The effect of natural organic matter (NOM) on the distribution and resources of mobile phosphorus in the bottom sediments of small retention reservoirs

The purpose of the work detailed here was to determine the impact of the distribution of natural organic matter and grain size on the resources and distribution of mobile phosphorus in the bottom sediments of small reservoirs located in catchments subject to different levels of anthropopressure. The research objects were five small reservoirs located in southeast Poland. In small retention reservoirs, it is the anthropogenic impact of the catchment and its geological structure, and to a lesser extent sediment granulation, that determine the distribution of phosphorus fractions (especially in inorganic compounds) and the share of total P they account for. In general, a higher level of contamination of sediments with organic matter (OSION increase) is shown to be associated with a higher content of the organic fraction of phosphorus and easily decomposable organic matter. Within small retention reservoirs under significant anthropogenic influence from the catchment, sandy sediments compared with silty sediments were significantly less loaded with potentially mobile phosphorus as well as with organic forms of phosphorus and (sometimes) nitrogen. The occurrence of humic-metal-phosphate complexes is determined by there being sufficient organic matter, especially humic fractions. Fractions of inorganic phosphorus compounds and organic matter have the terrigenous source of these pollutants in common. The organic sediment index can serve as an adjunctive indicator with which to assess the potential ability of the sediments in small retention reservoirs to internal supply of the water column in nutrients.


Introduction
Natural organic matter (NOM) accumulating in the bottom sediments of bodies of water consists mainly of fresh or partially decomposed plant and animal remains, metabolic conversion products of microorganisms, humic substances and other organic compounds belonging to many groups such as carbohydrates, carboxylic acids, amino acids, lipids, esters, dyes and lignins (vanLoon and Duffy 2007).Bodies of water are entered not only by the matter of natural origin but also by organic substances relating to human activities (including chemicals that are difficult to decompose and environmentally hazardous) (Hiller et al. 2009).In line with source, a distinction is drawn between material of allochthonous origin (terrestrial or terrigenous) and (autochthonous) matter produced inside the reservoir in the course of processes of primary and secondary production (Cieśla et al. 2020;Kurek et al. 2020;Li et al. 2018;Wauchope-Thompson et al. 2022).
It is possible for humic substances (HS) to account for even 90% of the total amount of organic carbon in the bottom sediments of lakes (Lipczyńska-Kochany 2018; Yuan et al. 2020).HS present in bottom sediments may be of aquatic (autochthonous) or edaphic (allochthonous) origin.The latter reflects the possibilities for even significant amounts of out-washing by surface runoff in a dissolved form (as humin and fulvic acids), or as suspended with soil particles (humins), where the sources are humus-rich soils, forest areas (forest litter) and peat bogs (Krupińska 2012;Smal et al. 2015).Given their molecular structure including 107 Page 2 of 13 a large number of functional groups, humic substances form connections with metals in the form of salts and complex compounds and have a high sorption capacity for both water and other chemical compounds (Ukalska-Jaruga et al. 2015).
The most mobile and most easily degradable fraction is soluble organic matter, which includes the residues of microorganisms at various levels of decomposition, and easily decomposing non-humic organic substances such as carbohydrates, polysaccharides, proteins, organic acids, amino acids, waxes and fatty acids (Poirier et al. 2005).Organic matter formed especially in forest catchments is less susceptible to decomposition and the release of mineral forms of nitrogen and phosphorus (Joniak 2018).Compared to land-based soluble OM, that leached from dead algae debris is considered to be decomposed more readily (Berberich et al. 2020;Gruca-Rokosz et al. 2020).Naturally occurring organic compounds are generally readily degradable biochemically.The exception concerns humic substances, which, due to their complex structure and high molecular weight, decompose very slowly.While the HS in forest soils may be > 200 years old, those in rivers and lakes are younger, though still perhaps 40 years of age (Puczko et al. 2017).Hard-to-decompose substances include many of anthropogenic origin (mainly organochlorine pesticides, PCBs, dioxins and PAHs) (Hiller et al. 2009;Ukalska-Jaruga et al. 2015).
