Samples were collected from 11 August to 5 September 2010 during the SoJaBio expedition in the Sea of Japan (RV “Akademik M.V. Lavrentyev”, Russian Academy of Science). Sampling stations are shown in Fig. 1. The samples were taken along four transects (A, B, C, D). The transect A includes four stations (A1, A2, A6, A7) (Fig. 1). Water depths ranged between 450 to 3370 m.
The transect B includes five stations (B1, B4, B5, B6, B7), water depths ranged between 495 to 3666 m. The transect C includes only two stations (Fig. 1) with water depth from 2700 to 3426 m, and transect D was most southern with two stations D1 and D2, water depths ranged between 2698 to 3358 m.
The sediment samples were collected using with a multicorer Barnett. For the meiofauna analyses the overlying water and the first 5 cm of sediment were taken. Each cylinder has an inner diameter of 10 cm, consequently a surface of 78.54 cm² and a height of 5 cm result in a volume of 392.7 cm³.
In total, 164 samples of meiofauna were taken during the SoJaBio expedition.
For the meiobenthic studies, the upper 5 cm sediment layer from cores was fixed with approxi- mately 5% formalin. All samples underwent further processing upon return to the laboratory. Sedi- ment samples were sieved through sieves with 1000 µm and 40 µm mesh sizes. Meiofaunal animals (only metazoan) were identified and counted to higher taxa under a microscope after staining with Rose Bengal.
Grain size analysis was carried out by combination of two methods. First of all, the sediments were dried in the oven at 105°C for 24 h and weigh it. The sediment particles >0.5 mm were sieved on a series of standard sieves, meshes of which decrease at regular size intervals. The fractions <0.5 mm were present as water suspension and were analyzed using a laser particle size analyzer Analysette 22 (FRITSCH firm). Finally, sediment fractions are normalized to 100% of the initial total amount of the sediment sample.
Yulia A. Trebukhova and Olga N. Pavlyuk
Results
The sediments in the north-western part of the Sea of Japan were presented by the mixed silts.
Grain size was characterised by the dominance of the silt-clay fraction at most investigated stations.
Transect “A”. The shallowest station A2 (depth 459–488 m) was characterized by silted mixed sand sediments with an admixture of clay. Median particle diameter ranged from 0.24 mm at 459-m depth to 0.005 mm at 3367-m depth. The organic matter content in deposits was the lowest – 0.9%.
Fig. 1. A schematic map of sampling area and station locations in north-western part of Sea of Japan.
The study of deep-sea meiofauna
The average density of meiofauna at station A2 was 125.75±40.1 ind/10 cm2. The taxonomic composi- tion of meiobenthos consisted of 18 groups (a class, order) (Fig. 2a). Nematodes were the numerically dominant taxon representing 55.9% of the total meiofauna abundance. Harpacticoid copepods were the second most abundant taxon, representing at most 13% of total abundance. Polychaetes dominated in pseudomeiobenthos with 14.5% (Fig. 2a).
At station A3 (depth 1584 m) the sediment has been presented by the mixed silt. The organic matter content was high enough. A station was characterized by much lower densities of meiobenthos (80.43±6.54 ind/10 cm2) compared to the previous station. A total of 18 different groups were found (Fig. 2a). Nematodes were the most abundant and their contribution to total meiofaunal density made 69.5%, the second taxon were Harpacticoids accounted for 12% of total density. Polychaetes domi- nated in pseudomeiobenthos – 6.7%.
Fig. 2. The percentage of major meiobenthic groups at transects A, B, C, and D. a. Transects A. Other groups:
Halacarida, Hydrozoa, Amphipoda, Cumacea, Gastropoda, Ophiuroidea, Sipuncula, Tanaidacea, Priapulida and Asteroidea. b. Transects B. Other groups: Ostracoda, Turbellaria, Hydrozoa, Amphipoda, Cumacea, Gas- tropoda, Ophiuroidea, Isopoda, Priopulida, Sipuncula, Nemertini, Echinoidea and Asteroidea. c. Transects C.
Other groups: Ostracoda, Bivalvia, Amphipoda, Cumacea, Ophiuroidea and Nemertini. d. Transects C. Other groups: Harpacticoida, Amphipoda, Cumacea and Ophiuroidea.
