Yasushi Kurihara

Stable isotopic structure of aquatic ecosystems

Isotopic, biogeochemical and ecological structure can provide a new dimension for understanding material flows, and the simultaneous function and structure of an ecosystem. Distributions ofδ13C andδ15N for biogenic substances in the Nanakita river estuary involving Gamo lagoon in Japan were investigated to construct isotope biogeochemical and ecological structure for assessing fate and transfer of organic matter, and food web structure. The isotopic framework of the ecosystem was successfully described in aδ15N–δ13C map. In this estuary the variations of isotope ratios of biogenic substances were clearly explained by the mixing of land-derived organic matter, and marine-derived organic matter.A trophic-level effect of15N enrichment was clearly observed. Organisms were classified into three groups depending upon the contribution of land-derived organic matter in a food chain. Almost all biota except mollusca in the lagoon depend on organic matter of marine origin. The contributions of both land and marine organic matter were comparable for mollusca in the lagoon.

Changes of traits in a bacterial population associated with protozoal predation

In an attempt to understand the significance of predation in the evolution of prey species, the ecological and morphological characteristics of bacterial species under predation by a ciliated protozoa,Cyclidium sp., were investigated. Serial transfer at 7 day intervals was applied to the bacterial populations in the presence or absence ofCyclidium. Although cells of the parental bacterial strain are typically short rods up to 1.5 μm long, cells of much greater length, up to 20 μm long (type L) were found in populations exposed to predation fromCyclidium. However, the wildtype, shorter length bacteria persisted even after the appearance of type L. Type L was not observed in the singl bacterial culture throughout the serial transfers. Type L appeared to improve the ability to escape predation by elongating cell size, but growth rate and saturation density were decreased.

The feeding habits and food sources of the deposit-feeding polychaete, Neanthes japonica (Izuka)

Abstract The food sources exploited by the deposit-feeding polychaete, Neanthes japonica (Izuka) have been studied. The viable bacteria growing on the surface of certain components of the deposit play an important role as a source of food for N. japonica . Furthermore, food consumption levels for N. japonica were higher for diatoms, blue-green algae, and fragments of marsh reed, which had been decomposed by bacteria, than for fresh ones. The importance of bacteria and protozoans as food for N. japonica was supported by the observation that sludge obtained from sewage aeration tanks, which consisted of bacteria and protozoa, was easily consumed. The faeces of N. japonica , which contain 3.8% total carbon and 0.3% total nitrogen, act as the substrate for bacteria, protozoans, and microalgae after decomposition by bacteria, and as food for N. japonica .

Removal of nitrogen, phosphorus and COD from waste water using sand filtration system with Phragmites australis

Abstract The removal efficiencies of nitrogen, phosphorus and COD from waste water were examined using sand filtration systems with Phragmites australis (Cav.) Trin. ex. Steudel. The quality of effluent waters from the system with plant were far better than those from the one without plant, implying Phragmites could incorporate nitrogen and phosphorus into its tissues and promote phosphorus absorption onto the sand by the release of oxygen from the roots. The P-pot provided with the influent containing 198 mg l - of total nitrogen and 21 mg l -1 of total phosphorus had the highest biomass of Phragmites . Harvestable above-ground biomass accounted for about 3.5 kg m -2 and removable nitrogen and phosphorus accounted for 69 and 6 g m -2 respectively. The removal rates of total nitrogen and phosphorus in the system with Phragmites receiving variable amounts of COD were almost at the same level and also much better than those of the systems without plant, implying that the different COD concentrations in the influent media do not impair the removal efficiencies of nitrogen and phosphorus. Also Phragmites was found to resist COD concentration as high as 128 mg l -1 , and signs of clogging were not detected in this system throughout the experiment.

The distribution and abundance of Helice tridens (De Haan) burrows and substratum conditions in a northeastern Japan salt marsh (Crustacea: Brachyura)

Abstract The effects of the physical condition of the substratum on the distribution and abundance of mud-crab burrows were investigated. In the salt-marsh habitats studied, where burrows were abundant, there was a high proportion of silt-clay. In such conditions, the diameters of openings and the depths of the burrows were greater than those when the relative proportion of silt-clay was low. In the laboratory, it was found that with a low proportion of clay, mud-crabs did not dig burrows when the water content was high and thus equivalent to that in the marsh. When the proportion of silt-clay was low, the larger mud-crabs did not make burrows at the depth of the water table in the marsh, because the layer above the water table was too thin to allow construction of the burrow. The field observations could thus be explained by the results of experiments showing that the depth of the burrows was controlled by the depth of the water table and the relative proportion of silt-clay in the substratum.

