User:LinusTheScientist/California Current

The California Current is an eastern boundary current in the Pacific Ocean that flows southward along the western coast of North America, from the Strait of Juan de Fuca to the southern tip of Baja California. It carries the southward flow of the North Pacific Gyre and is one of six major coastal currents affiliated with strong upwelling zones (including the Humboldt Current, the Benguela Current, the Canary Current, the Portugal Current, and the Somali Current). The current moves relatively fresh and cool surface water from northern latitudes southward and plays a key role in nutrient and sediment transport as well as plankton community composition. The current also displays strong seasonal, interannual, and decadal variability due to changes in atmospheric pressure, winds, and ocean properties.

Upwelling brings nutrients to the surface in the California Current region, fueling high productivity and supporting a diverse ecosystem. The current is economically important to coastal communities, linked to livelihoods through fisheries as well as shipping ports and ecotourism. Climate change is expected to impact the California Current in many ways, including intensified upwelling, increased seasonal hypoxia, acidification, increased harmful algal blooms, and disruptions to fisheries, marine mammals, and seabirds.

Physical properties
The California Current is 50-100 kilometers wide and flows at a maximum surface velocity of 40 to 80 centimeters per second, extending to a depth of about 300 meters. The current is made up of narrow areas of fast flow called jets within a broad area of slow flow, with two prominent zones of faster flow, a main branch offshore and a seasonal coastal jet. On average, the main branch of the current is located 200-300 kilometers offshore. The characteristics of the California Current vary along the coast, and it can be separated into a northern region north of Cape Mendocino, a southern region south of Point Conception, and a central region between the two. The transport of water by the current is a few Sverdrups, or million cubic meters per second.

The current has two components, both of which are ultimately driven by atmospheric pressure and winds. The first component is the southward flow of the North Pacific subtropical gyre, a large clockwise flow in the North Pacific Ocean. The California Current is fed by the North Pacific Current on the north side of the gyre. This part of the flow is driven by large-scale winds in the Pacific, which cause water to build up in the center of the gyre. This pressure distribution causes a surface flow to the south on the eastern side of the gyre through the geostrophic balance, which describes how pressure and the rotating earth combine to drive ocean currents. The buildup of water in the center of the gyre also drives downward motion of water called Ekman pumping. The squeezing of water columns by this downward motion causes southward Sverdrup transport throughout the gyre as the water columns move southward to maintain their angular momentum.

The second component of the California Current is the coastal upwelling system, driven by alongshore winds. These winds occur because the westerlies, the prevailing winds in the midlatitudes, are deflected to the south by the North American continent. In the Northern Hemisphere, water is transported to the right of the wind direction due to the rotation of the Earth in a process called Ekman transport. When winds blow to the south along the coast, this process moves water to the right (offshore). The movement of the water creates a pressure gradient with lower pressure along the coast, driving geostrophic surface flow to the south. The movement of water offshore also pulls water up from below, causing upwelling.

The movement of water from the north down the west coast of the United States, combined with upwelling in summer, results in much cooler ocean temperatures than at comparable latitudes on the East Coast, where ocean currents bring water from the tropics. Ocean surf temperatures are rarely above 66 F during the summer along the California coast south to San Diego, whereas they are often above 80 F on the East Coast from North Carolina southward. Cool surface water, along with the offshore movement of warm air on land, contributes to fog production in the coastal zone.

Related currents
The California Current is part of a system of currents that occur along the west coast of North America, including the California Current, the coastal jet, the California Undercurrent, the Davidson Current (sometimes called the Inshore Countercurrent), and the Southern California Eddy (also called the Southern California Countercurrent). The coastal jet is a strongly seasonal current occurring 5 to 25 kilometers from shore that contributes to the southward flow of the California Current. The jet flows southward in the spring and summer, with the strongest flow in spring and the maximum speed moving progressively offshore. In winter, flow along the coast tends to be northward. The strength and direction of the coastal jet are closely linked to winds. Especially in winter, the flow can vary greatly, even reversing, on a timescale of days as winds switch from north to southward.

