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.