Kohei Matsuno

Enhanced role of eddies in the Arctic marine biological pump

The future conditions of Arctic sea ice and marine ecosystems are of interest not only to climate scientists, but also to economic and governmental bodies. However, the lack of widespread, year-long biogeochemical observations remains an obstacle to understanding the complicated variability of the Arctic marine biological pump. Here we show an early winter maximum of sinking biogenic flux in the western Arctic Ocean and illustrate the importance of shelf-break eddies to biological pumping from wide shelves to adjacent deep basins using a combination of year-long mooring observations and three-dimensional numerical modelling. The sinking flux trapped in the present study included considerable fresh organic material with soft tissues and was an order of magnitude larger than previous estimates. We predict that further reductions in sea ice will promote the entry of Pacific-origin biological species into the Arctic basin and accelerate biogeochemical cycles connecting the Arctic and subarctic oceans.

Diets and body condition of polar cod (Boreogadus saida) in the northern Bering Sea and Chukchi Sea

To understand trophic responses of polar cod Boreogadus saida (a key species in Arctic food webs) to changes in zooplankton and benthic invertebrate communities (prey), we compared its stomach contents and body condition between three regions with different environments: the northern Bering Sea (NB), southern Chukchi Sea (SC), and central Chukchi Sea (CC). Polar cod were sampled using a bottom trawl, and their potential prey species in the environment were sampled using a plankton net and a surface sediment sampler. Polar cod fed mainly on appendicularians in the NB and SC where copepods were the most abundant in the environment, while they fed on copepods, euphausiids, and gammarids in the CC where barnacle larvae were the most abundant species in plankton samples on average. The stomach fullness index of polar cod was higher in the NB and SC than CC, while their body condition index did not differ between these regions. The lower lipid content of appendicularians compared to other prey species is the most plausible explanation for this inconsistency.

Seasonal changes in the population structure of dominant planktonic copepods collected using a sediment trap moored in the western Arctic Ocean

Winter ice cover of the Arctic Ocean makes year-round zooplankton sampling by plankton net a difficult task. Therefore, the collection of copepods with a sediment trap can be a powerful tool. In the present study, we analysed the seasonal changes in the population structures of five dominant planktonic copepods (Oncaea parila, Calanus hyperboreus, Metridia longa, Paraeuchaeta glacialis and Heterorhabdus norvegicus), which were collected using a sediment trap rotated at 10–15 day intervals moored at 184–260 m in the Northwind Abyssal Plain (75°00′N, 162°00′W) of the western Arctic Ocean from October 2010 to September 2012. Oncaea parila C6F with egg sacs occurred throughout the year, and the total abundance and composition of early copepodid stages (C1−C3) had two peaks each year. Calanus hyperboreus was dominated by C6F throughout the year, and their maturation was observed during February to May. Metridia longa C6F had a clear seasonality in lipid accumulation and gonad maturation: high lipid accumulation was observed from October to February, whereas gonad maturation occurred from March to September. Paraeuchaeta glacialis C6F also showed seasonality in lipid accumulation and gonad maturation, although their seasonal patterns varied from those of M. longa: high lipid individuals were abundant from February to April and mature individuals dominated from October to November. Heterorhabdus norvegicus showed seasonal changes in population structure as well: C1, C5, and C6M dominated from April to May, November to February and August to October, respectively. The life cycle patterns of these species are compared with those reported from other areas. While the results obtained by a sediment trap are inevitably subject to collection bias (i.e. passive collection at a fixed depth), a sediment trap should be considered as a powerful tool for the evaluation of the life cycle of planktonic copepods, especially in ice-covered oceans.

SEASONAL CHANGES IN BODY SIZE AND OIL SAC VOLUME OF THREE PLANKTONIC COPEPODS, PARACALANUS PARVUS (CLAUS, 1863), PSEUDOCALANUS NEWMANI FROST, 1989 AND OITHONA SIMILIS CLAUS, 1866, IN A TEMPERATE EMBAYMENT: WHAT CONTROLS THEIR SEASONALITY?

