Discussion:
The interquartile range (Q3-Q1) for the OM was limited and equal to 1.15 %. The difference between the interquartile range and the range of data along with box plot of data revealed that some river sediments had OM >2.5 content (figure 3). These values are higher than background OM content of soils in the upland area and agricultural soil around the Urmia Lake basin. It seems that organic matter enrichment in parallel with anthropogenic activity in one hand, and preferential soil OM erosion and degradation in upland area due to lower density of OM in other, are the main reason to this variations. By and large, the reaction of sediments was alkaline, which might be related to higher CCE in carbonate-rich rocks erosion in the basin and ACCE which may probably be due to biogenic carbonate genesis in the aquatic conditions of Urmia Lake ‘s river.
Overall, the PCA analysis revealed that variation of sediments properties is dominated by particle size distribution, and to a slightly smaller extent by EC, ACCE and OM. Particle size distribution varied, which was partly due to differences of geomorphological features of rivers, slope of terrain, and geological structure of region. It seems that higher CCE contents in the sediment reduced the fluctuation of pH in the studied sediments; however, the negative correlation OM and EC can reduce this fluctuation.
Downstream sediments had a higher Olsen-extractable P than the threshold amount of 20 mg kg-1, indicating possible release that may adversely affect the aquatic environments (McDowell et al., 2001; Sims et al., 2002). The strongest similarity is observed between sediment Mehlich III-P and Olsen-P (figure 8). Considering that the pH of most sediments in this study was less than 7.6 and negative effect CaCO3 on P extraction, perhaps Mehlich III might tend to be less effective in higher CaCO3 contents on the P extracting solution (Kuo, 1996; Wang et al., 2015). In Mehlich III, the extraction with nitric acid, and acetic acid, lead to the dissolution of P precipitates from sediment by the generation of complexes with iron and aluminum and calcium. As fluoride and calcium form precipitates of calcium fluoride, this leads to dissolution of calcium phosphates (Kamprath and Watson, 1980). The weakest correlation was found between sediment Olsen- P and Bray-II-P, likely due to small extraction by the Bray-II P test for sediments with pH >7 or sediments with a high degree of base saturation (Sims, 2000). However, the lack of correlation between Mehlich III and Bray II was not expected, because there is a common belief that these two methods extract similar P pools from soils and sediments by cation hydrolysis (Kleinman et al., 2001; Ebeling et al., 2008; Wang et al., 2016). We attribute the poor correlation between these two methods was to the high pH of some sediment samples (pH >7.3) (Sotomayor-Ramírez et al., 2004) and a range of sediment P levels studied with Mehlich III-P <50 mg kg-1. There was a significant positive correlation between all P extractions except with Bray II and Total-P contents. Zhou et al. (2001) expressed a good correlation between extracted Total-P with Olsen-P and 0.1 M NaOH. However, cluster analysis showed that similarity between NaOH 0.1 M and Total-P in sediments was weaker than those of other extractants.
Overall, the widespread distance among the P-extractant may attributable to contribution of P pools with various release potential. Despite the fact that AB-DTPA had lowest rank order of P extraction after Bray-II, it had strange coincidence with Total-P in the processes of removing P, indicating that the AB-DTPA-extracted P might not be in favor of exclusive P pools (figure 5). Therefore, to develop comprehensive P sediment testing between terrestrial and aquatic ecosystems, we need to be mindful of contribution different P pools in eutrophication and properties of sediment governing the P bioavailability in aquatic ecosystems.
Of the seven sediment P test measurements, Colwell-P correlated best with algal growth (r2=0.92,P <0.001). One of the important reasons for the high correlation between Colwell-P and Olsen-P with algal growth is that the pH of S. capricornutum growth medium, buffered against pH fluctuation with NaHCO3 at 7.5, was much nearer chemically to the Olsen and Colwell extracts of 0.5 M NaHCO3 at pH 8.5 than the other extractants. The solid : solution ratio (1:50) and the long extraction time of Colwell-P are more similar to the S. capricornutum growth conditions (1:100), which is the plausible reason that it outperformed the Olsen-P (1:20) extraction. McDowell et al (2020) predicted that for sedimentary soil P sorption strength or capacity is low, and it takes shortest time to reach the agronomic target 20 mg L-1.