A major ecological problem for bodies of water of various sizes is that posed by internal supply in nutrients originating in sediments.The intensity and further progress of their trophic degradation often depends on the intensity of this supply, especially when remaining external sources are largely eliminated (O'Connell et al. 2020;Lee and Oh 2018;Tammeorg et al. 2022;Yuan et al. 2020).An important mechanism for the internal supply of waters in nutrients is the biochemical decomposition of the organic matter contained in sedimenting particles and deposited in the bottom sediments (Aeriyanie et al. 2021;Hou et al. 2014;Zhang et al. 2022).As a result of aerobic decomposition, nutrients from such sources are introduced into the water in the form of phosphate (PO 4 3− ), nitrate (NO 3 − , NO 2 − ) and ammonium (NH 4 + ) ions.In the deeper sediment layers, organic matter is decomposed by fermentation because of a lack of oxygen.Mineral substances, including phosphates and ammonia, are then released into the deoxygenated interstitial water, from where they diffuse into the overlying water (Golterman 2005;Lee and Oh 2018;Wang et al. 2018).The processes of the oxidation of organic matter and release of nutrients at the sediment-water interface are both influenced by various conditions and environmental factors, such as oxygenation of water, the redox potential of sediments, temperature of water and sediment, composition of bottom sediments, quantity and quality of deposited organic matter, structure of benthic communities and depth of water (Lee and Oh 2018).The internal supply problem is more acute in the case of non-stratified bodies of water.On the one hand, a lack of thermal stratification promotes better oxygenation of the overlying water, while on the other, bottom sediments over the entire bottom surface take part in the internal supply process, while the mixing of water layers facilitates the diffusive transport of nutrients to the euphotic zone (Du et al. 2022;Tammeorg et al. 2022;Zamparas and Zacharias 2014).In addition, a common phenomenon in shallow lakes and dam reservoirs is the resuspension of bottom sediments, caused inter alia as waves are whipped up by the wind.For the time that sediment particles return to suspension, the surface area available for the exchange of substances between the sediment and water is greater, intensifying processes of exchange (release or retention), including in the oxidation of organic matter (Lee et al. 2019;Li et al. 2023;Wang et al. 2018).Much attention is paid to the release of nutrients into water due to their key role in advancing the trophic degradation of aquatic ecosystems (Yuan et al. 2020;Wang et al. 2022;Wang et al. 2024;Wauchope-Thompson et al. 2022).
The purpose of the work detailed here was to determine the impact of the distribution of natural organic matter and grain size on the resources and distribution of mobile forms of phosphorus in the bottom sediments of small reservoirs located in catchments experiencing different levels of anthropopressure, given the potential impacts regarding internal supply and further trophic degradation.A further relevant process here may be climate change, resulting in more prevalent weather events of an extreme nature, such as record heat, droughts or heavy rains and floods.In the face of that, the most basic and best form of limiting surface runoff of water in general and rainwater runoff in particular-into seas and oceans-is the construction of retention reservoirs.As objects supplied with low-level dams, such small retention reservoirs are considered environmentally harmless, unlike their much larger counterparts.There may be an estimated 16M + small reservoirs in the world as a whole (Mulligan et al. 2020).However, these objects are particularly susceptible to degradation, given their limited depths and consequent lack of thermal stratification.These facts, combined with serious problems with reclamation, encourage further research in this area.

Study area and sampling strategy
The research objects were five small retention reservoirs located in SE Poland (Fig. 1).Basic data on these reservoirs and their catchments are presented in Table 1, with the main differences seen to lie in the period of exploitation, time of retention of water and kind of management applied in the catchment area.Sediment samples were taken four times in 2013 and five times in 2014, between May and October (making a total of 9 samples).In Kamionka Reservoir, sediments were collected from three designated sites (near the inflow, at the centre and beside the dam) (Fig. 1).Otherwise, there were two designated sampling sites per water body (near the inflow and beside the dam).Laboratory testing was done on the upper (0-5 cm) layer of the sediment in each sample (three samples per site).Following averaging, samples were dried at 60 °C for 48 h, before being homogenised.

Sediment analysis
Sediment grain size analysis was performed using the sieveareometric method (Myślińska 2016).The content of the boundary fractions for particles of diameters 0.002-0.063mm was determined by areometric analysis.The fraction with particle diameters over 0.063 mm was determined by sieve analysis (Bartoszek et al. 2022).Content of organic matter (OM) was calculated for residue, following calcination of the sediment sample at 550 °C for 4 h.Sediment mineralisation was carried out in a microwave mineraliser  (UniClever II, Plazmatronika, Poland) in concentrated HNO 3 and at a high pressure of 2 to 4.5 MPa.Mineralisate contents of metals (iron, aluminium, manganese and calcium) were determined using an ICP spectrometer (Integra, GBC), while levels of phosphorus were determined spectrophotometrically on an Aquamate spectrophotometer (Thermo Spectronic).Organic matter fractionation included determination of the contents of fulvic acids (FA), humin acids (HA), humins (HU), total humic substances (HS) and easily decomposable organic matter (EDOM).The fractionation of humic substances in sediments was performed using the modified Griffith-Schnitzer method (Bartoszek et al. 2022;Griffith and Schnitzer 1975;Silva et al. 2016).The content of organic carbon (C org .)and total nitrogen (N tot .) in the sediment samples and the organic carbon of the HU fraction in the post-extraction residue were determined using a CNS elemental analyser of the Flash EA 1112 model (Finnigan Mat).The results regarding HS, OM and particle size have already been used in another work, in the context of the impact on the accumulation of heavy metals (Bartoszek et al., 2022).The content of easily decomposable (i.e.hot water extractable) organic matter was determined in line with the modified method used for soils (Strosser 2010); 2.5-5.0 g of the sediment was mixed with 50 cm 3 of hot water at 80 °C and heated in a water bath for 1 h.The carbon content of easily decomposable organic matter (EDOM) fraction was determined in the extract obtained in this way.Phosphorus fractionation in sediments was performed using the SMT (standards, measurements and testing) method.The appropriate scheme allows for the extraction from sediments of non-apatite, inorganic phosphorus, i.e. the NAIP fraction (in combination with iron, manganese and aluminium), apatite phosphorus (AP) (in compounds with calcium) and organic phosphorus (OP) (Bartoszek and Tomaszek 2008;Gonzáles Medeiros et al. 2005).Mobile phosphorus is that in a form moving easily between sediment and water, and vice versa, meaning constant circulation in the aquatic environment.Bioavailable phosphorus is in a form (phosphate ions) absorbed easily by aquatic organisms.Under the SMT method, mobile phosphorus is considered to be in compounds with iron, manganese and aluminium (the NAIP fraction) as well as organic compounds (as OP).
