Masumi Yamamuro

Chemical tracers of sediment organic matter origins in two coastal lagoons

The distribution of chemical tracers in two coastal lagoons was surveyed to assess changes in the contribution of autochthonous and allochthonous (terrestrial and marine) organic matter sources to sediments along a salinity gradient. C:N:P ratios of sediments from both lagoons were closer to the ratios of phytoplankton than to those of terrestrial plants, suggesting that the contribution of terrestrial organic matter was minor. A strong correlation between δ13C and the distance from the seawater entrance was attributed to changing δ13C of DIC sources used by autochthonous phytoplankton, because sediment N:P reflected the dominant phytoplankton inputs in each lagoon. These results suggest that autochthonous primary production controlled organic matter supply to sediments in both lagoons.

Dynamics of PCDDs/DFs and coplanar-PCBs in an aquatic food chain of Tokyo Bay.

Concentrations and accumulation profiles of PCDDs/DFs and coplanar-PCBs (co-PCBs) in aquatic biota (e.g., plankton, shellfish, benthic invertebrate, and fish) and sediment from Tokyo Bay were examined to elucidate the relationship between bioaccumulation and trophic level in the food web as determined by the stable nitrogen isotope analysis. Bioaccumulation patterns of PCDDs/DFs and co-PCBs varied greatly among congeners. Accumulation patterns of PCDDs/DFs and co-PCBs are not solely explained by their physicochemical properties. Biota-sediment accumulation factors (BSAFs) for co-PCBs in biota from Tokyo Bay were significantly greater than those of PCDDs/DFs. Furthermore, the slopes of the plots of delta15N and BSAF values and water solubility of 2,3,7,8-substituted PCDDs/DFs and co-PCBs were highly correlated. The results of our study would provide the valuable information to understand the accumulation properties of PCDDs/DFs and co-PCBs that can be used as a scientific basis to determine the sediment quality criteria of PCDDs/DFs and co-PCBs.

Role of the bivalveCorbicula japonica in the nitrogen cycle in a mesohaline lagoon

The effect of the bivalveCorbicula japonica on the nitrogen cycle in Lake Shinji, a mesohaline brackish lagoon in Japan, was examined quantitatively based on field surveys and laboratory experiments carried out in the summer of 1982 and 1983. The biomass of the flesh ofC. japonica comprised 97% of the total biomass of the macrozoobenthos in summer. Total biomass ofC. japonica in the lake was estimated 30 986 t fresh wt. The concentration of suspended solids immediately above the lake bottom, whereC. japonica filters the water, was 1.5 to 4 times higher than that in the surface water. In laboratory experiments, the filtration rate was 5.0 litres g dry flesh wt-1 h-1, the excretion rates of ammonia and of feces and pseudofeces ofC. japonica were 200×10-6 g N g dry flesh wt-1 h-1 and 33.4 mg dry wt g dry flesh wt-1 h-1, respectively at 27°C, the average summer water temperature in the lake. From our study, we estimated thatC. japonica filters almost the same amounts of particulate organic nitrogen produced in the lake by phytoplankton and that supplied from rivers, and that it excretes 30% of filtered nitrogen as feces or pseudofeces and 18% as ammonia.

Seasonal and vertical variations of sinking particle fluxes in the West Caroline Basin

Abstract A sediment trap experiment was carried out in the West Caroline Basin, located in the equatorial western Pacific between influences of the Asian monsoon and the open ocean. Annual mass flux at the shallow trap at Site 1 was 57.10 g m -2 yr -1 . Generally, the higher flux of organic matter was associated with higher activities of biogenic opal-producing and carbonate-producing plankton communities. In addition, as the organic matter content increases, the organic carbon/carbonate carbon ratio shows a tendency to increase. Carbonate-producing plankton was predominant during periods 1 and 3 (May to July and November to the beginning of December), which could be due to limited silica supply to the euphotic zone. On the other hand, surface sea water was more nutrient-rich during periods 2 and 4 (August to October and the end of December to April) at Site 1. These high total mass fluxes could be stimulated by wind. The amount of biogenic components collected in the sediment traps and the accumulation in surface sediments at Site 1 could be compared with primary productivity values. Carbonate and biogenic opal fluxes were 99% and 90% less, respectively, in the surface sediments compared to those in the shallow sediment trap. This could be due to the reaction of sinking particles with undersaturated deep sea water just above the sea floor, rather than with the water column during sinking. About 20% of the organic matter was decomposed between the shallow and deep sediment traps and more than 98% between the deep sediment trap and final burial in the surface sediments. The relative amount of organic carbon preserved in surface sediments was about 0.10% of annual primary productivity.

