Dissolved Oyxgen
The DO maps show areas and the months when monitors reported
low DO levels. These low levels usually occur in the warmer
months since the colder the water is the greater it's capacity to
hold oxygen. The warmer months also encourage greater growth of
aquatic plants and algae which can cause DO to drop sharply
during the night. Also salinity plays a factor since the more
slat in solution the less oxygen can be dissolved and the
wintertime is "often when the lagoon experiences its lowest
salinities and highest DO levels.
One reason why an area might experience a prolonged period of
low DO could be the lack of sufficient water circulation due to
its location in a protected cove or close to a causeway. Almost
every type of product we use on our lawns and in our automobiles
(pesticides, herbicides, detergents, antifreeze, lubricants,
etc.) eventually ends up in the lagoon and some of those
chemicals and the metals they contain can consume oxygen as they
break down (oxidize). One of the largest contributors to low DO
is excessive amounts of nutrients from septic systems or
fertilizers that can be introduced by storm water runoff and
groundwater seeping into canals, rivers and the lagoon. EPA
studies indicate that even properly sited and maintained septic
tanks contribute 10 pounds of nutrients per year into the ground
water for every person using it. Even undeveloped areas
contribute small amounts of nutrients as runoff. When an area is
cleared and used for agriculture the nutrient levels tend to go
up 100%. If an area is developed for our typical suburban
dwelling the nutrient levels tend to go up 500%. Fertilizer costs
are significant for any agricultural business and so a lot of
attention goes into using optimum amounts. Using excessive
amounts of fertilizer on our little quarter acres cost us pennies
but multiplied over the numbers living here the effect is huge.
The excess nutrients can result in an excessive growth of algae
that in turn can deplete the DO to levels fatal to other marine
life. After the algae complete their life cycle and die, bacteria
consume the dead algae and in the process consume more oxygen as
well.
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