Internal laboratory reference materials (LRM) were used for quality control, in which an accredited laboratory determined the total organic carbon content of HS and the total phosphorus content as part of comparative inter-laboratory tests.The recovery of the fractions obtained via the sequential extraction technique was evaluated by comparing the respective sums for the fractions with the total content specified in the internal standard.Five control samples yielded recoveries of 91.4-107.5% for HS and 84.4-95.3%for P tot .
The Organic Sediment Index (OSI ON )-after Ballinger and McKee (1971)-was determined as the product of the total organic-carbon content, C org .[%] and organic nitrogen content, N org .[%] in the bottom sediment (Ballinger and McKee 1971;Hou et al. 2014;Mansour 2020).The organic nitrogen content in sediments was in turn calculated from the 0.95•N tot .product (Hou et al. 2014).
OM enrichment/contamination of the bottom sediments of the analysed reservoirs was assessed via a classification developed for Polish dam reservoirs by reference to the value of the OSI index (OSI ON ).In line with that: level-I enrichment/contamination with organic matter corresponds to OSI ON values < 0.05, denoting an unpolluted sediment, whereas with level II (relatively uncontaminated) 0.05 ≤ OSI ON < 0.5, with level III (slightly contaminated) 0.5 ≤ OSI ON <1.5, with level IV (heavily polluted) 1.5 ≤ OSI ON <5.0 and level V (very heavily polluted) OSI ON ≥ 5.0 (Bartoszek 2019).

Estimation of phosphorus and nitrogen resources
The resources of potentially mobile phosphorus and nitrogen accumulated in (the upper 5 cm layer of) bottom sediments of the studied reservoirs were estimated by reference to the calculated average contents for the parameters mentioned above (average for three sites in the case of Kamionka Reservoir and two sites each in the remaining waters), in line with the formula as follows: where R is the estimated resource of the phosphorus or nitrogen fraction accumulated in bottom sediments (in the 5 cm uppermost layer) [t], A r is the surface area of the reservoir Sediment density and water content were determined using the gravimetric method: 1.67 t•m −3 ; 33.9% for Kamionka, 1.72 t•m −3 ; 30.7% for Cierpisz, 1.53 t•m −3 ; 42.5% for Brzóza Królewska, 1.21 t•m −3 ; 68.0% for Nowa Wieś, 1.40 t•m −3 and 51.2% for Blizne (average for reservoir, respectively).

Statistical analysis
Mean values for determined parameters and associated standard deviations were calculated using Microsoft Excel 2019 for Windows 10 (Microsoft Corp., Redmond, WA, (1) Page 5 of 13 107 USA).The relationships between the parameters (OM fractions and phosphorus fractions) were evaluated using a simple regression analysis with a predetermined significance level of 0.05.Use was also made of the Statistica 13.0 PL programme (from StatSoft Poland).

Results and discussion
Enrichment in organic matter and granulometry In the process of nutrient circulation in the aquatic environment, a key role is played by both the amount and type of organic matter accumulated in bottom sediments, with vulnerability to biochemical decomposition then being of particular importance (Ukalska-Jaruga et al. 2015).The sediments of dam reservoirs-as ecosystems with features intermediate between those of rivers and lakes-are generally characterised by a low content of organic matter, rarely exceeding 20% (Berberich et al. 2020;Lee and Oh 2018;Martins et al. 2021).In the small retention reservoirs analysed here, the total OM content ranged from 1 to 14% and was to some extent related to the content of the sand fraction (Table 2, Fig. 2).The share of the organic carbon accounted for by humic substances was in the range 59.2-94.7%, with humins usually being the dominant fraction (HU in the range 1.24-24.3mg•g −1 dw).Exceptions here were the sediments of the Cierpisz and Brzóza Królewska Reservoirs near the tributary (BK1).
