Verh. Internat. Verein. Limnol.

28

1–15

Stuttgart, February 2002

Lake Biwa watershed transformation and the changed water
environments
Masahisa Nakamura

Introduction
Lake Biwa is the largest and oldest lake in Japan,
located in the heart of Shiga Prefecture. The lake isestimated to have originated some 4 million years
ago and is one of the most ancient in the world. It
has developed a rich lake ecosystem, including over
50 indigenous species. For tens of thousands ofyears, humans have been a part of that ecosystem.
Endowed with the bountiful resource values of thelake, they have also developed a distinctive lake-community culture including that of wary respect for theoccasional dangers it presents, such as floods.
The Lake Biwa Comprehensive Development
Plan (LBCDP) was launched in 1972 as a national
program based on a special measures act of the Diet,and concluded in March 1997. The primary objectives of the plan were to develop additional water
resources for the downstream Osaka–Kobe (Hanshin) Region and to reduce the threat of floods in thelakeshore areas, while preserving the natural environment of the lake and restoring a water quality thatwas rapidly showing the effects of pollution. The
plan succeeded in providing more effective utilization of the water resources and in significantly reducing flood damage. Today, Lake Biwa has become avital water resource for the Keihanshin (Kyoto–
Osaka–Kobe) Region (Table 1). The lake also has
become a favorite recreational site for the region.
Another important aspect of the LBCDP, however, was the development of the Lake Biwa watershed. Over the 25 years of the LBCDP implementa-

tion, the lake watershed has undergone tremendous
transformation due to urbanization and industrialization. Despite the introduction of significant structural and non-structural environmental control measures, the water quality and ecosystem integrity of
the lake began to deteriorate. The transformation of
the watershed and coastal zone for housing, industrial and recreational development has resulted in theloss of paddy fields and wetlands, together with thedegradation of the upland forest areas. Despite somesigns of improvement in the nutrient concentrationlevel, as a result of the gradually expanding sewerage
networks, achievements have been considerably lessin the areas of restoration of the natural self-purification capacity lost through the transformed land uses,
and the conversion of forestlands and wetlands tofacilitate development activities. Consequently, during the last phase of the LBCDP, the prefectural government of Shiga began to consider how to ensure
the comprehensive conservation in the post-LBCDPera.
In March 1997, Shiga Prefecture compiled theresults of the deliberations of a national council
established for the purpose, and they prepared a proposal for the national authorities, resulting in theLake Biwa Comprehensive Conservation Plan, or
LBCCP, dubbed the “Mother Lake 21”. The planemphasizes that the ultimate solution to the problems facing Lake Biwa lies in the restoration of thenatural and ecosystem capacities of the coastal zone
and the watershed while pursuing the revival of anenvironmental culture to allow such re-transforma-

Table 1. Population served by Lake Biwa water, 1994.
Prefecture
Shiga Pref.
Kyoto Pref.
Osaka Pref.
Hyogo Pref.

Population withinjurisdiction
1,300,000
2,600,000
8,720,000
5,470,000

Population served by
Lake Biwa
1,000,000
1,790,000
8,530,000
2,610,000

Dependency on Lake Biwa
water (%)
79
69
98
48

0368-0770/02/0028-0001 $ 3.75
©2002 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

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Fig. 1. Downstream demand for water (m3/s, as of late 1990s).
tion to occur. The present paper gives an overview of
the transformation that has taken place in the Lake
Biwa watershed and of its impacts on the lake, which
led to the formulation of the LBCCP.

The Lake Biwa–Yodo River water system
Lake Biwa is Japan’s largest freshwater lake,both in terms of surface area (674 km2) and volume (27.5 × 109 m3). It receives water fromsome 120 rivers (over 400 if small streams areincluded) in its catchment basin, which is only
five times the area of the lake itself. It has only
one exiting river at the southern end – the Seta
River. The Seta River (called the Uji River in
Kyoto Prefecture) is joined by the Katsura andKizu Rivers some 30 km downstream of thelake to become the Yodo River (Fig. 1). The
flow contributions to the Yodo River from theUji, Kizu and Katsura Rivers are, respectively,
64.2%, 18.0%, and 15.0%. The official designation of the entire Yodo–Uji and Lake Biwa
water bodies is the Yodo River system. Itsannual average flow, its high flow and its low
flow are, respectively, 177.6 m3/s, 226.8 m3/s,
and 117.0 m3/s. The ratio of high to low flows,
0.52, is the highest among the major river systems in Japan, making the Yodo River a very
stable source of water.
The Lake Biwa–Yodo River system is of overriding importance in Kansai, the general designation of the western half of Japan. The waterfrom this system is supplied to such large cities
as Kyoto, Osaka, and Kobe, to the industries in
the Osaka Bay area, the urban and semi-urban
areas, as well as the extensive paddy agriculture
fields in the lake catchment area. A complex

