Sustained, high quality ocean measurements have been made in the California Current System for over 60 years, thanks to the far-sighted work of CalCOFI (the California Cooperative Oceanic Fisheries Investigations). This extensive program initially surveyed the ocean from Baja California, Mexico to the state of Washington, although today it samples a more restricted region from San Diego to San Francisco. The CCE-LTER site partners with CalCOFI to measure additional key ecosystem variables. This extensive time-series program has now identified long-term changes in the ocean environment that have important consequences for some of the services that the ocean provides.
CCE-LTER scientists, in concert with CalCOFI, have demonstrated long-term ocean warming over a large region of the California Current. Temperature influences rates of metabolism and affects the range limits of many marine organisms. Increased surface temperatures also increase the vertical stratification of the ocean, which often reduces the supply of deep nutrients from subsurface waters. In addition, CCE scientists, in collaboration with others, have shown that dissolved oxygen concentrations have varied substantially over the course of the past 60 years. Periods of lower oxygen (hypoxia) reduce the vertical extent of habitable ocean for a variety of marine fishes and invertebrates, while the habitable volume expands during intervals of higher dissolved oxygen (McClatchie et al. 2010). For the past 25 years dissolved oxygen has decreased progressively, which has raised concerns about habitat compression for some commercially important rockfish such as the cowcod (Bograd et al. 2008).
In addition, over the past 60 years, one group of marine zooplankton known as salps has shown a long-term decline in the California Current System, even as other groups have remained relatively unchanged (Lavaniegos and Ohman 2007). Salps are gelatinous, suspension-feeding zooplankton that are very efficient grazers of phytoplankton, including some of the very smallest cells. They are thought to be important vectors that help repackage organic carbon and other compounds into a form that sediments quickly to the deep sea floor. Hence, the decline in salps may have altered long-term carbon sequestration in the deep sea off California.
Another study has shown that the transparency of the California Current System has decreased slowly, but progressively over the past 6 decades (Aksnes and Ohman 2009). Seawater transparency affects the “encounter distance” at which visually-hunting predators like fish, seabirds, and some marine mammals can detect prey. Decreases in encounter distance may affect the sustainability of some of these consumers that occupy the middle and upper levels of ocean food webs.
Recent efforts in the CCE-LTER site have expanded scientists’ ability to measure such changes by adding continuous ocean measurements from both moored instruments and robotic ocean gliders, both of which telemeter ocean measurements ashore in near-real time. These data are publicly available (http://mooring.ucsd.edu/index.html?/projects/cce/cce_data.html, http://spray.ucsd.edu/pub/rel/index.php) and should improve our ability to detect and respond to alterations to ocean ecosystems that affect marine resources and human populations.