Biology Reference
In-Depth Information
Their movements dictate the behavior of many fish. From Pacific Her-
ring to young Longfin Smelt, many fish travel to the surface in search of
zooplankton, then sink back down into the water column to evade ma-
rauding seabirds.
Phytoplankton follow a cycle of bloom and bust. When conditions are
right, they explode into algal blooms that may cover hundreds of square
miles of ocean water. Any pond covered in green slime is the site of an
algal bloom in action. Algal blooms form the base of the aquatic food web.
But blooms that get out of hand can cause transient harm to estuary life.
Most harmful blooms are triggered by too many nutrients, too much
warmth, or relatively stagnant water. Filter feeders are unable to eat
enough to keep up with the supply. The excess algae decays, depleting oxy-
gen levels in the water and suffocating fish, crabs, and other species. Some
living algae can produce harmful neurotoxins potent enough to kill fish
and poison top predators such as pelicans and sea lions.
Not all blooms are harmful. Each year in the bay, phytoplankton
blooms convert sunlight and dissolved inorganic carbon into about
120,000 tons of organic carbon, roughly equivalent to the number of calo-
ries required to sustain over a million adult humans. Despite these peri-
odic blooms, San Francisco Bay is food-limited compared to other estuar-
ies. Two main factors help ensure that bay blooms are rare. Turbid water
blocks the sunlight required for photosynthesis, while the bacteria that re-
cycle decaying organic matter back into the food web suffer from low
abundance and growth rates.
Shifts in phytoplankton cycles are telltale signs of changes in bay con-
ditions. The first shift came in 1987, when a species of clam introduced
from Asia began invading the estuary's northern reaches. In Suisun Bay,
where the mix of fresh and salt water brings good bloom conditions each
spring, this exotic clam filtered phytoplankton out of the water column
faster than the algae could reproduce. Levels of chlorophyll A (a measure
of organic plant matter abundance) in the area, which once averaged
about a hundredth of a gram per quart of water, dropped to roughly a fifth
of that level after the clam's invasion. The virtual elimination of this bloom
cut the heart out of the estuarine food web: the clams gobbled up all the
plankton before the natives could get a mouthful.
The clam was not solely responsible for these abrupt shifts in bay plank-
ton availability. Between 1977 and 1997, the most explosive blooms always
occurred in spring, and remained relatively constant year after year. But
this 20-year-old pattern came to an end in 1998. Phytoplankton abundance
began exploding in one part of the bay and plummeting in another, and
blooms occurred at times of the year never seen before. “We find ourselves
in the midst of a big biological change, with biomass in the delta lower and
in the bay higher, and both gradual and abrupt changes in the last two
