Benthic Filter Feeding: A Natural Eutrophication Control
C.B. Officer, T.J. Smayda, and R. Mann
Paraphrased by S.A. Snyder for classroom use
From Marine Ecology – Progress Series
Volume 9:203-210
INTRODUCTION
The cause for water becoming rich in dissolved nutrients has received much attention
over the years. This is due to an increase of nutrients released into fresh water, estuarine
and salt water regions by humans. This increase in nutrients leads to a population boom
in phytoplankton. A variety of things control phytoplankton populations:
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Light and temperature
Turbidity
Phytoplankton self-shading
Nutrient concentrations and limitations
Trace elements and chemicals
Initial seed populations
Phytoplankton sinking and motility
Zooplankton grazing
Hydrodynamic effects
This study focuses on benthic animals that feed on suspended matter in the water.
These animals are often clams, mussels, and oysters. They feed by filtering the water
through special organs. The benthic animals remove planktonic matter from the system.
Scientists have studied the effects benthic animals have on regions of water. The depth
of water and the number of animals in the benthic community affect how much water is
filtered. Filtering is also controlled by the volume of water, it’s hydrodynamic
residence time, and time required for phytoplankton growth. Hence, filter feeders can
change phytoplankton populations.
Our interest in filter feeding animals was brought about by a study on the South
San Francisco Bay. This bay receives the bulk of its nutrients from the greater San
Francisco metropolitan area. The bay is relatively shallow. It is well mixed vertically,
and has a hydrodynamic residence time of a few months. The zooplankton population in
the bay is small. There is no light, temperature, turbidity, or nutrient limiting conditions.
From these conditions it is expected that South San Francisco Bay would have seasonal
phytoplankton blooms. This however is not the case. Phytoplankton populations remain
at a low level of a few g throughout the year. The exception is a brief period during
spring when an inflow of brackish water enters the Bay. This produces a vertical
stratification in the South Bay area. The benthic population in this area is mostly clams
and mussels. There can be up to several hundred grams per a one meter squared area.
Studies by Cloern show that the filter feeding capability of a large benthic population will
prevent the occurrence of phytoplankton blooms.
In our studies we consider the importance of benthic filter feeders as a control on
euthrophication. The conditions that are favorable for such a control are shallow water
depth and a good nutrient supply. Light, temperature, turbidity, poor hydrodynamic
water exchange, and a dense benthic filter feeding community are also favorable
conditions to this control.
We suggest that any modeling of phytoplankton or nutrient control in shallow
water regions look at the quantity and quality of the filter feeders. In regions for which
benthic filter feeding population is not as dense as that in the South San Francisco Bay,
the benthos may still play an important role in defining phytoplankton and nutrient levels
during part of the annual cycle.