No abstract

Iron availability directly affects photosynthesis and limits phytoplankton growth over vast oceanic regions. For this reason, the availability of iron is a crucial variable to consider in the development of active chlorophyll a fluorescence based estimates of phytoplankton primary productivity. These bio-optical approaches require a conversion factor to derive ecologically-relevant rates of CO2-assimilation from estimates of electron transport in photosystem II. The required conversion factor varies significantly across phytoplankton taxa and environmental conditions, but little information is available on its response to iron limitation. In this study, we examine the role of iron limitation, and the interacting effects of iron and light availability, on the coupling of photosynthetic electron transport and CO2-assimilation in marine phytoplankton. Our results show that excess irradiance causes increased decoupling of carbon fixation and electron transport, particularly under iron limiting conditions. We observed that reaction center II specific rates of electron transport (ETRRCII, mol e- mol RCII-1 s-1) increased under iron limitation, and we propose a simple conceptual model for this observation. We also observed a strong correlation between the derived conversion factor and the expression of non-photochemical quenching. Utilizing a dataset from in situ phytoplankton assemblages across a coastal – oceanic transect in the Northeast subarctic Pacific, this relationship was used to predict ETRRCII: CO2-assimilation conversion factors and carbon-based primary productivity from FRRF data, without the need for any additional measurements.

Droughts and climate-change-driven warming are leading to more frequent and intense wildfires [1][2][3] , arguably contributing to the severe 2019-2020 Australian wildfires 4 . The environmental and ecological impacts of the fires include loss of habitats and the emission of substantial amounts of atmospheric aerosols [5][6][7] . Aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals such as nitrogen and iron, respectively [8][9][10] . It has been suggested that the oceanic deposition of wildfire aerosols can relieve nutrient limitations and, consequently, enhance marine productivity 11,12 , but direct observations are lacking. Here we use satellite and autonomous biogeochemical Argo float data to evaluate the effect of 2019-2020 Australian wildfire aerosol deposition on phytoplankton productivity. We find anomalously widespread phytoplankton blooms from December 2019 to March 2020 in the Southern Ocean downwind of Australia. Aerosol samples originating from the Australian wildfires contained a high iron content and atmospheric trajectories show that these aerosols were likely to be transported to the bloom regions, suggesting that the blooms resulted from the fertilization of the iron-limited waters of the Southern Ocean. Climate models project more frequent and severe wildfires in many regions [1][2][3] . A greater appreciation of the links between wildfires, pyrogenic aerosols 13 , nutrient cycling and marine photosynthesis could improve our understanding of the contemporary and glacialinterglacial cycling of atmospheric CO 2 and the global climate system.Human activity is altering the global water and carbon cycles 14 . While the risk of drought associated with climate change varies regionally, warming and drying will increase the risk of more frequent and intense wildfires 1-3 . In turn, wildfires are increasingly viewed as a first-order control on climate. Among other things, wildfires change the Earth's radiative forcing by emitting greenhouse gases and aerosols 15 . The feedbacks between climate and wildfires are complex and often poorly represented in climate models, leading to high uncertainty in future projections.The austral summer of 2019-2020 was one of the most severe wildfire seasons in Australian history. Millions of hectares of vegetation were burned, having ecological, environmental and socioeconomical impacts 5,16 . It is estimated that nearly 3 billion animals may have died or been displaced 17 . According to a study by van der Velde et al. published in this issue of Nature 18 , approximately 715 million tonnes of CO 2 (195 Tg C) were released into the atmosphere during the fire period, exceeding Australia's 2018 anthropogenic CO 2 emissions of 537.4 million tonnes (147 Tg C) 19 .The 2019-2020 Australian wildfires (known in Australia as bushfires) also released an enormous amount of aerosols into the atmosphere 6,7 . Aerosols can influence terrestrial and marine biogeochemistry 20 , via supplying soluble forms of nitrogen 8 , ph...

The presence of algal pigments in sea ice alters under-ice irradiance spectra, and the relationship between these variables can be used as a non-invasive means for estimating iceassociated algal biomass on ecologically relevant spatial and temporal scales. While the influence of snow cover and ice algal biomass on spectra transmitted through the snow-ice matrix has been examined for the Arctic, it has not been tested for Antarctic sea ice at regional scales. We used paired measurements of sea ice core chl a concentrations and hyperspectral-transmitted under-ice irradiances from 59 sites sampled off East Antarctica and in the Weddell Sea to develop algorithms for estimating algal biomass in Antarctic pack ice. We compared 4 approaches that have been used in various bio-optical studies for marine systems: normalised difference indices, ratios of spectral irradiance, scaled band area and empirical orthogonal functions. The percentage of variance explained by these models ranged from 38 to 79%, with the best-performing approach being normalised difference indices. Given the low concentrations of integrated chl a observed in our study compared with previous studies, our statistical models performed surprisingly well in explaining variability in these concentrations. Our findings provide a basis for future work to develop methods for non-invasive time series measurements and medium-to large-scale spatial mapping of Antarctic ice algal biomass using instrumented underwater vehicles.KEY WORDS: Sea ice algae · Chl a · Bio-optics · Normalised difference index · Weddell Sea · East Antarctica Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 536: [107][108][109][110][111][112][113][114][115][116][117][118][119][120][121] 2015 ice coring (Meiners et al. 2012). Recent circumpolar estimates of ice algal biomass for Antarctica are limited to records from historical ice core data that are unevenly distributed in space and time (Meiners et al. 2012) and model-based estimates that may underestimate internal communities (Saenz & Arrigo 2014). Improved in situ data on temporal and spatial patterns of ice algal biomass distribution are needed for quantitative evaluation of sea ice primary production models and an improved understanding of the role of ice algae in Antarctic marine ecosystem function.Antarctic pack ice provides a habitat for iceassociated algae, which form distinct surface, interior and bottom communities (Arrigo et al. 2010, Meiners et al. 2012. Surface communities are promoted by snow loading, surface flooding by seawater and brine, and subsequent snow-ice formation (Ackley et al. 2008). Interior communities can form either through the rafting and ridging of ice floes or by scavenging of phytoplankton during ice formation (i.e. the uptake of algal cells from the water column as ice crystals form; Arrigo et al. 2010). Bottom communities thrive in the lowermost porous parts of sea ice floes, where brine salinities and high nutrient availability favour algal gr...