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San Francisco Bay

San Francisco Bay is a significant coastal estuary located in Northern California, recognized as the largest of its kind on the western U.S. coast. This inland body of water connects to the Pacific Ocean through the Golden Gate, flanked by the Marin Headlands and the Presidio of San Francisco. The bay's ecosystem is shaped by a mixture of salty ocean waters and freshwater from the Sacramento and San Joaquin rivers, with around 40 percent of California's freshwater flowing into it. Covering approximately 1,600 square miles, the bay includes various sub-regions, such as San Pablo Bay and South San Francisco Bay, characterized by rich biodiversity supported by extensive wetlands.

Despite its ecological importance, San Francisco Bay faces challenges from urban development, pollution, and climate change, which have led to habitat degradation and significant alterations to its natural environment. Conservation efforts have emerged in response, focusing on restoring the bay's wetlands and improving water quality. The area is also a critical habitat for numerous species, including various fish, migratory birds, and marine mammals, many of which have seen population recoveries after historical declines. The San Francisco Bay's diverse and complex ecosystem highlights the interplay between natural resources and human activities, making it a focal point for conservation and environmental efforts in the region.

Full Article

  • Category: Marine and Oceanic Biomes.
  • Geographic Location: California.

Summary: Among the largest coastal estuaries in North America, San Francisco Bay is the site of landmark efforts to restore and preserve a complex and biologically diverse bioregion.

San Francisco Bay is both a bay and part of the largest coastal estuary in the western United States. Its definition as a bay derives from its position as an inland body of water directly channeled to the Pacific Ocean. This Pacific entrance, dubbed the Golden Gate by early European explorers, is bound by the Marin Headlands to the north and the Presidio of San Francisco on the south; it opens into the deep waters of the central bay. The salty waters of the bay mix with abundant freshwater from the deltas of the Sacramento River to the north and the San Joaquin River to the south.

Dams along these two major rivers have substantially altered important freshwater inputs into the estuary system. The San Francisco Bay estuary drains water from a watershed that covers nearly 40 percent of California. The combination of freshwater and saltwater in the estuary creates a dynamic salinity gradient that varies across the estuary and is influenced by both river inflow and ocean tides. It is a very nutrient-rich habitat.

Covering a total area of approximately 1,600 square miles (4,149 square kilometers), the greater bay area features such contiguous components as San Pablo Bay, South San Francisco Bay, Suisun Bay, Carquinez Strait, and the extensive delta areas of its two main tributary rivers. Via the rivers, the bay is connected to the ecosystems of California’s great Central Valley, as well as the near-extinct Tulare Lake basin. A vital part of the bay area is its abundant wetlands, which support mollusks, crustaceans, amphibians, reptiles, and vast migratory bird populations. The marshes of the South Bay are high enough in salinity to support the commercial development of salt flats.

The maritime climate of the bay area is controlled by the southwestern flow of polar Pacific air, an effect that keeps water and land mass temperatures along the northern California coast cool and steady throughout the year. As a result of summer atmospheric inversions, the bay near the Golden Gate is characteristically shrouded in stratus cloud fog cover, particularly during the early part of the day during the summer months. These generally mild climate conditions and the voluminous interchange of marine and estuarine influences are the foundation of microclimates so extreme and diverse that unique classification systems have been developed to adequately describe some of the species that flourish within them.

Biota

The valleys and woodlands surrounding the bay typically receive about 20-30 inches (roughly 500–760 millimeters) of rainfall annually, depending on local topography and proximity to the coast, supporting a rolling landscape of shrubs, herbs, conifers, and broadleaf mixed forests of redwoods, oaks, maples, and alder. There is also extensive growth of eucalyptus, a nonnative species.

Phytoplankton are the primary producers of the food web that sustains the biodiversity of the bay. Elevated nitrogen levels and the turbidity of the bay’s waters have had negative effects on phytoplankton production; the introduction of the Amur River clam (Potamocorbula amurensis) further significantly reduced the biomass of phytoplankton, a condition that has continuing deleterious effects on the pelagic fish species that depend on the biomass for survival.

