Latitudinal and local variations of the life history characteristics of the thalassinidean decapod, Upogebia yokoyai: A hypothesis based on trophic conditions

Mean total length of the eldest cohort (MTLe) of the thalassinidean decapod, Upogebia yokoyai sampled at 16 sites showed a latitudinal variation from south-west islands of the Ryukyu Archipelago (24° N) to the Seto-Inland Sea of Japan (34° N). Local variations were however also found from sites in the same latitudes. The level of the organic content in each habitat showed a positive correlation with MTLe. We hypothesized that MTLe is strongly affected by the trophic condition, resulting in the local variations. To test this hypothesis, two estuaries with different organic content were compared; Kodono River (Kochi Prefecture, 33° N) which was relatively oligotrophic and Sakata River (Hiroshima Prefecture, 34° N) which was highly eutrophicated. The Kodono River population showed slow growth after settlement, and growth stopped in May to June. This population survived for only one or two years, resulting in small MTLe. On the other hand, the Sakata River population showed fast growth from August to December. This population survived for three years, resulting in large MTLe. Maturation was estimated to occur two years after settlement in both populations. Low salinities may partially affect MTLe. Habitats in the Seto-Inland Sea (33.5–34.8° N) were eutrophicated, but many habitats in the subtropical islands (24–31° N) were relatively oligotrophic. Latitudinal variation of MTLe was reflected by the geographical difference in trophic condition. A negative relationship between population density and growth was found, which was explained in terms of reduced survival of recruits in the eutrophicated habitats.     

亚洲通量观测网建立的回顾与展望

1IntroductionTheelucidationofthebudgetsofcarbon,waterandothermassesatvariouslandecosystemshasbecomeanimportantsubjectintheglobalwarmingissue.Forthisstudy,thedatafrompreciseandlong-termobservationsofcarbondioxide,watervapor,andheatfluxtakeninvariouslandsur…     

Emergence Patterns of Small Subtidal Arthropods in Relation to Day/Night, Tidal, and Surface/Bottom Factors: Investigations in the Boreal Sea, Japan (Akkeshi, Hokkaido)

The emergence of small arthropods was studied in the boreal sea, Japan (Akkeshi, Hokkaido). In the shallow subtidal zone, two impeller pumps were set in the surface and bottom waters. The pumps ran continuously for 25 days (22 August-16 September, 1998), and invertebrates were sampled using a nylon net (300 to 500 m in mesh size). The small arthropods collected with the net belonged to 16 orders comprising about 60 taxa. In the dominant 23 taxa, a two-way ANOVA was applied to determine whether there was any significant difference in abundance between day and night and between surface and bottom. While emergence of 15 taxa (65%) was significantly different with regard to the day/night factor, that of the other 8 taxa (35%) was not. As to the difference between the two depths, the distribution of 15 taxa (65%) was significantly different. Furthermore, in 25 taxa for which over 100 specimens were collected in each of the two depths of water, emergence patterns were examined with regard to the synchrony with day/night and tidal cycles. There were various nocturnal patterns, and the strength of the synchrony with the day/night cycle was different for each species or group. Within the same taxa, nocturnal patterns were more clearly manifested in the surface water than in the bottom water. A tidal rhythm of emergence was only seen in zoeas of shrimp. Variations of the emergence patterns of benthic crustaceans are accounted for by a hypothesis that the frequency of swimming during day versus at night is different in each species. On the other hand, the emergence patterns of some zooplankton reflect by the daily rhythm of vertical migration or dispersal in the water column.     