M. Nakamura, Changes in the Lake Biwa watershed

web of water supply and wastewater networks,
which support the region’s high level of municipal, industrial and agricultural activities, characterizes the region. This great metropolitan
complex within the Keihanshin region has been
almost totally dependent on the Yodo River, or
largely on Lake Biwa, for its water resource
needs. The water resource capacity had to be
gradually increased to meet the growing
demands.
The mountain ranges around Lake Biwa andthe tributaries of the Yodo River are mostly covered with dense forests. They function as a natural reservoir as well as a gigantic filter producing high quality raw water to replenish Lake
Biwa and the Yodo River. There are, however,
12 dams (including some under construction,but excluding agricultural irrigation dams) in
the watershed for the integrated managementof water resources, to make up for the fluctuations in precipitation. The Yodo River main
watercourse serves most of the downstream
needs. Waste water is collected by the sewerage
networks, treated, and discharged back into theBiwa–Yodo watercourse or directly into Osaka
Bay. The water systems of Lake Biwa, the Yodo
River, and Osaka Bay are all within a compact
geographical area of less than 10,000 km2. The
direct distance from the northernmost tip ofthe Lake Biwa watershed to the southern opening of the Osaka Bay estuary is less than 200
km.

The Lake Biwa Comprehensive Development Project (LBCDP)
Lake Biwa provides water supplies for municipal, industrial and agricultural purposes
amounting, respectively, to 6.6 million
tons/day, 1.2 million tons/day and 23.6 million
tons/day. It serves as many as 14 million people
living in the Keihanshin area, including Shiga.
The pollution of the lake has therefore been ofserious concern not only for those living aroundit but also for those receiving water from Yodo
River. The demand for water, particularly forindustrial uses in the downstream Yodo Riverand the Osaka Bay area, began to increase
sharply as the country entered an era of economic growth a decade or so after World War

3

II. Exploitation of the groundwater soon
became constrained due to competition
between industrial establishments and due toland subsidence caused by the overuse of thewater. Industries were then forced to look foralternative sources of water. Domestic watersupply needs also began to increase in the Yodo
River area after suburban cities joined Osaka inrequiring access to the Yodo River water. The
water situation in the downstream Yodo Riverculminated in the inception of a large-scalewater resource development – the LBCDP (Fig.
2).
The primary goal of the LBCDP was to allow
the lake to be converted into a ‘man-made’ reservoir (via a financial arrangement with thenational government), which would generate
additional water resources amounting to a maximum of 40 m3/s at times of drought, in order
to meet the ever-increasing downstream needsin the early 1970s. Flood control was an addedfunction achieved through the installation of
intricate flow control facilities around the lake.
The LBCDP, however, had another major

Fig. 2. Basic framework of the LBCDP.

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objective – the development of regional infrastructure systems for the Shiga Prefecture –which was included as a compensatory measurefor the prefecture so as to reach agreement on
the scheme of water resource development forthe downstream. The compensatory measures
included a wide range of lakeshore and watershed infrastructural development projects, suchas the construction of roads, a park, water supplies, sewerage, and many other facilities. These
provisions would improve the development
potential of the then rural and economicallydisadvantaged Shiga Prefecture, allowing people, industries, and commercial establishments
to relocate to the prefecture from the alreadydensely developed downstream regions.
When the LBCDP was inaugurated in 1972,
it was to be completed within 10 years, at theend of the 1981 fiscal year. Due to the economic slowdown in the 1970s, the project
period was extended for another 10 years,changing the target date of completion to theend of the 1991 fiscal year. Shiga Prefecture
insisted that not all of the regional infrastructure projects could be completed by the termination of the period so another 5-year extension was agreed upon. The final date ofcompletion therefore became the end of the1997 fiscal year, making the LBCDP a 25-year

Fig. 3. Component project budget of the LBCDP.

