Introduction:
Phosphorus is an essential macronutrient that is often limiting 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 increase 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 recognized 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.
Several chemical extraction methods have been developed in the field of
soil fertility to estimate bioavailable P 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 differ
regarding 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 NaHCO3and is more commonly used for pH-values exceeding slightly acid values,
as it was developed for alkaline soils. Phosphorus extracted by 0.1 M
NaOH 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) equilibrated
sediment suspensions with hydoxy-Al saturated cation exchange resin and
found that the P retained by resin correlated with Selenastrum
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-paper extracted P content of runoff sediment was closely correlated
(r2=0.96) to the growth of P-starved Selenastrum
capricornutum incubated for 29d with runoff as the sole source of P
(Sharpley, 1993).
To improve phosphorus management and deal effectively with adverse
legacy effects of sedimentary phosphorus, we investigate bioavailability
of P for the aquatic ecosystem. The scope of this research was to 1)
study the bioavailability P in sediments according to several single
chemical extractions and assessment of their relationship with
P-inhibited algal growth, and 2) recognize influencing factors in P
extraction in sustaining algal growth.