Marine phytoplankton are critical primary producers, contributing nearly half of the biosphere's net primary production. The impact of global warming on marine phytoplankton has become increasingly evident in the past few decades, but how phytoplankton is affected at different latitudes remains controversial. Ocean stratification enhanced by warming has been invoked as a major mechanism to explain phytoplankton decline on a global scale. Warming modifies the mixed layer depth and reduces the vertical mixing of the surface layer and underlying cooler nutrient-rich waters below the permanent pycnocline. Changes in these physical processes lead to a reduced supply of nutrients to the upper ocean and, consequently, have a negative impact on phytoplankton. However, although this might be true in temperate oceans, it is not always the case in the tropics and Polar Regions. Generally, the rise in temperature and changes in stratification in tropical oceans are smaller than in temperate oceans because of their larger heat capacity and thermal inertia. Moreover, warming-induced secondary climate effects, for example, increased tropical cyclones and upwellings, can compensate for nutrient depletion in the upper ocean by accelerating turbulent mixing and promoting rainfall. Phytoplankton metabolic capacity in the tropics is also higher than that at high latitudes. These factors can partially offset the negative effect of greater stratification and even lead to stage increase in phytoplankton biomass in tropical oceans.
In contrast to the impact of physical processes on phytoplankton in tropical oceans, the role of wildfires has received minimal attention. The risk and severity of wildfires in the southern hemisphere have greatly escalated on a global scale as a consequence of rising temperatures and more frequent heat waves, for example, there was a distinct lengthening of the fire weather season between 1980 and 2018 (Fig. 1a). The emissions and ash from wildfire are rich in biogenic elements, such as nitrogen, phosphate, silicate, and iron, and exert a distinct impact on atmospheric and aquatic environments. For example, particles emitted by wildfires account for approximately 62% of the global annual emissions of organic matter from biomass burning. The global flux of soluble charcoal from biomass burning is estimated to be 40-250 million tons y−1, and approximately 26.5 million tons enter the ocean every year. Despite their importance, our understanding of the effect of wildfires on the ocean is far less than our understanding of their role in terrestrial ecosystems.
Fig. 1: Maps showing changes in the length of the fire weather season on a global scale and sampling location. a Map showing the change in the length of the fire weather season between 1980 and 2018, data from the ERA5 dataset (Jolly et al., 2015; Vitolo et al., 2020); the green circle indicates the Kimberley coast, a fire-prone region in northern Australia). b Sampling sites of core 185, core 200, and core KGR (red dots) in the Kimberley coast.
Here, using geochemical palaeo-reconstructions, a century-long relationship between wildfire magnitude and MPP was established in a fire-prone region in northern Australia (Fig. 2). Wildfire magnitude was reconstructed using black carbon (BC) content in the sediment cores. BC is an organic, molecularly diverse product resulting from the incomplete combustion of biomass and fossil fuels and it decomposes slowly after burial in marine sediments as a component of total organic carbon. Biosilicate is a frustule component of diatoms and is often used as a proxy for diatom biomass in the ocean because of its major contribution to phytoplankton biomass. A positive correlation was identified between BC and BSi. The importance of wildfire on phytoplankton biomass was statistically higher than that of tropical cyclones and rainfall, when strong El Niño Southern Oscillation coincided with the positive phase of Indian Ocean Dipole (pIOD). Decadal chlorophyll-a variations along the Kimberley coast verified the spatial linkage of this phenomenon. Findings from this study suggest that the role of additional nutrients from wildfires has to be considered when projecting impacts of global warming on MPP.
Fig. 2: The profiles to illustrate the variations of El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and multiple geochemical proxies in the three sediment cores from the 1920s to 2010s. a Low-frequency signals of dipole mode index (DMI) and Niño 3.4 index showing the variations of IOD and ENSO, respectively. b Core 185: total organic carbon (TOC) and total nitrogen (TN). c Core 185: biogenic silicate (BSi) and black carbon (BC). d Core 200: TOC and TN. e Core 200: BSi and BC. f Core KGR: TOC and TN. g Core KGR: BSi and BC. Blue lines represent the shift changes assessed by sequential t test analysis of regime shift and numbers on lines were regime shift index (RSI) to show shifting magnitude.
The results highlight the need, when strong ENSO conditions coincide with pIOD phase, to consider the contribution of wildfire to the functioning of oligotrophic tropical oceans, not just the role of physical mixing mechanisms (e.g., upwelling, tropical cyclones). Such understanding will provide further knowledge on the asymmetry of phytoplankton community responses to climate change in the northern and southern hemispheres. This study clearly points out the importance of secondary environmental effect from land wildfire on marine ecosystems. The results are conducive for scientists to further use the global model to simulate and predict the trend of phytoplankton shift under future global warming.
Article source: Liu et al., 2022. Wildfires enhance phytoplankton production in tropical oceans. Nature Communications, 13(1348). https://doi.org/10.1038/s41467-022-29013-0