Diel variation of chlorophyll-a fluorescence, phytoplankton pigments and productivity in the Sub-Antarctic and Polar Front Zones south of Tasmania, Australia

Marine primary production is a fundamental measure of the oceanʹs capacity to convert carbon dioxide to particulate organic carbon for the marine foodweb, and as such is an essential variable used in ecosystem and biogeochemical models to assess trophic dynamics and carbon cycling. The Sub-Antarctic Zone (SAZ) is a major sink for atmospheric carbon and exhibits large gradients in ocean conditions on both temporal and spatial scales. In this dynamic system, an understanding of small-scale temporal changes is critical for modelling primary production at larger scales. Thus, we investigated diel effects on maximum quantum yield of PSII (FV/FM), photosynthetic pigment pools and primary productivity in the western (Diel 1) and eastern SAZ region (Diel 3) south of Tasmania, Australia, and compared this to a station at the polar front (Diel 2). Phytoplankton in the eastern SAZ had the greatest diel response, with cells showing decreased FV/FM and increased biosynthesis and transformation of xanthophyll and other photoprotective pigments during the day, but only in the surface waters (0 and 10 m). Diel responses diminished by 30 m. Cells in the western SAZ had similar responses across the depths sampled, increasing their FV/FM during the night and increasing their xanthophyll pigment content during the day. Phytoplankton at the polar front (Diel 2) showed intermediate diel-related variations in photophysiology, with xanthophyll conversion and increases in photoprotective pigments during the day but constant FV/FM. These diel changes at all sampling stations had little impact on carbon fixation rates, although cells sampled from the deep chlorophyll maximum at the polar front had significantly lower maximum carbon fixation and minimum saturating irradiance (Ek) compared to the other depths and stations. Considering the oceanographic context, cells at Diel 1 and 2 received less light and were more deeply mixed than cells at Diel 3, causing a dampening of the diel response. These results highlight that phytoplankton in the SAZ is regulated by the physical processes of mixing and light provision, but short-term diel effects on maximum quantum yield of PSII and photoprotective pigments may not propagate to changes in carbon fixation, particularly when cells are nutrient replete. If however, the more stratified eastern SAZ (which had the greatest diel responses) is indicative of how the SAZ region might respond to climate change, then diel effects may become more prominent in the future.