**3.5 Phosphorus cycle in water and the role of suspended particles and sediments in this cycle**

In aquatic ecosystems such as lakes and rivers, phosphorus is typically present in the pentavalent form. This includes various types of compounds such as orthophosphates, pyrophosphates, long-chain polyphosphates, organophosphate esters, and phosphodiesters, as well as organic phosphates. When phosphorus enters surface water, it can become attached to solid particles in the form of phosphates and organophosphates. Through chemical or enzymatic processes, this bound phosphorus may then be released into the water as orthophosphates, which are the only form of phosphorus that can be absorbed by plants, algae, and bacteria through hydrolysis. Phosphorus is highly dynamic in water environments and is biologically active. Once phosphorus reaches surface water, it may become attached to solid particles and settle at the bottom of lakes. Microbial communities can then gradually consume the organic parts of these particles, releasing the phosphorus in the form of orthophosphate back into the water, as shown in **Figure 5**. It is important to note that phosphorus bound to solid particles and dissolved organic phosphorus are not inert in

**Figure 5.** *Schematic diagram of the phosphorus cycle between water and bottom sediments [70].*

water conditions, as they can convert into soluble orthophosphate under certain conditions.

Phosphorus that enters lakes, reservoirs, and river estuaries is absorbed by various biological structures, sediments, and living organisms and deposited in the depths (as shown in **Figure 5**). This process helps prevent the widespread distribution of phosphorus in the water, but it also renders aquatic ecosystems highly sensitive to excessive phosphorus. In oligotrophic conditions, the deposited phosphorus may remain in sediments for years. However, in eutrophic conditions, the deep water is often in an anaerobic state, which can also occur in shallow waters on warm, windless nights. In this eutrophic state, most of the phosphorus deposited in sediments decomposes and spreads into the water.

Some researchers suggest that phosphorus attached to solid particles entering surface waters does not have much effect on the growth of aquatic algae [74]. However, several reports have indicated that dissolved orthophosphates in surface waters are linked to their dynamic relationship with particulate phosphorus and sediments at the lake bottom [75]. The dynamic balance between phosphorus bound to suspended particles and dissolved phosphorus is known as a phosphate buffering mechanism in water [76]. From a kinetic perspective, dissolved phosphorus reacts with suspended particles in water during two stages. The first stage, or fast stage, occurs in less than a few seconds, while the second stage, or slow stage, takes several days. During the fast phase, phosphorus attaches to the surfaces of suspended particles, but during the slow phase, phosphorus penetrates into the structure of the particles. When runoff with suspended particles is discharged into surface waters, exchanges occur between phosphorus attached to these particles and phosphorus dissolved in the water, creating a new balance. If the concentration of soluble phosphorus in the lake water is low, phosphorus is released from the suspended sediments into the water, and vice versa.

As suspended particles settle on the lake bottom, complex conditions arise. Bacterial activities cause gradual mineralization of organic phosphorus, releasing it into the water within the pores of lake bottom sediments. In the next step, dissolved phosphorus may spread into the lake water or be absorbed by the surface of the particles (sediments) before it can spread into the water. Phosphorus bonding with aluminum

hydroxides and ferric iron on the surface of sediments can be very strong. If the water in the sediments' pores undergoes regeneration (due to biological activities), ferric iron is converted to ferrous iron, and the bond between iron and phosphorus becomes very weak, facilitating phosphorus to enter the water column [50, 77, 78]. Therefore, the amount of phosphorus exchange between the water column and bottom sediments may change seasonally.