Keystone species here include the primary consumers of phytoplankton—copepods, rotifers, ciliates, and flagellates—as well as the macro-invertebrates (mysids, shrimp, and amphipods), and planktivorous fishes such as sturgeon, chinook salmon, and American shad. Larger fish common to the bay include various saltwater, freshwater, and some anadromous species of shark, ray, sunfish, tuna, surfperch, halibut, sole, flounder, smelt, pipefish, lamprey, carp, bass, and catfish. Some steelhead populations along the California coast are protected under the US Endangered Species Act; for example, the Central California Coast steelhead is listed as threatened. Frogs, toads, and salamanders are also common here, as are the leatherback sea turtle, the Pacific pond turtle, and a variety of lizards and snakes.

The diversity of habitat in the area sustains a thriving insect community including nearly 100 species of ants and bees; wasps, mud daubers, and yellowjackets; beetles, grasshoppers, and crickets; and dragonflies, moths and butterflies—all of which provide food for the millions of waterfowl that migrate to the bay via the Pacific Flyway of North America. Dabbling and diving ducks are common, as are the Canada goose, swans, terns, rails, egrets, great blue herons, clappers, pelicans, red-tailed hawks, American kestrels, quails, doves, hummingbirds, kingfishers, ring-billed gulls, sparrows, goldfinches, and blackbirds.

Many of the aquatic mammals native to the region are at last growing in population, after staggering decimations during the California Fur Rush of the nineteenth century. Once plentiful communities of beaver, marten, mink, fox, weasel, and river and sea otters were driven to near extinction by global fur traders. Recovery is finally taking hold after more than 150 years. For example, the California golden beaver is recolonizing in the North Bay area; the North American river otter has similarly been noted. Other marine mammals that appear both offshore and in the bay proper include a range of whales, dolphins, and seals.

Common land mammals can be found both in protected park areas and in urban environments here. These include opossums, shrews, jackrabbits, coyotes, a variety of squirrels, and mice. In the higher-elevation, more heavily wooded areas are found badgers, elk, deer, bobcats, mountain lions, and wild pigs. Bat communities are also common.

The San Francisco Bay biome is the site of one of the largest bayland restoration programs in the nation. The tidal flats and wetlands here are populated with sedges, rushes, cord grasses, and shrubs unique to salt marshes found along the banks of this estuary. These baylands provide essential nutrients that sustain shellfish and invertebrates, which in turn feed the fish and waterfowl that breed and spawn in its waters.

Human Impact and Conservation

Taken as a whole, the San Francisco Bay biome is one of the largest, richest—and heavily degraded—ecosystems in the world. It is also the site of landmark efforts to restore and preserve a complex and biologically diverse bioregion.

The urban, agricultural, and industrial geography of the San Francisco Bay biome was initiated in the early 19th century as fur traders recognized the bay and estuary for its ready access to contiguous rivers, and direct routes to the fecund valleys and mountains inland—ideal for commerce both before and after steamships and railroads were built. The bay was very quickly enlaced by early port settlements radiating from the San Francisco metropolitan core, all engaged in the trans-shipment of goods through the Port of San Francisco to markets in Asia, South America, and Alaska. The Oakland Pier in East Bay was built in 1863, accelerating patterns of settlement and trade. In the 2020s, the San Francisco Bay Area included nine counties—Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, Solano, and Sonoma—and supported a 2020 census population of 7.76 million people.

The bay area is somewhat unique in its underlying intimate relationship between urban, industrial, and agricultural lands that surround it. Of a total surface area of 4.5 million acres (1.8 million hectares), nearly 2.5 million acres (1 million hectares) are open space, with some 1 million acres (405,000 hectares) held in public trust. This is the birthplace of early conservation and environmental movements that include the Sierra Club, Marin Conservation League, Save San Francisco Bay Association (commonly called Save the Bay), San Francisco Estuary Project, State of California Coastal Conservancy, San Francisco Bay Area Conservancy Program, San Francisco Bay Conservation and Development Commission, the Audubon Society, the Bay Institute of San Francisco, and the Nature Conservancy. Other important science-based projects include the National Oceanic and Atmospheric Administration (NOAA) San Francisco Bay Watershed Database and Mapping Project and the Bay Area Ecosystems Climate Change Consortium.

