4:30 PM - 4:45 PM
[MIS05-16] Primary production and carbon export related to sub-surface chlorophyll maximum off Wilkes land during post bloom season
Keywords:Southern Ocean, phytoplankton bloom, seasonal ice zone, sub-surface chlorophyll maximum
In the Southern Ocean, ecological studies have mainly focused on ice edge phytoplankton blooms. This bloom is usually decline within 10-14 days, after then, sub-surface chlorophyll maximum (SCM) is often found widespread in the seasonal ice zone in austral summer. Structures and ecological roles of SCM have been reported especially in sub-tropical regions. Less is known in the seasonal ice zone in the Southern Ocean, although the important roles for trophic link and carbon cycling were implied by previous snap shots.
We conducted two time-series survey using a drifter array with series of ship based observation around the drifter to reveal the pelagic environment, net primary production (NPP) and export flux when SCM developed, during Southern Ocean cruises of T/RV Umitaka-maru of Tokyo University of Marine Science and Technology. The experiments were conducted off Vincennes Bay (63.5°S, 110°E) from January 17-18, 2017 and January 14-19, 2019. A drifter was composed of GPS buoy, 4 sensor frames (attached with CTD, Chl. sensor and PAR sensor), an upward looking ADCP (300 kHz) and sediment traps. Multiple CTD casts were also conducted to determine vertical profiles of physical parameters and to sample waters for measurements of macro-nutrients, chlorophyll a (chl. a), particulate organic matters (POC and PON) and phytoplankton abundance.
In 2017, SCM was found at 40-50 m depth with lower chl. a of less than 0.4 µg L-1. NPP was vertically even ranged 0.4-5.5 mg C m-3. Standing stock of POC highly decreased during 24 hours due to other than export flux, suggesting high microbial activity which was probably enhanced by small haptophytes dominant food web. On the other hand, marked high SCM, reached 2.0 µg L-1 was found in 2019. NPP was 4 times higher at 10 m depth than that at SCM depth, and surface (0-40 m depth) integrated NPP was higher in the latter period than the former. The temporal changes in NPP would be relation to increase in large diatom abundance (e.g. Chaetoceros spp.). Standing stock of POC was increased in the upper (0-60 m) layer in the latter period, and, at the same time, production-export (P-E) ratio also increased in the lower layer (60-150 m). Temporal changes in temperature at various depth indicates that nutrient supply by vertical mixing was higher in 2019 than in 2017, which would cause differences in biomass and NPP between two years. Increase in standing stock of silicic acid and low δ13C values in POM at SCM in 2019 also supported strong influence of deeper waters. Southern Annular Mode (SAM) from November to January was positive in 2018/19 and negative in 2016/17, respectively. This might cause the difference in pelagic system in two seasons. By the way, diel vertical migration of meso- to macro-zooplankton detected by an ADCP back-scatter suggested strong relationship between SCM and zooplankton communities in 2019.
We found a bottom up regulation of SCM structures. SCM was mainly formed by accumulation of phytoplankton at thermocline, which would play roles as a hot spot of herbivorous and omnivorous feeding of zooplankton community. Further studies focusing on biogeochemical contribution of SCM with physico-chemical conditions relation to SAM are needed for understanding changing ecosystems with climate changes in the Antarctic seasonal ice zone.
We conducted two time-series survey using a drifter array with series of ship based observation around the drifter to reveal the pelagic environment, net primary production (NPP) and export flux when SCM developed, during Southern Ocean cruises of T/RV Umitaka-maru of Tokyo University of Marine Science and Technology. The experiments were conducted off Vincennes Bay (63.5°S, 110°E) from January 17-18, 2017 and January 14-19, 2019. A drifter was composed of GPS buoy, 4 sensor frames (attached with CTD, Chl. sensor and PAR sensor), an upward looking ADCP (300 kHz) and sediment traps. Multiple CTD casts were also conducted to determine vertical profiles of physical parameters and to sample waters for measurements of macro-nutrients, chlorophyll a (chl. a), particulate organic matters (POC and PON) and phytoplankton abundance.
In 2017, SCM was found at 40-50 m depth with lower chl. a of less than 0.4 µg L-1. NPP was vertically even ranged 0.4-5.5 mg C m-3. Standing stock of POC highly decreased during 24 hours due to other than export flux, suggesting high microbial activity which was probably enhanced by small haptophytes dominant food web. On the other hand, marked high SCM, reached 2.0 µg L-1 was found in 2019. NPP was 4 times higher at 10 m depth than that at SCM depth, and surface (0-40 m depth) integrated NPP was higher in the latter period than the former. The temporal changes in NPP would be relation to increase in large diatom abundance (e.g. Chaetoceros spp.). Standing stock of POC was increased in the upper (0-60 m) layer in the latter period, and, at the same time, production-export (P-E) ratio also increased in the lower layer (60-150 m). Temporal changes in temperature at various depth indicates that nutrient supply by vertical mixing was higher in 2019 than in 2017, which would cause differences in biomass and NPP between two years. Increase in standing stock of silicic acid and low δ13C values in POM at SCM in 2019 also supported strong influence of deeper waters. Southern Annular Mode (SAM) from November to January was positive in 2018/19 and negative in 2016/17, respectively. This might cause the difference in pelagic system in two seasons. By the way, diel vertical migration of meso- to macro-zooplankton detected by an ADCP back-scatter suggested strong relationship between SCM and zooplankton communities in 2019.
We found a bottom up regulation of SCM structures. SCM was mainly formed by accumulation of phytoplankton at thermocline, which would play roles as a hot spot of herbivorous and omnivorous feeding of zooplankton community. Further studies focusing on biogeochemical contribution of SCM with physico-chemical conditions relation to SAM are needed for understanding changing ecosystems with climate changes in the Antarctic seasonal ice zone.