Yulia A. Trebukhova and Olga N. Pavlyuk
At the station A6 (depth 2511m) the average density of meiobenthos has strongly decreased (14.5±5.8 ind/10 cm2). Taxonomic meiobenthos structure was significantly reduced and consisted of 8 groups. The structure of meiofaunal assemblage is illustrated in Fig. 2a. Dominated nematodes – 66.8%, Harpacticoids have made 15.9% from total density of meiobenthos. In pseudomeiobenthos dominated Polychaeta – 10.3%. The sediments from the deepest-water station A7 (depth 3367 m) were represented by heterogenous silt. Meiobenthos abundance was lowest at the station A7 and has made 10.55±2.5 ind/10 cm2. Taxonomic structure was poor and consisted of only 6 groups (Fig. 2a).
Dominated nematodes 36.9%, other taxa were, in order of importance, harpacticoids, nauplii, turbel- larians. Polychaetes were the dominant group in pseudomeiobenthos (45.7%).
Transect “B”. The shallowest station B7 (depth 483–495 m) was characterized by silted mixed sand sediments with an admixture of clay. The average density of meiofauna at station B7 was 177.42±14.3 ind/10 cm2. The taxonomic composition of meiobenthos consisted of 19 groups.
The structure of meiofaunal assemblage is illustrated in Fig. 2b. Nematodes were the numerically dominant taxon representing 52.5% of the total meiofauna abundance. Harpacticoid copepods were the second most abundant taxon, representing at most 28.2% of total abundance, in pseudomeioben- thos dominated polychaetes – 9.62%.
The station B6 (depth 1040 m) was characterized by silt sediments with an admixture of sand and clay. The average density of meiobenthos has made 103.02±11.1 ind/10 cm2. A total of 18 major groups were found (Fig. 2b). Nematodes were the numerically dominant taxon representing 72.3%
of the total meiofauna abundance, harpacticoid copepods have made – 13.8%, in pseudomeiobenthos dominated Polychaetes – 9.7%. At the station B5 (depth 2666 m) the sediment has been presented by the mixed silt with clay. The average density of meiobenthos has strongly decreased and made 28.42±4.55 ind/10 cm2. The taxonomic composition of meiobenthos consisted of 18 groups (Fig. 2b).
Dominated nematodes 61.3%, the second group in density were harpacticoid copepods – 6.9%, in pseudomeiobenthos prevailed Polychaeta constituting of 23.4% of the total abundance. At the sta- tion B4 (depth 3381 m) the sediment was close in structure to the sediment from the station B5.
The organic matter content in a sediment was the highest – 2.54%. The average density of meio- benthos was low and made 8.82±0.80 ind/10 cm2. Taxonomic meiobenthos structure was poor and totaled only 6 groups (Fig. 2b). Dominated polychaetes 77.4%, in eumeiobenthos dominated nema- todes 15.9%. At the deepest-water station B1 (depth 3666m) the sediment has been presented by the mixed silt with an impurity of clay. The average density of meiobenthos was the lowest and has made 4.80±1.17 ind/10 cm2. Taxonomic meiobenthos structure was poor and totaled only 6 groups (Fig. 2b). Dominated polychaetes – 75.8%, nematodes have made 21%.
Transect “C”. At the station C1 (depth 2781m) a sediment has been presented by the mixed silt with an impurity of sand and clay. The average density of meiobenthos has made 42.73±7.54 ind/10 cm2. A total of 11 different meiofaunal taxa were found (Fig. 2c). Nematodes were the numerical dominant, representing 84.1% of the total abundance, polychaetes were the second most abundant taxon, repre- senting at most 11.8% of total abundance. At the station C3, which is located deeper (depth 3426 m), the sediment has been presented by the mixed silt with an impurity of clay. The concentration of organic matter was higher, than at the station С1 and has made 2.28%. The average density of meio- benthos was low: 14.66±2.86 ind/10 cm2. Taxonomic structure of meiobenthos totaled 10 groups (Fig. 2c). Dominated nematodes 49.6%, and polychaetes 43%.
Transect “D”. At the station D2 (depth 2698 m) the sediment has been presented by the mixed silt with an impurity of sand and clay. The average density of meiobenthos has made 21.24±1.60 ind/10 cm2. The structure of meiofaunal community is illustrated in Fig. 2d. Taxo-
The study of deep-sea meiofauna
nomic structure of meiobenthos totaled 10 groups. Dominated polychaetes 61.9%, nematodes has made 33.3%. At the deeper-water station D1 (depth 3358 m) the sediment has been presented by the mixed silt with an impurity of clay. The average density of meiobenthos was low and has made 12.89±2.51 ind/10 cm2. Taxonomic meiobenthos structure totaled 11 groups (Fig. 2d). Dominated nematodes 83%, polychaetes have made 12%.