Kanako Ishikawa

Accumulation and depuration of microcystin produced by the cyanobacterium Microcystis in a freshwater snail

Seasonal changes in microcystin concentrations in a resident snail (Sinotaia histrica) and an edible clam (Corbicula sandai) in Lake Biwa were surveyed. To clarify both the accumulation and depuration of microcystins, experimental studies with microcystin were also carried out on the snail. In the field investigation, microcystin was detected from the hepatopancreas and intestine of S. histrica (up to 3.2 µg g−1 dry weight and 19.5 µg g−1 dry weight, respectively); however, no microcystin was detected in the hepatopancreas of C. sandai. In the laboratory experiment, the microcystin-LR concentration in the hepatopancreas of S. histrica reached a value of 436 µg g−1 dry weight on day 10 of 15 days of uptake, and a high value persisted despite a depuration period of 15 days. The depuration rate constant of microcystin and its biological half-life were 0.0828 day−1 and 8.4 days, respectively. These results indicate that S. histrica has a high ability to accumulate microcystin in its tissue. Because S. histrica is predated by fish and water fowl, it is likely to play an important role as a vector for microcystin in lakes with dense blooms of toxic cyanobacteria.

Phytoplankton community reorganization driven by eutrophication and warming in Lake Biwa

We compiled and analyzed long-term data, including chemical, physical and phytoplankton community data, for the Lake Biwa ecosystem from 1962 to 2003. Analyses on environmental data indicate that Lake Biwa had experienced intensified eutrophication (according to total phosphorus concentration) in the late 1960s and returned to a less eutrophic status around 1985, and then exhibited rapid warming and thus increased water column stability since 1990. Total phytoplankton cell volume largely followed the trend of total phosphorus concentration, albeit short-term fluctuations existed. However, phytoplankton community shifted dramatically in response to those changes of environmental states. These shifts were cause by changes in trophic status driven by phosphorus loadings and physical properties in the water column driven by warming. Moreover, most phytoplankton species did not show a strong linear correlation with environmental variables, suggesting nonlinear transitions among different states.

Transport and accumulation of bloom-forming cyanobacteria in a large, mid-latitude lake: the gyre-Microcystis hypothesis

Abstract Toxic cyanobacterial blooms have occurred in the near-shore waters of the North Basin of Lake Biwa, Japan, since 1994, and have been attributed to deterioration of water quality in the enriched littoral zone of the lake. From 1997 onwards, the bloom-forming cyanobacteria have been observed with increasing frequency in the deep offshore waters of the North Basin. In the present study, we examined the mechanisms responsible for these bloom populations in the main body of the lake. Specifically, we addressed the hypothesis that buoyant, nutrient-replete colonies of cyanobacteria are generated inshore, are advected offshore by large-scale horizontal transport processes, and subsequently accumulate in the downwelling center of large surface gyres that characterize the overall circulation pattern in the epilimnion of the North Basin. Diel variations of Microcystis biomass at the center and the edge of the Lake Biwa gyre were monitored at 6-h intervals on August 23–24, 2000, and the horizontal distribution of buoyant Microcystis was determined on October 6. The hydrodynamic structure of the first gyre was determined over the preceding 2 days by an on-board Acoustic Doppler Current Profiler (ADCP). The gyre was characterized by a counterclockwise horizontal current that could potentially advect material large distances offshore, a downwelling current near the center of the gyre, and an upwelling current at the edge of the gyre, caused by the radial pressure gradients. The biomass of Microcystis near the water surface was greater at the center than at the edge of the gyre, and the biomass at 5 m depth at the edge of the gyre was greater than that at the water surface or at the thermocline near the edge of the gyre. The results are consistent with the gyre-Microcystis hypothesis, and show the potential for accumulation of large concentrations of cyanobacteria in deep offshore lake environments that are normally considered unsuitable for cyanobacterial blooms.

