According to an article by James Cloern, et al in the 2006 Pulse of the Estuary, Bay Area residents should become familiar with this sight. Algal blooms have been occurring with increasing frequency in the San Francisco Bay since the late 1990s, and the trend is likely to continue. The cause has been uncertain, however, because a host of factors promote the growth of algae. These include predators, nutrients supply, temperature, and metals. In every ecosystem one of these variables must be the limiting factor that controls algae growth and prevents bloom events. In many aquatic systems, such as the Chesapeake Bay, nutrients are the limiting factor. Given the excessive agricultural runoff in the Chesapeake Bay watershed, it is no surprise that algal blooms have been a serious problem. Despite the always reliable winter sewage spills, however, nutrient levels in the San Francisco Bay have been consistently low. So what is the variable that allows this unusual and unseasonable growth of algae? The upcoming 2009 issue of the Pulse of the Estuary will shed more light onto this question. The answer, in fact, is light.
The San Francisco Bay is becoming clearer! The concentration of suspended sediment in the Bay has been steadily decreasing since 1999, allowing sunlight to reach further below the surface of the water, stimulating algae growth and causing blooms. Incredibly, the reason for our water clarity today stems from human activities during the Gold Rush Era. In the late 1800s hydraulic gold mining sent tons of sediment, waste from the search for gold in the Sierra foothills and the Coast Range, down the Sacramento River and other Central Valley rivers. At the same time, development in the Bay Area caused the erosion of stream banks. Shoreline tidal marshes that were diked off to increase buildable and farmable land area could no longer capture this eroded sediment at the shore before reaching open water. As a result, the sediment settled on the floor of the Bay – so much sediment in fact, that the Bay became shallower. Bay Area residents are very familiar with the dredging platforms that regularly remove sediment, carving navigation channels into the floor of the Bay. In addition to dredging, natural wave patterns and burrowing wildlife can stir up sediment and re-suspend it in the water column. High concentrations of suspended sediment reduce the depth to which sunlight can penetrate the water, thus controlling algae growth and preventing most blooms.
Recent USGS data suggest that the Bay experienced a dramatic increase in clarity when this erodible supply of sediment was depleted in the late 1990s. In this year’s Pulse of the Estuary, David Schoellhammer of USGS offers an explanation as to how this may have happened. As long as the San Francisco Bay received sediment from upstream sources and held suspended sediment at capacity, erosion from the floor of the Bay was minimal. Although the Sacramento River delivered sediment, it also gently flushes the Bay and gradually pushed sediment through the Golden Gate. River banks in the Central Valley were protected during the 1900s to prevent erosion, and other sources of sediment are trapped behind dams. The remainder of the hydraulic mining supply slowly moved downstream until it reached the Bay. As a result the Sacramento River delivered clear water, which increased the erosion of sediment on the Bay floor. In 1998, a wet year during which the strong, clear flows from the Sacramento River persisted well into the summer, most of the remaining sediment supply was likely eroded pushed out of the Bay. The following year saw the suspended sediment concentration of the Bay waters decrease by 50%.
This great sweep of sediment through the Golden Gate did not unearth an ecological time capsule to the Bay’s pre-Gold Rush condition. The subsequent increase in clarity in the Bay is a major shift in water quality, which is causing a cascade of ecological and economic consequences in light of modern environmental stressors. As Bay Area residents have recently witnessed, the low concentration of suspended sediment in the Bay makes more light available to stimulate the growth of photosynthetic organisms – aquatic plants, algae, and other phytoplankton. As these organisms thrive they feed higher trophic levels, and the Bay food web becomes more robust. As Schoellhamer points out, the San Francisco Bay has crossed a threshold and become an estuary with a level of primary production that is more typical of temperate latitudes. This increased productivity has implications of its own. With a greater availability of light, nutrient inputs have a greater impact in the growth of algae. While the San Francisco Bay regularly receives nutrients from agricultural runoff or sewage spills, the low light has always prevented excessive growth of phytoplankton. Under current conditions, however, these inputs may trigger more intense bloom events and their associated problems.
The loss of sediments may also hinder coastal wetland restoration efforts. Wetland restoration usually involves opening up a previously diked area to the tides, so that suspended sediments in the water will naturally settle out along the shore, gradually building up until the land is high enough for plants to colonize. The lower the concentration of suspended sediments in the water, the longer it will take for the wetland to develop. Now with rising sea levels threatening to inundate our shorelines, the growth of new wetlands will likely be outpaced. To speed up the process, wetlands restoration projects may also utilize dredge spoils. With the loss of sediment from the Bay bottom, however, there is less of a need for dredging and a limit to sediment available for these restoration projects. Incredibly, the natural expulsion of sediments from the Bay, which caused supplies to shrink while demand has recently grown, has changed hidden Gold Rush waste into a valuable natural resource.
Learn more about Bay sediment in the 2009 Pulse of the Estuary, from the San Francisco Estuary Institute. The Pulse is the annual report for water quality in San Francisco Bay. You can find it at www.sfei.org
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