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Lake Ikeda

Lake Ikeda is a situated in the Ibusuki region of on Island, , approximately 40 kilometers south of City, and is the largest lake on the island with a surface area of 10.95 square kilometers, a circumference of 15 kilometers, and a maximum depth of 233 meters, ranking as the fourth deepest lake in . Formed by a massive volcanic eruption approximately 5,700 years ago during the Jomon period, which ejected about 5 cubic kilometers of volcanic material and created a 4-kilometer-wide , the lake features an active on its bed that rises 150 meters from the bottom and spans about 1 kilometer in diameter. As a warm with no major rivers flowing in or out, it maintains oligotrophic characteristics but has experienced declining water transparency—from 26.8 meters in 1929 to around 5 meters as of the early 2000s—along with occasional freshwater red tides due to nutrient inputs and climate influences. Ecologically, Lake Ikeda is renowned for its population of giant Japanese eels (Anguilla japonica), some reaching up to 1.8 meters in length and designated as a of Ibusuki City since 1969, alongside efforts to prevent through monitoring total nitrogen and phosphorus levels. The lake gained fame in 1978 for multiple sightings of a large aquatic creature known as Isshi, often compared to the and possibly linked to the giant eels, boosting its status as a for boating, scenic views, and resorts in the surrounding Kirishima-Kinkowan .

Geography

Location

Lake Ikeda is situated in Ibusuki, on Island, , approximately 40 km south of City. This positioning places the lake in the southeastern part of the Satsuma Peninsula, where it occupies a central role in the local landscape as Kyushu's largest caldera lake. The lake is bordered by the Satsuma Peninsula, with Mount Kaimon—a prominent rising to 924 meters—to the southeast, offering a striking scenic backdrop that enhances its appeal for visitors. This volcanic feature, often called the "Fuji of Satsuma," frames the lake's eastern horizon and underscores the area's dramatic topography. Accessibility to Lake Ikeda is facilitated by Japan National Route 226, which connects it to regional roadways, allowing for a drive of about 25 minutes from key intersections toward the lake's shores. Nearby towns like Ibusuki, located roughly 10 km to the west, provide essential infrastructure, including railway stations, bus services, and visitor facilities that support tourism and local travel. From Ibusuki Station, the lake is reachable in approximately 20 minutes by car or . In a broader regional context, Lake Ikeda forms part of a dynamic volcanic region in southern , influenced by the Ryukyu Arc—a tectonic system driven by the subduction of the beneath the . This arc contributes to the area's geothermal landscape, marked by ongoing volcanic activity and thermal features that define the environmental character of .

Physical Characteristics

Lake Ikeda covers a surface area of 11 km², establishing it as the largest lake on Island. The lake's maximum depth reaches 233 , ranking it as the fourth deepest in . The shoreline extends approximately 15 km around the lake, which exhibits a roughly circular shape characteristic of caldera lakes formed by volcanic activity. Situated at an elevation of 66 meters above , the lake features historically clear, oligotrophic waters known for high transparency, with visibility reaching up to 26.8 meters in early measurements before environmental influences reduced it.

Geology

Formation

Lake Ikeda formed as a caldera lake through the collapse of a volcanic following a major approximately 5,700 years ago. This event, known as the Ikeda eruption with a (VEI) of 5, involved the ejection of significant volumes of flows, , and bombs, leading to the structural that created the basin now occupied by the lake. The formation is part of the broader volcanic activity in southern , , driven by zone tectonics along the Ryukyu Trench, where the subducts beneath the , facilitating magma generation in the overriding plate. Specifically, Lake Ikeda resides within the Ikeda caldera, which is situated in the western sector of the larger caldera system, contributing to the regional volcanic chain that includes the nearby . The eruption's age has been determined through radiocarbon (14C) dating of associated materials, yielding an uncalibrated date of 2690 BCE ± 75 years (approximately 5,500–5,800 calibrated years BP), corroborated by tephrochronology using the widespread layer as a stratigraphic marker in southern sediments. These methods confirm the event's timing in the mid-Holocene epoch and its scale, with ash deposits providing evidence of the eruption's intensity and distribution. Following the caldera collapse, the basin filled primarily through precipitation and groundwater seepage, resulting in a freshwater lake isolated from marine influences despite its proximity to the Pacific Ocean. This infilling process stabilized the lake's hydrology over subsequent millennia, with no significant post-formational breaches reconnecting it to the sea.

Volcanic Features

Lake Ikeda's lake bed hosts a submerged volcano approximately 1 kilometer in diameter that rises 150 meters from the bottom, reflecting persistent volcanic dynamics within the caldera structure. This feature underscores the lake's origin as a volcanic crater filled by rainwater, with ongoing subsurface processes maintaining geothermal potential. Surrounding the lake, geothermal manifestations include active fumaroles, particularly along the eastern shore, where tracks gas emissions indicative of shallow magmatic heat sources. These fumaroles are part of broader hydrothermal activity in the Ibusuki , which encompasses the lake and features hot springs in the nearby Ibusuki area, driven by intrusions at depth. The hot springs exhibit boron isotope signatures consistent with volcanic fluid contributions, highlighting the region's linkage to active plutonic processes. Minor seismic activity accompanies these volcanic elements, with a swarm of felt earthquakes recorded in the Ibusuki area from August 5 to 8, 1967, attributed to volcanic-tectonic processes. Additional monitoring from 1997 to 2012 detected shallow volcanic-tectonic events and deeper low-frequency earthquakes, suggesting intermittent fluid movement and pressure changes beneath the lake. Volcanic influences extend to the lake's sediments, composed primarily of deposits from the caldera's formation, including and layers that reflect episodic eruptive history. Hydrothermal alteration in the surrounding rocks has generated deposits, such as small-scale gold-silver veins in the western Lake Ikeda area, formed through fluid-rock interactions tied to magmatic heating.

Hydrology

Water Properties

Lake Ikeda is classified as an oligotrophic lake, characterized by low concentrations that historically supported high and limited primary productivity. Total levels typically range from 0.22 to 0.38 mg/L at the surface and 0.24 to 0.27 mg/L in deeper waters, while total concentrations are even lower, between 0.002 and 0.008 mg/L at the surface and consistently around 0.003 mg/L at 200 m depth, based on measurements from 1985. These profiles reflect the lake's limited external inputs and internal , maintaining its oligotrophic status until at least the late , though management efforts have since aimed to prevent through controls on total and . The lake exhibits pronounced temperature , typical of deep lakes in subtropical regions. Surface waters vary seasonally from approximately 15°C in winter to 25°C in summer, while the hypolimnion remains stably near 10–11°C year-round due to the lake's depth exceeding 200 m, which limits vertical mixing. This thermal structure results in a meromictic , with no complete turnover observed since 1986, leading to persistent separation between the upper mixolimnion and the lower monimolimnion. Over the period from 1986 to 2005, deep-water temperatures rose by about 0.75°C, potentially weakening in response to regional warming. Water in Lake Ikeda ranges from 7.0 to 8.8, indicating slightly alkaline conditions influenced by dissolved volcanic minerals from the surrounding geology. Surface pH peaks in summer around 8.6–8.8, while deeper waters (e.g., 200 m) maintain values of 6.8–7.3, showing minimal variation with depth. Dissolved oxygen levels are relatively high in the , averaging 7.7–9.7 mg/L near the surface, but decrease below 100 m, with concentrations dropping to 5.8–7.6 mg/L at 200 m in the 1980s; anoxic conditions have since developed in the monimolimnion below approximately 130 m. Salinity in Lake Ikeda is characteristically low, with electrical conductivity around 50–70 µS/cm throughout most of the water column, classifying it as a freshwater system. A subtle increase in conductivity occurs at the chemocline around 130 m depth, suggesting minor accumulation of dissolved ions in the isolated bottom layer, which may stem from volcanic influences during the lake's formation. This faint salinity gradient contributes to the meromixis and supports a few euryhaline-tolerant species in the deeper waters, adapted to subtle ionic variations.

Inflows and Outflows

Lake Ikeda's is dominated by direct and subsurface inflows, with an average annual rainfall of approximately 1,962 mm recorded from 1983 to 1999, primarily concentrated in the summer months of and . The lake receives contributions from numerous small springs scattered around its perimeter, as well as diverted from three nearby river basins introduced since 1965 to offset withdrawals for and municipal supply; no major natural rivers directly enter the lake basin, which spans 41 km². The lake's sole natural outflow occurs through the Ikeda River on its eastern shore, which carries water toward the Ōsumi Peninsula and ultimately discharges into the . This unidirectional drainage maintains the lake's overall hydrological stability despite human interventions. With a volume of 1.47 km³ and the aforementioned inputs and outputs, Lake Ikeda exhibits a of about 1.7 years, which contributes to its gradual response to climatic or perturbations. Seasonal variations are pronounced, typically fluctuating by up to 4 meters annually, with peaks in late summer following heavy rainfall and events, and lows in early spring; contributes around 938 mm per year on average, with rates highest in autumn (peaking near 0.4 mm/day in ). These dynamics underscore the lake's sensitivity to regional patterns while highlighting the role of in its heat and budgets.

Ecology

Flora

Lake Ikeda supports a diverse array of aquatic flora, particularly in its shallower zones where light penetration allows for submerged species adapted to oligotrophic conditions. Surveys have identified 11 species of aquatic vascular plants, including the submerged perennial Hydrilla verticillata (known locally as kuromo) and Callitriche palustris (mizuhakobe), both of which are noted for their ecological importance and protection status due to their roles in stabilizing sediments and providing habitat. These plants thrive in the lake's clear, low-nutrient waters, with H. verticillata forming dense mats that contribute to oxygen production and nutrient cycling in littoral areas. The riparian zone along the lake's 15 km shoreline features emergent herbaceous vegetation dominated by (common reed), (cogongrass), and (Japanese silver grass), which form stabilizing buffers against and support transitional habitats. Surrounding the lake, broadleaf forests prevail, characterized by species such as Quercus phillyraeoides and , interspersed with pine forests of . These plant communities enhance shoreline stability and contribute to the overall biodiversity within Kirishima-Kinkowan National Park. Seasonal blooms of rapeseed (Brassica napus) in adjacent agricultural fields from December to February create vibrant yellow carpets that attract pollinators like bees and butterflies, bolstering local insect diversity and providing nectar resources during winter. This cultivated flora integrates with the natural ecosystem, supporting broader biodiversity without direct competition in aquatic zones. Phytoplankton in Lake Ikeda is primarily composed of diatoms, with Cyclotella sp. and Synedra sp. dominating during spring months, Mougeotia sp. in summer, and cyanobacteria such as Anabaena sp. and Microcystis sp. in autumn and winter, reflecting the lake's oligomesotrophic status and contributing to limited historical algal blooms. These microscopic algae form the base of the aquatic food web, with their silica frustules aiding in nutrient dynamics.

Fauna

Lake Ikeda supports a diverse adapted to its deep, oligotrophic environment, with notable thriving in oxygen-poor profundal zones. The lake is renowned for its population of giant mottled eels (Anguilla marmorata) and Japanese eels (A. japonica), which can reach lengths of up to 1.8 meters and weights of 20 kilograms, making them among the largest eels in . These eels, designated as a by Ibusuki City since 1969, inhabit the lake's deeper waters and form an economically important component of the fishery, with historical annual catches contributing to the lake's total fish yield of 379 metric tons in 1980. Other fish include (Cyprinus carpio), pond smelt (Hypomesus nipponensis), and (Plecoglossus altivelis), reflecting a moderate diversity suited to the lake's freshwater conditions. Invertebrates form a critical base for the lake's , particularly in the benthic layers. Zoobenthos communities feature species such as the Palaemon paucidens, the clam Pisidium parvum, and chironomid midge larvae (Prochladius sp.), which historically exhibited high densities in profundal sediments despite environmental stressors like low oxygen levels. However, recent studies as of 2024 have documented the disappearance of deep profundal zoobenthos, attributed to environmental warming and changes in . Zooplankton assemblages vary seasonally, dominated by cladocerans like Bosmina longirostris in spring, autumn, and winter, and rotifers such as Conochilus hippocrepis in summer, supporting higher trophic levels including fish. The lake's shoreline and surrounding forests host avifauna typical of coastal wetlands, including shorebirds like grey herons (Ardea cinerea) and overwintering waterfowl such as various duck species that utilize the area for . Mammalian presence is limited to the riparian zones, with occasional sightings of (Cervus nippon) and Japanese wild boar (Sus scrofa) in the adjacent woodlands, contributing to the broader ecosystem dynamics.

Environmental Concerns

Water Quality Decline

The water quality of Lake Ikeda began to decline in the mid-20th century, primarily due to agricultural runoff containing fertilizers and pesticides, as well as in the surrounding Ibusuki area. Development around the lake, including the expansion of resort facilities and increased tourism, contributed to nutrient inputs that accelerated starting around the 1950s and intensifying through the 1960s. The Nansatsu Field Irrigation Project, initiated in 1982, further exacerbated nitrogen loading from nearby farmlands, with elevated inflows observed particularly in 1998-1999. Key indicators of this include a marked in water transparency, which dropped from 26.8 meters in 1929 to approximately 5 meters by the late 20th and early 21st centuries, reflecting increased from algal growth and suspended particles. was evident in rising chlorophyll-a concentrations, signaling heightened activity. loading from agricultural sources drove nutrient enrichment that supported excessive algal proliferation. Monitoring efforts by have documented these trends through ongoing water quality assessments, revealing freshwater red tides—indicative of algal blooms—in the periods 1980-1981 and 1991-1993. While partial recovery occurred in the following natural lake overturns in 2011 and 2012, which reduced total levels and improved oxygen conditions, persistent challenges remain, including recurrent algal blooms and incomplete mixing events in years like 2013 and 2017 that restore anoxic bottom waters. These issues underscore the ongoing impact of historical nutrient accumulation on the lake's .

Conservation Measures

In response to declining water transparency and increasing nutrient levels observed since the mid-20th century, Kagoshima Prefecture established the Ikeda Lake Water Quality Environment Management Plan in March 1983 to prevent eutrophication and ensure long-term preservation of the lake's ecosystem. This comprehensive framework, updated through multiple phases—the second from 1991 to 2000, third from 2001 to 2010, fourth from 2011 to 2020, and fifth commencing in 2021—sets specific targets for chemical oxygen demand (COD) at or below 3 mg/L, total nitrogen (T-N) at or below 0.2 mg/L, and total phosphorus (T-P) at or below 0.01 mg/L, while addressing risks like incomplete lake overturn due to climate-induced warming. The plan covers the lake's direct watershed in Ibusuki City and indirect inflows from three rivers in nearby areas, guiding land use, development, and pollution control to minimize impacts on water circulation and quality. Key water treatment initiatives include a major irrigation diversion project launched in 1982, which reroutes from the three inflowing rivers (Magatagawa, Takatogawa, and Tsugawa) to reduce nutrient-laden agricultural runoff entering the lake, thereby stabilizing and levels. Additional measures encompass improved practices to limit organic waste discharge, advanced upgrades in surrounding communities, and enhanced to curb diffuse sources. These efforts have contributed to measurable progress, with targets for , T-N, and T-P fully achieved by 2022, alongside natural whole-lake overturn events in 2011 and 2012 that replenished bottom-layer dissolved oxygen and lowered concentrations. As of 2023, the fifth plan continues to monitor and adapt to influences on lake circulation. Biodiversity conservation focuses on the lake's endemic species, particularly the giant mottled eels (Anguilla marmorata), designated as a by Ibusuki City in 1969 to protect their habitat amid potential threats from water quality degradation and human activities. Ongoing monitoring under the management plan tracks ecological indicators, including oxygen levels critical for deep-water species, with the broader framework supporting habitat integrity through pollution controls that indirectly benefit aquatic life. The lake's inclusion in since its designation further enforces restrictions on shoreline development and promotes preservation. Community involvement is coordinated through the Ikeda Lake Water Quality Environment Conservation Measures Council, which engages local municipalities, businesses, and residents in implementing the plan's goals via public awareness campaigns on and sustainable . These collaborative efforts emphasize on climate adaptation, such as mitigating warming effects on lake circulation, and have fostered broader participation in watershed stewardship, contributing to the sustained improvements in water parameters observed into the 2020s.

Mythology and Culture

Issie Legend

The Issie legend originates from local in the region, where a white named Issie (or Isshi) lived peacefully on the shores of Lake Ikeda with her . When the foal was kidnapped by , the distraught mare desperately searched for her child before leaping into the lake in grief, transforming into a massive serpent-like creature that continues to surface in hopes of reuniting with her offspring. This tale, rooted in the emotional bond between mother and child, underscores themes of loss and eternal vigilance in Satsuma-area oral traditions. In the myths, Issie is depicted as a serpentine guardian spirit of the lake's waters, embodying protective yet mournful qualities drawn from regional . The creature's physical form is described as a long-necked, dark-bodied entity approximately 20 to 30 meters in length, with a black or reddish hue, - or snake-like body, and distinctive humps or projections that sometimes appear as if surfacing in pairs. These features evoke a saurian or dragon-like presence, often retaining a white reminiscent of the original . Cultural artifacts preserving the legend include references in 18th- and 19th-century records, such as the Edo-period geography text Sangoku Meisho Zue, which documents tales of a giant dragon god inhabiting Lake Ikeda as a powerful water deity. Oral traditions among local fishing communities have perpetuated these stories, linking Issie to the lake's mystical guardianship and passing down narratives of enormous aquatic beings through generations.

Modern Sightings and Impact

The phenomenon of Issie gained significant attention with the first widely reported modern sighting on September 3, 1978, when over 20 witnesses, including members of the , observed two dark humps, each approximately 5 meters long, emerging from the water near the western shore of Lake Ikeda and moving at high speed across the lake. Later that year, on December 16, 1978, local resident Toshiaki Matsuhara photographed what appeared to be two humped shapes with spinal ridges in the lake, an image that was officially recognized by authorities. In response to the surge in reports, the Ibusuki City tourism department offered a 100,000 yen reward for clear photographic evidence of the creature. Subsequent sightings continued through the and , with claimants reporting similar humped forms; notable examples include a 1988 observation of two black objects swimming rapidly and a 1990 video captured by Kazuo Kono showing a dark shape on the water surface, which was later broadcast on Japanese television. Photos and videos from this period were examined by local researchers and media, though analyses often concluded they depicted natural phenomena such as waves or optical illusions rather than an unknown . Over 20 such reports were documented during these decades, fueling public fascination but yielding no conclusive proof. Scientific scrutiny in the late , including ecological surveys of the lake, found no evidence of a large unknown ; instead, sightings were commonly attributed to the lake's population of giant mottled , which can reach lengths of 1.8 meters and were designated a local in 1969. A 1998 benthological survey confirmed the presence of these oversized in the lake's depths, supporting theories that their surface activity or groups could mimic the described humps. No dedicated expeditions were conducted in the , but wave patterns and were highlighted as likely explanations by investigators. The modern Issie reports have profoundly shaped local culture, inspiring Issie-themed statues and observation platforms along the lakeshore, as well as merchandise such as keychains, flavors, and souvenirs sold at roadside shops to draw visitors. These elements have integrated into regional festivals and events, enhancing community pride in the legend, while national media portrayals have popularized Issie as "Japan's ," appearing in television programs and articles that blend with contemporary mystery. This cultural resonance stems from the 1978 sightings onward, transforming an ancient tale into a symbol of local identity without verified biological discovery.

Human Activities

Tourism and Recreation

Lake Ikeda serves as a prominent destination for outdoor enthusiasts and nature lovers in , offering a range of water-based and land activities amid its scenic volcanic landscape. Visitors are drawn to the lake's calm waters and surrounding trails, which provide opportunities for relaxation and adventure throughout the year. Popular recreational pursuits include boating and watersports, with options such as paddle boats, banana boat rides, , and jetskiing available during the warmer months. Fishing is another favored activity, particularly for the lake's notable giant mottled eels, which can reach lengths of up to 1.8 meters, though they are a protected species requiring sustainable practices. Hiking along the approximately 15-kilometer shoreline trail offers panoramic views of the lake and its verdant surroundings, with the full loop rated as a challenging route spanning about 18 kilometers and taking 5.5 to 6 hours to complete. Kayaking and stand-up paddleboarding are also accessible through guided tours, allowing participants to explore the lake's deep blue waters up close. Seasonal events add vibrancy to visits, such as the Ibusuki Marathon held in early , where runners pass by the lake amid blooming fields that create a vivid backdrop. In summer, the lake's warm waters, reaching temperatures conducive to around 25°C, support additional activities like and leisurely dips, enhancing the appeal for families and adventure seekers. Key viewing spots include overlooks from Mount Kaimon, known as the "Fuji of ," which provide sweeping vistas of the lake framed by seasonal flowers and the . The lake's mythical allure, tied to the legendary creature Issie, inspires boat tours organized by local operators that combine recreation with exploration. Supporting infrastructure includes the Ikeda Paradise , which features exhibits on the lake's and to educate and engage tourists. These facilities, along with nearby cafes and sports areas, accommodate a steady flow of visitors seeking immersive experiences in this natural setting.

Economic Role

Lake Ikeda contributes to the local economy of Ibusuki City primarily through tourism and agriculture, with supporting roles in fisheries and research. The lake serves as a key attraction, drawing visitors for its scenic beauty, seasonal events, and natural features, which boost regional spending on accommodations, dining, and related services. Although specific revenue figures are not publicly detailed, tourism in Ibusuki, enhanced by Lake Ikeda's proximity to hot springs and hiking trails, forms a cornerstone of the area's economic activity alongside fishing and agriculture. In terms of fisheries, the lake is renowned for its population of giant mottled eels (Anguilla marmorata), designated as a protected by Ibusuki City since due to their exceptional size—up to 1.8 meters in length and 20 kilograms in weight. These eels do not support commercial harvesting owing to protection status, but the lake's ecosystem indirectly aids Ibusuki's broader sector, which focuses on eel farming and other freshwater in the region, valued as part of Prefecture's fisheries output. Agriculture around Lake Ikeda benefits from an extensive system developed since the , which diverts from the lake and nearby rivers to sustain farming on approximately 6,000 hectares of land. This infrastructure has transformed the volcanic soils of Ibusuki into productive fields, yielding high-quality crops such as sweet potatoes, mandarin oranges, and , making the area a significant agricultural hub in . The lake acts as a , regulating flow for and supporting livelihoods dependent on these exports. Research and education efforts centered on Lake Ikeda involve geological surveys of its formation and of and activity in the surrounding area. These studies, often funded through national geological institutions, employ local experts and guides, contributing to and scientific while informing strategies for the lake's .

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