national project.
As a result of the LBCDP, however, the area surrounding Lake Biwa has been increasingly
urbanized. The Lake Biwa system is now
strongly human-dominated. These developments have resulted in major ecosystem changes
in the lake, including littoral conversion, thedisappearance of some of the connecting
lagoons, called ‘Naiko’, the channelization andcontrol of stream flows, and the segmentationof coastal wetlands and shores, as describedherein in the section titled ‘ Watershed transformation over the past few decades’.
The rapid rate of water quality deterioration,
which occurred in the 1970s, called for agreater emphasis on environmental components in the 1982 revision of the project. In
1982, a substantial level of funding was
directed towards environmental protection andpollution control, e.g. from an annual expenditure of 10 billion yen/year for a sewerage andnight soil treatment system during the 1972–1981 period, to 26 billion yen/year during the
1982–1991 period (Fig. 3). The funds werealso made available for constructing dairy waste
treatment facilities, rural sewerage facilities,
refuse disposal facilities, and for upgrading thelake water quality surveillance and monitoring
system.

M. Nakamura, Changes in the Lake Biwa watershed

Nutrient control and lake water quality
Land-based nutrient loading and its control
Land-based pollution originates from point
sources (industries, commercial establishments,
households, etc.) as well as from non-point
sources (aerosols in precipitation, contaminants
in urban storm runoffs, forest and field runoffs
and irrigation return flows, etc.) Control ofsuch pollution is of primary concern for reducing the nutrient inputs into the lake.
Since the post-war industrialization policy inJapan encouraged heavy industries to locate
along the Pacific coast port cities, the inland
Lake Biwa region was left free from any significant industrial pollution. Nonetheless, somelarge industries, such as textile mills and electronic manufacturing firms, found the lakebasin area attractive for their water-intensive
operations, thus making the control of industrial pollution a top priority in the early years oflake pollution control. The Water Pollution
Control Law enacted in 1970, which included
stringent punitive provisions for offenders, wasan effective policy tool for the regulatory agencies, though the slowing down of the Japanese
economy in the mid-1970s also helped reducethe intensity of industrial activities in theregion. Today, the industrial pollution loads, interms of COD, TN, and TP, are 22.4%,
17.4%, and 29.0%, respectively (Fig. 4).
The rapid development in the Lake Biwa
basin prior to 1970 resulted in an increased
level of eutrophication. Specifically, the loads of
TP and TN to the lake became greatly elevated.
As a result, the lake suffered a major outbreak

Fig. 4. COD, TN, TP loads by source categories (asin 1999).

5of the flagellate, Uroglena americana (freshwater
red tides), in 1977 in the North Basin, and abloom of cyanobacteria – Anabaena and Microcystis – in the South Basin in 1983. These
blooms outraged the long-term Shiga residents
as they consider Lake Biwa the most valuablespiritual and cultural asset of the prefecture.
The citizens of Shiga Prefecture, particularlyhousewives, organized themselves immediately
after the red-tide incident to pursue the movement to “use soap to save the lake”, despite initial strong pressure from the detergent industry
against the movement. Prompted by the occurrence of freshwater red tides and the consequent citizen outcry, the Shiga Prefecture
enacted the Ordinance for the Prevention of
Eutrophication in Lake Biwa in 1980. This
Ordinance was effective in bringing about areduction in the level of TP in the lake, particularly in the early 1980s. Figure 5 shows a reduction in TP of more than 10 µg/L in the South
Basin after the implementation of the Ordinance and a gradual reduction in the North
Basin. This experience has been instrumental inensuring that the Shiga residents continue to be
actively involved in the citizen-initiated activities controlling land-based sources of pollution.
The municipal waste water (waste water fromcommercial establishments, public buildings,
small industries and general households) has
been another major source of lake pollution.
More than one-third of the TP and more than aquarter of TN and COD loads are attributed tothis source of input. Three major categories ofsewerage systems, aside from the public sewerage in the City of Otsu that has been operating
since the mid-1960s, have been instituted in
the Lake Biwa basin (Figs. 6 and 7). They are:Fig. 5. Gradually decreasing TP concentrations.

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the regional public sewerage program, which iscatered for by four regional service districts,
each with a large centralized treatment plant(57% population coverage as in 1998), therural sewerage program consisting of some 200
individual community service systems (12.3%),
and the on-site treatment technologies servicing
individual households not yet served by eitherof the above two programs (30.8%).
The categories of non-point sources of pollution in the Lake Biwa basin include: (1) precipitation directly onto the lake, and forest runoffs, (2) runoff from rice paddy and otheragricultural fields, (3) industrial run-offs, and(4) urban runoffs. The control of runoff fromurban and agriculture sources has already been
tried at a limited scale. For example, the runoffs

Fig. 6. Wastewater systems in the Lake Biwa watershed.

