Southern Ocean Phytoplankton Stimulated by Wildfire Emissions and Sustained by Iron Recycling

Abstract

Large ash plumes emitted by the 2019–2020 Australian wildfires were associated with a widespread phytoplankton bloom in the iron‐limited Pacific sector of the Southern Ocean. In this study, we used satellite observations and aerosol reanalysis products to study the regional phytoplankton community response to wildfire emissions. The bloom was stimulated by pyrogenic iron fertilization and coincided with elevated cellular pigment concentrations, increased photochemical efficiency, and apparent community structural shifts. Physiological anomalies were consistent with previously observed phytoplankton responses to iron stress relief and persisted for up to 9 months. Supported by a regional iron budget, we conclude that the bloom was sustained by iron recycling and episodic inputs of pyrogenic and dust‐borne mineral iron. The continuous regeneration of iron was likely facilitated by the bloom's large size, mitigating edge dilution effects, as well as enhanced bioavailability of pyrogenic and mineral iron due to atmospheric and chemical processing during long‐range transport.

What data are needed to detect wildfire effects on coastal ecosystems? A case study during the Thomas Fire

“…In this study, in situ measurements of 𝑅 𝑟𝑠 (𝜆), IOPs, and chlorophyll-a allowed for a system-wide consideration of the impacts of wildfire smoke and ash on ocean ecology and optics. In many recent studies examining wildfire impacts on the ocean, particularly in more remote or open ocean environments, in situ samples are not available (Ardyna et al, 2022;Wang et al, 2022;Weis et al, 2022). Inversion modeling and derived products are essential in these cases: all three studies used chlorophyll-a from remote sensing to consider the possibility of a phytoplankton bloom occurring in the wake of a wildfire smoke plume.…”

“…Inversion modeling and derived products are essential in these cases: all three studies used chlorophyll-a from remote sensing to consider the possibility of a phytoplankton bloom occurring in the wake of a wildfire smoke plume. The relationship between 𝑏 𝑏𝑝 (𝜆) and phytoplankton carbon (𝐶 𝑝ℎ𝑦𝑡𝑜 ) was also used to further describe the impact of wildfires on primary productivity beyond the chlorophyll-a concentration (i.e., photoacclimation, physiological stress; Wang et al, 2022;Weis et al, 2022). The accuracy of these two parameters (chlorophyll, 𝐶 𝑝ℎ𝑦𝑡𝑜 ) is paramount to the interpretation of results and significance.…”

“…In the coastal ocean, increased CDOM from ash leaching may impact both remotely sensed AOPs (as seen in the shape and magnitude of the MODIS-Aqua 𝑅 𝑟𝑠 (𝜆) spectra) and in situ IOPs and AOPs (as seen in the GIOP retrievals from C-OPS reflectances; Figure 5B-C). In situ measurements will also allow for an evaluation of the impact of these other optically significant materials on the estimation of chlorophyll-a from remote sensing, particularly if further conclusions about ecosystem impacts will be drawn from an enhanced chlorophyll signal (e.g., Ardyna et al, 2022;Wang et al, 2022;Weis et al, 2022).…”

“…Increasingly, satellite remote sensing provides a (near) real-time tool to monitor the impacts of these sporadic wildfire events on the open ocean, as has been done with volcanic impacts on the ocean (Westberry et al, 2019;Bisson et al, in review). Both ocean color remote sensing and autonomous profiling floats (e.g., Argo) have been used as tools to identify phytoplankton blooms following a wildfire, particularly in inaccessible, open ocean and/or highlatitude regions (Ardyna et al, 2022;Wang et al, 2022;Weis et al, 2022). All three of these studies examined the changes in chlorophyll-a concentration and phytoplankton carbon from optical proxies and determined that wildfire smoke and ash is potentially fertilizing the Southern and Arctic Oceans, similarly to the effects observed after volcanic eruptions.…”

“…There are no studies comparing in situ optical observations to satellite or in situ ocean color measurements during or immediately after wildfires, when smoke and ash inevitably influence the atmospheric correction via absorbing aerosols and also may impact the typical balance of colored dissolved organic matter (CDOM) and particulate ash (included as non-algal particles, or NAP) in the water. Furthermore, existing studies (e.g., Ardyna et al, 2022;Wang et al, 2022;Weis et al, 2022) use business-as-usual satellite products and assume those aerosol models and atmospheric corrections are appropriate, which may not be true during a wildfire.…”