Phosphates enter waterways from human and animal waste, phosphorus-rich bedrock, laundry and cleaning wastewater, industrial effluents, and fertilizer runoff. These phosphates become detrimental when they over-fertilize aquatic plants and increase the rate of natural eutrophication, which is sometimes called cultural eutrophication or accelerated eutrophication.
Eutrophication is the natural aging process of a body of water. As a lake or pond ages, the aquatic system tends to accumulate nutrients within the aquatic biota. This increased supply of nutrients can be made bioavailable to algal and other primary producers which increases the primary productivity of the system. This increased primary productivity has a direct impact and influence on secondary consumers like copepods and fish, but will also increase the amount of organic material or debris in the system and in its sediments. In systems with very low oxygen content in the deeper water, the plant and other organic organisms die quicker than they can be decomposed. This results in an increase in the carbon content and the amount of "mucky" or organic-laden sediments. This organic-rich sediment builds up on the bottom of the system and, together with sediment entering the water body, fills in the stream, pond, lake, or bay. Over time, the aquatic system becomes shallower and shallower and the system may be converted to a wetland complex or bog. Normally this process takes thousands of years.
Accelerated or cultural eutrophication is more of an unnatural process where human activities, i.e., excavation, farming, waste disposal, mining, or other efforts, have sped up the natural process. This tends to be associated with an increase in suspended materials, erosion/sedimentation, and increased levels of nutrients like nitrogen and phosphates whose lack are usually what limits aquatic plant growth. When humans get involved, we can speed up the natural process and make change happen in years and decades, rather than the normal time window of thousands of years.
When monitoring a water-based ecosystem, such as a stream, pond, lake, or a river, we attempt to gauge the "productivity" and "age" or "status" of these systems in a number of ways. In most cases, we may make observations on the size, watershed area, flow rate, depth, and configuration of the system, plus make observations on the observable properties of the water, such as the color or appearance of the water and the presence of any nuisance conditions, such as algal growth, and, in some cases, the odor of the water. In many cases, the water quality of the system will be documented and compared to a reference site or system and we would attempt to create both a water and a nutrient budget trying to track the ins and outs for the system.
When measuring water quality, the parameters may include pH, temperature, alkalinity, dissolved oxygen level (DO), biological oxygen demand (BOD), total suspended solids (TSS), nitrate, nitrite, ammonia, phosphate, turbidity / secchi disc depth, conductivity/total dissolved solids (TDS), and even the actual chlorophyll content of the water. For lakes, the primary productivity may be documented by evaluating the population of phytoplankton, zooplankton, and macrophytic vegetation and secondary productivity documented via a fishery assessment. For streams and rivers, a fishery or macroinvertebrate assessment may be conducted.
Phosphate comes in a number of different forms. In natural waters, it commonly presents as phosphate or the anion (PO₄ ⁻³). In the environment, phosphorus is an essential element needed for life and, in most cases, it is also a "growth limiting" nutrient, because bioavailable phosphate is naturally present at very low levels in the environment (which is why it is an important component of fertilizer). Phosphate is usually held (adsorbed) on soil particles, bound into organic material, or part of mineral compounds. In lakes that are unproductive, the level of ortho-phosphate (aka phosphoric acid, H₃PO₄ and its salts) is typically 0.005 to 0.007 mg P/L. The following is a general guide that relates the total phosphorus concentration to the "trophic status" of the lake:
< 0.01 mg Total Phosphorous per liter- Oligotrophic
0.01 mg to 0.03 mg Total Phosphorus per liter - Mesotrophic
> 0.03 mg Total Phosphorous per liter - Eutrophic ("Significant algal growth)
The symptoms of "cultural eutrophication" include:
Algal Blooms become more frequent and "blue-green algae" dominate; aquatic vegetation becomes very dense in shallow waters. Recreational uses of the water, such as swimming and boating, are associated with nuisance conditions. Over time - anaerobic conditions within the water column can be associated with odors, fish kills, and increased levels of cyanobacteria toxins.
Note: "Among cyanotoxins are some of the most powerful natural poisons known, including poisons which can cause rapid death by respiratory failure. The toxins include potent neurotoxins, hepatotoxins, cytotoxins, and endotoxins." (Source)
If too much phosphate is present in the water, the algae and weeds will grow rapidly, may choke the waterway, and will use up large amounts of precious dissolved oxygen which happens when,in the absence of photosynthesis, the algae and plants die and are consumed by aerobic bacteria. The result may be the death of many fish and aquatic organisms, not because of elevated phosphate or phosphate poisoning, but from the impact that phosphate has on algal growth and the consequent drop in dissolved oxygen.
