Iron availability allows sustained cyanobacterial blooms: a dual-lake case study

Abstract

Lakes dominated by cyanobacterial harmful algal blooms (CyanoHABs) are geographically widespread. These blooms are associated with fish kills and toxin production, which have negative impacts on human health. There is compelling evidence that iron (Fe) can also regulate CyanoHABs besides nitrogen and phosphorus. Low iron concentrations (tens of ng L -1 ) has been documented to limit CyanoHABs in freshwaters whereas high Fe concentrations (hundreds of μg L -1 ) can promote CyanoHABs. However, it is unclear the extent which Fe influences CyanoHABs in lakes that lack an extensive anoxic zone and where the magnitude of Fe concentration ranges from tens to < 400 μg L -1 . 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 utilized a multi-wavelength 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. Multi-wavelength fluorescence show cyanobacteria peaked (> 50% cyanobacteria Chl-a) during a DFe minimum. The drawdown in DFe 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 the CyanoHAB. This study suggest an underlying mechanistic role of Fe on CyanoHABs: the intermediate range of Fe concentrations investigated here may be sufficient to sustain CyanoHABs.

Statewide Assessment Reveals Spatiotemporal Variability of Iron in Iowa Lakes

“…Factors such as regional geology, watershed characteristics, and biological productivity, as well as mixing regime, influence the amount of DFe in freshwater systems (Gorham et al 1983; Davison 1993; Nürnberg 1995; Dillon and Molot 2005; Leung et al 2021). Dissolved Fe from ALM lakes was used to probe for spatial variability across lakes.…”

Non-Nitrogen-Fixers or Nitrogen-Fixers? Factors Distinguishing the Dominance of Chroococcal and Diazotrophic Cyanobacterial Species

“…It is known that iron (Fe 2+ ) regulates the efficiency of macronutrient use by cyanobacteria, as it plays a critical role in the uptake of nitrogen (N) and phosphorus (P) [ 23 , 54 ]. Since Fe 2+ is very difficult to measure in the field, we used total dissolved Fe (TDFe) as a proxy of Fe 2+ in anoxic waters [ 22 , 24 ]. Although all the studied waters are shallow, we found statistically significant differences between the upper and bottom layers, indicating weaker oxygenation of bottom waters.…”

“…The main macronutrients for which cyanobacteria compete are phosphorus (P) and nitrogen (N) [ 18 ]. However, some researchers extend the interaction to the N:P ratio [ 19 , 20 ] and also to iron and sulphate concentration [ 21 , 22 ]. Molot et al [ 23 ] published a hypothetical model that related anoxia and the concentration of P, N, Fe 2+ , and sulphate to the formation of cyanobacterial blooms and formulated a hypothesis based on a hypothetical decision tree.…”

Seasonal phytoplankton and geochemical shifts in the subsurface chlorophyll maximum layer of a dimictic ferruginous lake

“…Iron limitation has been described within marine SCML (Hogle et al, 2018), but iron bioavailability is generally not considered in investigations of SCML in lakes. This may be because iron is not frequently measured in lakes as it is not often found to be a limiting nutrient in lakes (Leung et al, 2021).…”