The sediments of the Nowa Wieś Reservoir, with its catchment supporting numerous wetlands and peatlands, proved to be the most enriched in organic matter (12 and 14% NW1 and NW2, respectively), HS (37.9 and 39.4 mg•g −1 dw), FA (6.77 and 7.25 mg•g −1 dw) and HU fractions (24.1 and 24.3 mg•g −1 dw) as compared with the sediments of the other reservoirs.In these sediments, silt was the dominant particle size fraction, at 50% (Fig. 2).The sand fraction was about 40% and the clay fraction about 10%.The predominance of clay soils, small forested areas and a large share of agricultural land, combined with a catchment urbanised to only a limited degree, ensured that the sediments of Blizne Reservoir+ are clearly distinguishable from the others.These sediments were less enriched in organic matter (7.16 and 5.83% B1 and B2, respectively), with a higher content of the fractions of silt (average 67%) and clay (19%).Within the clay fraction, there may be clay minerals, hydrated iron and aluminium oxides, finely divided primary minerals and organic matter (vanLoon and Duffy 2007).In addition, these were poor in humin acids, though they were not sandy (the sand fraction being of about 13%).The share  of HS in C org . in these sediments was also the lowest (59.2 and 77.1% of B1 and B2, respectively).The sediments of the other reservoirs-with the dominant share in the sand fractions-were generally characterised by a lower (1.00-2.75%)content of OM, as well as HS and FA and HU fractions, with the exception of the sediments near the tributary of the Brzóza Królewska Reservoir (BK1) with their relative enrichment in organic matter (at 6.48%) but also a slightly lower sand content of 77.8% (Fig. 2, Table 2) (Bartoszek et al. 2022).
Considering trophic degradation, easily degradable organic matter is of greatest importance to the aquatic ecosystem, as it is the main source of internal aqueous supply with phosphates and mineral forms of nitrogen.The carbon content of the EDOM fraction extracted with hot water points to the enrichment of bottom sediments with organic matter that can be easily decomposed by microbes (Strosser 2010).The bottom sediments of the reservoirs studied have relatively low contents of easily decomposable organic matter (in the range 0.17-1.41mg•g −1 dw) (Table 2).The highest contents of the EDOM fraction (1.38 and 1.41 mg•g −1 dw, respectively) were observed in the sediments of the Nowa Wieś Reservoir, which were most enriched in total OM.In the sediments of the remaining reservoirs, EDOM was usually at half the level or less.In the sediments of the Nielisz dam reservoir (also Poland), the average EDOM content (depending on zone) ranged from 0.39 to 1.01 mg•g −1 dw, while the HS ranged from 4.85 to 18.1 mg•g −1 dw (Gruca-Rokosz et al. 2020).In the bottom sediments studied in this case, the share of the EDOM fraction did not exceed 10% of organic carbon, with the range being 2.22-9.57%C org .The carbon of soil-labile fractions makes up 3-8% of C org .(Mclauchlan and Hobbie 2004).The low EDOM content may be related to the limited supply of matter from the catchment and the internal production of the reservoir but may also result from increased microbial activity and intense mineralisation (Hilli et al. 2008).Following its deposition in sediments, easily degradable organic matter undergoes continuous rapid decomposition via biotic and abiotic pathways, leaving stable aromatic plant material and lipids (Kurek et al. 2020).Organic matter mineralisation occurs to a greater extent in sandy sediments than in clay ones, with the former supporting a greater number and diversity of microorganisms.In clay sediments, the small diameter of the pores limits the life processes of microorganisms.In the aquatic environment, easily decomposing organic matter provides microorganisms with energy and nutrients, the excess being released into the water, where it has the effect of promoting phytoplankton growth (Wauchope-Thompson et al. 2022).
A classification based on the values of the Organic Sediment Index developed by Bartoszek (2019) was used to assess the degree of enrichment/contamination of the bottom sediments of the small retention reservoirs studied with organic matter.Application of the criteria led to the sandy bottom sediments of the reservoirs at Kamionka, Cierpisz and Brzóza Królewska (near the dam station BK2)-with their very low (0.02-0.25) values for the OSI ON index-being classified in the unpolluted Group I (in the case of BK2 and C2) or the "relatively uncontaminated" Group II (K1, K2, K3 and C1) (Table 2).The sediments of Brzóza Królewska Reservoir near the tributary (station BK1) (OSI ON = 1.15) and the sediments at both sites in Blizne Reservoir (OSI ON = 0.59 and 0.71) were assigned to the "slightly polluted" Group III.The highest values for the OSI ON index (of 1.77 and 1.58, respectively) characterised the sediments of Nowa Wieś Reservoir and resulted in their assignment to the "heavily polluted" Group IV.In comparison, OSI ON values for the sediments of Mariout Lake (Egypt) were found to range from 0.41 to 0.87 (near the sewage drain) (Mansour 2020), while Bargiela and de Iorio (2015) calculated the Organic Sediment Index to assess the degree of organic-matter contamination in the sediments of two South American rivers-obtaining a significant dispersion of OSI index values from 0.13 to 8.18 for sediments collected in various river cross sections.The sediments of Polish rivers contain up to 10% organic matter (Madeyski et al. 2008), with this explicable in terms of the unfavourable conditions for matter sedimentation due to turbulent movement and biodegradation processes taking place more intensively in better oxygenated river waters.

