Phosphorus transformations in water?
To understand the processes that take place in an aquarium, it is worth looking to natural environments. For this reason, as a continuation of the discussion on phosphorus in aquariums, I will now focus on the processes involving phosphorus that occur in natural water bodies. Although these processes do not translate directly to our aquariums, they help illustrate just how fascinating an element phosphorus is. You will learn how phosphorus transformations in water depend on a wide range of chemical, physical, and biological factors.
Sources of phosphorus in natural waters
On the one hand, phosphorus is essential for the functioning of living organisms; on the other, its excess in aquatic environments leads to eutrophication and changes in the functioning of entire ecosystems, including shifts in the plant and animal species that inhabit them.
Agriculture is considered the main source of phosphorus in aquatic environments. Phosphorus is a component of mineral fertilizers as well as plant protection products, and it enters surface waters mainly through surface runoff from agricultural areas. A very large phosphorus load also comes from treated domestic and municipal wastewater, not to mention discharges of untreated sewage into surface waters. Other sources of phosphorus include atmospheric deposition, soil erosion, and the decomposition of plant and animal remains.
Bottom sediments in water bodies
In aquatic environments, phosphorus accumulates in various components; however, bottom sediments play a key role in its transformation. It is estimated that nearly 90% of the total phosphorus in a lake ecosystem is found in the surface layer of sediments, down to a depth of about 10 cm.
Bottom sediments can function either as a sink for phosphorus or as its source, especially in highly eutrophic water bodies. Whether phosphorus is released or accumulated depends on many interrelated biological, physicochemical, and physical factors. These factors are further influenced by the morphology of the water body itself, its surface area, width, length, shoreline development, and depth. Shallow water bodies are more susceptible to internal nutrient loading than deeper ones. In deep, thermally stratified reservoirs, water layering and the frequency of mixing also play an important role. Like it or not, I need to explain the concept of stratification here. I think it is such an interesting phenomenon that you will read about it with curiosity.
Thermal stratification of lakes and the phosphorus cycle
Water temperature in water bodies depends not only on climatic conditions but also on basin depth and exposure to wind, which affects how water masses are mixed. In deeper water bodies, temperature decreases from the surface to the bottom, forming three distinct layers:
- Epilimnion – the temperature in this layer is relatively uniform due to wind-driven mixing, and photosynthesis occurs in the illuminated zone.
- Metalimnion – a transitional layer, also called the thermocline. Within this layer, a very sharp decrease in water temperature with depth is observed. It acts as a barrier, preventing mixing between the upper and lower parts of the lake.
- Hypolimnion – in temperate climates, the water in this layer usually comes into contact with the surface only in spring and autumn. For the entire lake to mix, the temperature and density of the water in the epilimnion and hypolimnion must equalize, causing the thermocline to disappear. When mixing does not occur, nutrients accumulate in the hypolimnion and oxygen deficits often develop.
In stratified lakes during summer stagnation, phytoplankton develops in the illuminated epilimnion. During this period, in clean water bodies, phosphate concentrations during are often very low or even undetectable, as they are completely taken up by algae, cyanobacteria, and higher plants. Dying phytoplankton is decomposed in situ, because circulation within the epilimnion and the presence of the thermocline hinder its sinking to the bottom. The phosphates released in this way re-enter circulation, and this cycle can repeat several times within a single season.
Of course, not all phosphorus remains in the epilimnion. Some reaches the bottom sediments as a result of organic matter sedimentation, adsorption, and the precipitation of insoluble phosphates onto settling clay particles.
Factors affecting phosphorus exchange between water and bottom sediments
The exchange of phosphorus between water and bottom sediments depends on conditions at the sediment–water interface, including:
- oxygen concentration,
- temperature (exchange is faster at higher temperatures),
- pH (exchange is faster at lower pH),
- redox potential,
- sediment type,
- activity of living organisms.
Sediments differ in their sorption capacity. Those rich in organic matter and clay minerals can bind more phosphorus. In aquariums, phosphorus binding may occur in tanks with active substrates or in low-tech aquariums where garden soil has been used.