Effect of temperature on growth of the cyanobacterium Aphanizomenon flos‐aquae in Lake Biwa and Lake Yogo

The water bloom‐forming cyanobacterium Aphanizomenon flos‐aquae Ralfs ex Bornet et Flahault (Nos‐tocales, Cyanophyceae) appeared in Lake Biwa and Lake Yogo in 1999 for the first time. The morphological characteristics were described using natural samples. In contrast to the other water bloom‐forming cyanobacteria such as Microcystis and Anabaena in Lake Biwa and Lake Yogo, the small summer population of A. flos‐aquae is apt to grow in winter, suggesting the low temperature preference or tolerance of this species. In order to clarify the effect of temperature on the growth, culture experiments were conducted using an axenic strain isolated from Lake Biwa. The strain could grow at above 8°C with an optimum temperature ranging from 23 to 29°C, and survived even at 5°C for at least 25days under low light conditions. Although these results confirmed the ability of the bloom formation during late autumn and winter, it is still unclear why the Aphanizomenon bloom occurred at temperatures of ca 10°C in December and not immediately after the disappearance of Microcystis and/or Anabaena bloom during autumn.

Effects of seston on ultraviolet attenuation in Lake Biwa

Abstract We examined the attenuation of underwater ultraviolet (UV) radiation and photosynthetically available radiation (PAR) in Lake Biwa, Japan, at offshore and inshore sites and under contrasting stratification and mixing regimes. There were large spatial differences in the water column transparency to both wavebands, despite little change in concentrations of dissolved organic carbon (DOC). The 1% of surface irradiance depth varied from 0.3 to 2.7 m at 305 nm, from 0.8 to 6.3 m at 380 nm, and from 2.3 to 12.8 m for PAR. Both PAR and UV transparency declined abruptly in the South Basin of the lake when a typhoon caused the resuspension of sediments. The water column ratio of UV to PAR increased by 30% at all stations over the course of a 3-week sampling period associated with the general increase in phytoplankton concentrations. At several sites, the diffuse attenuation coefficient for UV radiation deviated substantially from that predicted from UV-DOC models. A significantly positive linear relationship was found between UV attenuation (Kd determined with a profiling UV radiometer) and the beam attenuation coefficient at 660 nm as measured by transmissometer. These results indicate that scattering and absorption by particulate matter can reduce UV transparency to below that inferred from DOC concentrations, and that current UV-exposure models should be modified to incorporate this effect.

Fluorescence-assisted image analysis of freshwater microalgae.

We exploit a property of microalgae-that of their ability to autofluoresce when exposed to epifluorescence illumination-to tackle the problem of detecting and analysing microalgae in sediment samples containing complex scenes. We have added fluorescence excitation to the hardware portion of our microalgae image processing system. We quantitatively measured 120 characteristics of each object detected through fluorescence excitation, and used an optimized subset of these characteristics for later automated analysis and species classification. All specimens used for training and testing our system came from natural populations found in Lake Biwa, Japan. Without the use of fluorescence excitation, automated analysis of images containing algae specimens in sediment is near impossible. We also used fluorescence imaging to target microalgae in water samples containing large numbers of obtrusive nontargeted objects, which would otherwise slow processing speed and decrease species analysis and classification accuracy. Object drift problems associated with the necessity to use both a fluorescence and greyscale image of each microscope scene were solved using techniques such as template matching and a novel form of automated seeded region growing (SRG). Our system proved to be not only user-friendly, but also highly accurate in classifying two major genera of microalgae found in Lake Biwa-the cyanobacteria Anabaena spp. and Microcystis spp. Classification accuracy was measured to be over 97%.