Importance of epiphytic cyanobacteria as food sources for heterotrophs in a tropical seagrass bed

Abstract The natural carbon and nitrogen stable isotope ratios (δ13C, δ15N) of various autotrophs and heterotrophs were measured in a Syringodium isoetifolium-dominated seagrass bed at Dravuni Island, Fiji to define carbon and nitrogen sources for heterotrophic organisms in a system where few animals graze directly on seagrass leaves. The organic carbon, nitrogen, and phosphorus content of organisms was also determined. The δ13C and δ15N data suggest that herbivorous heterotrophs in this seagrass bed depend significantly on epiphytic cyanobacteria rather than seagrass leaves and its detritus. This can be attributed to relative differences in nitrogen content of those organic materials. The cyanobacteria nitrogen content (3.6–4.8% of DW) is nearly half that of heterotrophs (7.0–8.6% N of DW) while that of S. isoetifolium origin (0.6–1.1% N of DW) is less than one third of the cyanobacteria nitrogen content. Phosphorus content was similar among cyanobacteria (0.8–1.1 mg g-1) and S. isoetifolium (0.4–1.4 mg g-1). These results suggest that cyanobacteria are important food sources for heterotrophs at the study site, and that inorganic nitrogen released through breakdown of cyanobacteria by heterotrophs may support the continued production of S. isoetifolium.

Geographical and seasonal variations in mesozooplankton abundance and biomass in relation to environmental parameters in Lake Shinji–Ohashi River–Lake Nakaumi brackish-water system, Japan

Abstract We measured the abundance and biomass of the major taxonomic groups of mesozooplankton at six stations in Lake Shinji–Ohashi River–Lake Nakaumi brackish-water system, Japan, monthly for three full years (1995–1997), except for one station (for 1 year and 9 months). Over the entire area, copepods overwhelmingly dominated the zooplankton community both in terms of abundance (mean: 87.9%) and biomass (83.4%). The remaining taxa were cladocerans (i.e. Diphanosoma brachyurum , Evadone tergestina , Penilia avirostris , Podon leuckarti and Podon polyphemoides ), appendicularians ( Oikopleura dioica and Oikopleura longicauda ), chaetognaths ( Sagitta crassa ) and the larvae of benthos (e.g. polychaetes, bivalves, gastropods and malacostracans). The geographical and seasonal variations of the mesozooplankton community were therefore principally explained by the variations of the copepod community. The geographical difference in copepod species composition was associated with salinity preference or tolerance of respective species. In Lake Shinji, where the salinity was lowest (mean: 4.0), Sinocalanus tenellus was monospecifically abundant with sporadic occurrence of Pseudodiaptomus inopinus . In Ohashi River (mean salinity: 9.9), Acartia hudsonica , Acartia sinjiensis , Eurytemora pacifica and Oithona davisae added to the community. At central and southeast Lake Nakaumi and in Honjo District, where mean salinity ranged from 16.4 to 21.7, these four species became more important than S. tenellus and P. inopinus . At the entrance of Sakai Strait, where the salinity was highest (mean: 24.0), Paracalanus spp. constituted a significant component. Due to large temperature fluctuation with season, the copepods showed remarkable seasonal variations in abundance and biomass, with enormous annual peaks in winter–spring. These annual peaks might be attributed to scarce occurrence of predators.

