In the Shinkawa-estuary, located in the Bisan Seto Strait in the Seto Inland Sea, we examined the primary productivity and mineralization by long-term observation. Primary productivity caused mainly by diatoms in the water column and surface sediments was the highest in summer. It is already known that a high primary production rate can be obtained at the surface sediment. The primary production rate by area in the water column was similar with that observed in the coastal waters ; however, the volume was clearly higher during the period with a high water temperature（maximum rate418μgC L－1h－1）. The nutrient flux from the tidal flat sediment depended on the temperature, and the elution rate was high in the summer（for example, 14mmol-NH4m－2d－1）. Similarly, the excretion rate of NH4 by bivalves also increased during the period of high water temperature. The excretion rate in the summer of 1994/1995, when the abundance of these organisms was high, reached 27 mmol-NH4m－2d－1. Therefore, the mineralization rate in tidal flats depends not only on decomposition by bacteria in the sediment but also on the metabolic activity of dense patches of benthic organisms. Furthermore, the annual mass balance for nitrogen and phosphorus among rivers, the tidal flat and offshore was examined. As a result, both N and P had higher proportions as suspended particles when flowing from rivers to the tidal flat, but the ratio of dissolved inorganic substances was increased when flowing out from the tidal flat to offshore. This indicates that tidal flats function as“places of mineralization” even though they have high primary productivity.

View full abstract

This is a summary of published papers for secondary production of bivalve populations in the coastal region describing（1） regional trends of published papers, and（2）how to improve the empirical model for estimation of secondary production. Papers published in Europe and North America accounted for a greater proportion, but those in Asia, Oceania, and Africa were the minority during the 40-year period. The reason for the lack of published papers is that secondary production was considered a boundary area between population dynamics and material circulation studies. The empirical model, which was one of the simplified estimation methods for secondary production, was constructed using the annual mean biomass as an explanatory variable and was improved by adding a new data set（n＝231, r2＝0．876）

View full abstract

In this study, we observed spatial distributions of planktonic, benthic, and epiphytic microalgal biomass in the seagrass meadows of an estuary（Lake Furen, Japan）. Among planktonic microalgae, phytoplankton was particularly abundant in mesohaline water. On the other hand, suspended microphytobenthos was found to be decreased with increasing salinity and significantly increased with depth. Benthic and epiphytic microalgal biomass were 6．6－143- and7．9－20-times higher than planktonic microalgal biomass in the water column（32－200mg C m－2）, respectively. The sum of planktonic, benthic, and epiphytic microalgal primary production was ca.5，000mg C m－2 d－1 and considered to support fishery production. We therefore concluded that benthic microalgae mainly raised microalgal production throughout the lake, and that planktonic and epiphytic microalgae in particular contributed to microalgal production in oligo- to mesohaline and polyhaline water, respectively.

View full abstract

Seagrass meadows are one of the most productive ecosystems and play an important role in storing large amounts of organic carbon in the sediment as carbon reservoirs. Estuaries are considered to be a net source of atmospheric CO2 due to the inflow and mineralization of terrestrial carbon, but recent studies demonstrated that seagrass meadows in estuaries can be net sinks for atmospheric CO2. We here introduce recent studies on the dynamics of inorganic and organic carbon derived from multiple sources in seagrass meadows, and explain the important ecological functions, i. e. CO2 uptake and carbon storage.

View full abstract

The long-term variations in water quality in the Seto Inland Sea of Japan were examined based on the routine observation data from the local fisheries’ experimental stations from1972 to 2013. The water temperature increased due to global warming, and the increase rate in autumn was especially high. The transparency increased after the 2000s in Osaka Bay, whereas it increased after the 1990s in the Seto Inland Sea, excluding Osaka Bay. The nutrients decreased due to oligotrophication. In the Seto Inland Sea, excluding Osaka Bay, the DIN concentration decreased rapidly in the1970s and decreased again after the2000s. In Osaka bay, however, the DIN concentration did not decrease in the 1970s, but it markedly declined after the 1990s. The high concentration of DIP in the 1970s was decreased around 1980. Thereafter, the concentration remained stable in the Seto Inland Sea, excluding Osaka Bay, but it continued to decline in the surface layer in Osaka Bay. The reduction of DIN and DIP concentrations was largely affected by land load reduction. It is also thought that they were affected by a reduction in supply from the bottom mud and open sea, and increase in the total solar radiation in recent years.