The California Undercurrent flows northward underneath and inshore of the California Current, carrying tropical Pacific water with a warm, salty, low-oxygen signature. The undercurrent flows at speeds of 10 centimeters per second or faster, and it is about 20 kilometers wide. Its core is located at 250 meters depth and it extends to 1000 meters depth. The undercurrent occurs because the water along the coast has layers of density that change the pressure gradient at depth, leading to a reversal in the flow direction through a process called the thermal wind.

Inshore of the California Current is the Davidson Current, which flows to the north in the fall and winter. This current is the surface expression of the California Undercurrent, which moves closer to the surface during the winter and deepens in summer. The Southern California Eddy/Countercurrent originates at an eastward bend in the California Current near 32°N, where a northward flow splits off from the main current and moves into the Southern California Bight. This flow is seasonally variable, but there is northward flow near the shore most of the year in the northern part of the bight. The flow tends to recirculate within the bight in summer, when it is called the Southern California Eddy, whereas it can connect to the Davidson Current in winter, when it is called the Southern California Countercurrent.

Upwelling
Alongshore winds drive upwelling in the California Current system. Upwelled water comes from 150-200 meters depth and is defined by its high nutrient content and low temperature, producing a region of cool surface water 80-300 kilometers from shore and fueling high productivity in the coastal zone. The offshore motion of water is not uniform, but rather a complicated, swirling pattern of eddies and jets. These jets take the form of squirts, which end offshore, and meanders, which circle back to the coast. Two wind-driven processes control upwelling in the current. The first process is the Ekman transport of water offshore, driven by southward flow along the coast. Because the coast is a solid boundary, this transport requires that water move up from below to replace the water moved offshore. Similarly, if winds blow northward along the coast, water is transported towards the shore, causing downwelling. The second process leading to upwelling is the curl of the wind stress, which describes the degree of rotation of the winds. When the speed of the southward wind increases moving offshore, the curl of the wind is positive and the Ekman transport increases moving away from the coast, which enhances upwelling.

The extent and strength of upwelling varies strongly with the seasons, especially in the northern part of the current, with shifts in the atmospheric pressure patterns that drive winds. In the winter, north of Cape Mendocino, winds change from southward to northward and there is downwelling instead of upwelling. South of Cape Mendocino, winds support upwelling during the entire year. The upwelling system is more sensitive to local winds in the northern part of the current compared to the southern part. Overall, the strongest upwelling occurs from April to July, with the maximum occurring near 34°N, offshore of Point Conception. The shape of the coastline and the ocean bottom, including features like capes and canyons, also affects upwelling by causing jets of water to move offshore.

Upwelling indices are used to assess the strength of upwelling, and they are calculated based on winds or associated Ekman transport. The Bakun Index is based on the monthly mean Ekman transport, calculated from atmospheric pressure. The Coastal Upwelling Transport Index and the Biologically Effective Upwelling Transport Index provide improved estimates of vertical transport and vertical nitrate flux.

Variability
The California Current varies on multiple timescales, from seasonal to interannual to decadal. The major driver of seasonal variability is changes in winds and wind stress curl. In winter, the current is located farther offshore, with minimal upwelling and northward flow along the coast associated with the Davidson Current. In summer, an upwelling front develops at the coast, where there is a sharp change in water properties at the junction between upwelled water at the coast and surface water offshore. This front moves offshore as the season progresses. The onset of upwelling, called the "spring transition," and the change to downwelling, called the "fall transition," vary from year to year with winds. When upwelling starts later than usual, the water is warmer than usual and impacts are felt throughout the California Current ecosystem.

