The author compiled a pictorial tabular key of male imagoes and nymphs of seven Japanese genera of the family Baetidae (Ephemeroptera), Acentrella Bengtsson, Alainites Waltz and McCafferty, Baetiella Uéno, Baetis (s. str.) Leach, Labiobaetis Novikova and Kluge, Nigrobaetis Novikova and Kluge, and Tenuibaetis Kang and Yang. I also provided taxonomic information on species composition of each genus, diagnostic characters and locality types along with ecological information on the ranges of the distribution and species habitats.

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The red snow phenomenon called ‘Akashibo’ is usually observed from May to June during the melting snow season in Ozegahara, Japan. The red snow takes its color from the reddish-brown particle about 10 μm in diameter that were found to be the round resting spores of snow alga, Phacotaceae Hemitoma sp. They were covered by spiky lorice with Fe compounds as their major components. These results suggest that the red snow phenomenon is mainly caused by the spores of Hemitoma sp. The vertical distribution of spores in snow suggests that water movement from the marshland surface towards the snow surface causes on accumulation of spores in the snow, and thereby giving rise to the red snow phenomenon.

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The salinity tolerance of the mosquito-fish Gambusia affinis that is natived to Japan was investigated in tanks. The salinity of water in the tanks was raised to various levels, and the percentage of fish that survived the increase in salinity was investigated. When the salinity was raised to the sea water concentration (35 psu) within a 24-h period, almost all individuals (more than 90%) died. However, when the salinity was gradually raised over a period of three days or more, almost all survived. Thus, it was shown that the mosquito-fish can survive in sea water. We also considered the possibility of a migration of the mosquito-fish to other geographical areas via the sea.

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In the year 2002, from May to June and October to December, 10 gammaridean species (in total 3117 individuals belonging to seven families) were collected on a red alga Gracilaria asiatica and a green alga Enteromorpha prolifera in a brackish lake, Nakaumi, western Honshu, Japan. Eight of these gammaridean species are newly recorded in the lake: Eogammarus possjeticus, Apocorophium acutum, Monocorophium acherusicum, Monocorophium insidiosum, Paradexamine setigera, Pontogeneia rostrata, Melita setiflagella, and Melita shimizui. In the present study, gammarideans were collected from 9.9 to 25.9 psu in the salinity. Ampithoe valida was the dominant species in the gammarideans on G. asiatica, while M. insidiosum was dominant on E. prolifera in Nakaumi.

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Any one who studies the watershed environments cannot ignore the strong link between terrestrial and freshwater ecosystems through material cycling. Simulation models of material and water cycling are useful tools not only for planning and practicing sustainable uses of forest and water resources and the adaptive management of forested watershed environments, but also for scientific researches. At the 70th annual meeting of the Japanese Society of Limnology (Kashihara, Osaka, September 2005), we discussed the validity and limitations of watershed simulation models in the section “Watershed biogeochemistry: Simulation models for the watershed environment based on material cycling and hydrologic processes”. Biogeochemical and hydrologic models for forest, river and lake environments were introduced in the section. During the discussion, it was recognized that simulation models were valid and powerful for understanding and predicting environmental changes in the watershed. However, their applicability should be carefully checked when they are used for regions under different climatic and environmental conditions. We also recognized that the introduction of the general characteristics of each model would be helpful understand the significance of the simulation model in the limnological sciences. Titles of presentations were as follows: 1. Biogeochemical model in forest ecosystem; Application and problem of PnET model, 2. The influence of forest disturbance and examination of applying the PnET model for the long term influences, 3. Necessity for consideration on hydrological controls of biogeochemical cycling to develop a catchment scale ecosystem model, 4. Quantitative approach and problems of river hydrological simulation models, and 5. Biogeochemical model coupled with hydrodynamic model in lake environment. In this special issue, these five reviews are featured. Although each review is based on the presentation at the annual meeting, the content has been expanded and detailed.

