Overview of Current Californian Large Marine Ecosystem
The California Current Large Marine Ecosystem (CCLME) extends from Baja California, Mexico to Vancouver Island, British Columbia. It is characterised by a temperate climate and strong coastal upwelling, resulting in high levels of productivity (Fautin et al., 2010). Covering an area of over 2.2 million square kilometers and a shoreline of over 2 thousand kilometers, this ecosystem provides diverse coastal and open ocean habitats with an abundance of marine life, supporting a strong coastal economy (Risien, 2009).
Eastern boundary current (EBC) systems have high levels of biological productivity due to the increased supply of nutrients as a result of coastal upwelling. Upwelling is due to equatorward winds along the eastern side of the major ocean basins, which drive the broad and slow eastern boundary currents. Interaction of these winds with the Earth’s Coriolis Force and the coastal boundary, results in shallow offshore Ekman transport. This wind-driven flow is replaced by cool, nutrient-rich waters from below, reducing sea surface temperatures and increasing chlorophyll concentrations (Chavez and Messié, 2009). There is a rapid turnover of nutrients in EBC systems, due to the high rate of organic matter production and export by phytoplankton. This provides a mechanism for carbon sequestration, as inorganic carbon is taken up at the surface, organic carbon is formed and transported to the deep ocean – thereby acting as a sink for atmospheric carbon dioxide (Gruber et al., 2006). The California Current is an EBC system, making it a highly productive ecosystem. This productivity provides the base for food webs that support many economically important fisheries (Gruber et al., 2006).
The California Current System is made up of three main currents – the equatorward California Current, the poleward California Undercurrent and the seasonal poleward Davidson Current (King et al., 2011). The California Current is a broad surface (upper 300 meters) boundary current that makes up the eastern limb of the anticyclonic North Pacific Gyre. It transports cool, nutrient-rich, low-salinity, high-oxygen water from the North Pacific towards the equator (Lynn et al., 1987; King et al., 2011). The California Undercurrent originates in the eastern equatorial Pacific and flows poleward along the North American coast (Lynn et al., 1987). It transports warm, high-salinity and low-oxygen water from Mexico, poleward along the continental slope towards Canada (King et al., 2011). There is a seasonal change in surface flow during winter, forming the poleward Davidson Current. The California Current is strongest in spring and summer, while the Davidson Current dominates the flow over the shelf during the winter months (King et al., 2011). South of Point Conception (~34°N), the California Current turns northwards to form a countercurrent, but during the summer months it recirculates and forms the Southern California Eddy (King et al., 2011).
The bathymetry of this region is characterised by a narrow continental shelf and a deep ocean basin. As a result of this narrow continental shelf, productivity is lower than in other eastern boundary regions (Fautin et al., 2010). The coastline of this LME contains many large water bodies, such as the Columbia River Estuary, as well as numerous smaller river mouths, estuaries, bays, and lagoons. These water bodies serve as migratory and nursery areas for many marine species (Risien, 2009). The coastal region can be divided into three zones, which are both physically and ecologically distinct, the northern region is north of Cape Blanco, the central region is found between Cape Blanco and Point Conception, and the southern region is south of Point Conception (King et al., 2011). The northern region of this LME has the lowest sea surface temperatures, due to the influence of the cool California Current. It is characterised by a broad continental shelf, which is substantially narrower in the central and southern regions (King et al., 2011). There is a large amount of freshwater input to this region, predominantly due to runoff from the Columbia River, which provides buoyancy and nutrients (Checkley and Barth, 2009; Fautin et al., 2010). The central region has substantially less freshwater input than the northern region and upwelling is strongly seasonal. It consists of many diverse marine and coastal habitats, including islands, reefs, rocky cliffs, sandy beaches, estuaries, bays, and lagoons. The large habitat diversity supports a variety of marine life (Fautin et al., 2010). The southern region is characterized by warm sea surface temperatures due to the influence of the Davidson Current. Marine life in the southern region is typical of subtropical waters. Point Conception, an area of high species diversity, is where two water masses converge – it is therefore the northern / southern boundary for many marine species (Fautin et al., 2010).
