Hiroshi Kamiya

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.

Elemental (C, N, P) and isotopic (δ13C, δ15N) signature of primary producers and their contribution to the organic matter in coastal lagoon sediment

Estuarine ecosystems are easily deteriorated by organic pollution because of its high primary productivity. To identify chemical proxies for the possible sources of autochthonous organic matter [phytoplankton-derived particulate organic material (POM), macroalgae and seagrass], we measured C:N:P and the ratios of carbon and nitrogen stable isotopes (δ13C and δ15N values) in two estuarine environments, the polyhaline lagoon, Lake Nakaumi, and the oligohaline lagoon, Lake Shinji, in Japan. Due to vigorous photosynthesis, the δ13C of phytoplankton-derived POM in Lake Nakaumi was larger than what would normally be expected from estuarine salinity gradients. Concentrations of nitrogen and phosphorus did not affect the δ13C of phytoplankton-derived POM. The δ15N of all plants was uniform and was higher than the δ15N of sediments. The seagrass showed a higher C:N ratio than POM and macroalgae, while the macroalgae showed a higher N:P ratio. Thus, simultaneous evaluation of C:N and N:P ratios would distinguish these three plant groups, and it would be possible to identify the source plants from the elemental ratios of the sediments.

Observation of wind-induced two-layer dynamics in Lake Nakaumi, a coastal lagoon in Japan

Abstract A response in movement of a two-layered water body in Lake Nakaumi to a strong wind, which suddenly rose and continued for 15 h with nearly constant speed and direction, was observed using the current and salinity meters situated at two points in the lake. At the initial stage of water movement, large fluctuations in current velocity in reverse directions were observed in the upper and lower layers, accompanied by mixing between the two layers. At the stationary stage following the initial stage, current velocities in the upper and lower layers were kept constant, as was salinity. It was considered that a slab-type flow, which enhanced mixing between the two layers, occurred at the initial stage in water movement, then changed into a steady circulation flow at the stationary stage with completion of the set up in the two-layered structure.

Effects of water turbulence on plant, sediment and water quality in reed (Phragmites australis) community

Abstract Even though the interaction between water movements and aquatic plant is crucial for the aquatic ecosystem management, the importance of water turbulence in this regard is not well documented. To add to our knowledge on the interaction between aquatic plant communities and water turbulence, this study examined turbulence, plant, sediment and water quality at the reed community (Phragmites australis) in the Lake Shinji, Japan. Observations were conducted along transects perpendicular to the shoreline. For each transect, reed communities were observed at land ward side, centre, water ward and the outside of the reed community. An elevated level of turbulence was observed outside compared to inside reed community, where the magnitude of turbulence decreased with distance into the community interior. A significant positive correlation was observed for turbulence and surface-dissolved oxygen where the latter was negatively correlated to reed density. Sediment composition was affected by water turbulence where the content of coarse particles positively correlated to turbulence. Accumulation of organic matter in anoxic sediments together with fine particles was observed under low turbulence. Our findings can offer insight into understanding the interactions between turbulence and aquatic plant communities.

Sensitive Method for the Oxidation-determination of Trace Hydroxylamine in Environmental Water Using Hypochlorite Followed by Gas Chromatography

We developed a method for quantifying trace NH2OH in brackish- and sea-water samples. Previously reported methods applicable to fresh water cannot be applied to such samples. We determined that interference in seawater owing to the bromide ion can be removed by the addition of phenol. In our procedure, phenol and hypochlorite solutions were added to a sample solution to oxidize NH2OH to N2O. N2O in the sample was then quantified by headspace analysis. The method is not affected by the salt content or ammonia, nitrate, or nitrite at concentrations of 300 μgN L-1 or less. It has a limit of detection of 0.2 μgN L-1, and can quantify NH2OH in natural water samples with a wide range of salinity. It was applied to samples from Lake Nakaumi, a brackish lake located in the eastern part of Shimane Prefecture, Japan.

Determination of Trace Hydrazine in Environmental Water Samples by in situ Solid Phase Extraction

A simple and rapid in situ method for the determination of hydrazine based on the concentration of aldazine compound formed by the reaction of hydrazine with p-dimethylaminobenzaldehyde was developed. This method was based on solid-phase extraction using a Sep-Pak C18 cartridge, followed by the quantification of hydrazine using a spectrophotometric method. To a sample solution of environmental water, p-dimethylaminobenzaldehyde solution was added to form aldazine by the reaction with hydrazine. The solution was passed through a Sep-Pak C18 cartridge for the adsorption of aldazine. In the laboratory, the aldazine adsorbed on the Sep-Pak C18 cartridge was eluted by passing a hydrochloric acid-ethanol (1:10) solution through the cartridge, and the color intensity of the solution was measured at 457 nm. The limit of detection for the new method was 0.2 mgN L-1 of hydrazine. The determination of hydrazine in solution was not influenced even by hydrogen sulfide and organic matter. This method was then applied to the brackish water of Lake Nakaumi in the eastern area of Shimane Prefecture, Japan. This method was used to determine hydrazine in freshwater, seawater and wastwater.