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Phosphorus and Eutrophicaation of Aquatic Systems
Phosphorus (P) is an essential element for all life forms. It is a mineral nutrient. Orthophosphate is the only form of P. that autotrophs are able to assimilate. Extracellular enzymes hydrolyze organic forms of P. To phosphate. Eutrophication is the overenrichment of receiving aquatic systems with mineral nutrients. The results are excessive production of autotrophs, especially algae and cyanobacteria. This high productivity leads to high bacterial populations and high respiration rates, leading to low oxygen concentrations or anoxia in poorly mixed bottom waters and at night in surface waters during calm, warm conditions. Low dissolved oxygen causes the loss of aquatic animals and release of many materials normally bound to bottom sediments including various forms of P. This release of P. reinforces the eutrophication.
Excessive concentrations of P. is the most common cause of eutrophication in freshwater lakes, reservoirs, streams, and headwaters of estuarine systems. In the ocean, N becomes the key mineral nutrient controlling primary production. Estuaries and continental shelf waters are a transition zone, where excessive P. And N. create problems. It is best to measure and regulate total P. inputs to whole aquatic ecosystems, but for an easy assay it is best to measure total P. concentrations, including paniculate P, in surface waters or N/P atomic ratios in phytoplankton.
Characteristics of Phosphorus
Phosphorus is a needed component of nucleic acids and many intermediary metabolites, such as sugar phosphates and adenosine phosphates, which are an important part of the metabolism of all life forms. With the exception of trace emissions of phosphines from volcanoes, the P. compounds found on the surface of the Earth are not volatile and transport through the atmosphere is primarily in dust or aerosols. Atmospheric flux rates are slow compared with those in surface waters. With few exceptions surface waters receive most of their P. In surface flows rather than in groundwater, since phosphates bind to most soils and sediments. The exceptions are where watersheds are of volcanic origin or where soils are water-logged and anoxic. Phosphorus only occurs in the pentavalent form in aquatic systems. Examples are orthophosphate, pyrophosphate, longer-chain polyphosphates, organic phosphate esters and phosphodiesters, and organic phosphonates. Phosphorus is delivered to aquatic systems as a mixture of dissolved and particulate inputs, each of which is a complex mixture of these different molecular forms of pentavalent
However P. is a very dynamic, biologically active element. After these P. inputs arrive in a receiving aquatic systems, the particulates may release phosphate and organic phosphates to solution in the water column and various P. compounds may be chemically or enzymatically hydrolyzed to orthophosphate, which is the only form of P. that can be assimilated by bacteria, algae, and plants. Particulates may be deposited in the bottom sediments, where microbial communities gradually use many of the organic constituents of the sediments, ultimately releasing much of their P. contents back to the water column as orthophosphate. Hence, one should not assume that particulate P. Or dissolved organic P. are inert in these aquatic systems because under appropriate conditions these forms of P. can be converted to dissolved orthophosphate.
Once delivered to a lake, reservoir, or estuary, P is usually kept fairly efficiently by a combination of biological assimilation and the deposition of sediments and biota to the bottom sediments. This efficient trapping of P. inputs makes these systems sensitive to pollution with excessive amounts of P. If the system is oligotrophic (low primary production), the bottom waters will have oxygen throughout the year and most of this P. will be stored in the bottom sediments.
However, in eutrophic systems (excessive primary production), Bottom waters often become anoxic during the growing season and even shallow waters may become diurnally anoxic at night during warm, windless weather. When these conditions occur, much of this P. In bottom sediments is released and diffuses back into the water column.
Evidence of the Key Role of Phosphorus
Over time ecologists developed the concept that plant and bacterial growth in an aquatic system would ultimately become limited by the availability of an essential element. This would then constitute the limiting nutrient for that system at that time, and inputs of that nutrient could be managed to limit eutrophication. The term limiting nutrient has been used in somewhat different ways, sometimes meaning limiting the growth of the present population, sometimes the limitation of growth over time with species composition changes, sometimes limiting the ultimate primary or net production of an ecosystem. Here the ultimate limitation of ecosystem primary production is inferred.
