In situ fluorometry reveals a persistent, perennial hypolimnetic cyanobacterial bloom in a seasonally anoxic reservoir

Abstract

Cyanobacterial blooms are increasing in waterbodies worldwide because of anthropogenic forcing. Most blooms occur at the water's surface, but some cyanobacterial taxa, such as Planktothrix, are able to modify their buoyancy to access more favorable growing conditions in deeper waters. Here, we used in situ fluorometry to examine the vertical distribution and biomass of Planktothrix in a seasonally anoxic reservoir for 3 consecutive summers. We also collected depth profiles of photosynthetically active radiation, temperature, and nutrients to evaluate which environmental drivers were most important for predicting Planktothrix biomass. In all 3 summers, Planktothrix dominated the phytoplankton community, exhibiting a large (concentrations~100 lg/L), persistent (lasting~100 d) bloom below the thermocline. The bloom consistently exhibited maximum biomass at or below the depth reached by 1% of surface light. Light availability probably was the most important factor driving the vertical distribution of the stratified Planktothrix bloom, and light, temperature, and nutrients together were strong predictors of cyanobacterial biomass in the hypolimnion, explaining 71 to 93% of the variation in biomass. Our data suggest that Planktothrix remained in the hypolimnion where nutrient availability was maximized, while progressing slightly upward in the water column through each summer in response to light limitation. Our findings demonstrate that Planktothrix can dominate in low light and anoxic conditions and can form persistent blooms that last for multiple months. As cyanobacterial blooms become more prevalent, monitoring cyanobacteria at the surface and at depth will become critically important in freshwater ecosystems.

Toxins and Other Bioactive Metabolites in Deep Chlorophyll Layers Containing the Cyanobacteria Planktothrix cf. isothrix in Two Georgian Bay Embayments, Lake Huron

“…Consistent with our observations, others have observed metalimnetic DCLs most commonly occuring at depths of less than 15 m, particularly those dominated by bloom-forming cyanobacteria [ 4 , 5 , 7 , 31 , 34 , 38 , 44 , 45 , 46 ]. Although significant DCLs dominated by potentially toxic cyanobacteria such as Planktothrix have been reported at depths below 15 m (e.g., [ 47 ]), this appears relatively rare [ 7 , 46 ].…”

“…There is a considerable body of evidence to indicate that the development of opposing and intersecting vertical gradients of light and nutrients is strongly correlated with DCL formation in stratified, meso-oligotrophic systems [ 4 , 5 , 7 , 15 , 34 , 44 ]. Our survey showed that stratified Georgian Bay embayments lacking these opposing and intersecting vertical gradients also lacked discernible DCLs.…”

Relative importance of top‐down vs. bottom‐up control of lake phytoplankton vertical distributions varies among fluorescence‐based spectral groups

“…The majority of previous studies of DCMs examined only the vertical distributions of total phytoplankton biomass or of a single taxon (Christensen et al 1995;Jeppesen et al 2003;Kononen et al 2003;Hamilton et al 2010;Hamre et al 2018;Leach et al 2018), which could in part explain why there are so many different hypotheses and explanations for different DCM characteristics. Examination of total biomass obscures differences in the relative importance of top-down and bottom-up controls for different phytoplankton taxa.…”

“…The depth and magnitude of DCMs may be driven by multiple physical, chemical, or biological factors, as DCMs can exist due to phytoplankton migration, growth, settling, or entrainment (Moll et al 1984; Arvola et al 1992; Durham and Stocker 2011). DCM depth may be determined by light attenuation, in which water columns with greater light penetration display deeper, broader DCMs (Varela et al 1994; Hamilton et al 2010; Leach et al 2018), or by a combination of light attenuation and nutrients, in which phytoplankton select a depth to optimize both down‐welling light and greater nutrient availability in hypolimnetic waters (Klausmeier and Litchman 2001; Karpowicz and Ejsmont‐Karabin 2017; Hamre et al 2018). Motile or buoyant phytoplankton will migrate to take advantage of these gradients of light, nutrients, or temperature and develop DCMs in layers with optimal resources (Moll et al 1984; Camacho et al 2001; Sengupta et al 2017).…”

Cyanobacterial blooms in oligotrophic lakes: Shifting the high‐nutrient paradigm

“…In addition to efficient absorption in the visible light spectrum, cyanobacteria have a protein structure that allows them to use light in the far-red spectrum, giving them a further advantage for growth under severe light extinction (Kirk 1994). The capacity to effectively adapt to low irradiance at depth (often far <1% of incident light; Hamre et al, 2018) demonstrates that cyanobacteria are well adapted to not only the low light conditions of eutrophic lakes, but also the deep chlorophyll maxima that are often present in meso-and oligotrophic lakes (e.g. Planktothrix rubescens, Cyanobium sp., and Aphanizomenon flos-aquae; Scofield et al, 2017.…”