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California Current

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This satellite image shows the coast of California and the California Current system from space. Phytoplankton blooms are visible as intricate swirls of green in the blue ocean along the coast.
A satellite image of the California Current system reveals phytoplankton blooms along the coast fueled by upwelling.

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.[1] It carries relatively fresh and cool surface water from northern latitudes southward.[2] The current carries the southward flow of the North Pacific Gyre and is one of five major coastal currents affiliated with strong upwelling zones (the Humboldt Current, the Canary Current, the Benguela Current, and the Somali Current). Upwelling occurs along the coast because of southward winds that drive offshore Ekman transport.[1] The current displays strong seasonal, interannual, and decadal variability due to changes in winds and ocean properties.[3] Upwelled water brings nutrients to the surface, fueling high productivity in the coastal region and supporting a diverse ecosystem. The California Current region is economically important to coastal communities, supporting many commercially-important fisheries as well as shipping ports and ecotourism. Climate change may impact the California Current in many ways, including warming temperatures, intensified upwelling, seasonal hypoxia, increased harmful algal blooms, acidification, species distribution shifts, and disruption to marine mammals, seabirds, and fisheries.

Physical Properties

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The California Current is 50-100 km wide and flows at a maximum surface velocity of 40 to 80 cm/s, extending to a depth of about 300 m.[1][2] The current has two components. One is the southward flow of the North Pacific subtropical gyre, driven by Ekman pumping and southward Sverdrup circulation. The other is the California Current frontal system, driven by alongshore winds and coastal upwelling. The current is in geostrophic balance, with winds driving offshore Ekman transport, generating lower pressure along the coast, and driving surface flow to the south.[1] The characteristics of the 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.[3]

Upwelling

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Alongshore winds that originate in the deflection of midlatitude westerlies by the North American continent drive upwelling in the California Current system. Upwelled water comes from 150-200 m depth and is defined by its high nutrient content and low temperature, producing a region of cool surface water 80-300 km from shore and fueling high productivity in the coastal zone. Water that is upwelled along the coast is moved offshore by jets that take the form of squirts, which end offshore, and meanders, which circle back to the coast. The shape of the shoreline influences where offshore-moving filaments tend to occur.[1] Upwelling varies strongly with seasons, especially in the northern part of the current, as the atmospheric pressure patterns that drive winds move around.[4] In the winter, coastal waters usually flow northward, and there is downwelling instead of upwelling, because winds that blow northward cause Ekman transport onshore instead of offshore.[5]

This satellite image shows the temperature of the ocean surface along the coastline of California. Cooler temperatures occur along the coast, with warmer temperatures offshore and south of 34.5°N. The cooler temperatures are associated with upwelling.
Water temperature along the coast of California reveals the presence of cooler, upwelled water. The upwelled water moves offshore in filaments that occur near points in the coastline. Upwelling occurs north of Point Conception at 34.5°N.

Variability

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The characteristics of the California Current vary seasonally due to changes in alongshore winds. In winter, the current is located farther offshore, with minimal upwelling and northward flow along the coast associated with the Davidson Current, also called the Inshore Countercurrent.[1][2] In the summer, an upwelling front develops first at the coast, then moves offshore as the season progresses.[1] Upwelling indices are used to assess the strength of upwelling, and are calculated based on winds or Ekman transport. The strongest upwelling occurs from April to July, around 34°N, offshore of Point Conception.[1] The onset of upwelling, called the "spring transition", and the change to downwelling, called the "fall transition", vary from year to year with winds.[3] When upwelling starts later than usual, impacts are felt throughout the California Current ecosystem, including warmer water, reduced productivity, and lower recruitment of mussels and barnacles.[6] Climate change may lead to a delayed start to the upwelling season, as well as stronger upwelling later in the season.[7]

El-Niño Southern Oscillation (ENSO) also impacts the California Current. During a strong El Niño in 1997, productivity was lower than usual because of lower nutrient content in upwelled water, but then increased after the transition to La Niña in 1999.[8] El Niño affects the California Current because waves propagate northward along the coast from the equator, changing the vertical location of nutrient and temperature gradients, which changes the composition of upwelled water.[9]

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The California Undercurrent flows poleward underneath and inshore of the California Current, carrying tropical Pacific water with a warm, salty, low oxygen signature northward. The undercurrent flows at speeds of 10 cm/s or faster, with the core located at 250 m depth and extending to 1000 m depth, and a width of 20 km.[1] Inshore of the California Current is the coastal Davidson Current, also called the Inshore Countercurrent, which flows to the north in the fall and winter. This current is the surface expression of the undercurrent, which gets closer to the surface during the winter and deepens in summer.[2]

Biological Properties

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Microbial life

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Microscopic algae like diatoms are an essential part of the food web in the California Current system and beyond.

The California Current supports abundant microbial life, nourished by the upwelling of deep, nutrient-rich waters along the coast.[10] Multiple species of the algal groups diatoms, coccolithophores, and dinoflagellates form diverse and dynamic communities throughout the Current, and the size and composition of these communities is considered an essential ecosystem metric by researchers.[10] Phytoplankton are microscopic ocean plants (smaller than 200 μm) that influence food web dynamics by providing food for other species, influencing available nutrient concentrations, and fueling an ecosystem's primary production through process of converting light energy to biological matter.[11]

Ecosystem models show that large phytoplankton like diatoms comprise 90% of the California Current system's primary productivity. The Current's primary productivity is also influenced by other microbial organisms, including marine viruses, bacteria, and grazers.[12] 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.

Rapid, significant periods of phytoplankton growth (termed "blooms") fuel ecosystem productivity. They have also been linked to ocean oxygen-depletion events called hypoxia, which occur seasonally in the California Current system.[10] Some phytoplankton species found in the California Current, such as Psuedo nitchizia and Karenia brevis, can also produce toxins harmful to other organisms, like domoic acid and brevetoxins, which can impact other organisms including fishery species and humans.[13]

Zooplankton

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Phytoplankton are an essential food source for zooplankton in the California Current, including copepods, jellyfish, euphausiids, and crustacean larvae.[10] These small (<200 μm->20 mm[14]) animals provide food for larger animals, including economically-important fishery species like salmon, and macrofauna like whales. They also constitute an essential food web link between their phytoplankton prey and the higher trophic levels that eat them.[15]

Based on their life strategies, different species of zooplankton — and even different life stages of the same species— may live solely in one portion of the water column, or they may migrate from its sunlit surface to its depths, called the benthos.[14] In the California Current, environmental and climatic factors like upwelling and the Pacific Decadal Oscillation favor different types of zooplankton, resulting in shifts in zooplankton community structures that can further impact the food web.[16]

Zooplankton such as copepods shape the waters they live in by grazing on phytoplankton and providing food for larger animals in the California Current.

Zooplankton community structure changes significantly throughout the California Current system, based on the changing physical conditions that exist between the two ends of the Current. In addition, other oceanographic processes can impact zooplankton populations. The formation and maintenance of large eddies, for example, 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.[17]

Larval Transport

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Many fish species that live in the California current region have developed reproductive strategies around prevailing seasonal upwelling and downwelling cycles. Most open ocean (aka pelagic) species spawn during the wintertime, when prevailing winds force surface waters towards the coast. That way, larval and juvenile fishes can take advantage of protected habitats such as estuaries and kelp forests when they are most vulnerable to predation[18]. If these fishes spawned during the summertime, offshore winds would force surface waters and larval fish out to sea, where they would be less likely to survive. Offshore winds coupled with upwelling occurs year-round in some parts of the Northern California coast between Cape Blanco and Point Conception. In that area, local populations of marine species are less likely to broadcast spawn because their young would end up in the perilous waters offshore. Many pelagic species that inhabit that region spend their early life history phases in the Southern California Bight, where currents flow around in circles, and then migrate north as adults in search of food.[19]

Despite this timing, the California current can transport larval fish hundreds of miles from where their parents spawned over relatively over timescales of days to weeks.[20] As a result, the ranges and distributions of certain species can vary from year to year depending on the strength and timing of coastal wind patterns and resultant changes in the direction and intensity of the California current.[21] Although larval fishes and invertebrates are traditionally thought of as plankton that can't swim against the current, research suggests that the way these organisms swim up and down in the water column to regulate their depth can influence how far surface currents can carry them.[22]

Fisheries

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Nutrient upwelling within the California Current ecosystem creates the ecological conditions necessary to support a large number of commercially significant species. Nutrients support the growth of tiny marine plants (phytoplankton) that in turn support tiny marine animals (zooplankton) that provide food for a wide variety of species that are higher on the food chain, including many species that people like to eat. Prominent commercial fisheries within the California Current include groundfishes, coastal pelagic finfish, highly migratory species, shellfish, and salmon[23]. The West Coast grounfish fishery includes more than ninety species that are managed my the Pacific Fishery Management Council (PFMC) and the National Oceanographic and Atmospheric Administration (NOAA), including rockfish, halibut, Dover sole, and Pacific whiting. All of these species spend most of their lives on or near the seafloor, hence the name "groundfish[24]." Coastal pelagic species live in the open ocean nearshore, and include sardines, mackerel, krill and squid. These species tend to live in large schools or aggregations, and provide food for not only human beings but marine mammals and seabirds as well[25]. Highly migratory species (HMS) include yellowfin tuna, albacore tuna, and swordfish. These species also live out in the open ocean but tend to travel greater distances and are much larger than coastal pelagic fishes. Commercially significant shellfish include Dungeness crab, Oregon pink shrimp, oysters, clams, lobster, abalone, and urchin[26]. One of the most widely celebrated commercial fisheries to extend into the California Current ecosystem is the salmon fishery[27].

Marine Mammals and Seabirds

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High levels of nutrient upwelling and primary productivity within the California also current create food for large marine mammals, including whales, seabirds, seals, and sea lions, which call the California Current region home. One 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[28]. These dolphins can be divided into three primary subgroups: warm water, cold water, and cosmopolitan. 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 prefer to inhabit the colder regions of the current. The “cosmopolitan” dolphins, including bottlenose dolphins and killer whales, travel throughout the entire range of the current. Ranges of individual species vary from year to year as temperatures fluctuate, especially during El Nino and La Nina Pacific Decadal Oscillation (PDO) events, which bring warmer and colder waters to the West Coast of the United States, respectively.  

Large numbers of baleen and toothed whales can also be found in the California Current ecosystem, including humpback, grey, Minke, blue, and sperm whales. Baleen whales have a keratin-based comb-like structure inside their mouths called baleen instead of teeth. These whales feed by taking large mouthfuls of seawater and small prey like herring or sardines and strain their food out of the water using their baleen. Many species of baleen whales, including gray whales and humpbacks, migrate thousands of miles each year to travel between feeding ground and mating/birthing grounds. These migration patterns take many whales directly through the California current ecosystem.

- Seabirds

Seals and sea lions, which belong to a sub-order of meat-eating, semi-aquatic mammals with flippers known as pinnipeds[29], are a common sight in the California Current ecosystem. The six most frequently occurring species are California sea lions, Steller sea lions, Northern fur seals, Guadalupe fur seals, harbor seals, and northern elephant seals[30]. Several of these species were hunted to near

Biogeochemical properties

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Nutrient Flux

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The California current is a dynamic system; being an eastern boundary current there are unique physical drivers such as upwelling and downwelling that are inherent to other eastern boundary currents [31]. The biological response in the current are a result of these physical drivers interacting with the underlying biogeochemistry. Changes in nutrient fluxes on seasonal timescales provides distinct regimes for the region based on atmospheric conditions and ocean circulation. An important driver of nutrient transport is advection through upwelling and downwelling. Water transported by upwelling is rich in nutrients which are essential for primary productivity which then supports productive ecosystems [32]. Flux of nutrients is not consistent along the length of the current, coastal geomorphology strongly influences the intensity of upwelling in a particular region; this creates an inherent patchiness to the distribution of nutrients (and oxygen) along the coast [33]. Recent intensification of upwelling as a result of anthropogenic climate change has increased the nutrient flux to coastal shelf waters. This increased nutrient flux has created conditions favorable for harmful algal blooms, which have been reported more frequently off of Monterey, CA [34].

Limiting Nutrients

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Macronutrients such as nitrate and phosphate along with trace metals like iron are supplied to the coastal region through wind forced upwelling [35]. Nitrogen (N), phosphorous (P), and iron (Fe) are important limiting nutrients for the California current. Fe availability governs nitrate drawdown in many coastal upwelling systems and is strongly influenced by the physical drivers and bathymetry along the California current. Locations with narrow continental shelves can become Fe-limited due to the low suspended sediment levels and high nitrate concentrations from upwelling [36]. Further offshore, away from the freshly upwelled waters, there are regions that are high in nitrate, but Fe-limited. These areas are designated as high nutrient low chlorophyll (HNLC) regions, where given the abundance of nitrate, they should be producing more chlorophyll [33].

Oxygen

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This diagram shows the processes leading to hypoxia on the continental shelf. Water that is upwelled along the coast is low in oxygen but high in nutrients. Once the water reaches the surface, phytoplankton grow and die along the coast. When they sink to the bottom and decompose, more oxygen is used up and the water on the shelf becomes hypoxic.
Upwelling of low oxygen water can produce hypoxic and acidic conditions on the shelf due to the decomposition of plankton after a bloom. This process consumes oxygen and releases carbon dioxide in shelf waters that are already low in oxygen and high in carbon due to their source at mid depths.

Dissolved oxygen (DO) is a critical component of the California current. Oxygen dynamics play a significant role in not only the ecological and fishery components of the current, but the biogeochemical processes as well. The structure of oxygen is an important component for the current, with the oxygen minimum zone (OMZ) intersecting the continental slope at >600 meters depth [37]. Oxygen minimum zones exist at between depths of 100 and 900 meters, with minimum values experienced between 300 and 500 meters. These zones are formed due to a combination of poor ventilation with surface waters and respiration [38]. The biogeochemical cycling of many important inorganic compounds is highly oxygen dependent [39]. Variations in the oxygen dynamics in the California current are important, and recent evidence of strong hypoxia (DO <1.4 ml L-1) or even anoxia (DO = 0 ml L-1) over large expanses of the California current [40][37]. Oxygen concentrations are not homogenous across the California current; there is an inherent patchiness in oxygen that is the product of general ocean circulation , wind forcing, and bathymetry [31]. While oxygen changes on an inter-annual basis, with low DO typically observed during the boreal summer, between the spring and fall transition of prevailing winds. There is also evidence of oxygen changing on longer time scales; decadal variations to ocean gyres create conditions to transport more upwelled water onto large portions of the continental shelf through advection [41].

Climate change

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A growing body of evidence suggests that nutrient and oxygen dynamics are changing in the California current as the planet warms due to anthropogenic climate change [31][42]. Warming conditions in the California current are documented increasing the intensity of upwelling favorable winds, this drives more intense upwelling and subsequently increases the flux of nutrients onto the shelf. Increased nutrient flux has been attributed to harmful algal blooms and increased episodes of hypoxia [31][13][37]. In addition to intensified upwelling, climate change has also produced alterations to the chemistry of the current due to ocean acidification [42].

Geological Properties

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Geologic Context

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The California Current flows over or near two distinct tectonic plates boundaries: a continental transform boundary along the coast of Southern and Central California and the Cascadia Subduction Zone in northern California, Oregon, and Washington. The transform boundary occurs where the Pacific Plate grinds against the North America plate along the San Andreas Fault. The Cascadia Subduction zone occurs where the Juan de Fuca Plate actively slides under the North American Plates, creating an extremely tectonically active continental margin characterized by frequent earthquakes and tsunamis of geologic timescales. These active plate boundaries mean that there is a relatively narrow continental shelf off the West Coast of the United States. Essentially, as you travel from land out to sea along the West Coast, depth increases rapidly compared to other coastal zones, like the East Coast of the United States. This steep slope along the coastline contributes to the process of upwelling that define the California Current. Deep, nutrient rich water is forced to the surface in the coastal zone because of the tectonically formed continental margin.

Plate tectonics also influence to orientation of the coastline, which in turn influences the directionality of the California Current.

Sediment Transport

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Riverine inputs to the sea are important to physical and biogeochemical ocean processes, transporting carbon and sediment that settles on continental shelves and linking the ocean with the land. Rivers such as the Russian River and Columbia River carry sediments from land to 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.[43] The movement of sediments offshore is mediated by the California and Davidson currents, upwelling, seasonality, and wave patterns.[44] These processes impact coastal bathymetry, and they are important to multiple fields including geology and civil engineering.

Human Dimensions

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The existence of the California Current as an eastern boundary current in the North Pacific and it’s own unique features make it a highly valuable region for humans. Many individuals reside in the coastal region of the California Current or frequent it for work and pleasure. While a contentious topic, human interaction with the west coast of North America and subsequently the California Current dates back as far as 130,000 years[45]. The remains of numerous marine species native to California Current ecosystems have been found in pre-historic indigenous peoples archaeological sites inland and an extended distance away from their natural ranges[46]. The findings of 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 individuals enabling active fishing, tourism, and shipping industries. As a result of the high interaction of humans with the California Current, it in turn has been greatly impacted by human activity (see also Climate Change below).

Fisheries

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The commercial fishing industries with landings from the California Current region generates over $558 million in revenue while employing over 100,000 individuals[47] . Shellfish like crabs and shrimp make up the dominate source of commercial revenue while pelagic fishes make up the dominate catch by tonnage[48]. Fishes like groundfish, salmon, and tunas are more common recreational catches with over 1.97 million recreational fishers participating in harvest in 2006. Recreational anglers stimulate coastal California Current economies by spending  money on charter trips, boat rentals, gear, and other personal costs. Along with commercial and recreational harvests, aquaculture and sustenance fishing can also contribute greatly to non-harvest fishery levels[47]. The makeup and success of fishery catches are influenced by regional processes, interannual, annual and decadal climatic variability as well as long-term climate changes[48].

Other fisheries to maybe discuss- CA market squid (sustainable), pacific sardine (collapse), shellfish-crustacean/molluscs, rockfishes/groundfish

Shipping, Ports, and other Marine Operations

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Along with fishing, marine operations in coastal regions of the United States (WA, OR, and CA) 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 amounts of boat traffic through the California Current region. While this may generate income for the region, it also effects the natural state of the current. For example, major ports often require dredging and widening to enable deep draft vessels to enter that impacts sediment transport in the region and local benthic flora and fauna[49]. Significant interest has been raised by researchers and environmental groups into the impact of marine operations on the migratory species that visit the region (whales, sea birds, etc.) and the negative impact of invasive species and pollutants brought into the California Current by large vessels[50]. Marine oil and gas operations, like oil drilling rigs have had a long history within the offshore California Current. In some regions (like Oregon) drilling is banned  while others still have activity industries (Baja Mexico and California). While active drilling and oil exploration can generate income it also has many negative effects on the region (link offshore oil and gas in CA). Decommissioned rigs have been thought to create artificial habitats.

Ecotourism

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Decommissioned rigs have become popular diving destinations and act as a form of ecotourism in the California Current. The high biodiversity of the current system lends itself to ecotourism ventures with whale watching, fishing charters, and sea kayaks representing just some of the forms of ecotourism generated around the CA Current ecosystem. Such activities also often help people understand the CA Current region more.

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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 (Albert Brooks), Nemo (Hayden Rolence), and Dory (Ellen DeGeneres) join Crush (Andrew Stanton), Squirt (Bennett Dammannn) and a group of baby and adult sea turtles in using the California Current, to help them travel to Morro Bay, California to find her parents Jenny and Charlie.

Climate Change

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Physical Changes

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Ocean Acidification

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As the ocean takes up more CO2, which acts like an acid in water, it decreases in pH. This phenomenon is known as "ocean acidification." This decrease in pH negatively impacts a variety of organisms in the California Current system, but especially those with calcium carbonate shells such as mollusks and arthropods[51]. By 2050, benthic nearshore waters along the California Current are likely to be undersaturated year-round with aragonite saturation states below 1.5 in most regions[51]. Ocean acidification will likely have the greatest affects at higher latitudes where calcium carbonate concentrations and aragonite saturation states are already typically lower than at lower latitudes[52].

Deoxygenation

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As ocean temperatures and resulting stratification increase, waters of the California Current system are experiencing decreased dissolved oxygen levels due to decreased solubility and reduced ocean mixing[53]. Increased hypoxia and water-column anoxia observed throughout portions of the California Current are also closely tied to the increased upwelling of deoxygenated water, as well as the respiration of increased organic matter supported by this upwelling[53][54][55]. Because oxygen is a key component in the cycling of other elements, such as carbon, nitrogen, and iron, deoxygenation will have widespread biogeochemical effects[56]. Reduced oxygen levels will also cause ecological shifts, with an increase in those organisms which are tolerant of reduced oxygen levels[57].

Upwelling

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Changes in atmospheric CO2 and temperature will also affect the characteristic upwelling systems of Eastern Boundary Currents, like the California current[58]. These changes will mostly be due to shifts in the oceanic high pressure and continental low pressure zones which cause northerly (southward) winds and drive upwelling[57]. First, the Hadley Cells which influence the distribution of pressure systems between the tropics and mid-latitudes, are predicted to expand poleward[59]. It appears this shift is already driving oceanic high pressure zones further northward and causing California Current sourcewater to be increasingly polar[57][59]. Secondly, as stronger seasonal increases in temperature cause disproportionate warming over land, it is predicted that pressure differences between the ocean and land will increase, driving stronger southward winds and coastal upwelling along the US west coast[60][57]. This intensification of upwelling has already been observed over the past 60 years[61]. Wind-driven upwelling along the California Current is predicted to increase most intensely at higher latitudes where land-sea temperature differences will likely be most pronounced[60]. This upwelling may also be enhanced by increased eddy activity along the central and northern California Current[55]. Additionally, climate change has already influenced the seasonality of upwelling, particularly along the northern California Current, and climate models predict that seasonal upwelling will continue to begin, peak, and end later, with onset being delayed by up to a month[62]. Although upwelling is predicted to increase along the northern California Current, increased upper ocean temperatures also cause intensified ocean stratification which will counteract the effects of upwelling in some regions by reducing mixing[63][64]. Because primary productivity along the California Current is closely tied to upwelling and the nutrients it supplies, these changes will affect animals throughout the food chain.

Biological Changes

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The combination of increasing temperature, ocean acidification, decreasing oxygen, and changing nutrient availability will have widespread impacts on animals throughout the California Current system, including plankton, invertebrates, fish, marine mammals, and seabirds.

Primary Productivity

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Phytoplankton blooms in the California Current are closely tied to the increased nutrient availability affiliated with upwelling and will be affected by changes in this system[55]. Although upwelling typically increases primary productivity by carrying remineralized nutrients from the deep sea into surface waters, the response of phytoplankton to increases in upwelling along the California Current are complex[55]. Primary productivity is predicted to increase most along the central and northern California Current, with a potential "hot spot" in productivity near Cape Mendocino, CA, where wind and eddy forcing will combine to increase upwelling[55]. This increase in productivity is likely to be primarily due to an increase in diatoms, with smaller plankton playing a lesser role[55]. Factors, such as intensified stratification caused by warming surface ocean conditions, are predicted to limit the effectiveness of nutrient transport in some regions, especially along the southern California Current [55][64]. Regions near Baja California, Mexico may even see decreases in phytoplankton concentrations[55]. Changes in pressure-driven wind forcing may also shift the location of upwelling, causing phytoplankton blooms to be further off shore[65]. Additionally, as the seasonality of upwelling shifts, so too will the timing of phytoplankton blooms which support higher trophic levels[57]. Because primary producers form the basis of the food web, shifts to their community composition and abundance will have implications for organisms at higher trophic levels.

Zooplankton and Invertebrates

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Zooplankton, such as krill and copepods, and larger invertebrates play an important roll in transferring primary production to organisms higher in the food chain, and many of them will be impacted by changing upwelling, temperature, oxygen, and pH[57][66]. It is likely that increases in upwelling along the central and northern California Current, especially where diatom blooms increase, will fuel greater zooplankton abundance in these regions[55]. While this would be expected to increase energy transfer to higher trophic levels, other factors may negatively impact larger organisms, including shifts in the location, timing, and make-up of phytoplankton blooms[57]. Many invertebrates 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[57]. These ecological shifts have been compounded by the effects of ocean acidification, which causes calcium carbonate shell dissolution, impacting organisms such as pteropods, an important food source for fish, and oysters which provide benthic habitat and improved water quality[67][57]. As conditions such as temperature, pH, and oxygen availability change, invertebrate community shifts have already been observed, with cascading effects on upper trophic levels. These shifts include the recent range expansion of the Humboldt Squid[68] and increased abundance of jellyfish in recent years[69], both of which compete with other existing predators.

Fish

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Like invertebrates, many fish species within the California Current system have aligned their phenology with upwelling and productivity patterns, and will be influenced by changes in their seasonality[70]. A shift in upwelling and fish prey abundance to later in the year is likely to negatively impact fish which time spawning to match larval development to optimal upwelling conditions[71][57]. This, in conjunction with ecological and spatial shifts in the makeup and abundance of prey species, will affect fish species within the California Current[57]. Small pelagic fish, such as anchovies and sardines which constitute a large portion of mid-trophic level biomass, will likely be negatively impacted by upwelling that becomes either too strong, as expected in the northern California Current, or too weak, as expected further south[72]. These shifts in upwelling timing and prey are predicted to cause shifts in the ranges of various fish species including Albacore tuna, rockfish (Sebastes sp.), and sablefish, as well as declines in the populations of some fish species[73].

Marine Mammals and Seabirds

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It is predicted that marine mammals and seabirds which rely on the California Current ecosystem will experience decreased survival and reproductive success as a result of climate change-related shifts in prey location and abundance, in conjunction with other factors such as habitat loss[73][74]. Declines in anchovy and sardine populations, for example, are likely to negatively impact the populations of seabirds which rely on them as a food source[71][74]. Shifts in prey location are particularly detrimental to mammals and seabirds with specific and limited rearing and nesting sites[57][74]. As ocean temperatures warm, seabird species which typically inhabit cooler waters of the northern California Current are likely to decline, while those which favor warmer waters may expand their ranges, and overall biodiversity will likely decline[73][74]. Similarly, marine mammals typically inhabiting warmer waters of the California Current, such as California sea lions, common dolphins, and harbor seals, are likely to expand northward. Cold-water species such as the Dall's porpoise are likely to experience population declines[73].

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