The potential for airborne remote sensing of chlorophyll, suspended solids and transparency in Lake Biwa was investigated with a multispectral sensor flown in August 1993 in conjunction with surveys of limnological conditions. Georectified images calibrated to radiances were produced during BITEX'93. Linear regressions of band ratios (shown as midpoints of ranges in nm) versus limnological variables were developed as follows:
475/549 = 0.419 log (transparency as Secchi depth) + 0.997
549/385 = 0.029 (suspended solids) + 0.503
549/443 = 0.291 log (chlorophyll) + 0.431.
Images of the spatial distribution of transparency, suspended solids and chlorophyll were generated for portions of the South and North basins of Lake Biwa using these regressions.

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As a part of the Biwako Transport Experiments (BITEX'93), we did synoptic transects covering both the South Basin and the North Basin from 24 August-13 September, 1993 every day except two stormy days (4 and 9 September). Within a few hours before noon of each day, vertical profiles were conducted for temperature, chlorophyll a fluorescence (chl-a), turbidity, and dissolved oxygen concentration (DO) every 5 cm. Temperature profiles showed the surface mixing regime varied daily, influenced by fluctuations in surface cooling and wind forcing. In the North Basin, the metalimnion was 10-15 m below the lake surface, which was deeper than the euphotic zone. Chl-a tended to peak in the middle of the epilimnion, decrease rapidly in the metalimnion and was very low in the hypolimnion. A marked turbidity maximum was observed in the metalimnion after heavy discharge of turbid water from rivers. A pronounced feature was the metalimnetic DO minimum layer, which developed during warm and calm weather. Immediately after cool and windy weather, this layer became obscure, associated with a DO decrease in the epilimnion. This implies that the low DO water in the metalimnion was mixed into the upper layer due to the strong vertical mixing.

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Profiles of temperature-gradient microstructure were obtained from late August to mid-September, 1993, in Lake Biwa, Japan, as part of BITEX'93. These profiles were used to characterize the turbulence and extent of vertical circulations of phytoplankton in the euphotic zone of this mesotrophic lake. Overturning scales and the percentage of the upper mixed layer that was mixing were strongly dependent upon meteorological forcing. From mid-morning to mid-afternoon of days with maximum insolation exceeding 0.8 kW⋅m² and wind speeds less than 3 m⋅sec^-1, maximum overturning scales were on the order of I m in vertical extent and the average size of overturns was on the order of 0.2 m. Maximum overturns were on scales of 1 to 10 m in the North Basin and I m to the depth of the lake in the shallower South Basin when hourly wind speeds persistently exceeded 4 m⋅sec^-1. Overturning scales were similar to those in windy periods in measurements taken at night, in early morning, and late afternoon when irradiances were less than 0.5 kW⋅m^-2. Within the South Basin, lateral intrusions of cold water sometimes reduced the depth of penetration of turbulence due to heat loss at the air-water interface. In such cases, nutrient fluxes from waters near the sediment-water interface would be reduced. From late morning to midafternoon on calm days, phytoplankton would circulate over at most 10% of the euphotic zone in the North Basin, 20% in the South. During high winds, or when solar irradiance was low, phytoplankton were circulated in overturns of varying sizes with some equal to or exceeding the depth of the euphotic zone.

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Using an instrument with six vertical sonars (29 to 397 kHz) in Lake Biwa, Japan during August and September of 1993, measurements were made of acoustic backscattering due to microscopic air bubbles created by wave breaking processes. Due to the limited acoustic frequency range, these methods were focused on bubble sizes from 8 to 120um radius. From measurements of volume scattering strength at six frequencies, depth profiles of bubble size spectral densities, acoustic attenuation, and void fraction were extracted. In a typical example with 9m⋅sec^-1 winds, the volume scattering strengths reached peaks near 1.0m^-1 for the 51 to 397 kHz sonars at 0.45 m depth, with peak bubble spectral densities of 1.4×10⁶m^-3 (re 1μm spectral bins) near bubble radii of 16um. At depths from 0.4 to 1.7m, bubble size spectra decreased exponentially with increasing radius with e-folding scale of 10μm. Additionally, bubble concentrations were found to decrease exponentially with depth at all sizes, with averaged e-folding depth scale of 0.26 ±0.05 m. These freshwater bubble size spectral densities contained much smaller quantities of larger bubbles (>80μm radius) than oceanic spectra found in the literature. Finally, time-averaged bubble penetration depths were found to increase roughly parabolically with wind speed above a threshold of 4m⋅sec^-1, to typical maximum penetrations of 1.8 to 2.2 m under winds of 10 m⋅sec^-1.

