Control of Antarctic phytoplankton community composition and standing stock by light availability

Southern Ocean phytoplankton are especially subjected to pronounced seasonal and interannual changes in light availability. Although previous studies have examined the role of light in these environments, very few combined pigment-based taxonomy with flow cytometry to better discriminate the light response of various phytoplankton groups. In particular the different populations within the diverse and important taxonomic group of diatoms require further investigation. Six incubation experiments (9-10 days) were performed during the main productive period with natural seawater collected at the Western Antarctic Peninsula. Standing stock of Phaeocystis spp. cells displayed relatively fast accumulation under all levels of light (low, medium, high; 4-7, 30-50 and 150-200 mu mol quanta m(-2) s(-1)), whilst the small- and larger-sized diatom populations (4.5 and 20 mu m diameter) exhibited faster accumulation in medium and high light. In contrast, intermediate-sized diatoms (11.5 mu m diameter) displayed fastest net growth under low light, subsequently dominating the phytoplankton community. Low light was a key factor limiting accumulation and peak phytoplankton biomass, except one incubation displaying relatively high accumulation rates under low light. The 3-week low-light period prior to experimentation likely allowed adaptation to maximize achievable growth and seems a strong determinant of whether the different natural Antarctic phytoplankton populations sustain, thrive or decline. Our study provides improved insight into how light intensity modulates the net response of key Antarctic phytoplankton, both between and within taxonomic groups.

Automated summary

This study combined pigment-based taxonomy with flow cytometry to investigate the light response of Southern Ocean phytoplankton. The results showed that different-sized diatoms exhibit different responses to light, with intermediate-sized diatoms showing the fastest net growth rate under low light and dominating the phytoplankton community. Low light was found to be a key factor limiting accumulation and peak biomass of phytoplankton, and the preceding low-light period likely plays a crucial role in determining the sustainability and growth of different natural Antarctic phytoplankton populations.