The distribution of phosphorus in sediments
Bottom sediments can be an important source of internal supply in nutrients.As a result of the decomposition of the previously deposited organic matter, even in conditions of good oxygenation of the bottom zone, mineral forms of nitrogen and phosphorus are released into the water (Aeriyanie et al. 2021; Søndergaard and Jeppesen 2020).The stability of phosphorus accumulation in bottom sediments depends on the type of chemical compounds in which it occurs (Bartoszek and Koszelnik 2016;Tammeorg et al. 2022;Wauchope-Thompson et al. 2022).In the bottom sediments of four of the reservoirs studied [Kamionka, Cierpisz, Brzóza Królewska and Nowa Wieś (both sites)], NAIP was the dominant fraction of phosphorus in terms of content and ranged from 0.035 to 1.87 mg•g −1 dw (Table 3).Its percentage shares in P tot . in these sediments ranged from 43.8% (C2) to 71.1% (NW2) (Fig. 3).On the other hand, in Blizne Reservoir, the lowest contents noted for the NAIP fraction (of 0.065 and 0.098 mg•g −1 dw; 18.2 and 26.1% P tot .)were found in sediments in relation to the remaining phosphorus fractions.The distributions of the remaining fractions of P were diversified.In the sediments of Cierpisz and Brzóza Królewska Reservoirs, as well as the dam area at Kamionka, the lowest share of P tot .was accounted for by the AP (apatite) fraction.Equally, in the sediments of the near-tributary and middle parts of Kamionka Reservoir, as well as Nowa Wieś Reservoir, it was the organic fraction of phosphorus (OP) that proved to be most limited.
The sediments of Nowa Wieś Reservoir were ones most enriched in both total P and its individual fractions.The lowest content of total phosphorus and its fractions were in turn found in the sediments near the dams of Cierpisz and Brzóza Królewska Reservoirs (C2 and BK2).The distribution of phosphorus and its fractions did not always coincide with the share of individual granulometric fractions (Fig. 2).Sediments with a large (77.8-93.7%)sand fraction were poorer in terms of the contents of phosphorus and its different fractions, but only in comparison with Nowa Wieś Reservoir (sand fraction < 41%).Sandy sediments were also poorer in terms of their contents of iron, manganese, aluminium and calcium, i.e. metals forming sparingly soluble inorganic compounds with phosphorus (Table 3).
The different distribution of phosphorus in the sediments of Blizne Reservoir did not relate to grain size distribution.The sand fraction in these sediments was only about 13%, while the silt fraction was dominant (at 63.1 and 70.5%); even so, the contents of phosphorus and its fractions were not the highest, and the dominant fraction was apatite (AP 45.2%) near the tributary (B1) and organic P (OP = 38.3%) in the vicinity of the dam (B2) (Fig. 3).The bottom sediments of Blizne Reservoir distinguished it from other bodies of water given the highest contents of calcium (61.9 and 45.6 mg•g −1 dw).The content of organic phosphorus (OP) in the sediments ranged widely from 0.018 to 0.320 mg•g −1 dw (Table 3).The highest contents of OP (0.254 and 0.320 mg•g −1 dw) were found for the sediments of Nowa Wieś Reservoir, as were P tot ., OM, EDOM, etc.Although the sediments of this reservoir were the most enriched in OM, OP accounted for the smallest (13.0 and 12.5%) shares in P tot ., in relation to the remaining fractions (Fig. 3).The secondhighest OP contents in sediments were manifested by Blizne  Reservoir (0.113 and 0.147 mg•g −1 dw), which was deemed to be slightly polluted with organic matter (by reference to the OSI ON value).In sandy sediments, the OP fraction did not exceed 0.1 mg•g -1 dw.Organic phosphorus is commonly found in reservoirs.It occurs in phytoplankton, emergent and submerged macrophytes, particulate matter, bottom sediments and atmospheric deposits, and it can account for 46-70% of total P in phytoplankton and aquatic macrophytes (Ni et al. 2022;Wang et al. 2022).In sediments from Finnish lakes with OM contents in the 11-32% range and P tot .equal to 0.99-2.30mg•g −1 dw, the OP fraction ranged from 0.27 to 0.54 mg•g −1 dw (Tammeorg et al. 2022).In lakes, organic phosphorus may constitute the largest pool of sedimentary phosphorus, at up to 50% of P tot .(O'Connell et al. 2020).The higher content of NAIP is characteristic for the sediments of reservoirs with strong anthropopressure in the catchment area, as the main sources of this fraction are sewage and surface runoff (Ruban et al. 2001).In sediments enriched with iron, aluminium and manganese, compounds may arise with phosphorus from the decomposition of organic matter within the reservoir.Examples of reservoirs with a strong anthropopressure of the catchment area are Bort-Les-Orgues (France)-with respective contents of the NAIP, AP and OP fractions equal to 1.58, 0.40 and 0.65 mg•g −1 dw as well as Rzeszów (Poland)-with NAIP, AP and OP fractions, respectively, of 0.34, 0.27 and 0.17 mg•g −1 dw (Bartoszek et al. 2020;Ruban et al. 2001).Ca-P compounds (AP) are considered a stable fraction not participating in the release of P, especially in the case of unstratified shallow reservoirs of enriched trophic status, which usually experience alkalisation of the waters (Tammeorg et al. 2022).A greater