Since the mid-19th century, extensive filling, diking, and development have eliminated about 90 percent of the bay’s historic tidal wetlands. At the same time, dams along the two major rivers substantially altered important freshwater inputs into the estuary system. The National Estuary Program was established by the US Congress in 1987 as an amendment to the Clean Water Act; this program identified the San Francisco Bay as an estuary of national significance, and a Comprehensive Conservation and Management Plan was established in 1993, enumerating 145 actions to improve and restore the bay’s estuarine system.

Large amounts of sediment have entered the bay from the Sierra Nevada foothills since the 1800s; this has historically protected the bay from the algal blooms that usually result from nitrogen pollution. The blooms take up too much oxygen from the water, resulting in “dead zones” where other wildlife cannot live, and may be toxic, causing problems both for wildlife and human swimmers; however, they also require sunlight, which they cannot obtain in murky, sediment-filled waters. In 2022, a large harmful algal bloom (often described as a red tide) spread through parts of San Francisco Bay and caused widespread fish mortality. In 2024, regional regulators adopted new wastewater permit requirements calling for a 40 percent reduction in nitrogen discharges compared with 2022 levels.

In 1999, the Baylands Ecosystem Habitat Goals was published, with the recommendation that the bay’s tidal marshes should be increased by 100,000 acres. Referring to the 1993 Wetlands Conservation Policy, a benchmark was established for defining desirable tidal marsh and tidal flat sizes. These were defined as the percent similarity between their historical and present-day distribution (plus or minus 25 percent, according to the range of sizes in each category). By 2025, the bay had about 57,800 acres of tidal marsh, representing more than half of the regional 100,000-acre goal, while the flatlands are close to reaching their 30,000-acre benchmark. The South Bay Salt Pond Restoration Project is an ongoing effort to restore about 15,100 acres of former industrial salt ponds to tidal wetlands and other habitats. Climate change has become an issue in the area, leading to widespread drought into the 2020s, sometimes accompanied by severe rainstorms that have washed debris into the bay, and this and anticipated changes in sea level are ecological factors that will continue to challenge the health and diversity of the San Francisco Bay biome in future decades.


Bibliography

Beidleman, Linda H., and Eugene N. Kozloff. Plants of the San Francisco Bay Region: Mendocino to Monterey. University of California Press, 2003.

California Regional Water Quality Control Board, San Francisco Bay Region. “San Francisco Bay Nutrient Watershed Permit Adopted to Reduce Nitrogen Discharges.” California Water Boards, 10 July 2024, waterboards.ca.gov/press_room/press_releases/2024/pr20240710-sf-bay-permit-release.html. Accessed 6 Mar. 2026.

Cloern, James E., and Alan D. Jassby. “Drivers of Change in Estuarine-Coastal Ecosystems: Discoveries from Four Decades of Study in San Francisco Bay.” Reviews of Geophysics, vol. 50, no. 4, 2012, doi:10.1029/2012RG000397. Accessed 6 Mar. 2026.

Hart, John, and David Sanger. San Francisco Bay: Portrait of an Estuary. University of California Press, 2003.

Li, Roland, and Nami Sumida. “Census 2020: Bay Area Population Grew at Faster Rate Than California, With Big Asian and Latino Gains.” San Francisco Chronicle, 19 Aug. 2021, sfchronicle.com/bayarea/article/Census-2020-Bay-Area-population-grew-at-faster-16383491.php. Accessed 6 Mar. 2026.

Meadows, Robin. “Why Nutrient Pollution May Become a Threat to San Francisco Bay Health.” Water Deeply, 16 Aug. 2017, newsdeeply.com/water/articles/2017/08/16/why-nutrient-pollution-may-become-a-threat-to-san-francisco-bay-health. Accessed 6 Mar. 2026.

National Oceanic and Atmospheric Administration. “Central California Coast Steelhead.” NOAA Fisheries, fisheries.noaa.gov/west-coast/endangered-species-conservation/central-california-coast-steelhead. Accessed 6 Mar. 2026.

San Francisco Estuary Partnership. State of Our Estuary 2025: Executive Summary. Oct. 2025, ourestuary.org/wp-content/uploads/2025/10/State-of-Our-Estuary_Executive-Summary-2025_102725.pdf. Accessed 6 Mar. 2026.

Schoenherr, Allan A. A Natural History of California. University of California Press, 1992.