View full abstract

The decrease in nutrient concentrations and the role of bottom sediment on nutrient dynamics in the coastal water were discussed based on information obtained during our previous study. During the high economic growth from the 1960s onwards, the Seto Inland Sea became heavily eutrophicated. Since the enactment of the Law for Conservation of the Environment of the Seto Inland Sea in 1973, the nutrient concentrations gradually decreased. One fundamental explanation for the gradual decrease in DIN（Dissolved Inorganic Nitrogen）concentration from1970to the present is the enactment of the above law by Environmental Agencies. However, the reason for the recent decease in DIN concentration is unknown. Based on the information from our previous study, the upward nutrient flux across the overlying water-sediment interface was thought to greatly affect the water column nutrient concentrations in the coastal ocean. Although we had difficulties in measuring the upward nutrient flux, we demonstrated that the nutrient fluxes were regulated by the temperature and organic matter content in the surface sediment, and the effects of the benthic communities on measuring the fluxes were negligible.

View full abstract

The Goto Islands, located in the eastern-most part of the East China Sea, are one of the most famous fishing areas in Japan. In recent years, fishery yields and seaweed coverage have fallen across a wide area of the region. Moreover, coverage of seaweed beds consisting of mainly Ecklonia kurome and Sargassum macrocarpum has been markedly reduced. The present study conducted extensive spatial surveys（51 sampling stations）of inorganic macronutrients around the eastern area of the northern Goto Islands. Field surveys were conducted during four seasons : July（summer）and October（autumn）2012, and January （winter）and April（spring）2013. In summer and autumn, the PO43－ concentrations at all sampling stations were less than the detection limit（＜0．09μM）. In summer and autumn, opposite relationships between nutrients（NO3－＋NO2－ and S（i OH）4） and salinity were observed depending on the sources of the nutrients ; there were significantly negative relationships in bays that receive relatively large rivers, whereas there was a significantly positive relationship in the eastern part of the study area. These results suggest that nutrients were supplied by terrestrial runoff and regional upwelling, respectively. In winter, there was abundant NO3－＋NO2－（1．8－4．7μM）, PO43－（0．1－0．3μM）and S（i OH）4（3．9－6．7μM）, and these nutrients did not correlate with salinity. In spring, there were significantly positive relationships between salinity and nutrients（NO3－＋NO2－ and PO43－）, and a significantly negative relationship between salinity and S（i OH）4. In addition, S（i OH）4／PO43－ ratios were significantly negatively correlated with salinity, suggesting the relative importance of vertical mixing for PO43－, and terrestrial runoff for S（i OH）4. The present study suggests that although there were different nutrient supply processes depending on the season and area, continued nutrient deficiency（e.g. PO43－ throughout summer and autumn）is unfavorable for the formation of seaweed beds.

View full abstract

Diatoms Skeletonema spp. and Eucampia zodiacus, and dinoflagellates Akashiwo sanguinea and raphidophyte Fibrocapsa japonica are harmful phytoplankton and causative organisms in the bleaching of cultured nori（Phyropia）. The relationships between the population dynamics of these four phytoplankton and environmental factors were examined in the Shiota River estuary in the innermost part of the Ariake Sea, Japan between April 2008 and March 2013. Blooms of the diatoms Skeletonema spp. and E. zodiacus occurred during June to September and from January to March, mainly during February to March, respectively. Blooms of flagellate genera were also found : those of the dinoflagellate A. sanguinea mainly during September to November, and of the raphidophyte F. japonica during August to September and in November. This suggested that the winter blooms of Skeletonema spp. and E. zodiacus were triggered by an increase in transmitted light in the water column. Furthermore, the increase in transmitted light confirmed that stratification was formed due to flush and neap tide even in the mixing period. It was also suggested that A. sanguinea and F. japonica tended to bloom when diatom densities were low.

View full abstract