Fig. 7. The trend in number of agricultural community sewerage systems installed.

from rice paddies during periods of low flow or
no precipitation are treated using a combination of structural approaches (e.g. use of irrigation water recycling system) and non-structural
approaches (promotion of agricultural best
management practices (BMPs), which, together
with education and information dissemination
programs, entail the introduction of management practices proven to be most desirable interms of reducing wasteful use of water, fertilizer and pesticides). As the water resource andregional development objectives have been met
through the LBCDP, the Shiga Prefectural government, together with the central governmentand downstream governments, is now attempting to conserve the Lake Biwa environment via
the control of non-point pollution and theenhancement of ecological integrity.
Change in lake water quality
The quality of the water in Lake Biwa has not
improved much over the past few decades.
While the concentration of TP has shown slight
improvement in recent years (Fig. 5), the COD
and TN concentrations have been, if anything,creeping upwards (Figs. 8 and 9). The expansion of economic growth and the increase inpopulation are expected to prevail for somedecades ahead; therefore the pollution pressure
within the watershed is liable to increase. Thus,
the reduction in the influent point-source pollution load will continue to be a priority issue.
However, the institutional and managerial measures most suitable for the Lake Biwa non-point
sources control have not yet clearly emerged.

M. Nakamura, Changes in the Lake Biwa watershed

Fig. 8. Steadily increasing COD.

Fig. 9. TN concentrations have shown a slightly
decreasing trend since 1996.

Even more troublesome are the precipitationborne contaminants that result from thedensely laid network of highways and urban
roads, from the application of chemicals on
farmlands, as well as from the long-distancetransport of air pollutants from outside thewatershed, even from across the East Asian seas.
There is an urgent need to accelerate the movetowards the development of a sound water andmaterial cycle that would allow greater relianceon the self-purification capacity of the natural
ecosystem.

Watershed transformation over the past few
decades
Transformation of the littoral zone and aquaticvegetation
The rate of population increase in Shiga Prefecture in the past few decades has been one of thehighest in the nation, and the agricultural landhas been steadily converted for urban development. In the post-World War II era, much of
the littoral area and many attached lagoons, or

7‘Naiko’, around the lake were reclaimed, mainly
for increasing paddy fields. The 2900 ha of
Naiko area in the early 1940s has been reduced
to 425 ha (15%) today. By the time the reclamation projects were legally brought to a halt in
1972, 11 out of 33 attached lakes had been
totally or in part reclaimed, resulting in the disappearance of 75.8% of their combined surface
area. The coastal area around the lake, however,
was developed during the LBCDP era mainly
for recreational and commercial purposes. The
reed bed area has also been reduced from 260
ha in 1953 to 120 ha in 1990 (Fig. 10). The
loss of littoral area (Fig. 11) due to filling andthe construction of flood levies and embankments has caused a lowering of the natural purification capacity. The conversion of the littoralzone and the development of the coastal zone
have not only led to the disappearance of large
areas of aquatic plants in the lake, but have also
extensively impacted on fish production andwater quality. In fact, many recent changes in
the lake biota and the deteriorating water quality conditions may be traced to the conversionof the littoral area and the development of thecoastal zone (KIRA 1988, NAKANISHI & SEKINO
1996).
In addition to the changes in the vegetation
coverage, the dominant macrophytes have also
undergone major shifts. Two introduced species– Elodea nuttallii and Egeria densa – became
dominant. Elodea nuttallii, a native in North
America, was first found in November 1961 at
the Shiozu Bay, which is located at the northernend of the lake. This species was able to outcompete other species of submerged macrophytes using the fragmentation of its shoots tocolonize other parts of the littoral zones. The
increased level of eutrophication in Lake Biwa
may have been the reason for the leveling off in
the growth and colonization of this species. The
other species, Egeria densa, which is a native
species of South America, was first found in thelake in 1969 and became dominant in 1974. It
has now been found along all shorelines of thelake, including the western shore, constituting
about 6% of the total area of submerged macrophytes, with a biomass reaching 410 g/m.

8

Plenary lectureschannels. Other causes of land-use transformation due to the LBCDP include, for example,
the widening of roads for the construction oflarge trunk sewers, and the construction oflakeshore parks and embankment-motorway
around the lake for recreational and flood control purposes.
Fig. 10. Drastically reduced area covered by reed
bed.