content of the AP fraction is observed in the shallower zones of the reservoirs, where conditions are more favourable to the precipitation of CaCO 3 and, with it, phosphates (Bartoszek and Tomaszek 2008;Perrone et al. 2008).According to Kentzer (2001), a higher value for the AP fraction is associated with a higher trophic level of bodies of water because of progressing alkalisation of waters.However, the content of the AP fraction does not always correlate with the calcium content in sediments as well as the latter correlates with trophic level (Bartoszek and Tomaszek 2008).A dominant share for the apatite fraction has been observed in the sediments of mesotrophic lakes and dam reservoirs (Bartoszek and Tomaszek 2008;Kaiserli et al. 2002).Sediments of the mesotrophic Solina Reservoir-located in a weakly urbanised catchment of pastures and forestscontained NAIP, AP and OP at respective concentrations of 0.21, 0.32 and 0.29 mg•g −1 dw (Bartoszek and Tomaszek 2008).The apatite compounds were also dominant (at 57.5% P tot .) in the sediments of the mesotrophic Besko Reservoir located in an agricultural forest catchment (Piwińska et al. 2018).Four of the reservoirs analysed here are in essence eutrophic, with only Nowa Wieś Reservoir characterised by hypertrophy of the water (Bartoszek 2019;Bartoszek et al. 2017).The highest content of AP was found in the Nowa Wieś sediments but a largest share for the apatite fraction within P tot characterised the sediments of Blizne Reservoir.According to Ruban et al. (2001), the source of the apatite fraction is the bottom sediment decomposition of detritus, whose presence in a reservoir can be both autochthonous and terrigenous.In agricultural catchments, Ca-P apatite compounds can be formed where soil is deacidified using lime.They can also be washed directly out of minerals and thus delivered to a body of water along with surface runoff (Rafałowska and Sobczyńska-Wójcik 2014).
The potentially mobile phosphorus and nitrogen resources accumulated in the uppermost (5 cm) layer of bottom sediments of the reservoirs analysed are as presented in Fig. 4. The sediments of Nowa Wieś Reservoir had the largest resource of potentially mobile phosphorus (1.10 tons).The smallest amount of mobile phosphorus (0.10 ton) had in turn accumulated in the sandy sediments of Cierpisz Reservoir.For the remaining reservoirs, the load of mobile phosphorus deposited in the top layer of sediments was at a similar level in the 0.63-0.82-tonrange.In turn, the largest loads of organic phosphorus and organic nitrogen (0.38 and 5.82 tons, respectively) were accumulated in the sediments of Blizne Reservoir.Once surface area has been considered, the sediments of Nowa Wieś Reservoir can be seen to be most loaded with mobile phosphorus (threefold more), organic phosphorus and organic nitrogen (36.8, 5.55 and 70.7 g•m −2 , respectively).The sandy sediments of Cierpisz Reservoir were in turn the least loaded (on 4.43, 1.39 and 39.2 g•m −2 ).Nitrogen is part of the structure of HS molecules and may constitute 2 to 5% of them, but these substances (especially humins) are classified as not readily decomposable (vanLoon and Duffy 2007).Organic phosphorus (OP) is usually considered one fraction, but its nature and origin have already been analysed in other studies.Its pool in bottom sediments includes complex compounds with humin and fulvic acids, phosphate esters, sugar phosphates, phospholipids, nucleic acids, phosphoproteins, inositol phosphates, phytates and other compounds (Ni et al. 2021;Ni et al. 2022;Pu et al. 2023;Wang et al. 2024;Wauchope-Thompson et al. 2022).
The main source of internal supply, especially under aerobic conditions, is easily decomposable organic matter.The relatively small shares accounted for by the EDOM fraction (in the range 2-10% C org .)combined with the high (59-95%) shares of difficult-to-decompose humic substances [often including the largest (23-58%) shares of structurally complex humins, to indicate how a significant part of the organic matter accumulated in the sediments of the studied reservoirs] will be subject to slow decomposition, if at all.This means that the studied waters will not be resupplied with the entire pool of biogenic elements accumulated in the sediments, even where only the potentially mobile forms are taken account of.In small shallow reservoirs, there is usually good oxygenation of the water, such as can promote the aerobic decomposition of organic matter (Bartoszek 2019;Zhang et al. 2022).In such conditions, a significant share of the resources of the NAIP fraction can come be immobilised "permanently" in sediments (via stable Fe-P and Mn-P compounds).This leaves the resources of organic phosphorus (OP)-considered less biologically available in theory-as responsible for the internal supply of the water column in phosphates.Lee and Oh (2018) note that shallow bodies of water already have anaerobic conditions in the overlying water below a depth of 5 m.As other researchers also point to organic phosphorus playing an important role in the internal supply of bodies of water, increasing research is done into how organic matter controls the biogeochemical cycle for P (Pu et al. 2023;Yuan et al. 2020;Wang et al. 2022;Wang et al. 2024).