The northern region of the CCLME has the greatest productivity (Hickey and Banas, 2008), which is reflected in higher trophic levels, such as fish stocks (Ware and Thompson, 2005). Hickey and Banas (2008) suggested several mechanisms which may be responsible for the increased chlorophyll concentrations in the north, including upwelling enhancement by submarine canyons and iron input from the Columbia river. In addition, the northern section is known to have many physical features which cause phytoplankton retention on the continental shelf and therefore allow for the full development of phytoplankton blooms. These features include a wide shelf, coastlines lacking large capes and wind intermittency, as well as the stable water column and density fronts associated with freshwater input from the Columbia River.
This region is influenced by seasonal, interannual and interdecadal climate variability, including El Nino Southern Oscillation events and the Pacific Decadal Oscillation. Since upwelling results from alongshore winds, which are subject to seasonal variation, upwelling and subsequently primary production in the California Current Ecosystem (CCE) is seasonal and can be divided into three periods based on prevailing winds (King et al., 2011). The upwelling season occurs from February to September and is characterised by Northwesterly winds which drive surface waters offshore, increasing phytoplankton biomass and primary production. Decreased wind stress from September to October marks the oceanic season, where upwelling ceases, causing a decrease in primary production as phytoplankton sink below the euphotic layer (Ware and McFarlane, 1989). During winter, which occurs between November and February, southerly winds dominate. This results in a downwelling season where water from the open ocean is transported into the coastal area, before travelling equatorward along the continental shelf (King et al., 2011). Although the productivity of CCE communities is typically associated with the upwelling season, studies on CCLME species of groundfish, salmon, seabirds and rockfish suggest that reduced upwelling in the winter months has an important influence on ecosystem dynamics. This observed seasonality is essential to the physical and biological structure of this region and allows us to address both interannual changes and long-term trends associated with climate change (Black et al., 2011).
Due to bottom-up control, high coastal productivity leads to high marine biodiversity. As a result, there are many species of social and economic importance found in the CCLME (Fautin et al., 2010). In addition to the high productivity, the numerous and diverse habitats of the CCLME support a wide range of microbes, phytoplankton, zooplankton, nekton, mammals and birds (Checkley and Barth, 2009). Many invertebrate, groundfish, pelagic and migratory species are important resources for commercial fisheries in this region, including albacore tuna (Thunnus alalonga), Pacific hake (Merluccius productus), northern anchovy (Engraulis mordax), market squid (Loligo opalescens), sea urchins (Strongylocentrotus spp.), oysters (Crassostrea spp.) and salmon (Oncorhynchus spp.). Many shellfish aquaculture operations are also present in this region (Risien, 2009). A thriving Pacific sardine (Sardinops sagax) fishery, which began in the 1920s due to an increased need for food during World War I, became the largest fishery on the North American Pacific Coast. Catches peaked during the 1930s, however, poor recruitment due to overfishing and environmental forcing led to the fishery’s collapse in the 1940s with fish stocks reaching a minimum in the 1970s. Since the 1980s the fishery has slowly recovered (Hill et al., 2012). In addition to fisheries revenue, many species of marine mammals and seabirds generate revenue through tourism activities, while others, such as the killer whale (Orcinus orca), are of great cultural significance (Fautin et al., 2010). The CCLME contains the United States Exclusive Economic Zone, over 35 million residents of the United States live and work in the coastal areas and contribute to a coastal economy of approximately 32 billion USD annually (Risien, 2009).
There are, however, many factors threatening the health and diversity of this rich ecosystem. Approximately 25 species of marine organisms in the CCLME are listed as threatened or endangered as a result of overfishing or habitat loss. In addition, aquatic invasive species are a significant threat to biodiversity, leading to major environmental and economic issues. Other concerns include habitat alteration due to sea level rise, climate change impacts on global circulation and upwelling, shifts in abundance and distribution of many marine species, and hypoxic conditions due to excess anthropogenic nutrient input (Risien, 2009; Fautin et al., 2010).
The California Current Large Marine Ecosystem is a region of great biological, social and economic importance. It is a hotspot of marine biodiversity, supporting an abundance of marine life with its diverse habitats and highly productive coastal areas. Beyond this, it supports the regional economies of the U.S. west coast and provides significant cultural ecosystem services. Sustainable management and protection of these resources will ensure that this remains a healthy and thriving ecosystem in the future.
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