Toru Hirawake

Strategies of phytoplankton to perform effective photosynthesis in the North Water

Abstract During the 1999 expedition of the International North Water Polynya Study in northern Baffin Bay, the photosynthetic characteristics of phytoplankton were measured using pulse-amplitude-modulated (PAM) fluorescence methods. We focused mainly on the functioning of the diadinoxanthin cycle, which was expected to be an effective protection system of photosystems from high irradiance under low temperature. In spite of the low temperature (ca. 0°C), the diadinoxanthin cycle was very active, as monitored by the change of non-photochemical quenching of fluorescence (NPQ). The extent of NPQ became larger when phytoplankton were exposed to higher irradiance, and seemed to be in good correlation with the total amount of diadinoxanthin plus diatoxanthin, which are responsible for the diadinoxanthin cycle. The NPQ was more active in the surface phytoplankton community than in the phytoplankton living at the bottom of the euphotic zone, which had less xanthophyll pigments. These results indicate that the diadinoxanthin cycle actively drains out excess light energy as non-radiative heat and protects photosystems under high irradiance. The active NPQ reduced the effective irradiance; the effective quantum yield of photosystem II became smaller under higher irradiance. Reflecting these features, the resulting rate of photosynthesis in surface phytoplankton tended to be smaller than that of phytoplankton at the bottom of the euphotic zone. The irradiance at the onset of light saturation (Ik) was generally higher than the reported values for polar phytoplankton and ice algae, probably because of the presence of a water–water cycle and/or other metabolic pathways. Our work also shows the usefulness of PAM measurements for ecological and biochemical studies of psychrophilic phytoplankton in polar regions.

Eddies revealed by SeaWiFS ocean color images in the Antarctic Divergence zone near 140°E

[1]xa0Eddy-like distributions of chlorophyll a were clearly detected in the Antarctic Divergence zone around 140°E by SeaWiFS ocean color images. The distribution was also supported by in-situ measurements. The eddy-like structure was associated with changes in surface and subsurface water mass properties. Extremely high levels of chlorophyll in coastal areas were transported and redistributed horizontally offshore by these eddies, and a significant effect of the huge biomass on the biological processes in the investigated area was expected. The lateral exchange between coastal and offshore waters driven by these eddies, may be an essential factor in the observed patchy structures in this region.

An overview of the biological/oceanographic survey by the RTV Umitaka-Maru III off Adelie Land, Antarctica in January-February 1996

Abstract A biological/oceanographic survey was conducted around the Antarctic Divergence (AD) off Adelie Land, Antarctica by the RTV Umitaka-Maru III of Tokyo University of Fisheries in the 1995/1996 Austral summer. Investigations were included (1)xa0characteristics of oceanic structure, (2)xa0primary production and related upper water mass structure, (3)xa0phytoplankton community structure, (4)xa0zooplankton community structure, (5)xa0distribution of Antarctic krill, and (6)xa0optical properties and development of biooptical algorithms for future use of ocean color remote sensing in monitoring phytoplankton biomass. The existence of dense water on the continental slope suggests that the survey area is one of major sources of the Antarctic Bottom Water. We observed a southward meandering of ACC, which allowed the warmer, saline offshore water to intrude south in the eastern part of the area. This east–west gradient in water mass seems to influence the formation of the local trophic structure in the research area. In the western area, primary productivity was high, and diatoms and copepods dominated, particularly around the AD. The eastern part, however, showed low primary productivity, with a relative dominance of pico- and nanophytoplankton and high abundance of salps. Antarctic krill was distributed mostly in the south of the AD, and its biomass was low compared to previous records, both from adjacent areas and from other Antarctic regions. A biooptical algorithm obtained from this study did not agree with the conventional global algorithm, suggesting the need for constructing local algorithms appropriate for Antarctic Waters.

