Temporal variation in phytoplankton composition related to water mass properties in the central part of Sagami Bay

Temporal variations in water mass properties and the composition of phytoplankton pigments in the central part of Sagami Bay were investigated by monthly observations from June 2002 to May 2004. Eleven pigments were quantified using high-performance liquid chromatography (HPLC) from 100%, 20%, and 5% light depths relative to the surface; the class-specific composition of phytoplankton community was then obtained by CHEMTAX analysis. The study area was influenced by the Kuroshio water for most of the observation period. The mean contribution of diatoms in all samples was relatively low (29%), while that of flagellates, mainly chlorophytes or cryptophytes, was quite high (60%). The phytoplankton composition at the three depths was uniform throughout the observation period, indicating that the vertical structure of the phytoplankton community did not develop significantly over time. A distinct temporal pattern was observed: flagellates dominated during the summer of 2002 and the winters of 2002–2003 and 2003–2004, while diatoms dominated during the summer of 2003. This pattern was associated with water mass changes. The community in the summer of 2003 was influenced by coastal water. While no distinct spring bloom of phytoplankton was observed, a weak increase in chlorophyll a was observed during the spring of 2004. Ocean color satellite data showed that fluctuations in chlorophyll a concentrations at time scales much shorter than a month occurred during the spring of 2003 and that the elevations in chlorophyll a levels were not continuous. The fluctuations were probably associated with rapid flushing by the Kuroshio water, which has low chlorophyll a content.     

Interannual and Decadal Sea-Level Variations along the Japanese Coast

Interannual (an annual to a decadal) and decadal sea-level variations along the Japanese coast have been investigated on the basis of monthly mean sea level during the period from 1951 to 1995. For the interannual variation, the Japanese coast is divided into six regions according to a regional similarity of sea level by using the cluster analysis, which is close to Tsumura's (1963) classification. The first mode of the Empirical Orthogonal Function (EOF) of the Interannual variation is the simultaneous rising and falling of the sea level along the Japanese coast. The first mode shows the largest variation in winter and has a negative correlation with the wintertime monsoon index; this suggests that the first mode is associated with a steric height change depending on the wintertime cooling intensity. The EOF second mode represents the Kuroshio large meander because strong negative eigenvectors are seen on the southeast coast of Japan and the time coefficient shows a high correlation with the Kushimoto-Uragami sea-level difference. For the decadal sea-level variation, the EOF first mode is a bi-decadal variation exhibiting simultaneous rising and falling of the sea level for the entire Japanese coast. The time coefficient of the first mode shows the first minimum in 1965, reaches a maximum in 1975, and decreases to the second minimum in 1984. The decrease from the mid-1970s and the increase from the mid-1980s to early 1990s correspond to the climatic regime shifts occurring in the same periods.     

Verification of vertically generalized production model and estimation of primary production in Sagami Bay,Japan

The Vertically Generalized Production Model (VGPM) was verified by the primary production data of the Sagami Bay, Japan. The VGPM with open ocean parameters including P ^B _opt , maximum primary production per unit of chlorophyll a in the water column, explained only 40% of the variability of integrated primary production. Formulations of the open ocean P ^B _opt showed no correlation with in situ P ^B _opt . Adjustment of the parameters of chlorophyll a and temperature dependent P ^B _opt improved the estimation of integrated primary production to 47% of the variation. Vertical integration parameters of VGPM also have to be adjusted to improve the estimation. Integrated primary production calculated with a stronger light dependency and with the adjusted P ^B _opt model can explain 74% of the variation. This model was used to estimate primary production of the Sagami Bay during 2003 with satellite data. In situ measurements on cloudy days indicate that the use of satellite data from sunny days only overestimates primary production.     

Role of environment and hydrography in determining the picoplankton community structure of Sagami Bay,Japan

Seasonal variations in the picoplankton community were investigated from June 2002 to March 2004 within the photic zone of Sagami Bay, Japan. The study area was mostly dominated by coastal waters during the warm period (mixed layer water temperature ≥ 18°C). During the cold period (mixed layer water temperature ≤ 18°C), the water mass was characterized by low temperature and high saline waters indicative of the North Pacific Subtropical Mode Water (NPSTMW). Occasionally, a third type of water mass characterized by high temperature and low saline properties was observed, which could be evidence of the intrusion of warm Kuroshio waters. Synechococcus was the dominant picophytoplankton (5−28 × 10¹¹ cells m⁻²) followed by Prochlorococcus (1−5 × 10¹¹ cells m⁻²) and picoeukaryotes during the warm period. Heterotrophic bacteria dominated the picoplankton community throughout the year, especially in the warm period. During the Kuroshio Current advection, cyanobacterial abundance was high whereas that of picoeukaryotes and heterotrophic bacteria was low. During the cold period, homogeneously distributed, lower picophytoplankton cell densities were observed. The dominance of Synechococcus in the warm period reflects the importance of high temperature, low salinity and high Photosynthetically Active Radiation (PAR) on its distribution. Cyanobacterial and heterotrophic bacterial abundance showed a positive correlation with temperature. Prochlorococcus and picoeukaryotes showed a positive correlation with nutrients. Picoeukaryotes were the major contributors to the picophytoplankton carbon biomass. The annual picophytoplankton contribution to the photosynthetic biomass was 32 ± 4%. These observations suggest that the environmental conditions, combined with the seasonal variability in the source of the water mass, determines the community structure of picoplankton, which contributes substantially to the phytoplankton biomass and can play a very important role in the food web dynamics of Sagami Bay.     

Lagrangian observation of phytoplankton dynamics at an artificially enriched subsurface water in Sagami Bay,Japan

Phytoplankton dynamics in the lower euphotic zone were observed by tracking a subsurface water released at 20-m depth from Takumi, an artificial upwelling device. Takumi continually discharged seawater pumped up from a depth of 205 m: this water was mixed with 5-m depth water to adjust the density to that of 20-m depth water of Sagami Bay, Japan. The discharged water was pulse-labeled at Takumi with uranine and tracked for 63.9 h with a drifting buoy equipped with a drogue at 20-m depth. We present a simple model to estimate in situ phytoplankton net growth rates from temporal changes in phytoplankton abundance in the discharged water with correction for the influence of water exchange between the discharged water and neighboring layers. Lagrangian observation showed active growth of pico- and nanophytoplankton, especially cryptophytes and Synechococcus (Cyanobacteria), in the subsurface layer. In contrast, diatoms grew little in spite of micromolar concentrations of nutrients. The active growth of pico- and nanophytoplankton was in good agreement with shipboard serial dilution culture experiments. The low growth activity of diatoms was suggested to be related to low light availability in the subsurface layer.     

Education for sustainable development for Tokyo Bay: Developing a practice framework of university-based coastal ESD

This paper reviews the initial phase of a coastal education for sustainable development program for Edomae, the innermost reaches of Tokyo Bay. The program has been steered by a working group of Tokyo University of Marine Science and Technology faculty members from different academic backgrounds. Although the process began with conventional educational ideas, the ESD practice framework evolved to include more interactive activities. The overall goal is to pursue discussions of a plan for the sustainable use of Tokyo Bay in the coastal communities through a university–community partnership by developing Edomae ESD leaders in the coastal community.