Oxygen
In interstitial water, water filling the spaces between sand and gravel grains up to about 2 mm in size, under anoxic conditions, the concentration of dissolved phosphorus is several orders of magnitude higher than in the water above the sediment. To equalize concentrations, phosphorus diffuses from the sediments into the overlying water. If oxygen is present there, the phosphorus diffusing from the sediment encounters iron in its oxidized form (Fe³⁺) and forms insoluble complexes. Under oxygenated conditions, bottom sediments therefore act as a phosphorus trap. This phenomenon can certainly occur in aquariums.
The situation changes dramatically when the oxygen concentration in the water above the sediments drops to around 0.1 mg/L. At the same time, redox potential decreases. Under these conditions, iron is reduced from Fe³⁺ (insoluble in water) to Fe²⁺ (soluble), causing the insoluble phosphorus complexes formed under aerobic conditions to break down and release phosphorus into the water. In other words, under anoxic conditions, phosphorus is released from the sediments.
You’ve made it this far. Can you already see how remarkable an element phosphorus is, and how complex its transformations in aquatic environments truly are?
Anoxic conditions in the hypolimnion are common, because oxygen in the near-bottom layer is gradually depleted as a result of decomposition processes carried out by microorganisms. As you already know, hypolimnetic waters can be reoxygenated only during periods when the entire lake mixes, typically twice a year in temperate climates. Oxygen depletion causes the release of phosphorus accumulated in the sediments into the overlying water. During mixing, this phosphorus is transported to the photosynthetically active zone near the surface. This leads to blooms of algae and cyanobacteria, which, upon dying, supply bottom sediments with another dose of organic matter. This, in turn, further depletes oxygen and releases additional portions of phosphorus.
Bacteria
The breakdown of organic phosphorus by bacteria plays a very important role in releasing phosphorus into the water. This process can occur under both aerobic and anaerobic conditions and begins already in the water column during the sedimentation of organic particles. Different phosphorus compounds vary in their susceptibility to decomposition, with those bound to humic acids being particularly resistant. Bacteria can also store excess phosphorus in the form of polyphosphates; however, under stress conditions, such as sudden changes in water parameters or cell death, they can release it rapidly into the water.
High bacterial activity in bottom sediments leads to a decrease in oxygen concentration. Interestingly, even in oxygenated environments, anaerobic microzones can form, where aerobic and anaerobic decomposition processes occur simultaneously.
Microorganisms in an aquarium are our allies, they are responsible for the biological stability of the system. If plants are present, they also help maintain balance by utilizing nutrients released by bacteria. It is important to remember, however, that both organic matter and the products of its decomposition accumulate in an aquarium. This is why regular water changes are necessary, as they provide a simple way to remove excess compounds resulting from organic matter breakdown. Filters should also be cleaned periodically, and the substrate vacuumed. The more accumulated organic matter there is in an aquarium, the more oxygen bacteria consume to decompose it. This can pose a serious threat, especially during a power outage, in aquariums with large amounts of sediment, oxygen is depleted very quickly, and fish begin to suffocate.
Algae and higher plants
In shallow as well as deeper water bodies, the bottom can be colonized by algae. They take up phosphorus from bottom sediments and release it into the water when they die. Dying higher plants also contribute a certain pool of phosphorus to the sediments. During the growing season, an oxygenated zone with a higher redox potential forms around plant root systems, inhibiting anaerobic processes. Aerobic bacteria capable of decomposing organic matter can develop in this zone. Water pH may also change, either increasing or decreasing, depending on the dominant processes. All of this affects nutrient cycling, including that of phosphorus.
Animal organisms
Bottom-dwelling animals, including fish, disturb sediments as they forage, increasing sediment oxygenation and accelerating the mineralization of organic matter and the release of phosphorus. Mechanical resuspension of sediment particles further facilitates the release of phosphorus and other nutrients. For this reason, any disturbance of the substrate in an aquarium results in the release of organic matter and its decomposition products into the water. Consequently, vacuuming active substrates or those containing soil or nutrient-rich layers beneath gravel or sand is generally not recommended.
