Phosphorus Cycling
in the Estuarine Environment
Fall 97
Phosphorus:
It occurs in nature only in compounds called phosphates. And
because plants and animals must have it to live and grow,
phosphorus undergoes continuous transformation.
Phosphorus is the eleventh-most abundant mineral in the earth's
crust, and phosphorus deposits occur primarily as phosphate in
the mineral apatite. Phosphorus is a component of DNA, and plants
and animals must have it to live and grow.
Phosphorus in the estuarine system exists in either a particulate
phase or a dissolved phase. Particulate matter includes living
and dead plankton, precipitates of phosphorus and phosphorus
adsorbed to particulates. The dissolved phosphorus includes
inorganic phosphorus and organic phosphorus excreted by
organisms.
Sources of Phosphate:
In the estuarine setting, natural sources of phosphates
include weathering of the earth's crust and organic decay.
Draining wetlands are also potential phosphate sources for bays
and other coastal estuaries, although the marsh water may not be
completely exchanged on each tidal cycle resulting in little
phosphate exchange. Estuarine phosphates may come from domestic
waste and industrial effluent. Initial treatment of these wastes
removes only 10 percent of the phosphorus, and secondary
treatment removes only 30 percent. The remaining 60 percent is
discharged into the water system.
Accumulation of phosphates in the estuary may be regulated due to
channel characteristics--shallow broad channels have concentrated
phosphates at the head, and narrow steep-sided channels have
concentrated phosphates at the mouth.
Phosphorus Levels:
Many factors may affect the level of phosphorus in the
estuarine system. Estuarine water has close contact with
underlying sediments, thus phosphate exchange is more effective
in the estuary. Drastic changes in weather and sedimentation
rates over a short period of time also affect the rate at which
phosphorus is exchanged. Flood waters may reduce the salinity of
an estuary, and a heat wave may raise the temperature of the
estuary, both of which would enhance the inorganic control of
phosphorus. However, in a natural, undisturbed estuarine
ecosystem, the amount of phosphorus remains fairly constant due
to continuous transformation and recycling of phosphorus.
Inorganic Control of Phosphorus:
Mineral reactions with phosphorus in solution have been
observed to help control phosphorus levels in estuaries. This
"buffering effect"--maintaining a consistent phosphorus
level--occurs as a result of adsorption of phosphorus from
sediments. Phosphorus adsorption--the accumulation of phosphate
solutes on solid mineral surfaces--is regulated by several
factors.
First and most obvious, the greater the concentration of the
phosphorus in solution and the greater the amount of mineral
surfaces available, the greater the rate of phosphorus
adsorption. Second, adsorption is highest for the pH range
3-7--normal pH levels for both the fresh and brackish waters
found in the estuary. Next, phosphate adsorption increases with
higher temperatures. Because many estuaries are shallow,
estuarine temperatures may often be higher than surrounding water
bodies, thereby enhancing adsorption. Last, when pH and
temperature are constant, phosphate adsorption decreases with
increasing salinity. In the sometimes fresh, sometimes salty
estuarine environment, salinity may often be low and therefore
adsorption rates will be high.
Biological Cycling of Phosphorus:
Biological cycling begins with bacteria and
phytoplankton. Bacteria take up orthophosphates from the surfaces
of organic detritus and suspended mineral particles, and
phytoplankton assimilates it under appropriate lighting
conditions. Then the bacteria and phytoplankton are consumed by
filter feeding organisms. Over half of the organic phosphorus
consumed by the filter feeders is excreted as inorganic
phosphorus, which is assimilated by phytoplankton, and the cycle
repeats. In areas with high phosphate inputs, biological activity
will play only a minor role in phosphorus regulation.
Annual Phosphorus Cycle:
Studies have shown that in some areas phosphate
concentration varies in an annual cycle--phosphate concentrations
are highest in the summer and lowest in the winter.
As summer begins, the phosphates that have been
"remineralized"--reverted to inorganic
substances--react with iron to form an insoluble compound called
ferric phosphate and can remain suspended in interstitial waters
or as precipitates on the surface of the sediment. As summer
progresses, oxygen is removed faster than it is replaced. This
causes anoxic conditions, which enhances the reduction of iron
and the subsequent phosphate release from ferric phosphate into
the water.
As surface waters cool in the fall and winter, oxygen input to
the deep water exceeds demand, and oxygen levels rise. Because
anoxic conditions must be present for reduction to occur,
phosphate release from the bottom sediments is not measurable.
Physical circulation transports some phosphate to the euphotic
zone where it is taken up by phytoplankton to use in
photosynthesis. Phosphorus in this zone may be flushed out or
"biologically transported" out of the estuary, it may
be remineralized in the water column, or it may settle to the
bottom. As phosphates are taken out of the surface layers of
water, a decreased total phosphorus concentration is noted.
Effect of Increased Phosphorus Levels in the Estuary:
In most estuaries, nitrogen is the primary limiting
nutrient, and excessive nitrogen can lead to algal blooms.
Therefore, phosphorus levels must be considered in relation to
nitrogen levels. The recommended level of phosphorus in estuaries
is 0.01 to 0.1 mg/l and 0.1 to 1 mg/l of nitrogen (a 10:1 ratio
of nitrogen to phosphorus). Under these conditions, most algal
blooms may be avoided. When high levels of nitrogen result in low
phosphorus levels, effluent from local land areas--especially
highly urbanized and/or agricultural areas--can increase
phosphates found in estuaries as the land is bared to erosional
factors. These increased phosphorus levels may result in algal
blooms and eutrophy.
THREE COMPONENTS OF ESTUARINE PHOSPHORUS CYCLING
INORGANIC CONTROL
BIOLOGICAL CYCLING
Factors Affecting Biological Cycling:
The Biological Cycle:
SEASONAL VARIATIONS
Spring/Summer:
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