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The detailed structure and evolution of current and hydrography after a wind storm, typhoon T9313, is described from an intensive field study. The study was done using current meters deployed at 8 stations, as well as 15 thermistor chains, in the southern part of the North Basin of Lake Biwa in summer of 1993. In the morning of 4 September, strong southerly winds with the maximum speed of more than 20 m ⋅see^-1 blew over the lake. After the wind speed peaked, the cold water in the thermocline of the North Basin intruded into the South Basin. This intrusion of cold water was induced by the surface seiche which was generated by the winds of the typhoon. Then a large amplitude internal surge (a steepfronted depression wave on the thermocline) traveled from north to south along the west coast of the North Basin as an internal Kelvin wave. The internal surge shoaled over the southern slope of the North Basin and finally collided against the slope. When the surge shoaled over the slope, a strong jet of over 60 cm⋅s^-1 was generated in the lower benthic layer (benthic jet). Assuming the internal surge as a moving internal jump, we explain the mechanism which induced the benthic jet. When the surge collided against the slope, a homogeneous mixed water mass was generated in the thermocline. After the collision, the internal surge was reflected to travel north along the east coast of the North Basin.

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Benthic nepheloid layers and intermediate nepheloid layers in Lake Biwa were observed between 1991 and 1993, and the characteristics of both nepheloid layers are discussed using field data from Pre-BITEX in 1991-1993 and BITEX'93 in 1993.
The benthic nepheloid layer in Lake Biwa is formed during a stratified period from May to October. The top edge of the benthic nepheloid layer is very sharp as if it is a step function, and turbidity is higher in the benthic nepheloid layer than outside. The vertical distribution of turbidity in the benthic boundary nepheloid layer does not fit to the Rouse distribution. We showed that internal waves induce the oscillating flows in the benthic boundary layer, and it makes such distributions of suspended sediments as a step function.
The intermediate nepheloid layer in Lake Biwa was also discovered during Pre-BITEX and BITEX'93. The intermediate nepheloid layer observed during BITEX moved in one direction with a continuously horizontal velocity of 2.0 to 4.0 cm⋅sec-1, for almost 8 hrs. The intermediate nepheloid layer was about 800 m long and 5 m thick, and it brings suspended sediments rich in Mn.
By comparing these physical data with the results of chemical analyses, we can verify that the material in nepheloid layers originated from bottom sediments.

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An intermediate layer of highly turbid water was found at the seasonal thermocline in the southern part of the North Basin of Lake Biwa during synoptic observations conducted during late August to mid-September 1993. Vertical CTD, transmissometer and near-infrared backscatter profiles indicated that a turbid effluent from the Yasu River penetrated to the thermocline, where it achieved neutral buoyancy. The suspended matter was composed mainly of clay minerals with dimensions of 10 μm, peaking at 6 μm, suggesting a Stokes sinking rate of 1-2 m⋅d-1. Nevertheless, it was not likely that the suspensions were subject only to gravitational sinking, since they remained suspended at a constant isopycnal range in the lake. If the bottom of the turbid layer was cooled by the cold water beneath it, and convection was driven by the diffusivity of heat exceeding that of the suspended particles, the resulting double diffusion mixing phenomena would affect the vertical transport of materials in this freshwater environment.

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Nitrogen and carbon isotope ratios of surface sediments and benthic animals, and nitrogen isotope ratio of nitrate were measured to establish the isotopic biogeochemical structure in the Yodo River watershed. The δ¹⁵N of sediments in Lake Biwa, located in the middle reach of the Yodo River watershed, were the highest values in the watershed. The δ¹⁵N values of nitrate were also significantly higher in comparison with those of lakes in other watersheds. This fact suggested that denitrification was occurring on a large scale in Lake Biwa. Based on the isotope mass-balance, ca. 50% of nitrogen introduced into the North Basin of Lake Biwa seemed to be released as nitrogen gas through the denitrification process. δ¹³C values of benthic animals (aquatic insects) in the upper reach of the Yodo River watershed were 6 to 13‰ higher than those of sediments. This fact suggested that the aquatic insects in the upper reach foraged organic matter produced in the river ecosystem. The higher δ¹³C values of sediments and benthic animals in the South Basin of Lake Biwa and the Yodo River indicated that the middle to lower reaches in the Yodo River watershed were progressively influenced/transformed by human activities.

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An improved preconcentration method using activated carbon loaded with zirconium (Zr-C*) allows determination of low levels of phosphorus(P) in environmental waters. The Zr-C* complex was prepared by mixing activated carbon, which was purified beforehand by immersing in 3M hydrochloric acid solution, in zirconyl nitrate solution (20 gZr⋅1^-1). The Zr-C* complex was collected on a glass fiber filter and rinsed with 1M sodium hydroxide solution to decrease P in the blanks. The water sample (with pH adjusted to 2) was then filtered through the Zr-C* bed on the glass fiber filter and P in the water sample adsorbed to the Zr-C*. The adsorbed P was then recovered by eluting with IM sodium hydroxide solution. The P in the eluate was determined spectrophotometrically as molybdenum blue. The proposed preconcentration method enabled us to determine P down to 0.2μgP⋅1^-1. This method was successfully applied to the waters in Lake Biwa during BITEX'93, 24 August - 12 September 1993.