Water pH
An increase in the pH of the overlying water reduces phosphorus binding by iron and aluminum hydroxides, even in the presence of oxygen. Conversely, the mineralization of organic matter in sediments, under both aerobic and anaerobic conditions, leads to a decrease in pH due to CO₂ production. Low pH promotes the dissolution of carbonates and the release of phosphorus bound to them.
I am convinced this is the moment when you may have felt lost, with your brain trying to assemble all of this into a coherent whole. Don’t worry, you are not alone. It is enough to remember that phosphorus transformations in water consist of a series of complex processes dependent on a vast number of factors. That is why there are no simple, clear-cut answers.
Temperature
Temperature can be considered the primary factor controlling nutrient cycling in a lake. Biological processes such as nutrient assimilation, including phosphorus microbial activity, and bioturbation (sediment mixing by benthic organisms) all depend on it. Naturally, higher temperatures intensify all of these processes.
The impact of eutrophication on fish
By contributing to eutrophication, phosphorus strongly affects fish habitats. Above all, eutrophication leads to reduced oxygenation of the water. Fish, as mobile organisms, can seek areas with higher oxygen concentrations, for example by moving from deeper parts of a water body to shallower ones. However, this shift entails many consequences related to predation, changes in water temperature, food availability, and reproduction. Deteriorating water quality and oxygen conditions also affect aquatic plants and invertebrates, which form the food base for fish.
Conclusion
And so we have reached the end. I hope you now understand why I wrote in the opening paragraph that phosphorus is a fascinating element. It is involved in a great many reactions and processes, and even a basic understanding of how phosphorus behaves in aquatic environments allows for a much better understanding of how an aquarium functions.
Literature
- Boström B., Andersen J.M., Fleischer S. and Jansson M. (1988), Exchange of phosphorus across the sediment – water interface. Hydrobiologia, 170: 229-244.
- Forsberg C. (1989), Importance of sediments in understanding nutrient cyclings in lakes., Hydrobiologia 176/177, s. 263-277.
- Golterman H.L. (1995), The role of ironhydroxyde-phosphate-sulphide system in the phosphate exchange between sediments and overlying water. Hydrobiologia, 297: 43-54.
- Harper D. (1992), Eutrophication of freshwaters, Chapman & Hall, Great Britain.
- Hupfer M., Lewandowski J. (2008), Oxygen Controls the Phosphorus Release from Lake Sediments – a Long-Lasting Paradigm in Limnology. Internat. Rev. Hydrobiol. 93, 4-5, s. 415-432.
- Jones J.G. (1982), Activities of aerobic and anaerobic bacteria in lake sediments and their effect on the water column In: Sediment Microbiology Nedwell D.B. Brown C.M., (Eds), s. 107-145 Academic, London.
- Kentzer A. (2001), Fosfor i jego biologicznie dostępne frakcje w osadach jezior różnej trofii, Wydawnictwo Uniwersytetu Mikołaja Kopernika, Toruń, Rozprawy.
- Kleeberg A., Herzog Ch., Hupfer M. (2013), Redox sensitivity of iron in phosphorus binding does not impede lake restoration. Water Research 47: 1491-1502 (4): 177-182 [http://dx.doi.org/ 10.1016/j.watres.2012.12.014].
- Lampert W., Sommer U. (1996), Ekologia wód śródlądowych, Wydawnictwo Naukowe PWN: Warszawa.
- Søndergaard M., Jensen J.P., Jeppensen E. (2001), Retention and internal loading of phosphorus in shallow, eutrophic lakes. Review Article, The Scientific World, 1: 427-442.
- Wang H., Appan A., Gulliver J.S. (2003), Modelling of phosphorus dynamics in aquatic sediments: Imodel development. Water Research, 37 (16): 3928-3938 [doi: 10.1016/S0043-1354(03)00304-X].
She studied environmental protection (specializing in surface water protection) at the Agricultural University of Wrocław (now the University of Life Sciences), where she later obtained her PhD at the Faculty of Biology and Animal Husbandry.
Her research focuses on the effects of diet, supplementation, and natural additives on the health, condition, and reproduction of aquarium fish.
In her free time, she cares for her fish and cats, tends to plants, enjoys hiking, and stays active through sports.
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