Extreme rainfall drives early onset cyanobacterial bloom

The increasing prevalence of cyanobacteria-dominated harmful algal blooms is strongly associated with nutrient loading and changing climatic patterns. Changes to precipitation frequency and intensity, as predicted by current climate models, are likely to affect bloom development and composition through changes in nutrient fluxes and water column mixing. However, few studies have directly documented the effects of extreme precipitation events on cyanobacterial composition, biomass, and toxin production. We tracked changes in a eutrophic reservoir following an extreme precipitation event, describing an atypically early toxin-producing cyanobacterial bloom, successional progression of the phytoplankton community, toxins, and geochemistry. An increase in bioavailable phosphorus by more than 27-fold in surface waters preceded notable increases in Aphanizomenon flos-aquae throughout the reservoir approximately 2 weeks post flood and ~5 weeks before blooms typically occur. Anabaenopeptin-A and three microcystin congeners (microcystin-LR, -YR, and -RR) were detected at varying levels across sites during the bloom period, which lasted between 3 and 5 weeks. Synthesis and applications: These findings suggest extreme rainfall can trigger early cyanobacterial bloom initiation, effectively elongating the bloom season period of potential toxicity. However, effects will vary depending on factors including the timing of rainfall and reservoir physical structure. In contrast to the effects of early season extreme rainfall, a mid-summer runoff event appeared to help mitigate the bloom in some areas of the reservoir by increasing flushing.


Extreme rainfall drives early bloom
However, the degree to which those factors will affect primary production in a reservoir is 74 directly related to event timing (e.g. early vs. late summer) and the zonal gradient created by 75 basin morphometry differences (Kimmel & Groeger, 1984). 76 Reservoirs naturally exhibit spatial heterogeneity in physical, chemical, and biological following an extreme rainfall event that led to water column cooling and destratification in a 86 shallow, eutrophic New Zealand lake. Varying responses to extreme rainfall should be 87 anticipated given the importance of site and event-specific factors. 88 Changing cyanobacterial bloom dynamics and biomass are a great concern to water 89 managers. Several bloom-forming cyanobacterial species such as Aphanizomenon flos-aquae, 90 Microcystis aeriginosa, and Anabaena (Dolicospermum) flos-aquae synthesize an array of cyanobacterial species, successional patterns, and biomass, but also to various environmental 96 parameters including N, P, temperature, light, pH, salinity, and micronutrients (Chorus & 97 Bartram, 1999), which may all be altered by intense rainfall events (Reichwaldt & Ghadouani,98 2012). To date, only limited information exists regarding how toxicity may change as a result of 99 precipitation events. 100 Here, we document the effects of an extreme early summer rainfall event on 101 cyanobacterial bloom and toxin dynamics in a eutrophic reservoir and contrast these effects to a

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Sampling in the transitional zone began on 05 Jul, 2 weeks after the extreme rainfall event. 120 (Figure 1a). We measured water column profiles at 0.5 m increments using an EXO2 sonde 121 (YSI, Yellow Springs, OH) equipped with chlorophyll-a (µg L -1 ), pH, temperature (°C), and 122 dissolved oxygen (DO, mg L -1 ) sensors. Photosynthetically active radiation (PAR) in the water 123 column was also measured in 0.5 m increments using a LI-COR ® Underwater Quantum Sensor    Table   285 2). Total microcystin concentration did not exceed 1.0 µg L -1 and was comparable to previously , associated with an ~25 mm rainfall event occurring during already wet conditions (Fig. 1).

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Effects of this event were much more muted. Overall, there was a slight decrease in surface with minimal effects on nutrients (Fig. 2). TSS increased slightly in the lacustrine zone and 303 decreased in the transitional zone, suggesting the event was not of sufficient magnitude to lead to 304 substantive additional erosion in the upstream catchment. Light irradiation increased in both 305 areas (Fig. 4). In the transitional zone, this change was concurrent with a 66% decrease in total 306 phytoplankton biomass and ~50 % decrease in total suspended solid while the lacustrine showed  can also be important to transport of soluble P species, as P is desorbed from particulate forms, 336 or released from soils. change in N suggests that it was not likely to be the key factor leading to early bloom initiation.

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Instead, it appears that the excessive supply of P initiated the development of the 345 observed weakly toxic early-summer cyanobacterial bloom (Figs 5 and 6). Typically, previously considered non-toxic, they may represent a new class of emerging toxins, whose 384 potential impacts to human health and toxicity to aquatic organisms require immediate attention 385 and therefore, inclusion in risk assessment for lake mitigation and monitoring programs.

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While increasing bloom incidence globally has been associated with elevated nutrient      with values which exceeded the limit of detection (LOQ) presented as mean ng L -1 (standard 548 deviation ng L -1 ), those in italics exceeded the limit of detection (LOD) but not the limit of    most influential phytoplankton species (a, Goodness of fit > 0.6) and significant (P < 0.05) and