Seasonal patterns of coccolithophores in the ultra-oligotrophic South-East Levantine Basin, Eastern Mediterranean Sea

Abstract

Coccolithophore seasonality was examined in the southeastern Mediterranean Sea, both at the edge of the coastal shelf and in the open sea offshore Israel during 2018–2019. Oceanographic conditions varied seasonally between markedly stratified and ultra-oligotrophic from April to September and water column-mixing with relatively higher nitrate levels from October to February. Coccolithophores were quantified during the early (April), mid (July) and late (October) stratification period and in the mixing period (January). During stratification, cell densities progressively declined to <1.5 × 104 cells L−1, while both diversity and vertical differentiation of communities markedly increased. Emiliania huxleyi, Umbellosphaera spp., Syracosphaeraceae and Rhabdosphaeraceae as well as holococcolithophores were prevalent in the upper and mid-water layers. Florisphaera profunda characterized deeper sub-euphotic layers. During winter, mixing eroded the vertical zonation of species and coccolithophore density increased up to ~3.5 × 104 cells L−1. Communities became largely dominated by E. huxleyi, coincident with higher nutrient availability. The observed composition of coccolithophore assemblages and succession patterns support previous descriptions that the eastern Mediterranean largely resembles oceanic gyre systems. At the shelf station, the presence of higher fractions of r-selected species, and the rarity of oligotrophic coccolithophores (namely Umbellosphaera irregularis and holococcolithophores), suggest a somewhat greater influence of nutrients towards the shore. Finally, cells from different life-cycle phases were identified for a variety of species, highlighting the clear ecological divergence between coccolithophore life-phases. These data represent important baseline values for this area of the global ocean which is affected by major climate and environmental changes.