Significant nutrient consumption in the dark subsurface layer during adiatom bloom : a case study on Funka Bay, Hokkaido, Japan

We conducted repetitive observations in Funka Bay, Hokkaido, Japan, on 15 February, 4 and 15 March, and 14 April 2019. The diatom spring bloom peaked on 4 March and started declining on 15 March. Funka Bay winter water remained below 30 m depth, which was below the surface mixed-layer and dark-layer depth (0.1 % of the surface photosynthetically active radiation, PAR, depth) on 4 and 15 March. In the subsurface layer at depths of 30-50 m, concentrations of NO3-, PO43-, and Si(OH)(4) decreased by half between these dates, even in the dark. Incubation experiments using the diatom Thalassiosira nordenskioeldii showed that this diatom could consume added nutrients in the dark at substantial rates after pre-culturing to deplete nutrients. Incubation experiments using natural seawater collected in the growing phase of the bloom on 8 March 2022 also showed that nutrient-depleted phytoplankton could consume added nutrients in the dark. We excluded three physical process - water mixing, diffusive transport, and subduction - as possible main reasons for the decrease in nutrients in the subsurface layer. We conclude that the nutrient reduction in the subsurface layer (30-50 m) between 4 and 15 March 2019 could be explained by nutrient consumption by diatoms in the dark in that layer

Effects of Snow and Remineralization Processes on Nutrient Distributions in Multi-Year Antarctic Landfast Sea Ice

We elucidated the effects of snow and remineralization processes on nutrient distributions in multi-year landfast sea ice (fast ice) in Lutzow-Holm Bay, East Antarctica. Based on sea-ice salinity, oxygen isotopic ratios, and thin section analyses, we found that the multi-year fast ice grew upward due to the year-by-year accumulation of snow. Compared to ice of seawater origin, nutrient concentrations in shallow fast ice were low due to replacement by clean and fresh snow. In deeper ice of seawater origin (the lower half of the multi-year fast ice column), remineralization was dominated by the degradation of organic matter. By comparison between first- and muti-year ice, the biological uptake and the remineralization were dominated in relatively young ice and older ice, respectively, under the physical process of brine drainage

Seasonal Variations and Drivers of Surface Ocean pCO(2) in the Seasonal Ice Zone of the Eastern Indian Sector, Southern Ocean

To quantitatively assess the inorganic carbon cycle in the eastern Indian sector of the Southern Ocean (80-150 degrees E, south of 60 degrees S), we measured ocean surface temperature, salinity, total alkalinity (TA), the partial pressure of carbon dioxide (pCO(2)), and concentrations of chlorophyll-a (chl a), dissolved inorganic carbon (DIC), and nutrients during the KY18 survey (December 2018-January 2019). The sea-air CO2 flux in this region was -8.3 +/- 12.7 mmol m(-2) day(-1) (-92.1 to +10.6 mmol m(-2) day(-1)). The ocean was therefore a weak CO2 sink. Based on the DIC and TA in the temperature minimum layer, we estimated the change of pCO(2) from winter to summer (delta pCO(2)) due to changes in water temperature, salinity, and biological activity (photosynthesis). The spatial distribution of pCO(2) in the western part (80-110 degrees E) of the study area was mainly driven by biological activity, which decreased pCO(2) from December to early January, and in the eastern part (110-150 degrees E) by temperature, which increased pCO(2) from January to February. We also examined the changes in the CO2 concentrations (xCO(2)) over time by comparing data from 1996 with our data (2018-2019). The oceanic and atmospheric xCO(2) increased by 23 and 45 ppm in 23 years, respectively. These changes of ocean xCO(2) were mainly driven by an increase in CO2 uptake from the atmosphere as a result of the rise in atmospheric xCO(2) and increase in biological activity associated with the change in the water-mass distribution

Isoprene production in the water column and sediment in Funka Bay, Hokkaido, Japan

We conducted shipboard observations in Funka Bay, Hokkaido, Japan, in 2015, 2016 and 2019 to observe temporal variations of isoprene (C5H8) concentration in water and sediment. We found increases in C5H8 concentrations below and within the euphotic zone, which were associated with chl-a peaks and changes in nutrient and dissolved oxygen (DO). We found a C5H8 peak in the subsurface layer within the euphotic zone at 20-30 m depth in May-June 2019 that coincided with a decrease in DO, suggesting that there was net oxygen consumption. We hypothesize that the rapid cycle of regenerated production in the subsurface layer in early summer resulted in C5H8 production (1.00-1.75 pmol (mu g chl-a) d(-1) at 30 m), decreased DO, relatively high chl-a levels, and nutrient depletion. We propose that C5H8 can be used as a new chemical parameter reflecting primary production, in addition to the common parameters of chlorophyll-a, nutrients, and oxygen. We attributed a C5H8 peak at the surface in May-June 2019 to photosynthetic production of C5H8 associated with new production, likely stimulated by the sporadic input of nutrients to the sea surface. In addition, this is the first study to report the presence of C5H8 in seafloor sediments. We found very high C5H8 concentrations in the sediment from the surface to 5 cm depth. We propose that C5H8 production and degradation by the bacterial community in coastal marine sediments are approximately in balance

Saroma-ko Lagoon Observations for sea ice Physico-chemistry and Ecosystems 2019 (SLOPE2019)

Saroma-ko Lagoon, located on the Okhotsk Sea coast of Hokkaido, is seasonally covered by flat, homogeneous, easily accessible and safe sea ice. As such, it proves a very useful experimental site for the study of sea ice processes, the inter-comparison of methods, the testing of equipment, and the training of researchers new to the Polar regions. In this contribution, we describe a physical, chemical, and ecosystem survey at Saroma-ko Lagoon, conducted over February 23-28, 2019 under the auspices of the SLOPE2019 (Saroma-ko Lagoon Observations for sea ice Physico-chemistry and Ecosystems 2019) program. Sea ice cores were collected to examine temperature, salinity, oxygen isotopic ratio, thin sections, and chemical and biological parameters such as carbonate chemistry, CH4, nutrients, chlorophyll a concentrations, and ice algae community assemblage. Broadband and spectral irradiance measurements were carried out above/under the sea ice, and different sensors were inter-compared at close positions and environments. Equipment such as spectrometers, air-sea ice CO2/CH4 flux chamber, and under-ice turbulent heat flux systems were tested for future Arctic and Antarctic expeditions. Finally, an artificial pool was dug into the sea ice to understand the effect of snow particles on ice growth and to compare the gas exchange process over sea ice with an ice-free water surface. Our SLOPE2019 field campaign activities provided useful information for inter-comparison work and future sea ice research in the polar oceans