Seasonal changes in body size (prosome length: PL) and oil sac volume (OSV) of the three most numerically abundant copepods in Ishikari Bay, northern Sea of Japan, Paracalanus parvus (Claus, 1863), Pseudocalanus newmani Frost, 1989 and Oithona similis Claus, 1866, were studied using monthly samples collected through vertical hauls of a 100-μm mesh NORPAC net from March, 2001 to May, 2002. Seasonal changes in PL were common for the three species and were more pronounced during a cold spring. PL was negatively correlated with temperature, and this relationship was described well using the Bělehradek equation. Seasonal changes in OSV exhibited a species-specific pattern, i.e., OSV was greater during a warm summer for P. parvus and was greater during a cold spring for P. newmani and O. similis. The OSV peak period corresponded with the optimal thermal season of each species. The relative OSV to prosome volume of the small copepods (0.6-0.8%) was substantially lower than that of the large copepods (20-32%). These facts suggest that the oil sac of small copepods is not used for overwintering or diapauses or during periods of food scarcity, but is instead used as the primary energy source for reproduction, which occurs during the optimum thermal season of each species.

Distribution of viable resting stage cells of diatoms in sediments and water columns of the Chukchi Sea, Arctic Ocean

Abstract: The distribution of resting stage cells of diatoms in the surface sediments and water samples collected in the Chukchi Sea was investigated using the most probable number (MPN) analysis to understand the mechanism underlying diatom blooms in the Arctic region. High densities (3.5 × 105 to 6.8 × 106 MPN cells cm−3 wet sediment) of viable resting stage cells of typical Arctic diatom species were found in the surface sediments of the Chukchi Sea. Resting stage cells of the sea-ice–related diatom genus Fragilariopsis were more abundant (2.6 × 106 MPN cells cm−3 wet sediment) than Chaetoceros (2.18 × 106 MPN cells cm−3 wet sediment) or Thalassiosira (1.76 × 106 MPN cells cm−3 wet sediment) in the southern Chukchi Sea. This reflected a previous bloom of Fragilariopsis ahead of typical planktonic blooms of Chaetoceros and Thalassiosira and suggests that sea-ice–related blooms are a substantial factor for primary production in this area. During the sampling period, diatom assemblages in the in situ water columns were dominated by Pseudo-nitzschia delicatissima, Cylindrotheca closterium, Proboscia alata, and Thalassionema spp. After storage of water samples for 6 months in the dark, viable diatom resting stage cells of typical Arctic species such as Attheya longicornis and Chaetoceros socialis were detected. These autochthonous species formed resting stage cells that remained viable in the dark for more than 6 months, while diatom species of Pacific origins could not survive under conditions of extremely limited light. The resting stage cells provide an advantage for autochthonous diatoms to endure the unfavourable light conditions of the Arctic winter. In the Chukchi Sea, resting stage cells of diatoms were supplied by current inflows from shallow sea areas. Densely distributed viable resting stages at the bottom are also a possible source for seeding the diatom cells into phytoplankton communities and ice assemblages in shallow marginal ice zones.

Life history and production of pelagic mysids and decapods in the Oyashio region, Japan

Pelagic Mysidacea and Decapoda have important roles in marine ecosystems. However, information on their life histories is extremely limited. This study aimed to evaluate the life cycles of pelagic Mysidacea and Decapoda in the Oyashio region, Japan. Production of the four dominant species was estimated by combining body mass (DM) data and abundance data. Mysidacea belonging to 5 species from 5 genera occurred in the study area. Their abundance and biomass ranged between 11.7-50.1 ind. m−2 and 1.2-7.9 g wet mass (WM) m−2, respectively. Six species from 6 genera belonged to Decapoda, and their abundance and biomass ranged between 9.0-17.3 ind. m−2 and 3.0-17.3 g WM m−2, respectively. Based on body length histograms, there were two to four cohorts for the three dominant mysids and one dominant decapod on each sampling date. Life histories of the two numerically dominant mysids (Eucopia australis and Boreomysis californica) followed similar patterns: recruitment of young in May, strong growth from April to June, and a longevity of three years. Life cycles of the two minor species (the mysid Meterythrops microphthalma and the decapod Hymenodora frontalis) were not clear because of their low abundance. The timing of recruitment of the young and the strong juvenile growth for the two dominant mysids corresponds with the season when their prey is abundant. The annual production of the dominant mysid species was 14.0 mg DM m−2 (B. californica) and 191.8 mg DM m−2 (E. australis). Annual production/biomass ( P ∕ B ) ratios ranged between 0.242 (H. frontalis) and 0.643 (M. microphthalma). Compared with other regions, the Oyashio region showed high production and low P ∕ B ratios. The high production in the Oyashio region may be related to the high biomass of these species. Because of the low temperature conditions (3°C), pelagic mysids and decapods in the Oyashio region may have slower growth, longer generation times and lower P ∕ B ratios than in other oceans.