Phosphorus is the nutrient that often limits primary productivity
in aquatic ecosystems, while nitrogen limits terrestrial productivity.
Phosphorus (P) sources exist in both inorganic and organic forms.
Inorganic forms (e.g., phosphates) originate from apatite minerals.
Plant and organic biomass residues are the primary source of organic P.
Other important sources of P include commercial fertilizers and manure,
land application of biosolids, wastewater treatment plants (WWTP), livestock
grazing, non-agricultural fertilization, and septic systems.
The easiest and most common measurements of phosphorous are orthophosphate-phosphorus
(PO4 - P) and total phosphorus (TP). Water samples are
usually filtered through a 0.45 micron pore size filter to separate dissolved
from total P.
The filtered solution includes the orthophosphate-phosphorus (PO4 -
P), which is typically considered soluble and available for plant uptake.
Total phosphorus (TP) includes the phosphorus that is associated with inorganic
or organic particulate matter. Most of the TP is not readily available
for plant uptake. The total phosphorus concentration of a water sample
should always be greater than the dissolved orthophosphate concentration,
and is a common method to assess water quality lab performance.
Erosion transports phosphorous attached to soil particles. Unlike
nitrogen, phosphorus is not particularly mobile in soils, and orthophosphate
ions do not leach readily. Excess fertilization and manure productions
cause a P surplus, which accumulates in soil. Some of this surplus
is transported in soil runoff to aquatic ecosystems. Stream bank
erosion and re-suspension of P in stream sediments can contribute significant
portions of the overall P load of streams that drain both agricultural
and non-agricultural areas
The EPA has established a Maximum Contaminant Level (MCL) for nitrate
in drinking water of 10 milligrams per liter (mg/L) as nitrogen.
Phosphorus in water is not considered directly toxic to humans or animals
so no drinking water standards have been established for P. Any toxicity
caused by P pollution in fresh waters is indirect, through stimulation
of toxic algal blooms or resulting oxygen depletion. The EPA recommends
that total phosphorus concentrations should be less than 0.1 mg/L in rivers,
and less than 0.05 mg/L where rivers enter lakes and reservoirs because
concentrations greater than this could contribute to eutrophication (see
Table 2). The South Platte River does not always meet the EPA
recommendations (Figure 1).
Numerous water quality models use total phosphorus concentrations as
a measure or predictor of algal growth or biomass or concentration of chlorophyll.
The trophic status (health) of surface water has been categorized by the
concentration of phosphorus measured in the surface water (Table
2).
Excess nutrient concentrations may result in eutrophication. Eutrophication
means the fertilization of surface waters by nutrients that were previously
scarce. Over geologic time, eutrophication through nutrient and sediment
inflow is a natural aging process by which warm shallow lakes evolve to
dry land. Today human activities are greatly accelerating the process.
Excess nutrient concentrations can contribute to excessive growth of algae
and other aquatic plants that can cause destruction of habitat and depletion
of dissolved oxygen, which usually results in the disappearance of intolerant
aquatic insect species and fish.
In freshwater, blooms of cyanobacteria (blue-green algae) are a prominent
symptom of eutrophication. These blooms contribute to a wide range
of water-related problems including summer fish kills, foul odors, and
unpalatable tastes in drinking water. Furthermore, when such water
is processed in water treatment plants, the high load of organic detritus
reacts with chlorine to form carcinogens known as trihalomethanes.
Water-soluble compounds toxic to the nervous system and liver are released
when cyanobacterial blooms die or are ingested. These can kill livestock
and may pose a serious health hazard to humans.
The effect that reservoirs can have on water quality is evident when
nutrient concentrations are compared at sites upstream and downstream from
reservoirs. Nutrients in water often decrease significantly while
stored in reservoirs. The decrease is attributed to biological uptake
of nutrients, suspended sediment settling out of the water column, and
phosphorus sorption to sediments. In the South Platte River
basin, phosphorus does not significantly decrease after storage.
Concentrations of phosphorus, both orthophosphate and total phosphorus
in outlet waters approximate the inlet waters. The minimal retention
of phosphorus is probably due to the fact that much of the phosphorus load
in the South Platte is from WWTPs, and not associated with sediment.
The phosphorus inputs exceed plant uptake requirements. The existing
sediments in these reservoirs are probably near capacity in terms of phosphorous
fixation or sorption, since the inputs literally amount to thousands of
kilograms input of P per year. Phosphorus concentrations in
reservoir surface waters in the South Platte River Basin range from 0.05
to 1.0 mg/L, with means often above 0.1 mg/L.
If current practices continue, nonpoint pollution of surface waters
is virtually certain to increase in the future. Such an outcome is
not inevitable, however, because a number of technologies, land use practices,
and conservation measures are available that can decrease the flow of nonpoint
P into surface waters.
Phosphorus is a critical element for crop production. Proper management
can limit the amount of phosphorus reaching streams, wetlands, and lakes
from agricultural fields. The successful steward of land and water
resources will use crop selection, soil conservation measures, nutrient
management planning, knowledge of weather patterns and common sense to
limit phosphorus loss from agricultural fields.
Phosphorous concentrations in rivers, lakes and reservoirs are directly
related to eutrophication of waterbodies. Since most chemical forms
of phosphorous are not readily mobile in water, problems with phosphorous
and eutrophication can reoccur over several years due to the re-suspension
of P from the sediments. Since P cannot easily be removed from a
waterbody, the best method to help prevent eutrophication is to stop phosphorous
from reaching the waters through BMPs.
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