Photosynthesis-irradiance relationship of phytoplankton and primary production in the vicinity of Kuroshio warm core ring in spring

The light-saturated maximum value (P ^B _max) and initial slope (α) of the photosynthesis-irradiance (P-E) curve were examined in a warm streamer, a cold streamer and a warm core ring off the Sanriku area in the subarctic western North Pacific Ocean during an ADEOS/OCTS Sanriku field campaign in early May 1997. BothP ^B _max and α were within the ranges of temperate populations. A regional difference was apparent inP ^B _max: populations in the warm streamer tended to show higher value ranging between 1.92 and 4.74 mgC (mgChla)⁻¹h⁻¹ than those in the cold streamer and the warm core ring (1.35–2.87 mgC (mgChla)⁻¹h⁻¹). A depth variation was also observed in α in both the warm streamer and the warm core ring: shallow populations tended to have lower α than deep populations. The depth variations in bothP ^B _max and α resulted in a lower light intensity of the light saturation in a deeper population than that of a shallower one. These depth-related variations in the P-E parameters were likely a manifestation of “shade-adaptation” of photosynthesis. Photoinhibition was not observed over in situ surface light intensity varying below ca 1600 μmol photon m⁻²s⁻¹. Water-column primary productivity was biooptically estimated to be 233 to 949 mgC m⁻²d⁻¹ using vertical distributions of the P-E parameters, chlorophylla, phytoplankton light absorption and underwater irradiance. Applicability of surface data sets for estimation of water-column productivity is discussed.     

Phytoremediation of shallow organically enriched marine sediments using benthic microalgae

We examined whether replantation of benthic microalgae (BMA) can remediate shallow organically enriched sediment. Nitzschia sp., the dominant species in the tested area (Hiroshima Bay, Japan), was isolated and mass cultured, then replanted in the same area. Changes in the condition of the sediment were monitored for five months. During the study period, we observed an increase in redox potential (ORP) and a decrease in acid-volatile sulfide (AVS) in the experimental area, indicating that the sediment condition changed from reduced to oxic. Organic matter in the sediment, represented by chemical oxygen demand (COD), ignition loss (IL) and organic nitrogen (ON) decreased significantly, while inorganic nutrients (ammonia and phosphate) increased in the interstitial water. These changes imply that oxygen produced by the replanted BMA may have enhanced aerobic bacterial activity, accelerating the decomposition of organic matter. Thus, replantation of BMA shows potential as a novel and promising "phytoremediation" method for organically enriched sediment.     

Process of the Tsushima Current formation revealed by ADCP measurements in summer

In order to clarify detailed current structures over the continental shelf margin in the East China Sea, ADCP measurements were carried out in summers in 1991 and 1994 by the quadrireciprocal method (Katoh, 1988) for removing diurnal and semidiurnal tidal flows from observed flows, together with CTD measurements. We discussed the process of the Tsushima Current formation in the East China Sea. The Tsushima Current with a volume transport of 2 Sv (1 Sv=10⁶ m³s^–1) was found north of 31°N. A current with a volume transport of 0.4 Sv was clearly found along the 100 m isobath. Between the Kuroshio and the current along the 100 m isobath, southeastward component of velocity was dominant compared to northwestward one. Four eastward to southeastward currents were found over the sea bed shallower than 90 m depth. Total volume transport of these four currents was 1 Sv, and they seemed to be originated from the Taiwan Strait. Intrusion of offshore water into the inner shelf northwest of Amami Oshima was estimated to have a volume transport of 0.6 Sv. It is concluded that the Tsushima Current is the confluence of these currents over the continental shelf margin with the offshore water intruding northwest of Amami Oshima.     

Regionality and seasonality of submesoscale and mesoscale turbulence in the North Pacific Ocean

The kinetic energy (KE) seasonality has been revealed by satellite altimeters in many oceanic regions. Question about the mechanisms that trigger this seasonality is still challenging. We address this question through the comparison of two numerical simulations. The first one, with a 1/10° horizontal grid spacing, 54 vertical levels, represents dynamics of physical scales larger than 50 km. The second one, with a 1/30° grid spacing, 100 vertical levels, takes into account the dynamics of physical scales down to 16 km. Comparison clearly emphasizes in the whole North Pacific Ocean, not only a significant KE increase by a factor up to three, but also the emergence of seasonal variability when the scale range 16–50 km (called submesoscales in this study) is taken into account. But the mechanisms explaining these KE changes display strong regional contrasts. In high KE regions, such the Kuroshio Extension and the western and eastern subtropics, frontal mixed-layer instabilities appear to be the main mechanism for the emergence of submesoscales in winter. Subsequent inverse kinetic energy cascade leads to the KE seasonality of larger scales. In other regions, in particular in subarctic regions, results suggest that the KE seasonality is principally produced by larger-scale instabilities with typical scales of 100 km and not so much by smaller-scale mixed-layer instabilities. Using arguments from geostrophic turbulence, the submesoscale impact in these regions is assumed to strengthen mesoscale eddies that become more coherent and not quickly dissipated, leading to a KE increase.     

