Holocene size variations in two diatom species off East Antarctica: Productivity vs environmental conditions
Résumé
I here present a biometric investigation on two diatom species, Fragilariopsis kerguelensis (O’Meara) Hustedt and Fragilariopsis curta (Van Heurck) Hustedt, in Holocene samples from sediment core MD03-2601 from the Antarctic Continental Shelf off Adélie Land, East Antarctica. Apical valve length measurements of the two species are compared to their respective absolute and relative abundances as a proxy for the species productivity. Fragilariopsis kerguelensis valves were longer and more abundant during the warmer Mid-Holocene period and smaller and less abundant during the colder Late-Holocene period. Conversely, F. curta valves were smaller and less abundant during the warmer Mid-Holocene period and longer and more abundant during the colder Late-Holocene period. Mean apical valve length variations even follow centennial-to-millennial oscillations in the species abundances. Maximal valve length and minimal valve length were also larger during the warmer Mid-Holocene period and during the colder Late-Holocene period for F. keguelensis and F. curta, respectively. The observed positive size-abundance relationships are linked to the environmental conditions at the core location that stands today at the lower ecological limit for F. kerguelensis and upper ecological limit for F. curta. More favourable environmental conditions (warmer, less icy for F. kerguelensis and colder, icier for F. curta) allowed for sexual reproduction at the upper range of the sexually inducible size window and subsequent restoration of larger initial cells which, in turn, resulted in overall bigger size of the species populations, though vegetative multiplication was more frequent.