These land-use changes have provided material satisfaction and a convenient lifestyle, butthey have also led to the destruction of naturalenvironments as well as the physical and psychological alienation of humans from nature.

The change in lake water environments
Fig. 11. Transformation of the natural coastline to
artificial coastline.

Land use transformation in the Lake Biwa watershed
The coastal land of Lake Biwa has undergone asignificant transformation since World War II,
as described in the previous section. A dramatic
land-use transformation also took place in connection with the implementation of the
LBCDP. In addition to the reduction in coastal
wetland described earlier, the watershed forestland, including the riparian forests, has also
been reduced due to urban encroachment(Fig. 12). The deteriorating quality of management of Japan’s manicured forests, due to adwindling and aging forestry labor force, makes
the situation worse. Furthermore, the prefecture’s farmland decreased by 21.4%, from72,687 ha to 57,146 ha, between 1975 and1998 (Fig. 13), because of conversions to housing areas (42.7%), industrial estates (12.3%),
designated farm roads (9.8%), or roads and railroads (9.4%). The rice fields that cover 92% of
the prefecture’s farmland have also been extensively rearranged. Small, irregular plots havebeen reshaped into large rectangular units toallow the use of heavy planting and harvestingmachinery. The traditional gravity-fed channels
that were used for both irrigation and drainage
have been replaced with separate piped irrigation networks and straight concrete drainage

Impact of the altered precipitation pattern
One of the most critical climatological factors
affecting the limnological condition of Lake
Biwa is the amount of snow received by thewatershed, which is strongly affected by thestrength and position of the Siberia High. The
water from the melting snow in early spring
carries a substantial amount of dissolved oxygento the bottom of the North Basin, which determines the initial DO concentration each year in
the hypolimnion of the lake, prior to the onsetof the thermal stratification in May. There has
been a clearly decreasing trend in the amount ofprecipitation (Fig. 14) and snow falling in theLake Biwa region during the last few decades,
leading to a reduction in the supply of oxygen
and an increase in the hypolimnic water temperature (KUMAGAI & FUSHIMI 1995). The DOconcentration in the hypolimnion decreased
gradually from the 1960s to the 1980s, but it
began to recover unexpectedly in the early
1990s. The apparent oxygen consumption rate
began to decrease in the benthic boundary
layer, leading again to a decreasing trend in DO
concentration. Nonetheless, as Lake Biwa
becomes more eutrophic, an increasing amountof organic matter will settle in the hypolimnion, and the DO consumption by organic
degradation is expected to increase. The potential impacts from reduced snowfall, as per theeffects of global warming, could therefore significantly affect the hypolimnetic environments
in Lake Biwa.

M. Nakamura, Changes in the Lake Biwa watershed

9

Fig. 12. Rate of forest land conversion: Lake Biwa (Shiga Pref.) vs nationwide average.

Fig. 13. Rate of agricultural land conversion: Lake Biwa (Shiga Pref.) vs nationwide average.

Change in fish population
Lake Biwa has more than 60 fish species and 40
shellfish species. The total production of fish in
the 1990s has been stable, at a level of slightly
more than 4000 tons/year, but the fish yield has
been decreasing consistently and the species
harvested from Lake Biwa also differ greatly
from year to year. The fish yield from species
such as the only salmon species in Lake Biwa,
Oncohynchus rhodurus, a crucian carp, Carassius
auratus and a goby, Chaenogobius isaza, hasbeen drastically reduced in the last 10 years,
though with a dramatic increase in 1996 forunknown reasons, while the yield of ayu, Plecoglossus altivelis, a major source of income for thefishermen in the Lake Biwa area, has been
increasing since 1974 and now represents up to
60% of the total fish harvest from the lake.
The decreasing fish population in Lake Biwa
may be primarily due to habitat alterations, a
reduction in suitable spawning sites, over-fish-

ing, and insufficient prey. NAKANISHI & SEKINO(1996) indicated that the reductions in the Carassius carrassius grandoculis and Chanenogobius
isaza populations are mainly attributed to thedeterioration of the spawning sites, as a result of
the development of the littoral zone and coastal
area of the lake, which extensively altered thelittoral environments. Also, the introduced fish
species, i.e. large-mouth bass (Micropterus
salmoidea salmoides) and bluegill sunfish (Lepomis macrochirus), are driving these indigenous
species into collapse or extinction. The reduction in the population of the salmon, Oncohynchus rhodurus, could be due to the increased levels of eutrophication and the construction ofdams and weirs on streams, which prevent thesalmon from spawning in the river upstream.
Commercially important shellfish species,
including Corbicula sandai, are now maintained
through artificial stocking.
Incidents of mass die-off of ayu in Lake Biwa

