Lake Biwa is Japan’s largest freshwater lake, sitting in the west-central part of Honshu in Shiga Prefecture, just northeast of Kyoto. It is also one of the world’s oldest lakes, with a geological history stretching back roughly four million years. That combination of size, age, and location has made it a center of evolutionary novelty, a critical water source for millions of people, and a long-running natural laboratory for studying how freshwater ecosystems respond to human pressure and climate shifts.
One of the World’s Few Ancient Lakes
Most lakes on Earth are geologically young, carved or filled by glaciers during the last ice age and destined to fill with sediment within a few tens of thousands of years. Lake Biwa belongs to a much smaller group of “ancient lakes,” bodies of water that have persisted for hundreds of thousands to millions of years. Estimates place its origin at roughly four million years ago, putting it in the same class as Lake Baikal in Siberia and Lake Tanganyika in East Africa. Its survival owes to ongoing tectonic activity in the region: faulting has continued to deepen the lake basin even as sediment accumulates, preventing the lake from silting up and disappearing.
The lake’s present shape and depth, however, are more recent. About 400,000 years ago, a major expansion reshaped the basin into something close to its modern form, with a deep northern basin reaching roughly 104 meters and a much shallower southern basin. That expansion event turns out to be a pivotal marker in the biological and geological story of Lake Biwa, as several groups of organisms underwent rapid diversification around that time.
Endemic Fish and the Evolution They Represent
Ancient lakes are famous for harboring species found nowhere else, and Lake Biwa is no exception. The lake supports more than a dozen endemic fish species, most of them belonging to the carp and minnow family. A phylogenetic study of these fish found that their histories are surprisingly varied: some endemic lineages split from their closest relatives as long as one to thirteen million years ago, well before the lake reached its current form, while others diverged more recently, after the basin’s expansion around 400,000 years ago.1PubMed Central. Phylogeny and historical demography of endemic fishes in Lake Biwa: the ancient lake as a promoter of evolution and diversification of freshwater fishes in western Japan What unites them is that nearly all expanded their populations after the modern lake environment developed, suggesting that the lake’s enlargement opened new habitats and food sources that allowed older lineages to flourish alongside younger ones.
The same research pointed to something broader: Lake Biwa has served not just as a cradle for new species but as a reservoir for fish diversity across western Japan. Populations that evolved in or around the lake subsequently spread into surrounding river systems. In this sense, the lake’s role in regional biodiversity extends well beyond its shoreline.
Freshwater Snails and a Burst of Speciation
Fish are not the only group that radiated in Lake Biwa. The lake hosts about fifteen species of Biwamelania snails, a genus found exclusively in its waters. Research combining fossil evidence with molecular dating showed that two genetically distinct snail lineages independently colonized the lake and then underwent a rapid burst of speciation, each producing multiple new species at roughly the same time.2PubMed Central. Recent lake expansion triggered the adaptive radiation of freshwater snails in the ancient Lake Biwa That radiation was tightly linked to the lake’s expansion about 400,000 years ago. As the basin grew, new ecological niches appeared, driving the diversification of both lineages in parallel. It is a textbook example of how environmental opportunity can trigger adaptive radiation in freshwater systems.
A Four-Million-Year Climate Diary
Because Lake Biwa has existed for so long, its bottom sediments form a remarkably continuous record of environmental change. Researchers have drilled deep cores from the lakebed and analyzed them for chemical and biological markers that reflect past conditions. Geochemical work on these cores suggests the lake has remained relatively nutrient-poor since its formation, with ratios of aluminum, iron, and manganese in the sediment serving as indicators of past water depth and trophic state.3Palaeogeography, Palaeoclimatology, Palaeoecology. Geochemical history of ancient Lake Biwa in Japan—Chemical indicators of sedimentary paleo-environments in a drilled core
Different components of the sediment capture different climate signals. Quartz content tracks windblown dust, which reflects the strength of the Asian winter monsoon and the westerly wind belt. Meanwhile, organic carbon and biogenic silica content reflect biological productivity in the lake, which responds to precipitation driven by the summer monsoon.4Quaternary International. Last 40 ka climate changes as deduced from the lacustrine sediments of Lake Biwa, central Japan Together, these proxies allow scientists to reconstruct monsoon variability across tens of thousands of years, offering a window into East Asian climate that complements ice-core and ocean-sediment records from other parts of the world.