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Biogeochemical processes are closely related to ecosystem functioning and the change in stream chemistry against various environmental changes. A process-based model that replicates the interaction between biotic and abiotic factors is a powerful research tool in improring our understanding the environmental drivers of temporal and spatial fluctuations in biogeochemical cycling and predicting future changes in ecosystem functioning. In this paper, we review the general concepts, structure and applications of the existing process-based model, FOREST-BGC, CENTURY, TEM and PnET. We applied the PnET-CN model in a natural forest watershed in northern Hokkaido, Japan to assess the performance of the model and the problems encountered in improving its application. The PnET model is a generalized, lumped model based on the physiological processes of photosynthesis and evapotranspiration. Predicted seasonality in stream nitrate concentrations using the PnET-CN model was in general agreement with the observed values in Uryu Experimental Forest, Hokkaido University, despite a minor over-prediction during the winter and under-prediction in the non-snowy period. Analysis of the model's structure and validation suggested the need for modifications, including (i) the accumulation and melting process of atmospheric nitrogen deposition in the snowpack, and (ii) the effect of hydrological processes on the ground.

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The influence of forest disturbance was reviewed and examined applying the PnET model developed in the USA. Forest disturbance has a significant impact on nutrient cycling due to the interruption of plant uptake, which is one of the most important pathways for nutrients. Such an impact on nitrogen cycling is especially evident because the recycling, decomposition and uptake between plants and soil are the major pathways of nitrogen. The excess nitrogen produced by interrupted plant uptake is nitrified by soil microbes and leached out of the forest ecosystem to be carried off downstream. The impact of long-term forest disturbance on each process of nutrient cycling was estimated by applying the PnET-CN model. In the simulation results, the patterns of the biomass increment reaching a plateau and the decrease in leaf nitrogen concentration with forest age seemed to be well simulated. However, individual values of those simulated results varied significantly from those of the observed data in known studies. This finding suggests that the simulated results were false, and that the parameters need to be reconfigured based on soil fertility, soil moisture conditions, and root development processes. It appears to develop the new processes of replacement process of forest development. To apply PnET-CN to topographically heterogeneous Japanese forest ecosystems, it is necessary to first develop a hydrological model that can incorporate such heterogeneity. Furthermore, it is necessary to build long-term monitoring systems to provide the basic database needed for feature model construction in Japan

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In order to model the stream water chemistry in a forest catchment, the model should include functions describing both the biogeochemical cycle and the hydrological pathways as the transportation system for solutes and suspended substances. Although several catchment-scale models have been developed based mainly on the data collected under non-monsoon climate corditions in the northeastern United States and northern Europe, it is questionable whether these models are able to simulate the seasonal changes in streamwater chemistry in a forest catchment under monsoon climate conditions. This is because the monsoon-driven precipitation seasonality affects subsurface-groundwater mixing, and this in turn strongly influences the seasonal variation in streamwater chemistry. To simulate this effect in a catchment-scale response, it is important to combine biogeochemical and hydrological models that will be able to express the effects by soil and groundwater storage and their mixing.

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A distributed hydrological model instead of a lumped model has been proposed as the most important tool in enabling the sharing of water information. In essence, a distributed model consists of sub-basins of tributaries and a main river channel. As for the latter, commercial models are available based on advanced mathematical techniques for hydraulic calculations. However, the methodologies for the description of rainfall-runoff phenomena from subbasins have not been fully discussed. Especially for forested areas two key topics need to be addressed. The first is how to continuously describe runoff phenomena over several rainfall events and dry periods for further improving water quality simulation. The second is the serviceability of a model for the identification of its parameters via the measurable soil conditions of a sub-basin. To address these issues, the authors propose a new rainfall runoff model, the Yamashita Model, based on the assumption that the retention capacities of the soil have a significant effect on runoff phenomena. This model consists of a two-step retention and three-step runoff mechanism. This model is basic parameters includ the retention capacities of soil pores, which were identified by the measured volumes of the soil pores based on the classification by Takeshita, which distinguishes the retention capacities of a soil by its suction force. They examined this model using the actual rainfall-runoff data of two small catchments in the Uryu Experimental Forest of Hokkaido University, Japan. The simulation results generally agreed with the observed data. To improve rainfall runoff models, future long-term observations of rainfall and runoff should be conducted at various types and sizes of watersheds.

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To study the environmental impact of activities such as forest logging, especially the impact on a lake ecosystem, it is necessary to develop a combined biogeochemical and hydrodynamic model. Here the results of the application of a hydrodynamic model to Lake Shumarinai are shown, and the concept of a biogeochemical model is then introduced.

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