The loading of the bottom sediments of small retention reservoirs with potentially mobile phosphorus (P MOB .), as well as with organic forms of phosphorus and nitrogen, was definitely lower in the case of sandy as opposed to silty sediments.The only exceptions were those in Blizne Reservoir, whose location ensured its featuring a markedly different chemical and granulometric composition.According to Zhang et al. (2022), whose sediments had a P tot .content of 1.16-1.31mg•g −1 dw and an N tot .content of 3.83-5.87mg•g −1 dw, there was already a serious threat of a significant internal supply of nutrients to the water column.Nevertheless, sandy littoral sediments of a water body may be an important source of phosphate release, despite the low phosphorus retention capacity (Bartoszek and Koszelnik 2016;Lottig and Stanley 2007).

Influence of the distribution of organic matter on the accumulation of mobile phosphorus
Contamination of sediments with organic matter may be a factor significant in determining small retention reservoirs' enrichment with organic phosphorus, as is indicated by the comparison among the reservoirs studied here, in terms of higher average values of the OSI ON index being correlated with OP content (Fig. 5A).This trend was not found in the case of OSI ON and NAIP (or P MOB .)(Fig. 5B), given the very low contents of NAIP in sediments from Blizne Reservoir, as associated with limited anthropopressure in the catchment.Indeed, as Table 4 shows, correlation analysis confirms that the OM content exerted a statistically significant influence on the OP content in the sediments of all the analysed reservoirs, with the exception of Kamionka.In the case of the sediments of Cierpisz Reservoir, a significant but weak correlation was noted (lowest r = 0.48).It was also observable how the content of organic compounds of phosphorus in sediments may be influenced by the individual fractions of organic matter.In the cases of Brzóza Królewska, Blizne and Nowa Wieś Reservoirs, the OP fraction correlated in a statistically significantly manner with the content of the HS, FA, HA and EDOM fractions in the sediments.On the other hand, humins were only related in a statistically significantly way to the content of the OP fraction in the sediments of Brzóza Królewska and Blizne Reservoirs.Water-soluble and colloidal forms are fulvic and partially humin acids of lower molecular weight as well as their salts (most often with sodium and potassium).The insoluble fractions include humin acids of high molecular weight, humins and mineralorganic combinations of humic substances with suspensions (Krupińska 2012).
Metals can react with fulvic and humin acids and, at the remaining unbalanced positive charge, create slightly soluble complex compounds with phosphates (Smal et al. 2015;Trojanowska and Jezierski 2011).It is particularly possible for trivalent metals such as Fe(III) and Al(III), showing a tendency to form strong covalent bonds with HS to form chelate complexes with high stability constants (vanLoon and Duffy 2007).Stable in terms of durability, complex connections between phosphate anions and humic substances are formed by metal cation bridging.In addition to trivalent metals such as Al and Fe, other divalent metals (e.g.Ca, Mn, Mg, Cu and Zn) can bind humic substances with phosphates.The stability constants of humic-metal-phosphate complex connections are of the same order of magnitude as the adequate humic-metal complexes and humic substances themselves (O'Connell et al. 2020).The higher the pH of the environment, the easier it is to detach the H cations from carboxylic and phenolic groups of humic acids, thus increasing their negative charge and the possibility of binding metals and then forming complexes with phosphates.Tammeorg et al. (2022) found the important role of organic matter in the release of phosphorus from the bottom sediments of Finnish lakes (humic poor and humic rich).The Fe-P fraction turned out to be a significant source of sedimentary P release in the lakes studied (positive correlation between Fe-P and the phosphate diffusion flux).However, in the case of northern lakes rich in humic substances, they observed a lower content of Fe-P connections and a negative flux of phosphate diffusion from sediments.This indicates that humic substances can reduce the role of Fe-P in P circulation in the aquatic environment by forming complex compounds (Tammeorg et al. 2022).The formation of humic-Fe(III)-P complex compounds leads to the formation of a low-reactive pool of P and Fe in bottom sediments.The connections made between phosphorus and humic substances are considered more resistant to decomposition than other organic connections with phosphorus (O'Connell et al. 2020;Tammeorg et al. 2022).Due to the presence of reactive phosphate groups in the structure of the phytic acid molecule, phytates in sediments can also occur in the form of sparingly soluble, permanent, complex compounds with many metals, including iron, calcium and manganese (Golterman 2005).