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Fig. 14. Lowest precipitation records in the Lake
Biwa–Yodo River region.

have been seen occasionally since 1983. According to the studies conducted by the Shiga Prefectural Fisheries Station (SATOI 1995), Vibriosis, or infection by Vibrioanguillarum (A) was
identified as a major cause. The estimated scaleof fish die-off and the percentage of fish
infected during these incidents were estimatedat 300 million fish and 98% of fish sampled in
1983, 400 million and 88% in 1989, 160 million and 93% in 1990, 100 million and 90% in
1992, 70 million and 87% in 1993, and 6 million and 100% in 1994. Some of the more
recent die-off incidents, however, have beenfound to be due to infection by Favobacterium
psychrophilim.
Change in phytoplankton species
Lake Biwa has more than 90 species of algae,
including 12 species of Cyanophyceae, 41 of
Chlorophyceae, 6 of Chrysophyceae, 22 ofBacillariophyceae and 2 Dinophyceae. The seasonal changes in the dominant species since
1965 in the North Basin are as follows: Melosira
solida (November–April), Asterionella formosa(May), and Staurastrum dorsidentiferum, Closterium aciculare and/or Pediastrum biwae(June–October). In some years, Staurastrum
dorsidentiferum and Closterium aciculare areoccasionally abundant between October andJanuary. The increase of these species is often

associated with the decrease of Melosira solida.
The yearly pattern of seasonal changes in theSouth Basin is more variable. The dominant
species include Pediastrum biwae, Staurastrum
dorsidentiferum, Closterium aciculare and Melosira granulata.
A major outbreak of the flagellate, Uroglena
americana, was noted in 1977 in the North
Basin. This bloom gave the lake a reddishbrown color and foul smell, and was referred to
locally as ‘freshwater red tide’. Such blooms
have been seen each year since 1977, with the
exception of 1986 (Fig. 15). This red tide phenomenon usually appears at the end of April
and the beginning of June each year, when thewater temperature reaches somewhere between
15 and 20 °C. The occurrence of the red tide
was the beginning of a major change in the species composition of phytoplankton in Lake
Biwa.
In September 1983, the cyanobacteria, Anabaena and Microcystis, became dominant in abloom condition. Since then the blooms haveoccurred every year, except 1984 (Fig. 15).
However, it is now found more frequently
along the eastern shore of the South Basin. The
Microcystis bloom was found for the first timein the North Basin in late summer 1994, therecord drought year for Lake Biwa and for mostof the western part of Japan.
Dramatic declines in an endemic diatom species, Melosira solida (Fig. 16), and an endemic
green alga Pediastrum biwae, have beenreported (NAKANISHI & SEKINO 1996), and thisphenomenon is suspected to be due to a sudden
propagation of algal picoplankton consistingFig. 15. The incidences of cyanobacteria blooms
have surpassed those of red tide.

M. Nakamura, Changes in the Lake Biwa watershed

11

Fig. 16. Dramatic decline in the density of Melosira solida (data provided by S. ICHISE 2001, personal communication).

mainly of Synechococcus spp. The sighting oflarge-scale Synechococcus blooms was firstreported for the North Basin during 1989–
1991. This group of cyanobacteria are prokaryotic (0.4–1.5 µm) rod-shaped organisms withcell numbers reaching a maximum of 106
cells/mL, and the build-up of the bloom is dueto the fast growth of the species; it is later
destroyed by heterotrophic nanoflagellates andbacteriolysis by cyanophages (MAEDA 1993).
While the long-term trend of phytoplankton
in the North Basin of Lake Biwa has been moreor less constant in terms of chlorophyll a, there
has been a clear trend of increasing numbers of
total phytoplankton cell counts since 1995, asshown in Fig. 17. Furthermore, there has also
been a marked change in the species composition and the dominant species, and a rather
alarming trend of increase in the frequency of

cyanobacterial blooms and the associatedincrease in their cell count and total biomass (S.
ICHISE 2001, personal communication).
Changes in the zooplankton dynamics
In an earlier report (NAKANISHI 1984), 7 speciesof Copepoda, 18 of Cladocera, 80 of Rotifera
and 14 of Protozoa were reported for Lake
Biwa, but the identified number of species has
now grown to over 200 (S. ICHISE 2001, personal communication). The frequently occurring species in the North Basin include thecopepods, Mesocyclops leuckarti, Cyclops vicinus,
the cyclopoids, Mesocyclops and Cyclops, and thecladoceras, Daphnia biwaensis, Daphnia longispina and Bosmina longirostris, and the rotifers,
Eodiaptomus japonicus, a dominant andendemic species found in lakes and ponds insouthern Japan, and Kellicottia longispina. The

Fig. 17. Change in the total number of phytoplankton species (data provided by S. ICHISE 2001, personalcommunication).