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The nutritional state of phytoplankton communities in the South Basin of Lake Biwa was examined every other day from 24 August to 12 September, 1993. During this period, a windy typhoon (Typhoon 9313) and a rainy typhoon (Typhoon 9314) moved along Lake Biwa, on 4 and 9 September, respectively. The concentration of PO₄-P remained at a low level (<0.7 μgP⋅1^-1) from 26 August to 3 September, but it increased to 2.5 μgP⋅1^-1 on 4 September just after the arrival of the windy typhoon. NH₄-N varied in a similar way as PO₄-P, and the highest value, 64 μgN⋅1^-1, was observed on 4 September. NO₂ + NO₃ - N, which was 151 μgN⋅1^-1 on 24 August, decreased gradually until 10 September, reaching a concentration of 6 μgN⋅1^-1. It increased again to 76 μgN⋅1^-1 on 12 September after the arrival of the rainy typhoon, whereas concentrations of PO₄-P did not increase after that typhoon. Chl-a concentration also increased after the windy typhoon, and reached the highest concentration of the summer, 26.5 μg⋅1^-1 on 10 September.
The results of an in situ nutrient-addition experiment indicated that phosphorus was a limiting factor for phytoplankton growth in the South Basin of Lake Biwa before the arrival of the windy typhoon. The sestonic C: N: P ratios also indicated moderate phosphorus deficient phytoplankton from 24 August to 12 September, except on 4 and 5 September just after the windy typhoon. The phytoplankton were not nitrogen deficient at any time during our study. Thus, we suggest that the growth of phytoplankton communities from 5 to 10 September in Lake Biwa was mainly induced by an increase in available phosphorus, supplied from the disturbance of the bottom sediments by the windy typhoon.

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Changes in the abundance of phytoplankton and in the concentrations of nutrients and suspended solids (SS) were followed at a sampling station in the southern basin of Lake Biwa during the Biwako Transport Experiment (BITEX'93) from 24 August to 12 September, 1993. SS concentration increased rapidly when typhoon 9313 struck the lake on 4 September. On this occasion, concentrations of dissolved nitrogen and phosphorus increased, probably due to their release from the sediment resuspended by turbulent mixing. Between 3 and 10 September, the whole water column remained homothermal and was well mixed with the Brunt-Väisälä frequency remaining at O sec^-1. The predominant phytoplankton during the BITEX period was Aulacoseira granulata. Chlorophyll concentrations and algal densities started to increase before the passage of the typhoon (3 September). The alga proliferated rapidly to 7 September, and this was complemented by a decrease in nitrogen and phosphorus concentrations.

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The collaborative. research program BITEX'93 (Biwako Transport Experiment) examined how physical, chemical and biological processes interact in controlling cyanobacteria, blooms in the South Basin of Lake Biwa. During the course of BITEX, we tested the hypotheses that: (1) the growth and/or population size of phytoplankton in the South Basin are limited by nitrogen and/or phosphorus, (2) addition of these nutrients by Kelvin Wave sloshing (KWS) or Kelvin Wave breaking stimulates primary production, (3) during the typhoon period, dissolved reactive nutrients enter the South Basin of Lake Biwa by KWS, (4) these nutrients stimulate the growth of bloom-forming cyanobacteria, which are then advected into the North Basin, and (5) thus, during the typhoon events, biological responses of plankton in the two basins are strongly interconnected by KWS. We present evidence, from the 3-week sampling program, that 1) phytoplankton in the South Basin of Lake Biwa were nutrient limited, 2) KWS did not occur, 3) an internal wave from the North Basin did not enrich nutrient concentrations in the South Basin during typhoon forcing, and 4) there was no strong interconnection between the North and the South basins during the typhoon events. Biological responses provided conclusive evidence for the lack of such a mechanism at three different time scales. At the bacterial-generation time (a few hours), we examined the exchange of dissolved organic matter between basins. Responses of bacteria (abundance and production) to major changes in mixing conditions and primary production greatly differed between the two basins and showed no interconnection. At the phytoplankton-time scale (days), we tested nutrient exchanges by sloshing. Biomass (chlorophyll a, particulate carbon and nitrogen) and photosynthetic rates of the <2 μm and >2μm phytoplankton remained higher in the South than in the North Basin during and after the typhoon events, indicating no intrusion from the North into the South Basin. At the zooplankton-time scale (weeks), we tested the advective exchange of water masses. Concentrations of crustacean zooplankton and one developmental stage of E. japonicus were lower in the South than in the North Basin and showed no interconnection between basins during and after the typhoon events. Results indicate that the two basins act as separate entities even during high energy events, so that management strategies should be fine-tuned for each basin.