Phytoplankton functional group dynamics explain species abundance distribution in a directionally changing environment

The mechanism underlying species abundance distribution (SAD), particularly the characteristics of “excess of rare species,” remains controversial. The current equilibrium theory cannot explain the transient dynamics of SAD, which is essential for predicting biodiversity response to environmental changes. Using a unique 32-yr-long phytoplankton community data set from a pelagic site of Lake Biwa, Japan, we show that the dynamics of functional groups driven by environmental variation explain the excess of rare species over time. First, most of the rare species belong to the littoral group supplied through dispersal, whereas the common species belong to the pelagic group. Second, the littoral group was negatively influenced by environmental changes (i.e., lake warming, water-level manipulation, and partial re-oligotrophication), mechanistically explaining truncation of the excess of rare species in the SAD associated with biodiversity loss in Lake Biwa. Our findings imply the significance of an ecological t...

Lessons from Lake Biwa and Other Asian Lakes: Global and Local Perspectives

The freshwater resources of Asia are under enormous pressure given the high and increasing population densities of the region, and the serious degradation of many lakes and rivers due to agricultural, industrial and urban development. Global climate change is likely to exacerbate these pressures through changes in the hydrological balance; influences on stratification, algal blooms and deep water re-oxygenation; and for high altitude sites such as lakes on the Mongolian Plateau, accelerated permafrost melting and desertification. Eutrophication and other processes of water quality degradation are especially severe in many parts of China. At Lake Taihu, a drinking water supply for 40 million people in central China, toxic cyanobacterial blooms now occur throughout the year. In the Province of Yunnan, many of the shallow lake waters are highly polluted and are of human health concern. Lake Biwa, Japan’s largest freshwater body, has experienced many environmental problems over the last 50 years including loss of species habitat, changes in oxygen content, and blooms of noxious cyanobacteria. Two multi-disciplinary research programs involving specialists in hydrodynamics, bio-optics, biogeochemistry and freshwater ecology have generated new insights into the structure and functioning of the Lake Biwa ecosystem, and have contributed an improved understanding of the processes affecting water quality. The Biwako Transport Experiment (BITEX’93) provided new information about physical-biotic coupling in the lake, and revealed the stimulatory effects of typhoons on south basin populations of phytoplankton. Measurements of water currents also showed that toxic cyanobacteria can be transported via a reverse surface flow from the South to North Basins during typhoon events. A subsequent program, Cyanobacterial Risk Assessment at Biwako (CRAB), resulted in predictive bio-optical models and elucidated the importance of stratification and advection processes for cyanobacterial bloom development. The algal bloom populations are favored by diurnally stratified, nutrient-rich conditions inshore and are then advected via a gyre into the main basin of the lake where the cells can continue their growth on stored reserves. The successful management of Lake Biwa, and lakes elsewhere, will especially depend on an improved scientific understanding of the limnological controls on water quality. It will also be fostered by cross-disciplinary and international exchanges of information, and a commitment by scientists, residents and others with a vested interest in water quality, including regulatory authorities, to work together towards the common goal of long term, integrated protection of the lake and its surrounding watershed.

