Often-observed post-wildfire increase in nutrients—phosphorus in particular—commonly leads to increased biological activity in streams, starting with algal growth. Although phytoplankton and periphyton concentrations have been shown to increase in response to wildfires, literature claims that it could lead to—potentially harmful—algal blooms, but I cannot find an example of a (H)AB supporting this claim.
So, is the possibility of post-wildfire algal bloom a fact, a fiction, a minor concern, or a substantiated risk based on what we know from post-fire possible phosphorus releases and the triggers of algal bloom formation?
At this time, the possibility of post-wildfire algal bloom is perceived to be a minor concern. While there is evidence that nutrient concentrations can increase in streams following wildfires, there is no evidence to show that these increases lead to harmful algal blooms. Thus, it is not a substantiated risk at this time.
Diana, I went into our bibliography at wifri.org and found two articles when I searched "Algae" AND "bloom" in the abstracts. Access is free.
I've seen very abundant periphyton growth in creeks that drain burned peatlands, associated with elevated P loading. See study Burd et al., 2018 in Hydrol Earth Sys Sci. In a later study we looked at the food webs of these streams - and found that the food web was driven by recent algal productivity - not by ancient soil carbon released into the streams, see Tanentazp et al 2021 Glob Change Biol.
Even though we do not work on freshwater, we have done some research about this issue in the Ría de Vigo, an embayment in Galicia, NW Spain, a region affected by wildfires every summer, and where algal blooms are a major concern. We carried out two experiments (one in summer, and another one in winter), where ash leachate from a forest fire was added to microcosms containing water from the Ría de Vigo. The microcosms were incubated for 72 h under in situ light and temperature conditions. In the summer experiment, when the water was rather oligotrophic, the concentration of the ash leachate added was directly proportional to the response ratio of chlorophyll a (chl a) concentration (used as a proxy for phytoplankton abundance in an amended microcosm, with respect to the unamended control). Although we don't have taxonomic data about the phytoplankton, we do have 16S metabarcoding data from those experiments, which showed that some bacterial taxa, including potentially pathogenic Vibrio, increased their relative abundance after ash-leachate additions, with respect to the control microcosm. The reasons for the changes in bacterioplankton community composition are not very clear, as they might be either caused by nutrients contained in the ash directly, or by exudates from phytoplankton. Either way, phytoplankton seems to be directly affected by the ash leachates.
Furthermore, in October 2017 there was a massive wildfire around the Ría de Vigo, which was followed by a drought that lasted until December. We started sampling water in a time series, which started right after the fire and where water was sampled from different stations in the embayment, roughly every two weeks. When it finally rained in December, sampled water from the station closest to the mouth of a river, which might be the most affected by runoff, showed its greatest surface Chl a concentration, together with changes in bacterioplankton community composition. Please see:
Article Impact of wildfire ash on bacterioplankton abundance and com...
And the following paper suggests that wildfire ash and smoke might have contributed to a dinoflagellate bloom off the coast of California:
Kramer, S. J., Bisson, K. M., & Fischer, A. D. (2020).Fischer, A. D. (2020). Observations of phytoplankton community composition in the Santa Barbara Channel during the Thomas Fire. Journal of Geophysical Research: Oceans, 125, e2020JC016851. https://doi.org/10.1029/2020JC016851
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