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Size-fractionated distribution of hot-water extractable phosphorus (EP) and particulate phosphorus and their temporal changes were studied in the South and North Basins of Lake Biwa over 20 days as a part of an international cooperative survey of Lake Biwa (BITEX'93). In association with typhoon-induced physical turbulence of the lake water, pronounced increases and decreases of EP levels of total particulate matter were observed. Distinct peaks on or after the days of the typhoon occured in both basins. Ratios of EP to Chl-a, EP to particulate carbon and EP to particulate nitrogen for the < 2 μm fraction also changed with marked peaks on or after the days of the typhoons, while the ratios for the > 20 μm fraction did not display a significant fluctuation. These results indicated not only a considerable supply of available phosphorus in the lake water by typhoon-induced physical turbulence but also a size-dependent, sensitive response of plankton to the short term change of phosphorus supply.

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Concentrations, composition and bacterial utilization of dissolved free (DFAA) and combined amino acids (DCAA) were examined during the typhoon season (24 August - 12 September, 1993) in the South Basin of Lake Biwa (area 7.4 km²; avg. depth 4 m). Calm and stable conditions persisted during the first half (24 August - 3 September) of our study period, followed by substantial disturbance due to the passages of Typhoon 9313 (4 September) and 9314 (9 September). Despite massive resuspension of sediments due to mixing, both DFAA and DCAA concentrations and composition differed little between the calm and storm periods. Furthermore, we detected no significant increase in incorporation rates of DFAA and enzymatic hydrolysis rates of DCAA (potential peptidase activity) after the typhoon event. These results suggest that resuspended sediments were poor in labile DOM that enhances bacterial activities. Our data are contrasted with other results showing substantial increases in inorganic nutrients in response to sediment resuspension. We suggest that storm-induced mixing may result in differential supplies of dissolved inorganic nutrients and labile DOM, which may influence the pattern of primary and bacterial production in the South Basin of Lake Biwa.

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Production of bacteria and phytoplankton was examined over a three week period during the storm season (August-September) in the South Basin (water depth, 4 m) of Lake Biwa, Japan. Passage of two typhoons caused drastic changes in physical and nutrient conditions, including pulsed increases in the concentration of soluble reactive phosphorus (SRP) associated with sediment resuspension. In response to these meteorological events, phytoplankton increased substantially; up to 6-fold and 3-fold higher values of chlorophyll a and primary production relative to average prestorm levels were observed during the storm period. In contrast, bacterial abundance and production increased little after the storm events. The abundance, production rate, and specific growth rate of bacteria were not significantly correlated with primary production (p>0.4). Furthermore, the ratios of bacterial : primary production were high (maximum, 1.3), suggesting that bacterial carbon demand occasionally exceeded primary production. These results are consistent with the hypothesis that organic carbon from non-phytoplanktonic origins (i.e. allochthonous and littoral organic carbon) are important for bacteria in freshwater lakes. Notably, there was a highly significant correlation (p=0.003; r=0.627; n=20) between specific growth rates of bacteria and SRP concentrations, suggesting that bacterial growth was limited by P. We conclude that the coupling of bacterial and algal responses to the typhoon disturbance was weak in Lake Biwa. The results support a recent model of the lacustrine microbial loop, which suggests that bacteria are largely driven by external carbon supplies and that bacteria may compete with phytoplankton for P.

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As a part of Biwako Transport Experiment (BITEX'93) in the late summer of 1993, the population dynamics of planktonic crustacea in Lake Biwa were studied by a quantitative sampling regime. Crustacea were collected on a two-day interval at three stations in the two basins and also at the boundary between the basins. Two copepod species, Eodiaptomus japonicus and Mesocyclops dissimilis, were dominant and two branchiopod species, Daphnia galeata and Diaphanosoma brachyurum, were less abundant. Temporal changes in density of their life stages at a site were caused by horizontal transfer of patchy assemblages rather than by individual development. Whereas typhoons disturbed the lake ecosystem during the study period, no direct effect of typhoons was detected on crustracean distributions.

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We analyzed NOAA-AVHRR temperature data from Lake Biwa. These data are low in spatial resolusion but the observations are quite frequent. Ten sets of data during the BITEX'93 period showed that the MCSST (multichannel sea surface temperature) method for data correction is also applicable to Lake Biwa. They also showed that NOAA-AVHRR data is useful to distinguish day-to-day variation of surface temperature for Lake Biwa. However, we could not find any significant and systematic spatial temperature distribution on the lake.

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