Tsuneo Odate

DIMETHYLSULFONIOPROPIONATE PRODUCTION BY PSYCHROPHILIC DIATOM ISOLATES1

Dimethylsulfoniopropionate (DMSP) production by psychrophilic diatom strains, Chaetoceros sp., Navicula sp., and Nitzschia sp., were experimentally analyzed. All strains showed rapid growth (0.3–0.5u2003d−1) under cold culture conditions (1.7°C). DMSP concentrations (both as particulate, DMSPp, and dissolved, DMSPd) were negligible in cultures of Chaetoceros sp., whereas those for Navicula sp. and Nitzschia sp. increased toward the end of the 56‐day experiments. The ratio of DMSPp:chl a of the latter two species was approximately 5 in the early exponential growth phase and decreased slightly toward the early stationary phase. During the stationary phase, when chl a and fluorescence remained constant, the ratios in both species increased linearly by up to approximately 6.5 times the value at the start of the stationary phase. This growth‐related DMSP production by diatoms may result in the low concentrations of DMSPp observed in the early stage of diatom blooms under natural conditions.

Distribution patterns of micro- and meso-zooplankton communities in sea ice regions of Lützow-Holm Bay, East Antarctica

In the Southern Ocean, zooplankton research has focused on krill and macro-zooplankton despite the high densities of micro- and meso-zooplankton. We investigated their community structure in relation to different sea ice conditions around Japan’s Syowa Station in Lützow-Holm Bay, in the summers of 2011 and 2012. Zooplankton samples were collected using vertical hauls (0–150xa0m), with a closing net of 100-μm mesh size. The results of cluster analysis showed that the communities in this region were separated into fast ice, pack ice, and open ocean fauna. The fast ice fauna had lower zooplankton abundance (393.8–958.9 inds. m−3) and was dominated by cyclopoid copepods of Oncaea spp. (54.9–74.8xa0%) and Oithona similis (6.6–19.9xa0%). Deep-water calanoid copepods were also found at the fast ice stations. Pack ice and open ocean fauna had higher zooplankton abundance (943.6–2,639.8 inds. m−3) and were characterized by a high density of foraminiferans in both years (6.6–61.9xa0%). Their test size distribution indicated that these organisms were possibly released from melting sea ice. The pteropod Limacina spp. was a major contributor to total abundance of zooplankton in the open ocean zone in 2012 (26.4xa0%). The physical and/or biological changes between 2xa0years mayxa0affect the abundance and distribution of the dominant zooplankton taxa such as cyclopoid copepods, foraminiferans, and pteropods. Information on the relationships between the different species associated with sea ice will help to infer the possible future impacts of climate change on the sea ice regions.

Long-Term Trends of Nutrients and Apparent Oxygen Utilization South of the Polar Front in Southern Ocean Intermediate Water from 1965 to 2008

The variation of nutrients over decadal timescales south of the polar front in the Southern Ocean is poorly known because of a lack of continuous observational data in this area. We examined data from long-term continuous hydrographic monitoring of 43 years (1965–2008) in the Indian sector of the Southern Ocean, via the resupply of Antarctic stations under the Japanese Antarctic Research Expedition and Australian Antarctic Research Expedition. We found significant increasing trends in phosphate and nitrate, and a decreasing trend in apparent oxygen utilization (AOU) in intermediate water (neutral densityu200a=u200a27.8–28.1 kgm−3) south of the polar front. The rates of phosphate and nitrate increase are 0.004 µmol yr−1 and 0.02 µmol yr−1, respectively. The rate of decline of AOU was 0.32 µmol yr−1. One reason for this phosphate and nitrate increase and AOU decline is reduced horizontal advection of North Atlantic Deep Water, which is characterized by low nutrients and high AOU. The relationship between climate change and nutrient variability remains obscure, emphasizing the importance of long-term monitoring.

The vertical distribution and abundance of copepod nauplii and other micro- and mesozooplankton in the seasonal ice zone of Lützow-Holm Bay during austral summer 2009

The Antarctic seasonal ice zone (SIZ) is an ecologically important region of the Southern Ocean. However, the abundance and vertical distribution of micro- and mesozooplankton in this zone, in particular the contribution of nauplii of smaller copepod taxa, are poorly understood. We investigated the vertical distribution, abundance, and species composition of micro- and mesozooplankton in the SIZ of Lützow-Holm Bay, East Antarctica, in January 2009. Zooplankton samples were collected using a 60-µm mesh closing net (appropriate to estimate the quantitative abundance of copepod nauplii) vertically hauled from three depth layers (0–100, 100–200, 200–500xa0m). Small calanoid copepods, Oithonidae, and Oncaeidae were dominant among the zooplankton communities in this region. Cluster analysis revealed that the communities were separated by depth into epipelagic (0–200xa0m) and mesopelagic (>200xa0m) groups. Distribution patterns in the 0- to 100-m layer changed at the sea ice edge, but showed no clear relationship with frontal structures. Nauplii of copepods including Oithona spp., Oncaea spp., and Calanoida were major contributors to total zooplankton abundance. The abundance of these nauplii in the surface layer was high in open water and the marginal ice zone but low in the pack ice zone where chlorophyll a concentrations were high. Low water temperature associated with sea ice, rather than food availability, may have caused the low copepod nauplii abundance observed in the surface layer within the pack ice zone. These findings provide valuable background information to monitor variations in zooplankton community structure in response to changing environmental conditions.