Seasonal change in CO2 and H2O exchange between grassland and atmosphere

The seasonal change in CO₂ flux over an artificial grassland was analyzed from the ecological and meteorological point of view. This grassland contains C₃ and C₄ plants; the three dominant species belonging to the Gramineae; Festuca elatior (C₃) dominated in early spring, and Imperata cylindrica (C₄) and Andropogon virginicus (C₄) grew during early summer and became dominant in mid-summer. CO₂ flux was measured by the gradient method, and the routinely observed data for the surface-heat budget were used to analyze the CO₂ and H₂O exchange between the grassland and atmosphere. From August to October in 1993, CO₂ flux was reduced to around half under the same solar-radiation conditions, while H₂O flux decreased 20% during the same period. The monthly values of water use efficiency, i.e., ratio of CO₂ flux to H₂O flux decreased from 5.8 to 3.3 mg CO₂/g H₂O from August to October, the Bowen ratio increased from 0.20 to 0.30, and the ratio of the bulk latent heat transfer coefficient C_E to the sensible heat transfer coefficient C_H was maintained around 0.40-0.50. The increase in the Bowen ratio was explained by the decrease in air temperature from 22.3 °C in August to 16.6 °C in October without considering biological effects such as stomatal closure on the individual leaves. The nearly constant C_E/C_H ratios suggested that the contribution ratio of canopy resistance to aerodynamic resistance did not change markedly, although the meteorological conditions changed seasonally. The decrease in the water use efficiency, however, suggested that the photosynthetic rate decreased for individual leaves from August to October under the same radiation conditions. Diurnal variations of CO₂ exchange were simulated by the multi-layer canopy model taking into account the differences in the stomatal conductance and photosynthetic pathway between C₃ and C₄ plants. The results suggested that C₄ plants played a major role in the CO₂ exchange in August, the contribution of C₄ plants decreased in September, and daily variations of CO₂ exchange were mainly due to C₃ plants in October. The results also suggested that the decrease in the net canopy CO₂ exchange from August to October was induced partly by the decrease of net photosynthesis on the individual leaves in both C₄ and C₃ plants, which could be due to aging of the leaves.     

Continuous measurement of methane flux over a larch forest using a relaxed eddy accumulation method

We measured the methane flux of a forest canopy throughout a year using a relaxed eddy accumulation (REA) method. This sampling system was carefully validated against heat and CO₂ fluxes measured by the eddy covariance method. Although the sampling system was robust, there were large uncertainties in the measured methane fluxes because of the limited precision of the methane gas analyzer. Based on the spectral characteristics of signals from the methane analyzer and the diurnal variations in the standard deviation of the vertical wind velocity, we found the daytime and nighttime precision of half-hourly methane flux measurements to be approximately 1.2 and 0.7?μg?CH₄?m^?2?s^?1, respectively. Additional uncertainties caused by the dilution effect were estimated to affect the accuracy by as much as 0.21?μg?CH₄?m^?2?s^?1 on a half-hourly basis. Diurnal and seasonal variations were observed in the measured fluxes. The biological emission from plant leaves was not observed in our studies, and thus could be negligible at the canopy-scale exchange. The annual methane sink was 835?±?175?mg?CH₄?m^?2?year^?1 (8.35?kg?CH₄?ha^?1?year^?1), which was comparable to the flux range of 379–2,478?mg?CH₄?m^?2?year^?1 previously measured in other Japanese forest soils. This study indicated that the REA method could be a promising technique to measure canopy scale methane fluxes over forests, but further improvement of precision of the analyzer will be required.     

Hatching of an Estuarine Crab, Sesarma Haematochier: Factors Affecting the Timing of Hatching in Detached Embryos, and Enhancement of Hatching Synchrony by the Female

The zoeas of the estuarine terrestrial crab Sesarma haematocheir hatch in synchrony with nocturnal high tides. Hatching is also highly synchronized among embryos: i.e., female-attached embryos would all hatch within 1 h, if the female is maintained at about 23°C. This study examined the factors affecting the timing of hatching in female-attached and detached embryos. The timing of hatching in the embryos detached from the female was somewhat affected by continuous light, salinity, and cluster size, but it was much more strongly affected by low temperature. In addition, a notable feature was that the influence of low temperature was markedly different between female-attached embryos and detached embryos. Hatching of this crab occurs after a special developmental process for hatching ("hatching program") that requires 48–49.5 h. This program would be mediated by a temperature-dependent process. Hatching of this crab cannot be fully explained in terms of osmotic effects. Since hatching of female-attached embryos was highly synchronized, the female should release an (unknown) stimulus enhancing hatching synchrony among embryos. Highly synchronized hatching among embryos would enable the female to release hatched zoeas into the water all at one time.