NUTRIENTS - Marine Pollution: What Everyone Needs to Know - page 39

sewage. They turn the sewer mains into wastewater storage
sites; however, if the water level gets too high and threatens
to back up into homes or streets, sensors deflate the dam to
release some water. Each dam can retain about two million
gallons of water until the rain decreases and the dam deflates
to allow water to flow to the treatment plant. The dams are
expected to save about 100 million gallons of sewage from
flowing untreated into the harbor each year.
What techniques in the water can reduce effects of
eutrophication?
Marinas can provide pumpout facilities for boats so that they
do not discharge their wastes into the water. Techniques to
absorb N once it reaches the water are also possible using
biology—seaweed farms will absorb nutrients; culturing oys-
ters, clams, or other bivalves will consume large amounts of
phytoplankton, reducing eutrophication and at the same time
providing food. Oysters are powerful filter feeders that can
clear the water as they feed. One adult oyster can filter and
remove nutrients from 1.5 gallons of water in an hour. Oyster
populations have declined greatly along the East Coast of
North America in the past century from overharvesting, pol-
lution, and diseases. It is estimated that one hundred years
ago Chesapeake Bay was clear, because the oysters filtered it
every three weeks as opposed to every three years today. To
help overcome this loss of oysters, planting of oyster reefs has
become a very popular restoration procedure. Many pounds of
N can be removed by oyster reefs through the process of deni-
trification by associated bacteria—which returns the N to the
air in the form of N 2 gas. In the Chesapeake, scientists found
that one acre of oyster reef could remove 543 lbs of N in a year,
25% more than intertidal sediments without oysters. Oyster
reefs not only reduce eutrophication but also provide habi-
tat for many other organisms (at a site in Chesapeake, 24,000
organisms were living on one square meter of oyster reef!).