On interannual time scales, the El-Niño Southern Oscillation (ENSO) also impacts the California Current. During ENSO events, coastal Kelvin waves propagate northward along the coast from the equator. They change the vertical locations of sharp changes in properties like water temperature, described by the thermocline, and nutrients. This, in turn, changes the properties of upwelled water. When nutrient content is lower than usual, productivity is also lower than usual. El Niño events tend to cause warmer, saltier water and higher sea level than usual, with stronger flow to the north along the coast in the fall and winter and lower productivity. La Niña produces the opposite effect. ENSO can also change the California Current by affecting atmospheric pressure in the North Atlantic and by causing more "tropical" water to be carried northward along the coast. El Niño events vary in their impact on the current, depending on when they arrive and pre-existing water conditions.

Moving north along the coast, more of the variability in the current can be explained by decadal changes, although the dynamics of these changes are not fully understood. The Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO) describe patterns of variability in sea surface temperature and height that account for some of the changes in the California Current system. During the positive phase of the PDO, the California Current tends to be weaker, whereas the current is stronger and upwelling is enhanced during the positive phase of the NPGO.

Biogeochemical properties
In the California Current and other eastern boundary currents, upwelling brings water with a signature of high nutrients, low oxygen, and high dissolved inorganic carbon to the surface. The interactions between physical upwelling and advection of water and biological processes such as photosynthesis and respiration make the California Current a biogeochemically dynamic region with high variability.

Nutrient flux
Upwelling and downwelling in the California Current are important drivers of nutrient transport, and the biological response within the current depends on the interaction between these physical processes and underlying biogeochemistry. Water transported by upwelling is rich in nutrients that are essential for the growth of ocean algae, so upwelling supports productive ecosystems. Changes in nutrient fluxes on seasonal timescales cause distinct regimes based on atmospheric conditions and ocean circulation. Flux of nutrients also varies along the length of the current, because the shape of the coastline strongly influences the intensity of upwelling. This creates an inherent patchiness to the distribution of nutrients and oxygen along the coast.

Additional sources of nutrients from river and estuarine inputs along the US West Coast fuel ecosystem productivity. Notably, the Columbia River discharges an average of 3600 cubic meters of water per second. The nutrient signature from the Columbia River, which drains a total area of 660,480 square kilometers, can be tracked throughout the northern extent of the California Current.

Limiting nutrients
Nitrogen, phosphorus, and iron are important limiting nutrients for the California Current. Macronutrients such as nitrate, phosphate, and silicate, along with trace metals like iron, are supplied to the coastal region by upwelling. Iron availability governs nitrate drawdown in many coastal upwelling systems and is strongly influenced by physical drivers and bathymetry along the California Current. Locations with narrow continental shelves can become iron-limited due to low levels of suspended sediment and high nitrate concentrations from upwelling. Further offshore, away from the freshly upwelled waters, there are regions that are high in nitrate, but iron-limited. These areas are designated as high nutrient low chlorophyll regions, where given the abundance of nitrate, higher chlorophyll levels may be expected.

Oxygen
Dissolved oxygen (DO) dynamics play a significant role in the biogeochemical processes of the California Current. The oxygen minimum zone intersects the continental slope at more than 600 meters depth. Oxygen minimum zones exist between depths of 100 and 900 meters, with minimum values between 300 and 500 meters. These zones are formed due to a combination of poor ventilation with surface waters and respiration.

The biogeochemical cycling of many important inorganic compounds is highly oxygen-dependent. The California Current experiences significant variation in oxygen, and there is recent evidence of strong hypoxia (DO < 1.4 milliliters per liter) or even anoxia (DO = 0 ml l-1) over large expanses of the current. Oxygen concentrations are not uniform across the current; general ocean circulation, wind forcing, and bathymetry cause patchiness. Oxygen also changes on a seasonal basis, with low DO typically observed during the summer, between the spring and fall transition of prevailing winds. There is evidence of oxygen changing on longer time scales as well, because decadal variations in ocean gyres create conditions that transport more upwelled water onto large portions of the continental shelf.