Temporal and spatial patterns in the surface-water biomass of phytoplankton in the North Water

Temperature, salinity, and in vivo fluorescence of surface seawater in the North Water were recorded continuously, using a CTD+fluorometer, in August 1997, April–July 1998 and August–October 1999. The phytoplankton bloom started in the polynya on the Greenland side in April. In April and May, high phytoplankton biomass coincided with saline water on the Greenland side, while biomass was low on the Ellesmere Island (Canada) side where a deep mixed layer prevailed. High phytoplankton biomass extended over the whole polynya in June, when surface temperature increased due to solar heating and salinity decreased due to freshwater input. The initiation of the bloom was about 2 months earlier on the Greenland than the Canadian side. In July and August, phytoplankton biomass became low in the southern survey area, indicating that the phytoplankton bloom had ended. In September, relatively saline and warm water occurred in the southeastern part of the study area where, consistent with the change in water properties, high concentrations of chlorophyll a were observed again. These results imply that both the earlier start of the algal bloom in spring and the eventual increase in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland side, thus influencing the structure and biological productivity of the entire North Water ecosystem.

Long‐term variation of surface phytoplankton chlorophyll a in the Southern Ocean during 1965–2002

[1]xa0The variation in the phytoplankton biomass over a decadal time scale, and its relationship with the Antarctic Circumpolar Wave (ACW) and climate change, has been poorly interpreted because of the limited satellite chlorophyll a (chl a) data compared with the physical parameters from satellite. We analyzed a long-term chl a data set along the Japanese Antarctic Research Expedition (JARE) cruise tracks since 1965 to investigate inter-annual variation of phytoplankton biomass. In the Southern Ocean, increasing trends of chl a and the spreading of higher chl a area to the north with 3–7 year cycles were found. Although relationships between the decadal change in chl a and climate change such as variation of sea ice extent and the El Nino are still obscure, large variation of primary production in proportion to the chl a is implied.

Inter-annual variability of chlorophyll and sea-ice in the Antarctic Divergence region : an attempt to derive their quantitative relationship

Inter‐annual variability in chlorophyll a (chl a) and sea‐ice concentration in the Antarctic Divergence (AD) region near 140°u2009E was examined using satellite sensor data in order to derive a quantitative relationship between the two. Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) derived chl a and Special Sensor Microwave/Imager (SSM/I) derived sea‐ice concentration data during 1997–2003 were analysed. Mean chl a concentration south of 64°u2009S during the period between September and April (from austral spring to autumn) of each year was calculated. The mean chl a in 2001/2002 was extremely high (1.08u2009mgu2009m−3) within the six periods between 1997/1998 and 2002/2003. Simply integrated indices on sea‐ice amount, such as the maximum sea‐ice extent in the winter season, annual integration of coverage area over ice edge to coast, were not able to explain inter‐annual variation of chl a, especially the highest chl a value in 2001/2002. We found a high correlation between the chl a south of 64°u2009S and the sea‐ice index, which might be due to the surface meltwater of the sea‐ice from the AD zone as a result of eddies. The quantitative relationship might have contributed to the prediction of phytoplankton blooms in this coastal region and demonstrated the impact of the sea‐ice extent on the Antarctic marine ecosystem.

Spatial Patterns in Nutrient and in vivo Fluorescence Distributions in the Marginal Ice Zone and the Seasonally Open Oceanic Zone in the Indian Sector of the Antarctic Ocean, in Austral Summer

Surface temperature, salinity, concentrations of silicate (Si) and nitrate + nitrite (N), and in vivo fluorescence (Fluor) were investigated in the marginal ice zone (MIZ) and the seasonally open oceanic zone (SOOZ) (32–40°E, 64–69°S) from February 23 to 28 1992. In the MIZ the mean Si and N were 67.8 ± 2.2 μM and 32.5 ± 1.7 μM, respectively. There was a trend that low N values coincided with high Fluor values. Observation conducted at one point (64°S, 38°E) revealed a diel variation pattern in Fluor. Applying this pattern of deviation from noon value, all Fluor data were normalized to value at local noon. In the MIZ a significant negative correlation was observed between the normalized Fluor and N but not Si. On the other hand, Si decreased continuously from south to north in the SOOZ and was negatively correlated with the normalized Fluor. Difference in Si concentration was about 30 μM between the sea around 64°S and the MIZ, while the difference in N concentration was estimated as less than 10 μM. If diatoms take up silicate and nitrogen at an approximate ratio of 1:1, additional nitrogenous nutrients other than nitrate and nitrite (e.g. ammonia, urea etc.) would be required. In this case, an f-ratio of lower than 33% is obtained. It is suggested that in the MIZ abundance of phytoplankton community dominated by non-diatom increases utilizing nitrate while in the SOOZ abundance of phytoplankton community dominated by diatoms increases consuming Si and regenerated nitrogen.