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most common species in the South Basin are
crustaceans, Bosminopsis deitersi and Chydorus
sphaericus, and the rotifers, Synchaeta stylata,
Polyarthra vulgaris and Keratella cochlearis.
Although there are reports of long-term trend
data and episodic observations made on Lake
Biwa zooplankton in the past, the overall extent
of change in the food-chain dynamics involving
autotrophic algae and heterotrophic planktonic
organisms in Lake Biwa is still not very clear.
Recent studies on the subject, such as thatreported by URABE et al. (1996), are expected toshed some light on this important subject.
In the pelagic systems, zooplankton play various roles in community organization and material flow, which are coupled with each other,although not perfectly. One study (URABE et al.
1995) indicated that zooplankton grazing in
Lake Biwa is a major loss process in the elimination of seston from the epilimnion from summer to fall. This indicates that the zooplankton
consumption can promote phosphorus (P) limitation for phytoplankton by fixing a large fraction of the P in the system into their mass, andby recycling a substantial amount of the N to adissolved form.
The preferred prey of ayu (Plecoglossus altivelis), a commercially valued freshwater fish in
Lake Biwa, is Daphnia galeata, which is also anefficient predator preying on phytoplankton
species. The recent report indicated that the
selective feeding by ayu fish on Daphnia galeata
may result in a predatory effect, in which thephytoplankton population increases in summer in the epilimnion as a result of the reduced
numbers of Daphnia (ANONYMOUS 1997). Since
ayu is an important commercial fish, theincrease in the ayu population in the lake, andits selective feeding, will greatly impact thezooplankton community in the North Basinand can cause a significant increase in phytoplankton biomass.
Changing benthic conditions
Lake Biwa has 152 identified benthic animal
species including insect larvae, mollusks, tubificids and chironomids. The first two are foundin large numbers in Lake Biwa while the latter
two are generally small in number. Crustaceans

and insect larvae are bottom inhabitants and area major part of the epifauna. Shrimps andprawns are mostly found in areas with submerged aquatic plants. Since the shallow littoralzone of Lake Biwa is composed of several different substrates and provides a variety of habitats
to benthic animals, insect larvae and rudineans
dominate in this zone. The substratum in the
profundal region is uniform with no substantial
growth of aquatic plants, and species such as
oligochaetes are found in large numbers (58%)
in this region (OHTAKA & NISHINO 1995).
There are 44 species of mollusks in Lake
Biwa. Of these species, Asian clam, Corbicula
sandai, is the most important species in termsof economic value in Lake Biwa. The young
Corbicula prefers the sandy substrates and primarily lives at depths of between 2 and 8 m.
The adults feed on sandy or sandy mud substrates up to depths of 40 m (HARADA & NISHINO 1995). This species is endemic and is
widely distributed in the lake. Habitat alterations, increased eutrophication, and over-fishing of this species have resulted in a major
reduction in its yield. The catch of the species
has plummeted from a peak of 4225 tons/year
in 1969 to less than 400 tons/year in early
1990s, but it recovered to 1319 tons/year in
1995 (TAKAHASHI et al. 1999).
Indications of rather drastic changes in the
distribution and densities of benthic animals
taking place over the past few decades in the
profundal bottom of Lake Biwa have beenreported (e.g. NISHINO & HARADA 2001). The
total disappearance of an endemic leech, Ancyrobdella biwae, first found in 1915 at a depth of
80 m in the North Lake, is a well-establishedfact among Lake Biwa scientists. On the other
hand, the recent discovery (in 1992) (NISHINO
et al. 1998) of a dense mat of sulfur-oxidizing
and sheath-forming filamentous bacteria, Thioploca spp., was a surprise as it was only the
fourth reported sighting in the world of such a
dense mat in lakes. The sighting is consistent
with observation on the increasing accumulation of organic matter due to eutrophication,
together with the decreasing concentration ofDO, due to the afore-mentioned decrease in
winter precipitation.

