Motohiro Shimanaga

Seasonal phytodetritus deposition and responses of bathyal benthic foraminiferal populations in Sagami Bay, Japan: preliminary results from “Project Sagami 1996–1999”

The seasonal carbon cycle was studied in the bathyal environment of Sagami Bay, Japan, to determine whether “benthic–pelagic coupling” takes place in this eutrophic marginal oceanic setting. Both Japanese sea color observation satellite (ADEOS) photography and sediment trap moorings have been used since 1996 for monitoring sea surface primary production. Video records at a real time deep-sea floor observatory off Hatsushima Island in Sagami Bay were also used to monitor the deposition of phytodetritus on the sea floor. At this location, a spring bloom starts in mid-February and ends in mid-April. About 2 weeks after the start of the spring bloom, phytodetrital material is deposited on the sea floor. Video records clearly show that phytodetritus deposition has taken place in the spring of every year since 1994, even though the exact timing is different from year to year. The population size of benthic foraminifera is highly correlated to this phytodetritus deposition. The phytodetritus triggers rapid, opportunistic reproduction of the shallow infaunal taxa, Bolivina pacifica, Stainforthia apertura and Textularia kattegatensis. Shallow infaunal species mainly occur in the phytodetrital layer or just below this layer during the spring. This indicates that such opportunistic species are key indicators of phytodetrital deposition. The deep infaunal taxa Globobulimina affinis and Chilostomella ovoidea show less pronounced seasonal fluctuations in population size, but nevertheless exhibit some response to phytodetrital deposition. Thus the seasonal flux of organic matter is the most important determinant of population size, microhabitats and reproduction of benthic foraminifera in Sagami Bay.

Response of benthic organisms to seasonal change of organic matter deposition in the bathyal Sagami Bay, central Japan

Abstract An interdisciplinary research project was carried out to understand seasonal carbon cycling in a deep-sea ecosystem in Sagami Bay, central Japan. Temporal changes in the chloroplastic pigment (CPE) concentration in the sediment, as well as the abundance, the biomass and the metabolic activity of benthic organisms were studied. CPE was detectable throughout the year, and its amount increased in the spring when a fluffy layer was observed on the surface of the sediment. Significant seasonal fluctuation of bacterial abundance was found, but the range was small, the maximal value being only 1.6 times larger than the minimal one. Metabolic activity did not show significant temporal difference. The abundance and biomass of metazoan meiofauna seemed to fluctuate seasonally, but ANOVA did not confirm it statistically. Our results suggest that if enough organic matter is supplied constantly, the deep-sea benthic community will be stable even though seasonality of organic matter flux associated with the spring bloom exists.

Long-term monitoring of the sedimentary processes in the central part of Sagami Bay, Japan: Rationale, logistics and overview of results

Abstract Deep-sea benthic ecosystems are mainly sustained by sinking organic materials that are produced in the euphotic zone. “Benthic–pelagic coupling” is the key to understanding both material cycles and benthic ecology in deep-sea environments, in particular in topographically flat open oceanic settings. However, it remains unclear whether “benthic–pelagic coupling” exists in eutrophic deep-sea environments at the ocean margins where areas of undulating and steep bottom topography are partly closely surrounded by land. Land-locked deep-sea settings may be characterized by different particle behaviors both in the water column and in relation to submarine topography. Mechanisms of particle accumulation may be different from those found in open ocean sedimentary systems. An interdisciplinary programme, “Project Sagami”, was carried out to understand seasonal carbon cycling in a eutrophic deep-sea environment (Sagami Bay) with steep bottom topography along the western margin of the Pacific, off central Japan. We collected data from ocean color photographs obtained using a sea observation satellite, surface water samples, hydrographic casts with turbidity sensor, sediment trap moorings and multiple core samplings at a permanent station in the central part of Sagami Bay between 1997 and 1998. Bottom nepheloid layers were also observed in video images recorded at a real-time, sea-floor observatory off Hatsushima in Sagami Bay. Distinct spring blooms were observed during mid-February through May in 1997. Mass flux deposited in sediment traps did not show a distinct spring bloom signal because of the influence of resuspended materials. However, dense clouds of suspended particles were observed only in the spring in the benthic nepheloid layer. This phenomenon corresponds well to the increased deposition of phytodetritus after the spring bloom. A phytodetrital layer started to form on the sediment surface about two weeks after the start of the spring bloom. Chlorophyll-a was detected in the top 2 cm of the sediment only when a phytodetritus layer was present. Protozoan and metazoan meiobenthos increased in density after phytodetritus deposition. Thus, “benthic–pelagic coupling” was certainly observed even in a marginal ocean environment with undulated bottom topography. Seasonal changes in features of the sediment–water interface were also documented.