Eutrophication and a Grassroots Soap Movement
Lake Biwa supplies drinking water to about 14 million people in the Kansai region, including much of Osaka and Kyoto. That role made its water quality a matter of intense public concern when conditions began to deteriorate in the late twentieth century. Starting in the late 1960s, rapid population growth and industrial expansion around the lake, especially near the shallow southern basin, poured nutrients into the water. By 1977, the consequences were impossible to ignore: the lake experienced its first large-scale red tide, a bloom of plankton fed by excess phosphorus.5PubMed Central. Community capability building for environmental conservation in Lake Biwa (Japan) through an adaptive and abductive approach – Section: The problem: overgrown aquatic weeds in Lake Biwa
The response came partly from an unexpected direction. Local homemakers, alarmed by the algal blooms, launched what became known as the “Soap Movement,” urging households to stop using synthetic detergents that contained phosphorus. The grassroots campaign gained enough momentum to push the Shiga Prefectural government to enact the Ordinance for the Prevention of Eutrophication of Lake Biwa in 1980, one of Japan’s earliest local environmental regulations targeting a specific pollutant.6PubMed Central. Community capability building for environmental conservation in Lake Biwa (Japan) through an adaptive and abductive approach – Section: The problem: overgrown aquatic weeds in Lake Biwa Parallel to this, the national Lake Biwa Comprehensive Development Project worked to manage urban, industrial, and agricultural pressures on the lake through infrastructure and land-use planning.7Lakes & Reservoirs: Science, Policy and Management for Sustainable Use. Lake Biwa: Have sustainable development objectives been met?
These efforts did reduce nutrient loading. Phosphorus inputs dropped, and the lake’s overall trophic status shifted back toward less enriched conditions. But the story did not end there, because a new complication emerged: the lake was now changing in ways that nutrient management alone could not address.
Climate Change, Mixing, and the Oxygen Problem
Lake Biwa’s deep northern basin normally mixes from top to bottom during winter, when cold surface water sinks and carries dissolved oxygen to the deepest layers. This annual overturn is critical for bottom-dwelling organisms and for keeping nutrients and pollutants in check. Warming winters have begun to weaken that process. When winter mixing is incomplete, the deep water can become depleted of oxygen, a condition that releases phosphorus stored in the lakebed sediments and degrades habitat for fish and invertebrates on the bottom.
Numerical modeling of the lake’s mixing behavior and nutrient cycling has shown that although wastewater treatment improved conditions over the decades, climate-driven changes in how and when the lake overturns may independently worsen water quality. After about 1980, the statistical relationship between traditional nutrient-loading indicators and deep-water oxygen levels weakened considerably, suggesting that climate was becoming the dominant factor in bottom-water hypoxia.8Scientific Reports. Numerical analysis of the relationship between mixing regime, nutrient status, and climatic variables in Lake Biwa The same research noted, however, that controlling phosphorus inputs remains beneficial: even under climate stress, less phosphorus in the system means less severe oxygen depletion when mixing does fail.
At the surface, climate change and the return to lower nutrient levels have together reduced the lake’s overall biological productivity since the 1990s. The total amount of new organic matter produced by phytoplankton has declined, and modeling suggests the production of small, readily edible phytoplankton has fallen along with it, which could ripple upward through the food web by reducing the energy available to zooplankton and fish.9Science of the Total Environment. Impact of climate change and oligotrophication on quality and quantity of lake primary production: A case study in Lake Biwa For zooplankton specifically, long-term monitoring has documented large fluctuations in the seasonal dynamics of key species, with food availability and predation pressure interacting in ways that shift from year to year depending on temperature and other environmental conditions.10Journal of Great Lakes Research. Disrupted seasonal cycle of the warm-adapted and main zooplankter of Lake Biwa, Japan
Vanishing Reed Beds
The fringe of reed beds along Lake Biwa’s shoreline once covered about 5.2 square kilometers, based on aerial photographs from 1948. By 2000, that area had shrunk to roughly 3.2 square kilometers.11Springer Nature. Ecological Changes in Lake Biwa – Section: The Changes in Lakeshore Landforms of Lake Biwa The loss matters because these reed stands serve as spawning habitat for many of the lake’s native and endemic cyprinid fish. As lake water levels declined and small satellite lagoons along the shore became stagnant, the spawning areas degraded and fish catches dropped.
Reed bed loss is a problem that intersects with several other pressures on the lake. Development along the shoreline, water-level management for flood control and irrigation, and changes in sediment transport all contribute. Efforts to replant reeds and restore shoreline habitat have been ongoing, but restoring a naturally functioning littoral zone in a landscape that also serves millions of people as a water source is an inherently slow process.