Carbonate chemistry
Coastal water in the California Current system is naturally more acidic and susceptible to undersaturation of carbonate minerals like aragonite than other ocean regions because upwelling moves comparatively low pH waters onto the continental shelf. Aragonite saturation state, which depends on the relative abundance of carbonate ions in the water, impacts the ability of calcium carbonate shell-forming organisms such as coccolithophores to produce their shells.

Surface waters also exchange carbon dioxide (CO2) with the atmosphere, and the amount of CO2 in ocean water affects its pH and saturation state. Water that upwells from depth tends to be high in CO2 because these waters have been isolated from the surface, and CO2 produced by respiration builds up over time. When this water is brought to the surface, CO2 is released to the atmosphere, but the availability of nutrients also supports high productivity. This productivity takes up CO2, and can cause the coastal region to become a net carbon sink rather than a source. The carbonate chemistry of surface water is quite variable because of these dynamics.

Geologic context
The geological setting of the California Current helps shape its flow dynamics as well as seafloor structures and habitats. Events like submarine earthquakes can alter the seabed, disturbing biological communities and altering food webs.

The California Current flows over a tectonically active region with multiple plate boundaries, including the Cascadia Subduction Zone, San Andreas Fault, and the Juan de Fuca and Gorda Ridges. The Cascadia Subduction Zone sits between Vancouver Island and Cape Mendocino, where the Juan de Fuca Plate slides under the North American Plate, creating an active continental margin characterized by frequent earthquakes and tsunamis. The San Andreas Fault is a continental transform boundary along the coast of Southern and Central California, where the Pacific Plate grinds against the North America Plate. The Juan de Fuca and Gorda Ridges are divergent plate boundaries where the Pacific Plate spreads away from the smaller Juan de Fuca and Gorda Plates.

The active Cascadia Subduction Zone creates a relatively narrow continental shelf along the US West Coast, with depth rapidly increasing away from the coast. This quick drop-off is in contrast to passive continental margins where there is no subduction and depth drops off more gradually, as is the case along the East Coast of the US. The steep seafloor topography along the US West Coast contributes to characteristic upwelling along the California Current by forcing deep, nutrient-rich water upward.

Plate tectonics also created the Southern California Bight, the portion of the California coastline that runs east to west, south of Point Conception. This bend in the coastline creates a large eddy off the California Current called the Southern California Eddy. The Channel Islands off the coast also impact the flow of surface currents in the Southern California Bight region.

Finally, features like capes, submarine canyons, and banks influence the flow of the California Current near the coast, as well as the intensity of upwelling. Cape Blanco causes a jet of water to separate from the coast, carrying coastal water across the shelf. Heceta Bank produces an offshore movement of the coastal jet, recirculation inshore of the bank, and a "retention" region with higher productivity.

Sediment transport
The complex interactions between plate tectonics, waves, discharge of sediments from rivers, coastal erosion, and sea level change define the geomorphological shape of the coastline that parallels the California Current. Coastal geology determines the boundaries of features called littoral cells, which are complete systems of sediment cycling.

Particles of rock, sand, and silt make their way into these oceanic littoral cells from sources including crumbling sea cliffs and rivers. In the ocean, the fate of these particles depends on factors like the angle of incoming waves relative to the coastline, the bathymetry of the sea floor, and the relative position of sediment discharge to major geological features like headlands and submarine canyons. Particles will often flow along the coastline in the direction of longshore drift, forced by incoming waves, until they are blocked from moving further by headlands. Particles may escape littoral cell systems by sinking into deep submarine canyons, or be carried further offshore by periods of intensified river discharge. Sediments that make it into the California Current can be carried southward for great distances, depending on factors such as sediment grain size and the seasonal intensity of winds.