Phosphorus is one of the key elements necessary for the growth of plants and animals. Phosphates PO₄⁻³ are formed from this element. Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate), and organically-bound phosphate. Each compound contains phosphorus in a different chemical formula. Ortho forms are produced by natural processes and are found in sewage. They are based on phosphoric acid, H₃PO₄, and its derivatives: H₂PO₄⁻, HPO₄=, PP₄⁻³, and their salts. Polyphosphate (aka metaphosphate) is a polymer of linked phosphoric acid groups such as (P₂O₇)⁻⁴ which is a polymer of two phosphoric acid groups. Poly forms are used for treating boiler waters and in detergents. In water, they change into the ortho form. Organic phosphates are important in nature. Their occurrence may result from the breakdown of organic pesticides which contain phosphates. They may exist in solution, as particles, as loose fragments, or in the bodies of aquatic organisms. Phosphorus can also be found in minerals like the apatite family [Ca₅(PO₄)₃(F,Cl,OH)] which is very insoluble in water; it is converted into water-soluble phosphate salts by treatment with sulfuric (H₂SO₄) or phosphoric acid (H₃PO₄) to produce ‘superphosphate’ fertilizer.
Methodology: The analysis of phosphorus uses an atomic absorption spectrophotometer. Phosphorus is oxidized to the phosphate ion (PO₄⁻³), reagent dye is added, and the absorbance read to get the concentration of phosphorus.
Rainfall can cause varying amounts of phosphates to wash from farm soils into nearby waterways. Phosphate will stimulate the growth of plankton and aquatic plants which provide food for fish. This may cause an increase in the fish population and improve the overall water quality. However, if an excess of phosphate enters the waterway, algae and aquatic plants will grow wildly, choke up the waterway, and use up large amounts of dissolved oxygen. This condition is known as eutrophication or over-fertilization of receiving waters. This rapid growth of aquatic vegetation eventually dies and, as it decays in the water, it uses up dissolved oxygen. This process, in turn, causes the death of aquatic life because of the lowering of dissolved oxygen levels.
Phosphates are not toxic to people or animals unless they are present in very high levels, i.e., > 1000 mg/L. Digestive problems could occur from extremely high levels of phosphate, leading to Hyperphosphatemia.
"Oligotrophs are characterized by slow growth, low rates of metabolism, and generally low population density. The adjective oligotrophic may be used to refer to environments that offer little to sustain life, organisms that survive in such environments, or to the adaptations that support survival." (Source) Typically, a lake will have a total phosphorus concentration of < 0.010 mg/L, a chlorophyll concentration from < 0.0025 mg/L to < 0.008 mg/L, and a mean secchi-depth of 4 to > 8 meters. The Trophic State Index (TSI) is < 40.
"Mesotrophic lakes are lakes with an intermediate level of productivity. These lakes are commonly clear water lakes and ponds with beds of submerged aquatic plants and medium levels of nutrients. The term mesotrophic is also applied to terrestrial habitats." (Source) Typically, a lake will have a total phosphorus concentration of 0.010 mg/L to 0.035 mg/L, a chlorophyll concentration from > 0.008 mg/L to < 0.025 mg/L, and a mean secchi-depth of > 2 to 4 meters. The Trophic State Index is from 40 to < 50.
Eutrophication (from Greek eutrophos, "well-nourished"), or hypertrophication, is when a body of water becomes overly enriched with minerals and nutrients which induce excessive growth of algae. This process may result in oxygen depletion of the water body. (Source) Typically, a lake will have a total phosphorus concentration of > 0.035 mg/L, a chlorophyll concentration > 0.025 mg/L, and a mean secchi-depth of from 0.5 to 2 meters. The Trophic State Index is from 50 to < 70. A system with a TSI of 70 or more is classified as hypereutrophic.
Note - "Hypereutrophic lakes are very nutrient-rich lakes characterized by frequent and severe nuisance algal blooms and low transparency." (Source)
The excessive algal blooms can also significantly reduce oxygen levels and prevent life from functioning at lower depths, creating dead zones beneath the surface; the blooms can be associated with cyanotoxins.
The following criteria for total phosphorus were recommended by US EPA (1986):
1. No more than 0.1 mg TP (total phosphorus)/L for streams which do not empty into reservoirs,
2. No more than 0.05 mg/L TP for streams discharging into reservoirs, and
3. No more than 0.025 mg/L TP for reservoirs.