A dynamic subtropical coastal hotspot of benthic foraminifera in the Southeastern Mediterranean indicates early-stage tropicalization.

Due to ongoing ocean warming, subtropical environments are becoming accessible to tropical species. Among these environments are the vermetid reefs of the Southeastern Mediterranean (SEM). In the last decades, these valuable coastal habitats witnessed the proliferation of numerous alien species of tropical origin. Among the meiofauna thriving on these reefs are benthic foraminifera, single cell marine organisms that make a significant contribution to global carbonate production. It has been widely recognized that benthic foraminifera among are invasive species thrive in the macroalgal cover, and it has been suggested that their populations are becoming a significant new source of sediment substrate. Here, we report on the first systematic assessment of the population size of the benthic foraminifera, allowing a comparison with data from the native tropical habitat of these species. Our study is based on a seasonal sampling of benthic foraminifera from confined sampling areas at four sites along the vermetid reef platforms of the Israeli SEM coast. Our survey reveals a patchy distribution of each species with peak population densities exceeding 100,000 specimens per m2, making the SEM a hotspot of benthic foraminifera, with population densities comparable to tropical coral reef environments. The assemblages of the SEM hotspot are dominated by cosmopolitan foraminiferal taxa and tropical invaders from the Indo-Pacific (e.g., Amphistegina lobifera, Pararotalia calcariformata, soritids, and Hauerina diversa). In contrast to foraminiferal hotspots in the tropics, which are completely dominated by larger symbiont-bearing taxa, the SEM hotspot stands out due to high abundances of non-symbiont-bearing species Textularia agglutinans and small miliolids. An intriguing observation is the significant heterogeneity in composition and density of foraminiferal assemblages between the vermetid reefs' southern and northern areas (Israel), indicating that the productivity of the dominant species are also modulated by local yet unknown environmental factors.

Epiphytic benthic foraminiferal preferences for macroalgal habitats: Implications for coastal warming.

Considering the thermal limits of coastal macroalgae habitats in the South-Eastern Mediterranean, it is important to study the response of the associated meiofauna to better understand the expected feedback of ecosystems to future warming. In this study, we compared benthic foraminiferal assemblages from two common macroalgal habitats, Turf and Coralline algae, based on ecological monitoring of a thermally polluted station representing near future warming, and an undisturbed environment. None of the common local species is confined to a specific algal habitat. This implies that their existence is not threatened by the disappearance of the Coralline algae. However, most likely their community structure will be impacted with coastal warming. Species that are more affiliated with Coralline algae are highly thermally tolerant, thus their proliferation might be reduced with warming. Specifically, the negative response of Coralline algae to warming may limit the contribution of invasive species such as Pararotalia calcariformata.