These sediment inputs to the sea are important to physical and biogeochemical ocean processes. Riverine inputs are an important link between the ocean and the land, transporting nutrients and sediment that settles on continental shelves. Major rivers such as the Russian River and Columbia River carry large volumes of sediment from land into the California Current system. Local waves, bottom currents, and storms influence the volume of sediment delivery, as well as the suspension of particles near the bottom. The movement of sediments offshore is affected by the California and Davidson Currents, upwelling, seasonality, and wave patterns.

Sediment transport processes impact coastal bathymetry, and they are important to multiple fields including geology and civil engineering. Studying past sedimentation events and processes in the geologic record gives scientists insight into what the California Current and broader region looked like thousands of years ago, including past levels of biological productivity, upwelling force, and changes to atmospheric circulation and current flow.

Biology
The California Current is a highly biologically active region, home to a range of species diversity spanning many taxa. Eastern boundary currents like the California Current support many different species, a quality termed species richness, as well as high productivity relative to other ocean regions. The scale of organisms in the current spans from single-celled bacteria up to the largest animal on earth, the blue whale.

Despite their small size, microscopic organisms have a large impact on the region. Primary productivity in the current is influenced by microbial organisms, including marine viruses, bacteria, and grazers. Although they occur on microscopic scales, the interactions between these groups constitute important ecosystem controls for the broader California Current that change on daily, seasonal, and interannual time scales.

Basin-scale climatic forces such as El Niño, La Niña, and Pacific Decadal Oscillation (PDO) events combine with seasonal variations to create dynamic changes in water properties to which the biology of the California Current responds. For example, biological communities in the North Pacific experience shifts on a 50-year timescale related to the PDO. A warm "sardine" regime favors these fish during periods when the California Current is weaker, and a cool "anchovy" regime results when the current is stronger and upwelling is enhanced.

Primary producers
Multiple species of the algal groups diatoms, coccolithophores, and dinoflagellates form diverse and dynamic communities throughout the California Current. The size and composition of these communities is considered an essential ecosystem metric. Phytoplankton are microscopic ocean plants (smaller than 200 micrometers) that shape food web dynamics by providing food for other species, influencing available nutrient concentrations, and fueling primary production through photosynthesis. Ecosystem models show that large phytoplankton like diatoms comprise 90% of the California Current system's primary productivity. Rapid, significant periods of phytoplankton growth, or blooms, fuel ecosystem productivity. Blooms have also been linked to ocean oxygen-depletion events, which occur seasonally in the California Current system. Some phytoplankton species found in the California Current, such as Pseudo-nitzschia and Karenia brevis, produce toxins harmful to other organisms, like domoic acid and brevetoxins.

Macroscopic brown algae of the family Phaeophyceae are abundant throughout the California Current. Common brown algae include bull kelp and giant kelp. Large aggregations of kelp form dense forests along the coastal region of the current. These kelp forests are important habitat for thousands of marine species.

Consumers
Consumers, or organisms that are unable to produce their own food, must rely on predation to acquire energy. They exist across a range of trophic levels, which describe an organism's position in the food web. The following sub-sections outline a few ecologically-important consumers found in the California Current.

Zooplankton
Zooplankton in the California Current include copepods, jellyfish, euphausiids, and crustacean larvae. By consuming phytoplankton and other zooplankton, they facilitate the transfer of energy and nutrients through the food web and ecosystem. Environmental and climatic factors like upwelling and the PDO favor different types of zooplankton, resulting in seasonal shifts in zooplankton community structures that further impact the food web. The copepod community, for example, includes more cold-water species when the California Current is strong, and more warm-water species when it is weak.

Zooplankton community structure also changes significantly along the length of the California Current because of changes in water characteristics. In addition, the formation and maintenance of large eddies can isolate zooplankton and influence survival of their larvae, as has been observed at Point Conception in southern California and Punta Eugenia in Baja California.