Similarly, 0.1 M NaOH had a significant relationship with algal growth. Phosphorus associated with Fe/Al-P minerals or adsorbed onto Fe and Al oxides which can be dissolved and exchanged with OH¯and soluble in bases (Wu et al., 2011). Dorich et al. (1984) also found that 0.1 M NaOH-extractable phosphorus was significantly correlated with 2-day and 14-day available phosphorus for an alga (S. capricornutum ). The high correlation between 0.1 M NaOH and algal growth indicated the sensitivity of P bioavailability to redox conditions in river system.
Morgan-P (NaActate pH = 4.8) was expected to extract P from Ca-P pools bound to the authigenic carbonate fluorapatite (CFAP) + biogenic apatite (Ruttenberg, 1992). The correlation between Morgan-P and algal growth was therefore significant. In fact, the presence of CaCO3 as a controlling solid of the activity of Ca2+ in solution is a plausible reason for lower P mobility in aquatic environment rather than that of carbonate anions, that affects P transfer in leaching column (Jalali and Jalali., 2107). Torbert et al. (2002) studied relationship of soil depth and phosphorus runoff losses in calcareous and non-calcareous soils. They denoted that P losses by runoff in calcareous soil was decreased because of the reaction of Ca2+ with orthophosphate to form insoluble Ca-P minerals. William et al. (1980) reported that absorption of P byScenedesmus quadricauda differed from 8 to 50% of total P and from 38 to 83% of nonapatite inorganic P when measured directly. The assimilation of P from Ca-P pool by Microcystis aeruginosa has been reported under sediment resuspension condition (Xiao-Fei et al, 2015).
Overall, bioavailable-P measurements were consistently high in sediments with high total-P concentrations, however, in some sediments with low Total-P content, the response was very high during the two-week growth period and even continued for a long period of time. A mechanistic explanation tells us that the presence of internal conversion of P from recalcitrant P pool to bioavailable P is another reason for occurring eutrophic condition in river system. The Colwell extractant separated sediments well against the algal population. It means that sediments with high bioavailable P had high algal growth. Some sediment P can be easily desorbed and released, particularly when the P is depleted from water column. It is expected that Colwell-extracted P would separate from potentially to immediately available P in natural condition. Therefore, the drop in water quality is common phenomenon even in sediments below the threshold values. The proportion of Colwell-P to Total-P in the sediments of rivers was the most consistent of all the extractions used.
The sediment of the present investigation had an EC of only 0.23 to 1.22 dS m-1 and our data suggest that an increasing EC might lead to larger P bioavailability and eutrophication (figure 7). This larger growth could be related with greater ionic pair’s formation of orthophosphate, but it might also be due to other factors such pollution entering the river system. Several earlier studies also showed a rise in P bioavailability with increasing salinity (Jordan et al., 2008; Hartzell et al., 2010; Li et al., 2015); however, this salinity is in the threshold that does not hinder organism survival (EC = 0.46 ± 0.28). From the negative relationship between algae growth and both CCE and pH could be deduced that these sediment properties restricted the degree of eutrophication (Figure 5). Therefore, Selenastrum capricornutum is not calcium-loving and occurs in calcareous streams with deposits of travertine and tufa. Drylie et al (2019) reported that CaCO3 dampened the effects of eutrophication-induced acidification and OM enrichment in the costal sediments. There is also a positive relationship between OM and the algae growth, which might represent the availability of organic P for the growth of algae thus decreasing resilience of river system by creating hypoxic condition in water (Kortinger et al., 2001)
The P extractants were negatively correlated with pH and the correlation was strongest with 0.1 M NaOH. Wang et al. (2012) concluded that soil pH was master variable in estimating P mobility and leaching by single P extraction indicators. The study by Arfania et al. (2018) revealed that sediment chemical properties such as electrical conductivity, organic matter, pH and calcium carbonate, had effects on the distribution and bioavailability of P pools. Calcium carbonate equivalence (CCE) was negatively correlated with Colwell and NaOH 0.1 M. Olsen, Colwell and 0.1 M NaOH extractant were positively correlated with organic matter content Jalali and Jalali. (2017) reported that pH, and EC made important contributions to most P release from leaching column, while OM and silt content was less contributed on P availability. A positive correlation of Olsen and Mehlich III with the silt fraction signified the influence of sediment particle size on P bioavailability.