M. Nakamura, Changes in the Lake Biwa watershed

The Lake Biwa comprehensive conservation
plan
The efforts to develop a conservation plan forthe lake ecosystem began in 1995. The originalidea of the Prefectural government was to solicit
the central and downstream governments tomaintain the LBCDP framework for another10 years or so, in order to be able to continue
investing in environmental conservation
projects. Neither the central government nor
the downstream governments were willing to go
along with such an idea, because it was not clear
how much restoration of ecological integrity
and how much improvement of the lake water
quality were to be considered adequate, or how
much more cost they would be expected to bearin the implementation of what later turned out
to be the LBCCP (Fig. 18).
After 4 years of preparatory studies and deliberations by the officially commissioned expert
panel, a plan has been developed for the comprehensive conservation measures of Lake Biwa,
which consists of (1) maintaining water quality,

Fig. 18. A general conceptual sketch of the LBCCP.

13(2) improving the recharge capacity of the soiland (3) preserving the natural environment andscenic landscapes, together with the measuresneeded to promote these activities: participation and practice, exchange and information
access, and research. The issues contained in thethree primary conservation domains are interrelated in a complex manner. This complexity
stems from the wide range of targeted phenomena, as well as the diversity of causal factors andtheir interactions. It is further compounded by
the impacts of the lifestyles of the people who
enjoy Lake Biwa. The first order of businessstipulated in the plan was for all those with astake in the lake to develop a multi-tiered collaboration between various entities. The prefectural government will continuously evaluate theresults of this activity to ensure its constantimprovement.
The plan will run for 22 years, from April
1999 to March 2020, and will comprise twostages, the first lasting from 1999 to 2010, andthe second covering the decade from 2010 to2020. By the end of 2020, the plan aims to14

Plenary lectures

have the lake in a suitable condition to leave tofuture generations, and the achievement of the
50-year long-term vision (ca. 2050) will be wellunder way. By concentrating on continuously
updating existing measures and implementing
them in a consistent manner, in the 12-year
first stage, the plan aims to (1) seek to coordinate different policy measures, and (2) carry out
continuous surveying and monitoring while
new policies and measures are formulated.
Applying the knowledge and experience thus
amassed, in the second stage, the plan aims to(3) further promote conservation measureswhile shifting the focus toward more forwardlooking activities.
Because progress will only be evident in thelong-term, and the future developments are difficult to predict, the plan must be flexible. In
many senses, the best results produced by various measures in the earlier stages have to beincorporated into the plan and its execution inlater stages. New technologies and other pertinent developments will also have to be properly
accommodated.

Summary and conclusion
The Lake Biwa watershed has undergone tremendous transformation in the past few decades fromwhat used to be an agricultural and rural region conducive to the survival of the lake and its surrounding
environment to what is now a rapidly urbanizing
and industrializing region attracting populations andindustries from across the western part of Japan, particularly from the Osaka, Kyoto and Kobe areas.
This transformation took place mostly over the pastfew decades, during which the Lake Biwa Comprehensive Development Project (LBCDP) was implemented. As this was a mega-scale regional project
aimed at achieving water resource control, flood control, environmental conservation and economic
infrastructure development, LBCDP implementation was accompanied by large-scale land-use
changes, a loss of coastal wetlands, the introductionof highly mechanized irrigation systems over the vast
paddy fields, and the loss of soil recharge capacity inthe surrounding forests, each of which is likely toresult in significant impacts on the integrity of thelake–watershed system. Despite enormous efforts
made by the national and prefectural governments tocontrol the pollution, particularly through theimplementation of regional wastewater systems, thedeteriorating trend of lake environments has not yet

been reversed.
This large-scale transformation of the watershed islikely to continue for decades. Having realized theserious future implications of the resulting alterationof the lake ecosystem characteristics, the prefectural
government, together with downstream and national
governments, developed a comprehensive plan (theLBCCP) for the conservation of the lake environments over the next 50 years (ca 2050), with implementation, in the next two decades, of measuresproven successful so far, as well as new measures for
exploratory purposes. The proposed plan is expected
to evolve over years of trial and error, and thus therole of science and scientific approach in lake management will become even more crucial for bringing
the plan to success.

References
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15

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Author’s address:
M. NAKAMURA, Lake Biwa Research Institute, 1-10
Uchidehama Otsu, Shiga, Japan 520-0806.
E-mail: nakamura@lbri.go.jp