Two-year seasonality (2017, 2018), export and long-term changes in coccolithophore communities in the subtropical ecosystem of the Gulf of Aqaba, Red Sea

A two-year (2017, 2018) study of coccolithophores was undertaken in the Gulf of Aqaba in the northern Red Sea to 1) determine the local diversity and community succession patterns with high temporal and taxonomic resolution, 2) determine the contribution to exported inorganic carbon and 3) evaluate changes in communities relative to those in the mid-1970's reported in Winter et al. (1979). As typical, the oceanographic conditions in the Gulf alternated between stratified, oligotrophic through spring/summer, and mesotrophic during winter due to deep convective mixing. Noticeably, the second summer of 2018 was warmer resulting in a more pronounced thermocline. Overall, the coccolithophore dynamics followed the seasonal changes in oceanographic conditions. During the mixing period, coccolithophores were denser (>25 coccospheres mL⁻¹) and were estimated to account on average for 3.46% of the total Chl-a. The communities were fairly homogeneous through depths and over time with a marked dominance of Emiliania huxleyi and Gephyrocapsa ericsonii. During the stratified season, coccolithophore densities were lower, particularly at the surface, such that coccolithophores were estimated to represent on average 0.72% of the total Chl-a. However, the diversity was higher and a clear vertical zonation developed with well-defined upper, intermediate and deep-dwelling communities. These included Umbellosphaera spp., Rhabdosphaera clavigera, a variety of Syracosphaera spp., holococcolithophores and Florisphaera profunda, respectively, as typical in oligotrophic systems. This further underscores the pelagic nature of the Gulf of Aqaba. Noticeably, however, coccolithophores declined in abundances and diversity during 2018, possibly due to the warmer conditions. The relative export of inorganic carbon by coccolithophores largely correlated with the density of living communities, representing <4% and <10% of total inorganic carbon exports during the stratified and mixing periods, respectively. A marked differentiation between different coccolithophore life phases was also detected. Broadly, holococcolithophores inhabited preferentially shallow, oligotrophic water layers in the stratified season, while heterococcolithophores prevailed in deeper water layers or in winter. Yet, species-specific deviations to this pattern were also detected. Finally, comparisons with the 1970's revealed marked changes compared to current communities. G. ericsonii appeared less prevalent and Umbellosphaera spp. displayed a wider temporal residence through spring/summer. Moreover, F. profunda and other deep dwelling species absent in the past are now common during the peak of summer. These observations are indicative of changes in local microbial communities, possibly linked to the ongoing rise in sea temperatures. This study provides a detailed baseline information on coccolithophores for future reassessments of community changes in the GoA.