Introduction:
Phosphorus is a vital macronutrient that is frequently restricting primary production in rivers and lakes. In terms of the Redfield ratio (106C:16N:1P), even a small addition of P under P-limited conditions could lead to a significant growth in the primary production of aquatic ecosystems. This may lead to eutrophication and hypoxia, a water dead zone, with significant loss of biodiversity and changes in the food web (Orihel et al., 2017; Dijkstra et al., 2018). As algae may bind nitrogen, an increase in P-availability may trigger algal outbreaks. This has been observed in many cases to occur due to excessive fertilizer and manure applications
For many aquatic systems, soil and particulate-P erosion together with runoff water are main sources of inputs of P to surface water (Brendel et al., 2019). Sediment pore water and desorption from its solid phase replenish P in overlying waters, and become available for consumption by aquatic organisms. It has long been acknowledged that soil and sediment bound P is only partly biologically available to plants and aquatic organisms as algae (Van Rotterdam et al., 2012). The bioavailable fraction of P can be evaluated by algal bioassays (Liu et al., 2016). Although algal assays are difficult to conduct and time-consuming, the bioavailable P estimated by assays is considered well to quantify the potential of sediment P to cause eutrophication (Young et al., 1985; Ekholm and Krogerus, 2003). Indeed, the quantification of bioavailable phosphorus for algal growth was shown to be essential to estimate eutrophication risks (Ellison and Brett, 2006; Okubo et al., 2012). However, it is attractive to predict bioavailable P using a chemical measure that is simple, routinely applicable, and cost-efficient. A single chemical extraction that allows to screen a large number of samples and monitor the health of aquatic ecosystems requires that the relationship between bioavailability of P from algal bioassay experiments and chemically extracted P is clear and unambiguous.
A great deal of chemical extraction approaches have been evolved to estimate bioavailable P roughly in soil and sediments such as those developed by Morgan (1954), Mehlich (1983), Bray-Kurtz (1961), Colwell (1952) and Olsen et al. (1954). Chemical extractants are based on different mechanisms and abilities to extract P and may diverge from the chemical used for extraction as well as e.g. sediment: water ratio, equilibration (shaking) time, and targeted P pools. For instance Bray-P may be useful for acid, but not for calcareous materials. Also Mehlich-P is based on extraction by acid, but performs better for high pH soils. Olsen-P extracts with NaHCO3 and is more commonly used for pH-values exceeding slightly acid values, as it was developed for alkaline soils. The well-known 0.1 M NaOH extractant is one of first methods developed to quantify algal available P (Young, 1982). Anderson and Magdoff (2005) reported that Olsen-P had the highest correlation with Selenastrum capricornutum growth in comparison with other extractants. Huettl et al, (1979) suspended sediments with hydroxy-aluminum modified cation exchange resin and detected that the P adsorbed onto resin had good magnitude of agreement withSelenastrum capricornutum (r2 = 0.98; n = 5). The well-known 0.01M CaCl2-extraction (Boekhold et al., 1993) provided excellent results for agricultural soils that are regularly limed. Sharpley (1993) used a selective extraction that was based on establishing contact between a soil suspension and Fe-oxide coated paper strips that were developed by Van Der Zee et al. (1987). The so-called Fe-oxide paper strips significantly estimated bioavailable P from runoff sediment for P-deficient Selenastrum capricornutum(Sharpley, 1993).
Rivers are the main components of Urmia Lake ecosystems and flow in circuitous path from cities, agricultural lands, industrial area, etc., and then end up in Urmia saline lake. They are also perceived as delivering nutrient to the Lake and wetlands around it by biogeochemical processes. However, eutrophication processes are becoming dominant wetlands degradation around the Lake (Arfania et al., 2018), which is a prominent habitat of migrant birds. Soil degradation is gradually becoming the main obstacle to provide ecosystem services for irresistible Urmia Lake basin. Therefore, to develop phosphorus management and deal effectively with adverse legacy effects of sedimentary phosphorus, we investigate bioavailability of P for the aquatic ecosystem. To verify our argument this study was conduct to 1) develop P bioavailability assessment in sediments according to several single chemical extractions and with P-inhibited algal growth, and 2) recognize influencing factors in P management in sustaining aquatic life and deteriorate sediment ecosystem service.