Particle dynamics in the deep water column of Sagami Bay, Japan. I: origins of apparent flux of sinking particles

Abstract Temporal variations of sinking particle flux, together with their organic chemical properties, were monitored in the deep basin of Sagami Bay, Japan, using sediment traps with very high time resolutions from March 1997 to August 1998. At a height of 350 m above the bottom (about 1200 m water depth), the averaged total mass flux was more than 1000 mg/m2/day, which is about 10 times higher than those obtained for open ocean regions near Sagami Bay. While large amounts of phytodetritus, derived from phytoplankton blooms in the surface water, were transported downward in spring, the following extraordinary patterns in the temporal variability of sinking particle flux were also observed: (1) A sustained large flux of sinking particles during low productive periods from summer to winter in 1997. (2) An episodic increase of sinking particle flux in June 1998. (3) A difference in the temporal variability of sinking particles between the spring bloom periods of 1997 and 1998. The content of total organic carbon (TOC) and the stable carbon isotopic ratio (δ13C) of TOC demonstrated that the large fluxes observed in (1) and (2) could be attributed to the resuspension of phytodetritus deposited on the sea floor during the spring bloom period, and the abrupt erosion of surface sediment on the continental slope, respectively. The concentration of suspended particles in the deep water column affect the apparent flux of sinking particles. At the same time, sinking particles exported from surface waters during the spring bloom both decrease and increase suspended particle concentration through scavenging and rebound processes, respectively. Finally, the apparent difference in sinking particle flux between 1997 and 1998, (3), could be explained by differences in the extent of the scavenging process, which depend on the flux and quality of exported particles from the surface waters.

Seasonal patterns of vertical distribution between meiofaunal groups in relation to phytodetritus deposition in the Bathyal Sagami Bay, Central Japan

Temporal changes in the vertical distribution of metazoan meiofaunal groups were examined at a bathyal site (depth 1450 m) in Sagami Bay from December 1996 to August 1998. Among major meiofaunal groups, the vertical distributions of copepods (adults and copepodites), and kinorhynchs changed seasonally and they concentrated in the shallower part in the sediment profile when CPE concentrations were higher. Seasonal changes in the vertical distributions of ovigerous females and nauplii of copepods were not related to CPE concentrations, but related to that of total copepods. On the other hand, nematodes, ostracods, and polychaetes did not show seasonal migrations. These results suggest that there is a difference in the response to the sedimentation of organic matter between metazoan meiofaunal taxa.

210Pb and 137Cs in sediments from Sagami Bay, Japan: Sedimentation rates and inventories

Abstract Profiles of the radioisotopes 210 Pb and 137 Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of 210 Pb ex (unsupported) in core top sediments increased with water depth from 25 dpm g −1 on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g −1 at the deep-sea station SB. The high 210 Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective 210 Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, 137 Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced 137 Cs fallout. The mean sedimentation rates estimated by the 210 Pb ex inventory method, rather than using 210 Pb ex profiles, ranged from 0.06 g cm −2 y −1 to 0.14 g cm −2 y −1 . The average value of the rates in SB cores was calculated to be 0.11 g cm −2 y −1 , which was similar to that calculated under the assumption that the age of the 137 Cs peak corresponds to its maximum fallout year in 1963. Although 137 Cs inventories represented one tenth of the anthropogenic fallout of 137 Cs until 1997, they correlated with the increase in 210 Pb ex inventory. This suggests that the scavenging of refractory 137 Cs as well as 210 Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.

