The Gulf of the Farallones National Marine Sanctuary, along with its sister sanctuaries to the north and south, Cordell Banks and Monterey Bay, are sentinels for the effects of global warming on ocean waters. And, as documented in a report released at an event at the California Academy of Sciences on June 3, Central California’s offshore waters and coastline are already showing the effects of global warming: warmer surface waters, higher sea level, stronger winds, more intense upwelling, increased acidification, and accelerated shoreline erosion. As a result, offshore ecosystems and food webs are being reshuffled as all organisms, from zooplankton to sperm whales, adapt (or don’t) to these new conditions.
The study was authored by a working group of marine scientists from California universities and resource agencies. (Download full report and executive summary.) Maria Brown, Farallones sanctuary superintendent, said because this study is the first to compile the effects of global warming on a national marine sanctuary, it will become a model for similar work at other national sanctuaries. William Douros, regional director for the National Oceanic and Atmospheric Administration, which has jurisdiction over marine sanctuaries, said he hopes to see new climate change studies that link together all five West Coast sanctuaries, from the Olympic Coast in the north to the Channel Islands in the south, to provide full context for the climate change effects on the California Current ecosystem.
In the panel discussion at the California Academy of Science that accompanied the release of the report, UC Davis researcher John Largier, chair of the science working group, emphasized that although the study identifies and synthesizes climate change impacts for the two sanctuaries, it does not predict future changes. Instead, it provides a foundation of scientific insight to enable staff at each of the two sanctuaries to develop and prioritize more detailed strategies for monitoring the effects of climate change. Later, with more specific and localized information in hand, the sanctuaries can then develop appropriate resource management plans.
- The new report focuses on the impacts of ecosystems at Cordell Bank and the Gulf of the Farallones, both highly influenced by the California Current. Click map for larger version. Map courtesy NOAA.
The report states that the most severe ecological disruption will be associated with changes in upwelling, ocean temperature, sea level rise, and ocean pH (acidification). Currently, upwelling appears to be increasing along the north-central California coast due to the more rapid increase in land temperatures contrasted with those of the ocean, creating atmospheric pressure gradients that produce stronger northerly winds, which, in turn, drive coastal upwelling. Given that the California Current ecosystem is characterized by upwelling-dependent food webs, the impact of any change in the intensity of upwelling will effect its nutrient delivery system and subsequently resonate through the whole coastal ecosystem. For example, more intense upwelling produces stronger eddies that then carry the larvae of fish and other organisms farther west out to sea, disconnecting populations and disrupting their developmental timing, leading to higher mortality. This unusual larval transport may explain the low number forage fish (rockfish, anchovies, and sardines) responsible for the high chick mortality among some kinds of seabirds in 2009.
In contrast to cooler waters in the upwelling area, surface waters farther offshore are warming as the ocean absorbs heat and carbon dioxide from the atmosphere. The shallow waters of coastal bays and estuaries such as Tomales and San Francisco bays have also warmed, resulting in steep temperature gradients from east to west. While spatial and vertical patchiness has always been a characteristic of the California Current ecosystem, these new extremes may be causing the change now being documented offshore.
Warmer surface waters also inhibit the vertical mixing of ocean waters that is the foundation of nutrient production. Without vertical mixing, nutrient-rich cold waters can produce blooms of zooplankton that absorb nutrients and create zones of low dissolved oxygen, which can both kill off certain native species and invite in species that thrive in low-oxygen conditions. This probably explains the recent invasion of the Humboldt squid, a voracious subtropical predator that is now disrupting local ecosystems. Bill Sydeman, an ecologist with the Farallon Institute and member of the science working group, calls the squid an invasive species on par with scotch broom, pampas grass, and other well-known terrestrial weeds because of its rapid spread and the havoc it’s causing to endemic ecosystems.
- Ochre seastars might expand their range due to changing conditions. Photo by Kibuyu, used under Creative Commons.
Warming ocean waters also account for most of the eight-inch rise in sea level measured at the San Francisco tidal gauge over the last 100 years because of thermal expansion. Not only has this sea-level rise accelerated shoreline erosion, but it has also changed intertidal communities as tidepool fish, crabs, and other shellfish relocate along rocky shores. Some species apparently find this reshuffling to their advantage; the ochre seastar, for example, is expanding its range at the expense of the slow-moving California mussel. Higher sea levels are also reducing accessibility to traditional haul out and nursery areas for pinnepeds (seals and sea lions). While the agile California sea lion can scramble to higher ground, harbor seals may suffer adverse effects because they have such restricted mobility out of the water.
Finally, ocean waters continue to become more acidic as they absorb more and more carbon dioxide. This issue, which has been called “the other CO2 problem” because of its severe threat to marine life, may seriously alter marine ecosystems by preventing organisms from forming calcium carbonate structures, like shells and exoskeletons (obviously critical for hundreds of species, from coral to crabs). Because deep waters are more acidic than those on the surface, the report warns, the California coastal region may suffer double jeopardy: More intense upwelling could bring deep, highly acidic water forming a surface layer that’s uninhabitable for many key invertebrate plankton populations that form the basis of the California Current food web, fueling everything from tidepool anemones to blue whales.