. The aim of this study was to assess Fe trends, measured as dissolved Fe (DFe), associated with cyanobacteria dynamics in lakes in northwestern Iowa. We utilised a multiwavelength fluorometer to monitor and classify phytoplankton based on their unique fluorescence spectra from chlorophyll a (Chl-a) and accessory pigments. Here we report a declining DFe trend (drawdown) potentially due to phytoplankton uptake in East Okoboji and its effect on CyanoHABs. Multiwavelength fluorescence data show cyanobacteria peaked (>50% cyanobacteria Chl-a) during a DFe minimum. The DFe is inversely correlated with phosphorus, suggesting decoupled Fe and phosphorus cycling. The strong positive correlation between phosphorus and cyanobacteria Chl-a supports a role for phosphorus in triggering CyanoHABs. This study suggests an underlying mechanistic role of Fe on CyanoHABs; the intermediate range of Fe concentrations investigated here may be sufficient to sustain CyanoHABs.

摘要

以蓝藻有害藻华（CyanoHABs）为主的湖泊在地理上分布广泛。这些水华与鱼类死亡和毒素产生有关，并对人类健康产生负面影响。有令人信服的证据表明，除了氮和磷，铁 (Fe) 还可以调节 CyanoHAB。已证明低 Fe 浓度（数十 ng L ^-1）可限制淡水中的 CyanoHAB，而高 Fe 浓度（数百 μg L ^-1）可促进 CyanoHAB。然而，在缺乏广泛缺氧区的湖泊中，Fe 对 CyanoHABs 的影响程度以及 Fe 浓度范围从几十到 <400 μg L ^-1不清楚。本研究的目的是评估与爱荷华州西北部湖泊中蓝藻动态相关的铁趋势，以溶解铁 (DFe) 来衡量。我们使用多波长荧光计根据浮游植物来自叶绿素a ( Chl- a ) 和辅助色素的独特荧光光谱对浮游植物进行监测和分类。在这里，我们报告了可能由于东 Okoboji 浮游植物吸收及其对 CyanoHAB 的影响而导致的 DFe 下降趋势（下降）。多波长荧光数据显示，在 DFe 最小值期间，蓝藻达到峰值（>50% 蓝藻Chl- a）。DFe 与磷呈负相关，表明铁和磷循环解耦。磷与蓝藻 Chl- a呈强正相关支持磷在触发 CyanoHAB 中的作用。这项研究表明 Fe 对 CyanoHAB 的潜在机制作用；此处研究的中等 Fe 浓度范围可能足以维持 CyanoHABs。