The diatom Cyclotella nana, grown in P-limited chemostats could only reach biomass atomic ratios of C. To P. Of 480 and N. To P. Of 35. This diatom had reached its limits of growth with the available P. In a series of bioassays of lake waters from the Great Lakes region of the USAu sing the Provisional Algal Assay Procedure (USDA1, 969), Selenastrum capricornutum cell number was found most often to respond to the addition of phosphate, rather than N, indicating that most of these lake waters contained limiting concentrations of P. Mesocosmex periments in which 320 L. Of Minnesota or Oregon lake water were enclosed in clear plastic bags and then enriched with various nutrients, found that P. was the primary controlling nutrient when positive responses were found (Powers et al., 1972). Mesocosm experiments in which 1000 to 4000 L. Of water from Lake Michigan were enclosed in clear plastic bags found that when P. was added, silica was reduced to levels that limited algal growth but N. was not (Schelske and Stoermer, 1972). They concluded that P. was the limiting nutrient in Lake Michigan, but that silica was becoming limiting for diatoms.
Somewhat later Lean and co-workers introduced the concept of an "index of P. deficiency." They used radioactive tracers to measure the turnover times of dissolved orthophosphate in lake surface waters. High turnover rates (short turnover times) indicated more P. limitation. This was further developed by measuring the ratio of C. fixation to phosphate uptake under various conditions. Atomic ratios of C. fixation to phosphate uptake varied from 1.2 to 206 depending on the degree of P. deficiency prior to the measurement. If the algae had previously been highly P. limited, they would fix a higher amount of C. per P. fixed.
A more direct measure of the key importance of P. In lake eutrophication was the work at the Experimental Lakes research area in northwestern Ontario. Whole lakes were enriched with P. For a period of years. These P-enriched lakes used atmospheric N. And C. For algal production and this resulted in significant increases in ecosystem primary production. Phosphorus additions triggered undesirable cyanobacterial blooms unless N. was also added. However, if C. Or N. were added, in the absence of P. enrichment the effects were minor ( Schindler, 1974, 1975, 1977). In another "whole lake" experiment, Lake Washington near Seattle, WA, had been heavily loaded with nutrients in sewage outfalls for many years and had become severely eutrophic.
Then, in 1963 the sewage effluent was diverted away from the lake. By 1969, chlorophyll in the summer and phosphate in the winter had declined to only 28% of previous years, but nitrate declined by only 10 to 20%. Lake Washington returned to a mesotrophic status. This was interpreted to mean that P. was the key limiting nutrient (Edmondson, 1970). Another example is Lake Erie that began experiencing dissolved oxygen depletion due to eutrophication. In 1968, its annual P. input was estimated to be 20-000 t and surface waters had an average of 22 ~zg of total P/L. By 1982, improved wastewater treatment had reduced annual P. inputs to 11-000 t and surface waters averaged only 12 I~g of total P/L (Boyce et al., 1987).
Many years of research on the effects of nutrient additions on lake productivity have led to a simple model that related algal biomass (Cla in mg/m3) t2o total P. input rates (Lp in g/m d-l), mean water depth (z in m), and outflow per unit l "a5k]e surface area (Qs in m/acre); Cla = (Lv/Qs)/[1 + (z/Qs) (according to Vollenweider model) This model fit the data from most the lakes and reservoirs that had been studied in the world and accurately predicted trophic status based only on input rates of one nutrient (P). This was very strong support for the importance of P. In the eutrophication of lakes. The Vollenweider model is still widely used by lake water quality managers, partly because of its simplicity. More sophisticated models often require more data than is available for a given lake.
If we accept that P. is usually the limiting nutrient in lakes, the next question is what is the relationship between P. enrichment and primary productivity. Prairie et al. analyzed data from 133 lakes for overall chlorophyll a relationships with total N. And total P. In the surface waters. The 133 lakes were selected from a larger set so that there was an even distribution of lakes that had ratios…[continue]
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