Growth of bloom-forming cyanobacteria under low nutrient conditions

In 1994, toxic Microcystis aeruginosa (cyanobacteria) blooms occurred for rhe first time in rwo harbors in the Norrh Basin of Lake Biwa, Japan (YosHIDA et al. 1996). Since then, there have been many other reports of inshore blooms of this species, and low numbers of Microcystis colonies are now found throughout the Norrh Basin, the main body of the lake, during late summer each year (KUMAGAI et al. 1999). Since Lake Biwa supplies drinking water to almost 14 million people, it is importanr to assess the risk of toxic cyanobacterial blooms. This requires an improved understanding of the mechanisms of Microcystis bloom initiation an d developmenr. The ability of Microcystis colonies to migrate vertically rhrough the water column is well esrablished (e.g. R.EYNOLDS & WALSBY 1975, R.EYNOLDS et al. 1987). The colonies are able to photosynrhesize at the surface during the daytime and move inro nutrient-rich deeper waters at night (GANF & ÜLIVER 1982). This migration abiliry gives Microcystis a competitive advantage relative to other phytoplankton during periods of thermal stratification in temperate lakes. Because of rhis association with srratification, it was thought thar such blooms occurred only in shallow stratified eutrophic lakes and nor in deeper oligotrophic lakes (see R.EYNOLDS 1973). The deep North Basin of Lake Biwa thus seems an unusual habitat for Microcystis. lt is obvious that Microcystis colonies overwinrer on the sediment and recruit inro the warer column as rhe water temperature increases in spring or summer (PRESTON er al. 1980). Nevertheless, colonies o f this bloom-forming species are unlikely to be recruited from the bottom sedimenr in rhe offshore region of the Norrh Basin of Lake Biwa because rhe depth is more than 50 m (TSUJIMURA er al. 2000) and the pressure effects on gas vacuoles would be considerable. When a Microcystis bloom occurs, i t tends to remain near rhe warer surface and is more srable (KuMAGAI er al. 2000). lt is more likely that offshore populations are recruited from enriched, shallow inshore sites and it is therefore important to know to what extenr these inshore popularions can conrinue to grow once they are transported into offshore waters by the wind and currenrs (ISHIKAWA er al. 1999). The relationship berween the phytoplankron growth and ambienr nurrient supply is commonly described by rhe Monod equation (MüNOD 1949). However, luxury nutrienr uptake is well known in phytoplankton, whereby the cells can store nutrients in excess of rheir immediate growth requirements, for example nitrogen as cyanophycin and phosphorus as polyphosphate bodies (KRoMKAMP 1987, SIMON 1987). Under poor nutrienr conditions the phytoplankton use rhese stores to maintain biomass and to continue to grow (RHEE 1973). The Droop equation (0ROOP 1973, ÜKADA et al. 1982, ISTVÁNOVICS er al. 1993) describes growth rate as being dependenr on rhe stored nutrienr quota in the cell. However, this model cannot be directly applied to field data since phytoplankton are seldom under conrinuous steady-state condirions. If there are time lags berween nutrient uptake, storage, remobilization of stored reserves and cell division, the growth rate will be slower than that predicted by either the Monod or Droop models. Recently an Artificial Neural Nerwork model successfully predicted cyanobacterial blooms in Lake Kasumigaura, Japan (R.ECKNAGEL 1997, YABUNAKA et al. 1997). The novel back-propagation method may be useful in simulating furure phytoplankton growth in that eutrophic lake. However, the suirabiliry of this method in predicting blooms in lakes with ongoing eutrophication, as is the siruation in the North Basin ofLake Biwa, is uncertain since nutrient distributions in such lakes are variable and unsteady. The overall aim of the present research project was to evaluate rhe transporr and growth of Microcystis populations that originate in the inshore waters of Lake Biwa under high nutrienr conditions and rhat are subsequently advected offshore where nutrienrs are low. The first step of the study was to estimate

Modern lake ecosystem management by sustainable harvesting and effective utilization of aquatic macrophytes

There are many problems related to overgrowth of aquatic macrophytes in many lakes and rivers throughout the world; for instance, the harvesting costs in Lake Biwa have been increasing by 200 million Japanese yen a year (equivalent to 1.8 million USD). Historically, aquatic macrophytes were harvested for use as fertilizer in agriculture in Japan, but are no longer in use because chemical fertilizers promote plant growth more effectively and are easier and cheaper to use. Thus, developing effective ways to utilize aquatic macrophytes is important to resolve this issue. In addition, sustainably harvesting macrophytes is also important for aquatic ecosystem management because macrophytes play a key role in aquatic ecosystems as nursery grounds and refuges for other small organisms living in the littoral area. Therefore, management and effective utilization of macrophytes through sustainable harvesting may play an important role in the conservation of lake ecosystems. In this short review, a recycling system using anaerobic digestion (AD) of submerged macrophytes, which were sustainably harvested from lakes, and microalgal mass culturing with AD effluent were introduced as a new technique for the conservation of lake ecosystems.