Microplastics in the Water
Like freshwater systems worldwide, Lake Biwa has not escaped microplastic contamination. Surveys of both the northern and southern basins found that polyethylene and polypropylene together made up about 90% of the microplastic particles detected.12Researchmap / Independent Academic Repository. The abundance and composition of Micro Plastics in the North and South Basins of Lake Biwa, Japan These are the same plastic types dominant in consumer packaging and single-use products. Given that the lake feeds the drinking-water supply for the broader Kansai region, the presence of microplastics adds another dimension to the monitoring challenges already posed by nutrients, oxygen, and warming.
A Ramsar Wetland for Waterbirds
Lake Biwa and its surrounding wetlands are registered under the Ramsar Convention, the international treaty protecting wetlands of global importance. The designation reflects the lake’s role as habitat for a diverse community of waterbirds, including migratory species that use the lake and its coastal lagoons as stopover and wintering sites along the East Asian flyway.13Ecological Research. Habitat use and diversity of waterbirds in a coastal lagoon around Lake Biwa, Japan The satellite lagoons and marshes surrounding the main lake are especially important, offering sheltered foraging areas distinct from the open water. Research on habitat use in these lagoons has emphasized that bird diversity depends on the variety of habitats available, not just the lake’s total area, which is another reason the loss of reed beds and shoreline wetlands matters beyond its effect on fish.
Microbial Communities and the Deep Water
The lake’s biology extends far below what is visible on the surface. Year-round sampling of the water column has revealed that the bacterial communities living in the deep hypolimnion differ markedly from those in the sunlit upper layers. While some bacterial groups are found throughout the water column, others are specialists confined to the cold, dark, deep water. Members of the Chloroflexi and Planctomycetes groups are heavily represented among these deep-water specialists, along with organisms involved in nitrogen cycling such as nitrifying bacteria and archaea.14PubMed Central. Vertical partitioning of freshwater bacterioplankton community in a deep mesotrophic lake with a fully oxygenated hypolimnion (Lake Biwa, Japan) These communities play a role in nutrient cycling that researchers are still working to quantify. Their dependence on the deep oxygenated environment connects them directly to the mixing and warming trends described earlier: if the lake stops mixing fully in winter and the deep water loses its oxygen, these microbial communities could be reorganized in ways that alter how the whole system processes nitrogen and carbon.
Groundwater, Seiches, and the Hidden Plumbing
Lake Biwa is not a closed bowl of water. It exchanges water continuously with the surrounding groundwater through its lakebed, and the rate of that exchange fluctuates in a surprising way. When wind pushes surface water to one end of the lake and it sloshes back, the resulting oscillation is called a seiche, a standing wave that can cause the water level at any given point to rise and fall rhythmically. Measurements of groundwater seeping into the lake showed that the seepage rate fluctuates with the same dominant periods as the lake’s seiches.15Water Resources Research. An Effect of Seiche on Groundwater Seepage Rate into Lake Biwa, Japan In other words, the lake’s internal waves are pushing and pulling groundwater in and out of the sediment like a slow, invisible pump.
This interaction matters because groundwater carries dissolved chemicals from the surrounding watershed into the lake. If seiche activity modulates how fast groundwater enters, then it also modulates the delivery rate of nutrients, minerals, and potentially pollutants from subsurface sources. It is one of those connections that is easy to overlook when thinking about a lake’s health but that shapes water chemistry at the most fundamental level.
Traditional Fisheries and Food Culture
Lake Biwa has supported fishing communities for thousands of years, and its food traditions reflect that long relationship. Funazushi, a fermented crucian carp preparation, is one of the oldest forms of sushi in Japan, predating the vinegared-rice style familiar worldwide today. The dish involves packing the fish in rice and allowing it to lacto-ferment for months or even years, producing a sour, intensely flavored food that remains a regional specialty in Shiga Prefecture. Eri fishing, using large fixed nets that guide migrating fish into enclosures, has been practiced on the lake for centuries and still operates on a smaller scale.
These traditions depend on healthy populations of native fish, which ties them directly to the ecological pressures the lake faces. As reed beds shrink and invasive species like largemouth bass and bluegill compete with or prey on native fish, the cultural practices built around those fish come under strain alongside the ecosystems themselves. The intertwining of ecology and food culture gives conservation efforts around Lake Biwa a dimension that purely biological arguments sometimes lack: when a species declines, a piece of culinary heritage goes with it.