Biology Reference
In-Depth Information
plankton. The dramatic switch in San Francisco Bay species was most pro-
nounced in the area most closely connected to the ocean—the Central Bay.
Another boost to biological change and diversity comes from the fact
that the bay is an outlet for so many rivers and creeks. These tributar-
ies bring species that prefer fresh and brackish waters into the realm of
the bay.
Geologic history, too, has helped shape the bay's present biodiversity.
The 10,000-year-old bay is a recent feature on the Pacific Coast. The bay's
geologic youthfulness means that relatively few clams, worms, and other
species that live in the mud have had a chance to establish themselves in
local waters. By the same token, the tides have had fewer seasons to carve
out marshes, making bay wetlands less extensive than in older estuaries
such as Chesapeake Bay on the East Coast. The relative rarity of local
marshes further limits the amount of food available, because decaying
plants and wetland ecosystems are important sources of estuary nutrients.
The last 200 years of human habitation have left a sizable mark on bay
health and richness. During the Gold Rush, miners in the Sierra foothills
unleashed tons of soil into the estuary's rivers, which eventually wound up
on the bay floor as a thick layer of sediment. The sudden influx of mud
smothered sedentary creatures such as the native Olympia Oyster, and
bottom-dwelling crustaceans and worms, and changed the consistency of
the bay floor. Because these species aren't very mobile, they had difficulty
repopulating the new layers of mud. Eventually, foreign species filled the
vacuum. To this day, scientists still characterize the bay floor as having low
species diversity.
Bay Habitats
Habitats in and around the bay belong to one of three groups: open water,
wetland, and upland. Open water areas are always submerged, and they
range from the deepest depths of the bay to shallower patches near
marshes that are never exposed to air. Beneath their watery surfaces lies
the
substrate
, or bottom
type, which can consist of soft sand, silt, rock,
hard shells, and even peat in the delta. Shrimps and crabs, worms and
clams live here. The water column itself is divided by depth into
deepwater
,
midwater
, and shallow or
subtidal
zones. Subtidal areas include most of
the bay floor and shallows that are always submerged.
Wetland habitats ring the shores of the bay. These marshy areas are
washed by tides or runoff that periodically leave them high and dry.
Sea-
sonal wetlands
flood only during winter and spring rains. Many consist of
flooded corn fields and other agricultural lands that provide important
feeding and resting places for migrating waterfowl. The
tidal flats,
, which
include the broad mudflats that ring much of the bay, are only exposed at