Upwelling dynamics also impact the transport of zooplankton, including the larval stages of marine organisms. The distribution of transported spawn is an important component of recruitment (the process by which new individuals are added to a population) for many species, and it has implications for future fish abundance throughout the California Current. The relative timing of spawning events and the spring and fall transitions in upwelling cause significant variations in the ranges of many species.

Shellfish
Shellfish live in a variety of habitats within the California Current, such as kelp forests, rocky intertidal zones, sandy bottoms, and even open water habitats. Echinoderms, including sea urchins and sea stars, commonly inhabit California Current kelp forests and rocky intertidal zones, although sea star wasting disease has ravaged populations in recent decades. Filter-feeding mollusks like abalone, snails, oysters, clams, and mussels are also widespread within the California Current region, as are shrimp and crab species. Many of these shellfish species are commercially valuable, most notably Dungeness crab and Oregon pink shrimp (Pandalus jordani).

Fish
Fish at lower trophic levels feed on zooplankton and transport energy up the food chain. Examples of forage fish with varying ranges throughout the California Current include Whitebait smelt, Pacific sardine, Pacific herring, and Northern anchovy. These species constitute important food sources for higher tropic levels in the ecosystem. The California Current is also home to many types of predatory fish. Rockfish (Sabastes) is a genus with over 100 species present along the US West Coast. These commercially-important fish (see Fisheries section below) rely on kelp forests for habitat. Other demersal (groundfish) predatory fishes important to the California Current ecosystem include hake, cod, and spiny dogfish. Predators also include a number of shark species including the salmon shark, which migrate throughout the California Current.

Seabirds
A wide variety of seabirds take advantage of the productive waters within the California Current. Notable species include Cassin's auklets, Rhinoceros auklets, common murres, pigeon guillemots, brown boobies, Brown pelicans, sooty shearwaters, and several species of cormorants and gulls. Sooty shearwaters migrate 40,000 miles annually from their breeding grounds in New Zealand to feed in the California Current.

Marine mammals
High levels of nutrient upwelling and primary productivity within the California Current create food for large marine mammals, including whales, seals, and sea lions. Some of the most visible groups of organisms within the current fall into the order Cetacea, which includes dolphins, porpoises, toothed whales, and baleen whales.

Dolphins are by far the most numerous cetaceans, with an estimated population size of 540,000 individuals within the California Current ecosystem. Species that prefer warmer conditions, including common dolphins and short-finned pilot whales, tend to stay in the southern section of the Current off the coast of California. Cold-loving species like Pacific white-sided dolphins tend to stay farther north, off the coast of Northern California, Oregon, and Washington. Dall’s porpoises also inhabit the colder regions of the current. The “cosmopolitan” dolphins, including bottlenose dolphins and killer whales, travel the entire range of the California Current. Individual species ranges vary from year to year as temperatures fluctuate, especially during El Niño and La Niña events.

Baleen and toothed whales also inhabit the California Current ecosystem, including humpback, gray, minke, blue, and sperm whales. Many species of baleen whales, including gray whales and humpbacks, migrate thousands of miles each year between seasonal feeding grounds and mating/birthing grounds. These migration paths take many whales directly through the California Current ecosystem.

Seals and sea lions are also common in the current region. The six most frequently occurring pinniped species are California sea lions, Stellar sea lions, northern fur seals, Guadalupe fur seals, harbor seals, and northern elephant seals. Whales, seabirds, and pinnipeds often congregate around schooling fish, creating "feeding frenzies" in coastal waters, especially during periods of intense upwelling.

Human dimensions
The productive nature of the California Current creates numerous opportunities for industry, recreation, and tourism. Human habitation on the west coast of North America dates back as far as 130,000 years. The remains of marine species native to California Current ecosystems have been found in pre-historic archaeological sites, inland and distant from their natural species ranges. Such remains suggest that humans have actively harvested organisms from the California Current and relied on them as a food source and means of trade since at least pre-historic times.