Relationship between burrowing activity of the polychaetous annelid, Neanthes japonica (Izuka) and nitrification-denitrification processes in the sediments

The influence of the benthic organism, Neanthes japonica (Izuka) on nitrification-denitrification processes has been studied in experimental aquaria. The aquaria were filled with diluted sea water with or without nitrate, and contained or did not contain N. japonica. In the series supplied with diluted sea water without nitrate, extremely high concentrations of nitrite + nitrate nitrogen were found in the inner layer of the burrow wall, and denitrification activity in the surface layer (0–0.5 cm) containing N. japonica was three times that of control. In the series supplied with nitrate, there was no significant difference in the activity between surface layers with and without N. japonica. The influence of bioturbation of sediments by N. japonica on nitrification-denitrification processes and its mechanisms have been discussed.

The effects of burrowing of Helice tridens (De Haan) on the soil of a salt-marsh habitat

Abstract The physical and chemical effects of the burrowing activity of the mud crab Helice tridens (De Haan) on the soil of a salt-marsh habitat were investigated. Soil-turnover rate caused by burrowing activity was found to be ≈ 3% of the soil from the surface to a depth of 40 cm every day during the summer. The vertical distributions of leaf and stem fragments of the salt-marsh plant Phragmites australis (Trin.) and the vertical distribution of ammonium N concentration in the soil were also investigated. At locations in the marsh where there were many large burrows, numerous leaf and stem fragments were recognized in the soil, while in areas in the marsh containing only a few small burrows these fragments were scanty. The soil depths at which leaf and stem fragments were abundant, corresponded to the depths of the burrows. These results show that mud crabs bury fallen plant fragments in the soil by their burrowing activity. Ammonium N in the soil was also abundant at locations in the marsh where there were many burrows, indicating that organic matter, such as fallen leaves and stems, may be decomposed to inorganic nutrients which are useful to the salt-marsh plants.

Effect of the feeding behaviour of macrobenthos on changes in environmental conditions of intertidal flats

Abstract This experimental study was conducted to investigate how macrobenthos feeding behaviour changes the environmental conditions of intertidal flats. Three representative species living in Nanakita River Estuary, Miyagi Prefecture, Japan were studied. Both the deposit-feeding polychaete Neanthes japonica (Izuka) and the bivalve Nuttallia olivacea (Jay) feed on fine particulate deposits containing rich organic materials on the surface of the flats. Thus, they decrease the organic matter content of the substratum and cause a sandy condition. The suspension-feeding bivalve Laternula limicola (Reeve), however, does not feed on fine particles deposited on the flats and, therefore, does not play an important role in changing the organic matter content of the substratum. When organic matter accumulates gradually on the surface, Eh potentials fall and reducing conditions occur in the substratum. Benthic animals living under these conditions may increase Eh potentials by their feeding behaviour and movements.

Effect of Cu2+ stress on an aquatic microcosm: a holistic study.

Cu2+ stress was exerted on an aquatic microcosm containing green algae (Chlorella and Scenedesmus), a filamentous blue-green alga, ciliata protozoa (Cyclidium), two rotifers (Philodina and Lepadella), aquatic oligochaetes (one species), and bacteria (five species or more), and the changes in population density and community metabolism were observed. Exposure to Cu2+ in a static system in the early and middle phases of heterotrophic succession showed that the shift thus induced was greater in the early phase than in the middle phase. This shows the variations of the response to stress at successive stages. The response of the system was compared in a continuous-flow system on the basis of the amount of Cu2+ added, at two concentration levels of organic matter (1 and 50 ppm). The shift thus induced was greater at the high nutrient level than at the low nutrient level. This indicates that in a given system containing the same biological species, the response to stress varies with the nutrient level. These characteristic responses of an ecosystem to chemical stress are one of the factors to be taken into account when the toxicity of a chemical agent on a natural ecosystem is predicted.

The effects of the oligochaete Branchiura sowerbyi Beddard (Tubificidae) on the biological and chemical characteristics of overlying water and soil in a submerged ricefield soil system

Submerged ricefield soils, outdoors, and with and without the tubificid Branchiura sowerbyi Beddard, were used to study the effects of tubificids on some biotic and abiotic variables. Measurements of chemical and biological features showed that the tubificids accelerated the diffusion of dissolved substances, increased the release of soil bacteria and enhanced the production of algae and a duckweed (Lemna paucicostata) in the overlying water. In the presence of tubificids, increased numbers of Cladocera (Moina sp., Simocephalus sp.) and ostracods in the overlying water, and higher number of sulfate-reducing bacteria in the soil were observed. The tubificids further enhanced the microbial activity in the upper soil layer, which in turn produced large amounts of organic matter.