Seasonal change in a filter-feeding bivalve Musculista senhousia population of a eutrophic estuarine lagoon

Filter-feeding bivalves often predominate the benthic biomass of estuaries, although their population size may drastically fluctuate due to physical and biological disturbances. To examine the recovery of a mussel population after periods of severe predation and anoxia, and to estimate the amount of nutrients removed from the system through mussel production, we surveyed, over 2 years, the Musculista senhousia population in the estuarine lagoon, Lake Nakaumi, Japan. Predation by diving ducks (Aythya fuligula, Aythya ferina and Aythya marila) during winter dramatically reduced the mussel biomass in both years, but recruitment of juvenile mussels sustained the population. Anoxia during the second summer severely reduced the mussel population, resulting in less biomass than in the autumn of the previous year. Potential annual removal of nitrogen and phosphorus from the lagoon water through burial of M. senhousia shells under oxic conditions was estimated to be 7.1 and 5.1 tons, respectively. These are equivalent to 0.7% and 4.9% of the nitrogen and phosphorous annual load entering the lagoon via the main river. Under anoxic conditions, removal would decrease to only 5.6% of the potential amount.

Model study of Lakes Shinji and Nakaumi : a coupled coastal lagoon system

Abstract To understand the physical and biological processes that occur in Lakes Shinji and Nakaumi, a coastal lagoon system, a hydrodynamic–ecological coupled model was applied. The model was run to hindcast these processes from March 1, 1995 to the end of September 1995. The simulation of physical processes suggests that the meteorologically induced sea level variation and a large episodic amount of freshwater discharge effectively caused the water exchange between the open sea and this lagoon system. The simulated results of the biological processes using the ecological model show good agreement with field data except for the phosphate concentration in the bottom water. It was also found that the microbiological processes in the water column, as well as in the sediment, contribute largely to the formation of oxygen-depleted water in the bottom layer. It was also clarified from the simulation that the two species of benthic bivalve, which have dense populations in this lagoon system, play important roles on removing organic particulate matter and nutrients from the system.

Abundance and size distribution of sublittoral meiobenthos along estuarine salinity gradients

Abundance and size distribution of the sublittoral meiobenthos in the upper 3 cm of the sediment were surveyed in the eutrophic, estuarine lagoons, Lakes Nakaumi and Shinji, Japan. Samples were taken in late spring from 10 stations located along salinity gradients. The density of meiobenthos was at a minimum in the intermediate salinity zone. Nematodes in L. Shinji were significantly larger in size than in L. Nakaumi, due to the size reduction of marine species in the latter. The biomass of co-existing bivalves, which would contribute more food supply to the meiobenthic fauna, did not affect the abundance and size of meiobenthos. It was suggested that the salinity gradient was the most influential factor on the abundance and size structure of meiobenthos in estuarine lagoons where salinity is fairly constant.

Comparison of fish fauna in three areas of adjacent eutrophic estuarine lagoons with different salinities

Variation in fish assemblages was investigated to clarify the migration patterns and growth rates of marine, brackish and freshwater species in three adjacent areas with different salinities. In the polyhaline Lake Nakaumi, Japan, benthic fishes and crustaceans were negatively affected by oxygen depletion in the lower water under stratification during the warm seasons. On the other hand, marine and brackish species that once migrated into the Honjo Area, which is isolated from L. Nakaumi and has a homogeneous salinity structure, remained and grew there until winter. In the mesohaline Lake Shinji, two types of fishes were distinguished: one that migrated into the lake during the warmer seasons; and the other which stayed in the lake during winter. Fauna and production in L. Shinji were compared with those of a comprehensive study carried out approximately 35 years ago. For 35 years, Konosirus punctatus and Sardinella zunasi have increased in abundance, while Carassius sp. and Hypomesus nipponensis have decreased. Even with the recent acceleration in eutrophication of the lake, fish production during the warmer seasons has not declined for 35 years, but the fish fauna have drastically changed.