In modern times, the California Current plays an important role in the livelihoods of many people, enabling active fishing, tourism, and shipping industries. These interactions between humans and the California Current have also led to significant human impacts on the current (see Climate Change section below).

Fisheries
Commercial fishing in the California Current region generates over $558 million in annual revenue while employing over 232,000 people. Shellfish like crabs and shrimp make up the dominant source of commercial revenue, whereas pelagic fishes make up the dominant catch by tonnage. Fishes like groundfish, salmon, and tunas are common recreational catches, with over 1.97 million recreational fishers participating in harvest in 2006. Recreational anglers stimulate coastal economies by spending money on charter trips, boat rentals, gear, and other personal costs. Along with commercial and recreational harvests, aquaculture and subsistence fishing also contribute greatly to non-harvest fishery levels. The makeup and success of fishery catches are influenced by regional processes and interannual, annual, and decadal climatic variability as well as long-term climate changes. Harvest of California Current species is managed by various organizations such as the Pacific Fishery Management Council and the National Oceanographic and Atmospheric Administration. Management strategies are often organized by the location and life histories of the species, such as groundfishes, highly migratory species, and coastal pelagic species that live in the water column. In recent years, effort has been made to implement ecosystem-based management approaches.

Some species are more strictly managed due to historic exploitation in the region, or based on life history and desirability. For example, the California market squid has a short life-span and high fecundity, so fishing pressure has little effect on the population. Comparatively, the Pacific sardine has experienced historic overexploitation, and the population is subject to large natural fluctuations. As a result, the fishery is heavily monitored and frequently closed. In many instances, management strategies involve numerous stakeholder groups because of the important roles the species play in communities and ecosystems. For example, salmon populations fuel key commercial, recreational, and subsistence fisheries, and salmon also play a critical role in California Current and freshwater ecosystems. Adaptive management strategies have been implemented to reduce human impact on salmon and associated environments.

Shipping, ports, and other marine operations
Along with fishing, marine operations on the west coast of the United States and Mexico are impacted by the properties of the California Current and vice versa. Some of the busiest ports in the world, such as the Ports of Los Angeles and Long Beach, bring heavy ship traffic through the California Current region. While this generates income for the region, it also impacts the area and current itself. For example, major ports often require dredging and widening to enable deep draft vessels to enter. This impacts sediment transport processes, as well as bottom-dwelling producers and consumers. Researchers and environmental groups have highlighted the impacts of marine operations on migratory animals like whales and seabirds that visit the region, and the negative impacts of invasive species and pollutants brought into the California Current by large vessels.

Marine oil and gas operations, like oil drilling rigs, have a long history within the offshore California Current. Some places, like Oregon, have banned drilling, while others still have active industries, such as Baja Mexico and California. While active drilling and oil exploration generate income, they may also have many negative environmental impacts on the region.

Ecotourism
The high biodiversity of the California Current lends itself to ecotourism ventures including whale watching, fishing charters, SCUBA diving, and sea kayaking. In some places, decommissioned oil rigs have been transformed into artificial reef habitats, which now serve as popular diving destinations.

In popular culture
In the Disney/Pixar animated films Finding Nemo and Finding Dory, the California Current is portrayed as a superhighway that fish and sea turtles use to travel to California. The characters Marlin, Nemo, and Dory join Crush, Squirt, and a group of baby and adult sea turtles in using the California Current to help them travel to Morro Bay, California to find Dory's parents.

Climate change
Anthropogenic climate change has introduced additional variability in the physical and geochemical properties of the California Current system. These changes have ramifications for the organisms and coastal communities supported by the current.

