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Anthropogenic Changes to Factors Controlling Cyanobacterial Blooms Along a Chain of Fresh to Hypersaline Water Bodies

Raymond Lee,S. Collins,E. Jones,Gabriella M Lawson,Sarah Chan,Rachel L. Wood,Neil C. Hansen,Benjamin W. Abbott,Greg Carling,Michelle A. Baker,Z. Aanderud

Published 2025 in Freshwater Biology

ABSTRACT

Cyanobacterial proliferation in inland waters can degrade water quality and harm human health. Cyanobacterial proliferation is determined largely by nitrogen and phosphorus supply, though nutrient conditions in the impacted lake or reservoir and its inflow(s) are not always sufficient for explaining/mitigating the bloom. Other factors may include fluvial seeding of cyanobacteria, in‐lake sediment release then entrainment of nutrients and species preference for nutrients (e.g., diazotrophic vs. non‐diazotrophic). We studied diverse aquatic systems (drinking water reservoir to throughflow and endorheic lakes along a freshwater to hypersaline gradient) to investigate (a) how nutrients supported cyanobacterial blooms (blooms were defined by the outsized relative abundance of a given cyanobacterial species); (b) which factors besides external nutrient loading supported blooms; and (c) if/how all these processes changed across limnologic conditions in successively downstream systems. Results showed that blooms occurred in different patterns despite being located in the same basin with the same regional climate. Blooms were controlled by nitrogen at the freshwater reservoir ( Phormidium spp. succeeded by Microcystis spp.), phosphorus at the brackish lake ( Aphanizomenon flos‐aquae ) and inflow at the hypersaline endorheic lake ( Nodularia spp.). At the reservoir, there were spatial and temporal shifts in dominance from diazotrophic (historically) to non‐diazotrophic species (currently) that occurred likely due to successful watershed management of phosphorus loading; at the endorheic lake, the bay dried up completely, likely due to high water consumption, which reduced inflow over time. Results suggested that bloom characteristics and controls changed at each study lake due to different local human activities, so strategies for nutrient and bloom mitigation should be site‐specific rather than uniform for the region. The concept of a cyanobacterial ‘bloom’ is ambiguous because it can be defined by different thresholds of different specimens (e.g., chlorophyll concentration, cyanobacterial cell count/density, cyanotoxin concentration or perceived/actual harm or risk thereof). Here, we defined a bloom as excessive cyanobacterial relative abundance, which is a novel way to track the life cycles of dominant cyanobacterial species in the context of limnologic and nutrient controls. This approach allowed us to identify significant and different anthropogenic changes to controlling factors of blooms in diverse settings.

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