Paper Highlights Decades of Change in San Francisco Estuary

A recent paper entitled “Drivers of Change in Estuarine-Coastal Ecosystems: Discoveries from Four Decades of Study in San Francisco Bay,” published in Reviews of Geophysics shows how San Francisco Bay exemplifies the fast-paced change occurring in many of the world’s estuaries, bays, and inland seas in response to human and natural forces.

The paper, by Jim Cloern of the U.S. Geological Survey, who received the Brown-Nichols Award in 2012, and Alan Jassby from the Department of Environmental Science and Policy at U.C. Davis, uses observations from the San Francisco Estuary (also known as the Bay-Delta System) to illustrate responses to six drivers that are common agents of change where land and sea meet. Those drivers are:

  • Water consumption and diversion

  • Human modification of sediment supply

  • Introduction of nonnative species

  • Sewage discharge

  • Environmental policy

  • Climate shifts

As further described below, responses to the six drivers include: shifts in the timing and extent of freshwater inflow, salinity intrusion, decreasing turbidity, restructuring of plankton communities, nutrient enrichment, elimination of hypoxia and reduced metal contamination of biota, and food web changes that affect the estuary’s response to nutrient pollution.

  1. Water consumption and diversion

    The Central Valley Project and State Water Project are two of the largest water diversions in the world; their operations have changed the quantity and seasonal pattern of freshwater inflow to San Francisco Bay. Delta exports now divert about 30 percent of freshwater inflow from the estuary. They are a direct source of mortality to fish (with unknown population effects), remove about a third of the phytoplankton food resource produced in the Delta, and have allowed salt to move further upstream, most significantly during autumn.

  2. Human modification of sediment supply

    Hydraulic mining between 1849 and 1884 resulted in the delivery of nearly a billion cubic meters of sediments to San Francisco Bay by 1914. Since then, erosion of the hydraulic-mining debris, diking rivers and disconnecting them from their floodplains, and retention of sediments behind large dams have reduced the amount of sediment delivered downstream. Sediment supply to the estuary has been halved since the mid-20th century. The consequence is reduced concentrations of suspended particles and turbidity in the upper estuary.

    Reduced sediment supply to San Francisco Bay has important ecological implications including: reduced sediment for tidal wetlands, increased light availability for primary producers, increasing vulnerability of some native fish species to predation, and habitat expansion for some introduced species.

  3. Introduction of nonnative species

    Biological invasions challenge the integrity of natural plant and animal communities and confound conservation plans to preserve endangered species. The paper highlighted one of the most far-reaching of these invasions; a restructuring of the Suisun Bay planktonic food web following introduction of the nonnative clam Corbula amurensis. This is a contributing factor to collapses of fish abundances in the low-salinity regions of the estuary.

  4. Sewage discharge

    San Francisco Bay has been described as “The Urbanized Estuary” since it is surrounded by such Bay Area communities as San Francisco, San Jose (Silicon Valley), and Oakland. As a result of this urbanization, the Bay receives wastes from human activities through atmospheric deposition, runoff, and discharges from municipal sewage treatment plants.

    One of the major issues associated with sewage discharges is nutrient pollution. Sewage discharges deliver tons of nitrogen and phosphorus to South San Francisco Bay annually. South San Francisco Bay has the potential to produce phytoplankton biomass at levels that severely impair other estuaries such as Chesapeake Bay. However, the South Bay currently is resistant to the harmful effects of nutrient enrichment because of its physical and biological characteristics.

  5. Environmental policy: The U.S. Clean Water Act (CWA)

    In 1972 the U.S. Congress unanimously passed the Clean Water Act, landmark legislation that established the first federal regulation of sewage disposal by requiring secondary treatment of municipal wastewater to reduce inputs of solids, oxygen-consuming chemicals, and pathogens to the nation’s waters. Formerly heavily impacted by organic matter and heavy metal pollution, conditions improved in South San Francisco Bay after the CWA and local policies mandated enhanced sewage treatment. For example, in the 1950’s, oxygen depletion was common in South Bay but by the late 1970’s, the discharge of oxygen consuming organic matter had decreased tenfold.

    Although there has been substantial progress, the paper notes that standards prescribed in the CWA have not been fully met. Further rehabilitation, or even maintenance of the status quo, will require innovative strategies to solve the much more difficult problems of nonpoint sources of nutrients and toxic contaminants, legacy contaminants from the past, and new contaminants of the future.

  6. Shift in the ocean-atmosphere system

    Fishermen have known for centuries that fish abundance in the sea fluctuates between eras of good and poor catch that are tied to climate variability-not climate change but natural cyclical patterns of changes in the regional climate. “Scientists have discovered that population fluctuations of fish and their food resources are synchronized with shifting patterns of atmospheric pressure over ocean basins that are represented by climate indices,” the paper says. For example, salmon stocks in Alaska and California fluctuate with the Pacific Decadal Oscillation.

    A major climate shift in the North Pacific Ocean in 1998-1999 is reflected in changes in the populations of crabs, fish, shrimp, clams, and phytoplankton in South San Francisco Bay. This linkage between the ecosystem inside SF Bay and North Pacific Ocean conditions is a new discovery.

    The paper concludes with a discussion of the importance of monitoring and its interpretation and use. Because of the “breathtaking pace of change” occurring in the world’s estuarine-coastal ecosystems (mostly caused by human activities), monitoring is “essential for managing the human dimension of ecosystem dynamics because it detects environmental changes, provides insights into their underlying causes, can provide early warning signs of impending state shifts, prompts mitigation and adaptation policies (such as the Clean Water Act), and measures outcomes of those policies.”

    One problem highlighted in the paper is the reluctance to fund long term ecosystem monitoring programs because their benefits might not be immediately apparent.

    Yet as the abstract concludes, “The many time scales of variability and the multiplicity of interacting drivers place heavy demands on estuarine monitoring programs, but the San Francisco Bay case study illustrates why the imperative for monitoring has never been greater.”

To read the paper, click here.