Physical changes
Increasing atmospheric CO2 and temperature will alter the characteristic physical properties of the California Current region in multiple ways. Climate change has already influenced the seasonality of upwelling, and climate models predict that upwelling will begin, peak, and end later, with a delay in its onset of up to a month. Additionally, shifts in atmospheric circulation and temperature will cause regional shifts in upwelling throughout the California Current system. Upwelling in northern regions of the current, from Southern British Columbia to northern California, may be enhanced by stronger southward winds. These increased winds are caused by the disproportionate warming of land relative to the ocean, especially at higher latitudes where temperature differences are most pronounced. Upwelling along the northern and central California Current may also be further enhanced by eddy activity. In contrast, the southern California Current region may experience decreased upwelling due to intensified ocean stratification. Induced by warming of the upper ocean, intensified stratification may counteract the effects of upwelling by reducing mixing in this region.

As ocean temperatures and stratification increase, lower dissolved oxygen levels are already occurring due to decreased solubility and reduced ocean mixing. Increased hypoxia and water column anoxia observed throughout portions of the California Current are also closely tied to increased upwelling of deoxygenated water, as well as the respiration of increased organic matter introduced by this upwelling. Because oxygen plays a key role in the cycling of other elements, such as carbon, nitrogen, and iron, deoxygenation will have widespread biogeochemical effects.

Human-induced increases in atmospheric CO2 since the Industrial Revolution, primarily from the burning of fossil fuels, have greatly increased the uptake of CO2 by the ocean. This process has lowered seawater pH in various regions, a phenomenon known as ocean acidification. This process is exacerbated in upwelled waters, like those that occur in the California Current, which are typically higher in CO2 than overlying waters. Ocean acidification will likely have the greatest effect in northern regions of the current, where upwelling is greater. Increased ocean acidification also results in reduced carbonate saturation state.

Biological ramifications
The combination of increased temperature, ocean acidification, decreased oxygen, and nutrient availability changes will have widespread impacts on living things throughout the California Current system, from microbial life all the way up the food web to humans.

As a result of upwelling-affiliated nutrient availability, primary production is predicted to increase most along the central and northern California Current, supporting higher zooplankton abundance in these areas. Increasingly favorable conditions for phytoplankton blooms may also increase the frequency of harmful algal blooms and hypoxic events, as has already been reported off of Monterey, California and along the Oregon coast. In contrast, intensified stratification along the southern California Current may limit nutrient transport, causing potential phytoplankton declines near Baja California. Changes in upwelling-favorable winds may shift phytoplankton blooms further offshore in some regions of the current, influencing food web dynamics.

Many organisms in the California Current ecosystem have evolved the timing of their developmental and reproductive stages, known as phenology, to align with optimal seasonal upwelling and prey availability. As upwelling conditions change, the mismatch between evolutionary timing and actual seasonal upwelling and prey availability will cause ecological shifts with cascading effects on upper trophic levels including invertebrates, fishes, marine mammals, and seabirds. For example, small forage fishes throughout the current will be negatively impacted by changing upwelling and prey conditions. Declines and shifts in these important prey species will in turn influence the success of larger marine predators like seabirds and marine mammals.

Shifts to upwelling and prey species will be compounded by the effects of ocean acidification, temperature changes, and oxygen availability, inducing diverse ecological shifts in the California Current. Ocean acidification will impact the ability of many marine invertebrates and phytoplankton to produce protective calcium carbonate shells because of reduced carbonate saturation state. This threatens prey availability, water quality, and contribution of calcium carbonate shells to seafloor habitat. Reduced oxygen levels will likely support an increase in organisms that are tolerant of lower oxygen conditions.

Changing biogeochemical conditions in the California Current have also already induced range shifts in a number of species. For example, the range of the Humboldt Squid has recently expanded, while seabird and marine mammal ranges have shifted latitudinally with temperature, leading to a decline in biodiversity. In comparison, gelatinous organisms such as jellyfish or pyrosomes, which have lower nutrient and oxygen demands and are more tolerant of warming conditions, have experienced population increases in recent years. Given the socioeconomic link between the California Current ecosystem and adjacent coastal communities, these ecological shifts and their cascading trophic effects may render coastal communities vulnerable to the impacts of climate change.