Full annual monitoring of Subantarctic <i>Emiliania huxleyi</i> populations reveals highly calcified morphotypes in high-CO₂ winter conditions

<p>Ocean acidification is expected to have detrimental consequences for the most abundant calcifying phytoplankton species <i>Emiliania huxleyi</i>. However, this assumption is mainly based on laboratory manipulations that are unable to reproduce the complexity of natural ecosystems. Here, <i>E. huxleyi</i> coccolith assemblages collected over a year by an autonomous water sampler and sediment traps in the Subantarctic Zone were analysed. The combination of taxonomic and morphometric analyses together with <i>in situ</i> measurements of surface-water properties allowed us to monitor, with unprecedented detail, the seasonal cycle of <i>E. huxleyi</i> at two Subantarctic stations. <i>E. huxleyi</i> subantarctic assemblages were composed of a mixture of, at least, four different morphotypes. Heavier morphotypes exhibited their maximum relative abundances during winter, coinciding with peak annual TCO₂ and nutrient concentrations, while lighter morphotypes dominated during summer, coinciding with lowest TCO₂ and nutrients levels. The similar seasonality observed in both time-series suggests that it may be a circumpolar feature of the Subantarctic zone. Our results challenge the view that ocean acidification will necessarily lead to a replacement of heavily-calcified coccolithophores by lightly-calcified ones in subpolar ecosystems, and emphasize the need to consider the cumulative effect of multiple stressors on the probable succession of morphotypes.</p>