South Bay Salt Pond Restoration Project. “About the South Bay Salt Pond Restoration Project.” southbayrestoration.org/about/. Accessed 6 Mar. 2026.

Vance, James E., Jr. Geography and Urban Evolution in the San Francisco Bay Area. Institute of Governmental Studies, University of California, 1964.

Wolfrom, Jessica. “Can San Francisco Save the Bay?” The San Francisco Examiner, 22 Mar. 2024, sfexaminer.com/news/climate/can-san-francisco-save-the-bay/article_53f6e53a-ad85-11ed-8221-7b88023d1b77.html. Accessed 6 Mar. 2026.

Full Article

  • Category: Marine and Oceanic Biomes.
  • Geographic Location: California.

Summary: Among the largest coastal estuaries in North America, San Francisco Bay is the site of landmark efforts to restore and preserve a complex and biologically diverse bioregion.

San Francisco Bay is both a bay and part of the largest coastal estuary in the western United States. Its definition as a bay derives from its position as an inland body of water directly channeled to the Pacific Ocean. This Pacific entrance, dubbed the Golden Gate by early European explorers, is bound by the Marin Headlands to the north and the Presidio of San Francisco on the south; it opens into the deep waters of the central bay. The salty waters of the bay mix with abundant freshwater from the deltas of the Sacramento River to the north and the San Joaquin River to the south.

Dams along these two major rivers have substantially altered important freshwater inputs into the estuary system. The San Francisco Bay estuary drains water from a watershed that covers nearly 40 percent of California. The combination of freshwater and saltwater in the estuary creates a dynamic salinity gradient that varies across the estuary and is influenced by both river inflow and ocean tides. It is a very nutrient-rich habitat.

Covering a total area of approximately 1,600 square miles (4,149 square kilometers), the greater bay area features such contiguous components as San Pablo Bay, South San Francisco Bay, Suisun Bay, Carquinez Strait, and the extensive delta areas of its two main tributary rivers. Via the rivers, the bay is connected to the ecosystems of California’s great Central Valley, as well as the near-extinct Tulare Lake basin. A vital part of the bay area is its abundant wetlands, which support mollusks, crustaceans, amphibians, reptiles, and vast migratory bird populations. The marshes of the South Bay are high enough in salinity to support the commercial development of salt flats.

The maritime climate of the bay area is controlled by the southwestern flow of polar Pacific air, an effect that keeps water and land mass temperatures along the northern California coast cool and steady throughout the year. As a result of summer atmospheric inversions, the bay near the Golden Gate is characteristically shrouded in stratus cloud fog cover, particularly during the early part of the day during the summer months. These generally mild climate conditions and the voluminous interchange of marine and estuarine influences are the foundation of microclimates so extreme and diverse that unique classification systems have been developed to adequately describe some of the species that flourish within them.

Biota

The valleys and woodlands surrounding the bay typically receive about 20-30 inches (roughly 500–760 millimeters) of rainfall annually, depending on local topography and proximity to the coast, supporting a rolling landscape of shrubs, herbs, conifers, and broadleaf mixed forests of redwoods, oaks, maples, and alder. There is also extensive growth of eucalyptus, a nonnative species.

Phytoplankton are the primary producers of the food web that sustains the biodiversity of the bay. Elevated nitrogen levels and the turbidity of the bay’s waters have had negative effects on phytoplankton production; the introduction of the Amur River clam (Potamocorbula amurensis) further significantly reduced the biomass of phytoplankton, a condition that has continuing deleterious effects on the pelagic fish species that depend on the biomass for survival.

Keystone species here include the primary consumers of phytoplankton—copepods, rotifers, ciliates, and flagellates—as well as the macro-invertebrates (mysids, shrimp, and amphipods), and planktivorous fishes such as sturgeon, chinook salmon, and American shad. Larger fish common to the bay include various saltwater, freshwater, and some anadromous species of shark, ray, sunfish, tuna, surfperch, halibut, sole, flounder, smelt, pipefish, lamprey, carp, bass, and catfish. Some steelhead populations along the California coast are protected under the US Endangered Species Act; for example, the Central California Coast steelhead is listed as threatened. Frogs, toads, and salamanders are also common here, as are the leatherback sea turtle, the Pacific pond turtle, and a variety of lizards and snakes.