Middle Layer Intrusion as an Important Factor Supporting Phytoplankton Productivity at a Tidal Front in Iyo Nada, the Seto Inland Sea, Japan

Field observations were conducted to examine the processes governing the phytoplankton distribution and photosynthetic activity in and around a tidal front formed in Iyo Nada, the Seto Inland Sea, Japan. The existence of a middle layer intrusion, which, it has been suggested, moves from the mixed region to the stratified region of the tidal front, was ascertained by the phytoplankton distribution in addition to a T-S diagram. Skeletonema costatum, which originally inhabited the mixed region, was used as the indicator to reveal the intrusion. However, the tip of water containing the S. costatum population did not extend deeply into the stratified region. The velocity of the intrusion seemed to be slow enough to make biological processes, such as nutrient uptake by phytoplankton and subsequent growth, as well as the decrease in cell density due to zooplankton grazing, dominate during the transportation. The patchy distribution of copepod nauplii implied that grazing has an influence on the distribution pattern of phytoplankton. The location of high photosynthetic activity did not coincide spatially with the center of high phytoplankton biomass, suggesting the importance of these biological processes. Therefore, it is considered that the middle layer intrusion plays a role as an inducer of subsequent biological processes at the tidal front by not only supplying nutrients from the mixed region but also by increasing the vertical diffusivity.     

The Role of Suspended Oyster Culture on Nitrogen Cycle in Hiroshima Bay

The predominance of bivalves affects the cycle of materials in the coastal ecosystem. In the present study, the role of suspended oyster culture on the nitrogen cycle was demonstrated for the northern Hiroshima Bay. The nitrogen cycle was considered as two systems, (1) the primary production (PP) system and (2) the oyster culture (OC) system. The results show that about 26% of N productivity was supplied to process by cultured oysters in the OC system. This process varies seasonally due to the seasonal variations of PON, physiological activities and biomass of oysters. The N processing rates were found to be high in summer and low in winter. The biodeposition and excretion of N in the OC system are 3.0 and 2.1 ton N d^–1, while the natural sedimentation rate and N regeneration in the PP system are 8.3 and 18.0 ton N d^–1, which indicates that the PP system is a major system regenerating N in the water column. The release of total dissolved N from the bottom to the water column is about 8.3 ton N d^–1. The amount of N harvested as oyster product was about 1.3 ton N d^–1, which is about 10% of daily N loading in north Hiroshima Bay. According to the N cycle developed in the present study, the results suggest the significant role of suspended oyster culture on the nitrogen cycle in Hiroshima Bay. In addition, our results indicate that oyster production was efficiently harvested, suggesting that oyster culture could probably be used as a tool to remove N from Hiroshima Bay.     

Determination of cation distribution in (Co,Ni,Zn)2SiO4 olivine by synchrotron X-ray diffraction

 The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal diffraction method. The crystal data are (Co_0.377Ni_0.396Zn_0.227)₂SiO₄, M _r  = 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm³, Z = 4, D _x  = 4.864 g cm⁻³, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)_M1/(A/B)_M2] = −0.272 (IR _A -IR _B ) + 3.65 (EN _A −EN _B ), where IR (pm) and EN are ionic radius and electronegativity, respectively. Received: 8 April 1999 / Revised, accepted: 7 September 1999     

Surface current structure of the Tsushima Warm Current region in the Japan Sea derived by satellite-tracked surface drifters

We examined the surface current structure in the Tsushima Warm Current (TWC) region of the Japan Sea based on a dataset from satellite-tracked surface drifters, including our new deployments during 2004–2008. The gridded mean current velocity and mean kinetic energy (KE) fields calculated from all available drifter data revealed the structure and seasonality of the three branches of the TWC. By comparing the eddy kinetic energy (EKE) field to the KE field, we found four regions where the ratio of EKE to KE was high. In particular, this ratio became extremely high east of the Oki Islands located between the first branch of the TWC (FBTWC) and the second branch of the TWC (SBTWC). Our analysis suggests that the generation of alternate warm and cold eddies east of the Oki Island induced this high ratio. The occurrence of a warm or cold eddy was related to the route—either nearshore or offshore—of the SBTWC. When the offshore route of the SBTWC became dominant, a warm eddy was frequently generated east of the Oki Islands. In contrast, when the nearshore route of the SBTWC became dominant, a cold eddy was more likely to be generated.     

Comparing management of recreational Pagrus fisheries in Shark Bay (Australia) and Sagami Bay (Japan): Conventional catch controls versus stock enhancement

This paper compares the management of recreational fisheries for pink snapper (Pagrus auratus) in the inner gulfs of Shark Bay (Australia) and the closely related red sea bream (Pagrus major) in Sagami Bay (Japan). Fishing and other factors have resulted in population declines of these species in both regions. In response, fishery managers have employed contrasting management, more conventional catch controls in Shark Bay and stock enhancement in Sagami Bay. Although recreational harvest levels were higher than commercial levels in both fisheries, the driving mechanisms are comparatively different due to historical, social, economic and political issues in the respective locations.     

Geochemical diversity in submarine HIMU basalts from Austral Islands, French Polynesia

We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the ⁴⁰Ar/³⁹Ar ages of the submarine basalts show no significant age gaps between the submarine and subaerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low ¹⁷⁶Hf/¹⁷⁷Hf decoupled from ¹⁴³Nd/¹⁴⁴Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd–Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation.