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Ocean Acidification
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. 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. 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.

Deoxygenation
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. 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. Because oxygen is a key component in the cycling of other elements, such as carbon, nitrogen, and iron, deoxygenation will have widespread biogeochemical effects. Reduced oxygen levels will also cause ecological shifts, with an increase in those organisms which are tolerant of reduced oxygen levels.

Upwelling
Changes in atmospheric CO2 and temperature will also affect the characteristic upwelling systems of Eastern Boundary Currents, like the California current. 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. First, the Hadley Cells which influence the distribution of pressure systems between the tropics and mid-latitudes, are predicted to expand poleward. It appears this shift is already driving oceanic high pressure zones further northward and causing California Current sourcewater to be increasingly polar. 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. This intensification of upwelling has already been observed over the past 60 years. 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. This upwelling may also be enhanced by increased eddy activity along the central and northern California Current. 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. 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. 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
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
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. 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. 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. This increase in productivity is likely to be primarily due to an increase in diatoms, with smaller plankton playing a lesser role. 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. Regions near Baja California, Mexico may even see decreases in phytoplankton concentrations. Changes in pressure-driven wind forcing may also shift the location of upwelling, causing phytoplankton blooms to be further off shore. Additionally, as the seasonality of upwelling shifts, so too will the timing of phytoplankton blooms which support higher trophic levels. 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
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. 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. 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. 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. 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. 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 and increased abundance of jellyfish in recent years, both of which compete with other existing predators.

Fish
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. 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. This, in conjunction with ecological and spatial shifts in the makeup and abundance of prey species, will affect fish species within the California Current. 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. These shifts in upwelling timing and prey are predicted to cause shifts in the ranges of various fish species including Albacore tune, rockfish (Sebastes sp.), and sablefish, as well as declines in the populations of some fish species.

Marine Mammals and Seabirds
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. 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. Shifts in prey location are particularly detrimental to mammals and seabirds with specific and limited rearing and nesting sites. 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. 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.