The diversity of habitat in the area sustains a thriving insect community including nearly 100 species of ants and bees; wasps, mud daubers, and yellowjackets; beetles, grasshoppers, and crickets; and dragonflies, moths and butterflies—all of which provide food for the millions of waterfowl that migrate to the bay via the Pacific Flyway of North America. Dabbling and diving ducks are common, as are the Canada goose, swans, terns, rails, egrets, great blue herons, clappers, pelicans, red-tailed hawks, American kestrels, quails, doves, hummingbirds, kingfishers, ring-billed gulls, sparrows, goldfinches, and blackbirds.

Many of the aquatic mammals native to the region are at last growing in population, after staggering decimations during the California Fur Rush of the nineteenth century. Once plentiful communities of beaver, marten, mink, fox, weasel, and river and sea otters were driven to near extinction by global fur traders. Recovery is finally taking hold after more than 150 years. For example, the California golden beaver is recolonizing in the North Bay area; the North American river otter has similarly been noted. Other marine mammals that appear both offshore and in the bay proper include a range of whales, dolphins, and seals.

Common land mammals can be found both in protected park areas and in urban environments here. These include opossums, shrews, jackrabbits, coyotes, a variety of squirrels, and mice. In the higher-elevation, more heavily wooded areas are found badgers, elk, deer, bobcats, mountain lions, and wild pigs. Bat communities are also common.

The San Francisco Bay biome is the site of one of the largest bayland restoration programs in the nation. The tidal flats and wetlands here are populated with sedges, rushes, cord grasses, and shrubs unique to salt marshes found along the banks of this estuary. These baylands provide essential nutrients that sustain shellfish and invertebrates, which in turn feed the fish and waterfowl that breed and spawn in its waters.

Human Impact and Conservation

Taken as a whole, the San Francisco Bay biome is one of the largest, richest—and heavily degraded—ecosystems in the world. It is also the site of landmark efforts to restore and preserve a complex and biologically diverse bioregion.

The urban, agricultural, and industrial geography of the San Francisco Bay biome was initiated in the early 19th century as fur traders recognized the bay and estuary for its ready access to contiguous rivers, and direct routes to the fecund valleys and mountains inland—ideal for commerce both before and after steamships and railroads were built. The bay was very quickly enlaced by early port settlements radiating from the San Francisco metropolitan core, all engaged in the trans-shipment of goods through the Port of San Francisco to markets in Asia, South America, and Alaska. The Oakland Pier in East Bay was built in 1863, accelerating patterns of settlement and trade. In the 2020s, the San Francisco Bay Area included nine counties—Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, Solano, and Sonoma—and supported a 2020 census population of 7.76 million people.

The bay area is somewhat unique in its underlying intimate relationship between urban, industrial, and agricultural lands that surround it. Of a total surface area of 4.5 million acres (1.8 million hectares), nearly 2.5 million acres (1 million hectares) are open space, with some 1 million acres (405,000 hectares) held in public trust. This is the birthplace of early conservation and environmental movements that include the Sierra Club, Marin Conservation League, Save San Francisco Bay Association (commonly called Save the Bay), San Francisco Estuary Project, State of California Coastal Conservancy, San Francisco Bay Area Conservancy Program, San Francisco Bay Conservation and Development Commission, the Audubon Society, the Bay Institute of San Francisco, and the Nature Conservancy. Other important science-based projects include the National Oceanic and Atmospheric Administration (NOAA) San Francisco Bay Watershed Database and Mapping Project and the Bay Area Ecosystems Climate Change Consortium.

Since the mid-19th century, extensive filling, diking, and development have eliminated about 90 percent of the bay’s historic tidal wetlands. At the same time, dams along the two major rivers substantially altered important freshwater inputs into the estuary system. The National Estuary Program was established by the US Congress in 1987 as an amendment to the Clean Water Act; this program identified the San Francisco Bay as an estuary of national significance, and a Comprehensive Conservation and Management Plan was established in 1993, enumerating 145 actions to improve and restore the bay’s estuarine system.