Significant correlations could be noted between the contents of various forms of organic matter and the contents of inorganic fractions such as NAIP and AP in the sediments of practically all the reservoirs studied.Both phosphorus   (Martins et al. 2021), which tend to be more enriched in organic matter.In the sediments of Nowa Wieś Reservoir (i.e.those most enriched in organic matter), the lack of a correlation between OP and HU points to different origins.Over a lengthy (> 40-year) period of exploitations, the Nowa Wieś Reservoir sediments will mainly have generated their own humins, while unfavourable conditions for phytoplankton growth (a short water retention time of 1.3 days) may ensure that organic P is of predominantly terrigenous origin.
The relationship shown between NAIP and FA may result from an inaccurate distinction being drawn between organic and inorganic phosphorus compounds (separate extraction schemes under the SMT method).Due to good solubility in water over the entire pH range, fulvic acids remained in the extract containing NAIP.However, it is more likely that inorganic phosphorus compounds (NAIP and AP) are related to HS fractions via a common source.Surface runoff from the catchment area may introduce into the aquatic ecosystem both inorganic phosphorus compounds and fractions of organic matter in dissolved and suspended form as well as sparingly soluble humic-metal-phosphorus complex combinations with soil particles.In Blizne Reservoir, the lack of correlation between NAIP and HU (also HS and OM) is indicative of the different origin of these substances.In the silty sediments of this reservoir, humins may be of indigenous origin, as well as enter the reservoir along with the surface runoff, while the NAIP is exclusively terrigenous.Due to the limited degree of urbanisation and hence mainly agricultural use of this area, as well as the low content of this fraction, the source could simply be dust fall.Statistically significant and strong correlations between OSI ON and OM, EDOM and OP (r = 0.96; 0.92; 0.91, respectively) show that the Sediment Organic Index can be considered a tool by which to evaluate the potential of sediments for supplying waters with nutrients internally, where these waters are small retention reservoirs.

Conclusions
The sediments of Nowa Wieś Reservoir (displaying the highest values for OSI ON ) were classified as heavily contaminated with organic matter, while remaining reservoirs were found to be slightly or relatively polluted, or else unpolluted.
The distribution of phosphorus fractions (especially in inorganic compounds), and the shares in total phosphorus accounted for, were shown to relate mainly to anthropogenic impact in the catchment as well as the latter's geological structure.Only to a lesser degree was sediment grain size of importance.It is usual for a higher level of contamination of sediments with organic matter to be associated with greater contents of the organic fraction of phosphorus and easily decomposable organic matter, the latter being important to the internal supply of small, shallow bodies of water in phosphates.Within retention reservoirs of small size and significant (anthropogenic) influence of the catchment, the loading of bottom sediments with potentially mobile phosphorus, as well as with organic forms of P, and (sometimes) nitrogen, proved to be significantly more limited where sediments were sandy as opposed to silty.
In the humic-metal-phosphorus complex compounds, P is present when the sediments are sufficiently rich in organic matter, especially in HS, as is indicated by the confinement of significant relationships between organic phosphorus and humic fractions to the sediments of Reservoirs at Nowa Wieś, Blizne and Brzóza Królewska.The observed relationships between inorganic phosphorus fractions and OM fractions-present in practically all reservoirs' sediments-may reflect common terrigenous source of these pollutants.
Due to severe organic matter pollution, the decomposable form in which this is present and the significant content of organic phosphorus compounds, the sediments of Nowa Wieś Reservoir may represent a critical internal source of phosphates for the water column.This idea gains confirmation in the classification of these sediments as "heavily contaminated" by reference to the Organic Sediment Index criterion.Sediments in Blizne Reservoir, with their very marked enrichment in calcium compounds and small amounts of mobile phosphorus, should not pose any threat of trophic degradation of this water progressing further.However, the sandy sediments of the remaining reservoirs (under aerobic conditions) may provide a limited internal supply of water in phosphates.

Fig. 3
Fig.3The share of phosphorus fractions in P tot .[%] in the sediments of the small retention reservoirs studied.P MOB .= NAIP + OP

Fig. 5
Fig. 5 Summary for the reservoirs in terms of the relationship between higher values of OSI ON and OP (A) and OSI ON in relation to NAIP (B) (average for the reservoir)

Table 3
The content of total phosphorus and its different fractions, as well as iron, manganese, aluminium and calcium, in the bottom sediments of the reservoirs analysed (average ± SD)

Table 4
Relationships between OM and phosphorus fractions in the bottom sediments of reservoirsA significance level below 0.001 is indicated by double underlining r = Pearson correlation coefficient; p < 0.001; p < 0.01; p < 0.05; -non-significant correlation; n = 18, for Kamionka n = 27 correlated with the EDOM, FA and HA fractions in all reservoirs.Humins did not correlate with NAIP and AP in the sediments of Nowa Wieś Reservoir, or with NAIP at the Blizne site, where HS (and OM) also failed to correlate with NAIP.The lack of relationship between OP and HS, FA, HA and HU in the sandy sediments of the Kamionka and Cierpisz Reservoirs results from their only slight enrichment with organic matter, including humic substances.Metals associate preferably with finer fractions of sediment, such as silt and clay fractions