Assemblages of Harpacticoida (Crustacea: Copepoda) from the Ryukyu and Kuril Trenches, north-west Pacific Ocean

To examine the specific features of trench communities, spatial changes in the assemblages of harpacticoids were investigated at the family level around the Ryukyu Trench (the Ryukyu region) and Kuril Trench (the Kuril region). In the Ryukyu region, there were high average dissimilarities in the harpacticoid assemblages among the trench, trench slope and abyssal plain, indicating that the assemblage structures differ substantially between these topographic settings at the family level. Conversely, in the Kuril region, the average dissimilarities in harpacticoid assemblages between the trench and the trench slope and between the trench and the abyssal plain were lower than that between the slope and the abyssal plain. This result suggests that the hadal assemblage is a transition zone between the slope and the abyssal plain in this region. In addition, the analyses indicate that the composition of harpacticoid assemblages is influenced by the quantity of organic matter in the Ryukyu region, while sediment properties play a key role in the Kuril region. Comparisons of the assemblages between the two regions, however, revealed that the average dissimilarities between the trenches and between the abyssal plains were higher than that between the adjacent slopes. This result suggests that interchange among regions is difficult for deep-sea benthic animals, including harpacticoid copepods, likely due to the presence of physical barriers around trenches.

Sex ratio and reproductive activity of benthic copepods in bathyal Sagami Bay (1430 m), central Japan

Sex ratios and reproductive activity of benthic copepod assemblages were investigated at the bathyal site (depth 1430 m) in Sagami Bay, central Japan. The ratio of adult females to adult males was approximately 3.5:1, significantly different from 1:1, although this parameter did not show a seasonal pattern. On the other hand, the percentage of ovigerous females among adult females and the ratio of nauplii to total copepods appeared to fluctuate seasonally in 1997 and 1998. Statistical tests, however, could not detect significant difference in either parameter. We discuss the possibility that the reproductive activity of copepods was enhanced by the increased supply of fresh phytodetritus to the sea floor.

Sex Ratio and Gut Contents of the Deep-sea Harpacticoid Neocervinia itoi and Other Cerviniids: A Possibility of Reduced Foraging among Males

Abstract We investigated the sex differences in the spatiotemporal distribution, body length, and gut contents of adult Neocervinia itoi, the most abundant cerviniid harpacticoid in Sagami Bay, Japan. Two other cerviniid species, Cerviniopsis sp. A and Cervinia bradyi, were also examined for comparison. The density of N. itoi tended to be higher at stations in or below sea bights with steep slopes in the bay, although there was no correlation between their distribution and chloroplastic pigment equivalent (CPE) in the sediment. In general, the sex ratio was strongly biased toward females in this species; the overall mean percentage of males was approximately 23% of adults. The sex ratio seemed to fluctuate temporally, even at the same site, although this fluctuation was not statistically significant. Comparisons of body size and sex ratio between the three species suggest that sampling bias was not the only cause for the observed skewed sex ratio. Distinct sex differences were observed in gut contents of N. itoi. Detritus-like gut contents were clearly observed in the urosomes of almost all females, whereas few males had gut contents. This result suggests that males do not forage during the adult stage. This non-feeding habit of males may result in a shorter lifespan than that of females and may be one cause of the skewed sex ratio. It remains unknown, however, whether this is a general trend among cerviniid species.

Deep-sea meiofauna off the Pacific coast of Tohoku and other trench slopes around Japan: a comparative study before and after the 2011 off the Pacific coast of Tohoku Earthquake

We compared meiofaunal assemblages obtained from the landward slope of the Japan Trench off the Sanriku region of Honshu Island before the 2011 off the Pacific coast of Tohoku Earthquake, 4.5 months and 1.5 years after the earthquake. Sediment samples were collected after the earthquake along two transects. Meiofaunal density after the earthquake did not differ from that before the earthquake. However, meiofaunal composition after the earthquake was different from that before the earthquake, and the composition dispersion after the earthquake was lower than that before the earthquake. These results suggested that the turbidity current may have affected meiofaunal composition and reduced its variability. These changes were probably caused by the seismic motion and displacement of the substratum after the earthquake, rather than sediment redeposition induced by the earthquake. To assess the effect of the earthquake more thoroughly, we compared meiofaunal assemblage in the Sanriku region before and after the earthquake with that in the Kuril Trench (comparable productivity to the Sanriku region) and Ryukyu Trench (lower productivity). Differences in meiofaunal composition before and after the earthquake within the Sanriku region were lower than those between the Sanriku and Ryukyu regions. These results suggested that deep-sea meiofaunal assemblages are influenced by large-scale disturbances, but changes are within the range of variation generated by surface productivity levels.