Large amounts of sediment have entered the bay from the Sierra Nevada foothills since the 1800s; this has historically protected the bay from the algal blooms that usually result from nitrogen pollution. The blooms take up too much oxygen from the water, resulting in “dead zones” where other wildlife cannot live, and may be toxic, causing problems both for wildlife and human swimmers; however, they also require sunlight, which they cannot obtain in murky, sediment-filled waters. In 2022, a large harmful algal bloom (often described as a red tide) spread through parts of San Francisco Bay and caused widespread fish mortality. In 2024, regional regulators adopted new wastewater permit requirements calling for a 40 percent reduction in nitrogen discharges compared with 2022 levels.

In 1999, the Baylands Ecosystem Habitat Goals was published, with the recommendation that the bay’s tidal marshes should be increased by 100,000 acres. Referring to the 1993 Wetlands Conservation Policy, a benchmark was established for defining desirable tidal marsh and tidal flat sizes. These were defined as the percent similarity between their historical and present-day distribution (plus or minus 25 percent, according to the range of sizes in each category). By 2025, the bay had about 57,800 acres of tidal marsh, representing more than half of the regional 100,000-acre goal, while the flatlands are close to reaching their 30,000-acre benchmark. The South Bay Salt Pond Restoration Project is an ongoing effort to restore about 15,100 acres of former industrial salt ponds to tidal wetlands and other habitats. Climate change has become an issue in the area, leading to widespread drought into the 2020s, sometimes accompanied by severe rainstorms that have washed debris into the bay, and this and anticipated changes in sea level are ecological factors that will continue to challenge the health and diversity of the San Francisco Bay biome in future decades.


Bibliography

Beidleman, Linda H., and Eugene N. Kozloff. Plants of the San Francisco Bay Region: Mendocino to Monterey. University of California Press, 2003.

California Regional Water Quality Control Board, San Francisco Bay Region. “San Francisco Bay Nutrient Watershed Permit Adopted to Reduce Nitrogen Discharges.” California Water Boards, 10 July 2024, waterboards.ca.gov/press_room/press_releases/2024/pr20240710-sf-bay-permit-release.html. Accessed 6 Mar. 2026.

Cloern, James E., and Alan D. Jassby. “Drivers of Change in Estuarine-Coastal Ecosystems: Discoveries from Four Decades of Study in San Francisco Bay.” Reviews of Geophysics, vol. 50, no. 4, 2012, doi:10.1029/2012RG000397. Accessed 6 Mar. 2026.

Hart, John, and David Sanger. San Francisco Bay: Portrait of an Estuary. University of California Press, 2003.

Li, Roland, and Nami Sumida. “Census 2020: Bay Area Population Grew at Faster Rate Than California, With Big Asian and Latino Gains.” San Francisco Chronicle, 19 Aug. 2021, sfchronicle.com/bayarea/article/Census-2020-Bay-Area-population-grew-at-faster-16383491.php. Accessed 6 Mar. 2026.

Meadows, Robin. “Why Nutrient Pollution May Become a Threat to San Francisco Bay Health.” Water Deeply, 16 Aug. 2017, newsdeeply.com/water/articles/2017/08/16/why-nutrient-pollution-may-become-a-threat-to-san-francisco-bay-health. Accessed 6 Mar. 2026.

National Oceanic and Atmospheric Administration. “Central California Coast Steelhead.” NOAA Fisheries, fisheries.noaa.gov/west-coast/endangered-species-conservation/central-california-coast-steelhead. Accessed 6 Mar. 2026.

San Francisco Estuary Partnership. State of Our Estuary 2025: Executive Summary. Oct. 2025, ourestuary.org/wp-content/uploads/2025/10/State-of-Our-Estuary_Executive-Summary-2025_102725.pdf. Accessed 6 Mar. 2026.

Schoenherr, Allan A. A Natural History of California. University of California Press, 1992.

South Bay Salt Pond Restoration Project. “About the South Bay Salt Pond Restoration Project.” southbayrestoration.org/about/. Accessed 6 Mar. 2026.

Vance, James E., Jr. Geography and Urban Evolution in the San Francisco Bay Area. Institute of Governmental Studies, University of California, 1964.

Wolfrom, Jessica. “Can San Francisco Save the Bay?” The San Francisco Examiner, 22 Mar. 2024, sfexaminer.com/news/climate/can-san-francisco-save-the-bay/article_53f6e53a-ad85-11ed-8221-7b88023d1b77.html. Accessed 6 Mar. 2026.

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