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Phytoplankton Response to the Invasion of the Asian Clam in North and South San Francisco Bay, California

North Bay

Current Conditions - After the invasion of the Asian clam - Corbula amurensis (previously known as Potamocorbula amurensis)

Winter/ Spring

Low phytoplankton
consumption rate

+

Low phytoplankton
productivity

=

No phytoplankton
bloom

  • Annual winter migration of ducks and shorebirds increases predation on clams
  • Clam population at lowest annual abundance due to predation
  • Few clams are present to eat phytoplankton so consumption rate is low
  • High freshwater inflows increase turbidity of water
  • High-turbidity water limits the amount of light penetrating into the water
  • Phytoplankton need light to grow so the lack of light limits phytoplankton growth
  • The bay's water is turbid due to sediment-laden freshwater inflows so phytoplankton cannot grow due to lack of light

Summer/ Fall

High phytoplankton
consumption rate

+

High phytoplankton
productivity

=

No phytoplankton
bloom

  • Lower number of ducks and shorebirds are present to eat clams so clam populations increase
  • Higher number of clams eating phytoplankton keep phytoplankton populations in check
  • Lower freshwater inflows result in lower turbidity water
  • Lower turbidity water allows more light into the bay’s water, which promotes growth of phytoplankton
  • Phytoplankton can grow but the Asian clams are such voracious feeders that the phytoplankton is eaten before a bloom can occur

North Bay

Historical Conditions - Before the invasion of the Asian clam - Corbula amurensis

Winter/ Spring

Low phytoplankton
consumption rate

+

Low phytoplankton
productivity

=

No phytoplankton
bloom

  • Clams are not consistently present due to large annual range in water salinity
  • Annual migration of ducks and shorebirds results in fewer clams when they are present, and thus lower phytoplankton consumption
  • High freshwater inflows increase turbidity of water
  • High-turbidity water limits the amount of light penetrating into the water
  • Phytoplankton need light to grow so the lack of light limits phytoplankton growth
  • The bay's water is turbid due to sediment-laden freshwater inflows so phytoplankton cannot grow due to lack of light

Summer/ Fall

Low phytoplankton
consumption rate

+

High phytoplankton
productivity

=

Phytoplankton
bloom

  • Clams when present are not abundant in most years and phytoplankton is not heavily fed upon**
  • The number and feeding rate of clams was not sufficient to reduce phytoplankton
  • Lower freshwater inflows result in lower turbidity water
  • Lower turbidity water allows more light into the bay’s water, which promotes growth of phytoplankton
  • Freshwater inflows are no longer making the water turbid so phytoplankton receive enough light to grow
  • Clams don’t consume phytoplankton at a rate that prevents a bloom
**An exception to this pattern was seen during the drought of 1977-1978 when the consistently high salinity resulted in a phytoplankton-consuming clam from the more saline portion of the bay settling into the North Bay. It's feeding is believed to have limited the phytoplankton bloom in a manner similar to what we see today. This clam's residence in North Bay was short-lived, unlike that of the Asian clam, because it was unable to withstand the salinities caused from normal freshwater inflows.

South Bay

Current Conditions - After the invasion of the Asian clam - Corbula amurensis

Winter

Low phytoplankton
consumption rate

+

Low phytoplankton
production rate

=

No phytoplankton
bloom

  • Annual winter migration of ducks and shorebirds increases predation on clams
  • Clam populations at lowest annual abundance due to predation so phytoplankton consumption rate is low
  • Winter winds along with high tidal energy and freshwater inflows keep the turbidity of the water high
  • High turbidity during winter limits the amount of light in water
  • With limited light, phytoplankton growth is very slow
  • Phytoplankton growth is limited by light so bloom is not possible

Spring

Low phytoplankton
consumption rate

+

High phytoplankton
production rate

=

Phytoplankton
bloom

  • Clam populations have not recovered from winter predation by ducks and shorebirds
  • Few clams are present to eat phytoplankton so consumption rate is low
  • Decreasing wind and low tidal energy at the spring equinox decreases mixing and thus the turbidity of the water
  • Clearer water allows more light to enter the bay’s water
  • With more light phytoplankton growth can occur
  • Clams are not present to eat phytoplankton and water has cleared, which permits growth of phytoplankton

Summer

High phytoplankton
consumption rate

+

Low phytoplankton
production rate

=

No phytoplankton
bloom

  • Bird predation rates remain low so clam populations recover
  • Higher number of clams eating phytoplankton keep phytoplankton populations in check
  • Summer diurnal winds stir up bottom sediment during the day, which increases the turbidity of the water
  • High turbidity of water in summer limits phytoplankton growth
  • Combination of diurnal winds stirring up sediment and high clam feeding rates prevent the phytoplankton from growing

Fall

High phytoplankton
consumption rate

+

High phytoplankton
production rate

=

No phytoplankton
bloom

  • Clam populations are high
  • High number of clams eating phytoplankton keep phytoplankton populations in check
  • Summer diurnal winds die out, winter storm season has not yet arrived, and the fall equinox tidal energy is low, which results in a decrease in the turbidity of the water
  • Water is quiet so turbidity is low, which should allow phytoplankton to grow
  • The fall like the spring is a time when the water is clear (lower turbidity); however, unlike spring the clams have recovered from winter feeding by ducks and shorebirds and are now consuming phytoplankton, which prevents a bloom

South Bay

Historical Conditions - Before the invasion of the Asian clam - Corbula amurensis

During the period of historical record there has not been a fall phytoplankton bloom in the San Francisco Bay southern embayment, but since there have been so many invasive species in the ecosystem for so long, some scientists conjecture that there might have been a phytoplankton bloom in the fall before the invasive species arrived.

Many phytoplankton consumers are food-limited in San Francisco Bay because there is so little phytoplankton

More Information

References

General Information on Invasive Species

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Page Last Modified: Thursday, 19-Dec-2013 16:46:43 EST

Created on November 16, 2005