Biology Reference
In-Depth Information
times before settling out. Multiply that by the billions of specks suspended
in the bay at any given moment and it's clear just how solid the liquids are
in this estuary.
“Typically in estuaries the finest material is up near the rivers,” explains
Schoellhamer. “Things like the two-layer flows and the asymmetry of tides
generally move the finer material preferentially toward the land, and the
coarser material out toward the ocean. In this estuary, we have the large
sandbars out on the continental shelf, and those may act as a source of
sand moving into Central Bay. Also, near the mouth of the estuary at the
Golden Gate, we have generally the biggest tides and biggest water veloc-
ity, which again applies the most force, so the smaller rocks are not going
to settle, as they are constantly picked up and moved somewhere else with
every tide. Only the heavier stuff—the sand—can actually stay down there.
That's why it's clearer down at the bottom of the Central Bay than in the
shallower bays.”
Sediments are good for the bay in several ways. First, the bay's natural
turbidity protects it from the harmful algae blooms that plague other estu-
aries. Not enough sunlight penetrates into the water to drive phytoplank-
ton growth. Second, contaminants introduced into the water often attach
themselves to sediments and settle out, thereby getting removed either
temporarily or permanently from circulation. Third, the estuary's wet-
lands and beaches need a steady supply of sediments to sustain places for
Erosion of the San Pablo baylands shoreline. (Jude Stalker)
