DISTRIBUTION OF PHOSPHORUS FORMS IN WATER-PARTICULATE MATERIALS-BOTTOM SEDIMENTS SYSTEM OF THE PRUT RIVER DURING 2009-2012 YEARS

Phosphorus is the important nutrient which stimulates the growth of aquatic organisms in water bodies, but in excessive quantities phosphorus has a fertilizing effect that affects both the ecosystem and water quality as whole. A completed scheme for determination of phosphorus forms in water, particulate materials and bottom sediments was applied. The results obtained during of 2009-2012 years were analyzed and systematized, identifying the seasonal and spatial dynamics of phosphorus forms in water, particulate materials and bottom sediments of the Prut River.


Introduction
Phosphorus is an essential nutrient for living organisms.In order to assess the anthropogenic impact on the chemical composition of surface water, the hydro-chemical indicators have been estimated, including either the content of phosphorus-orthophosphates [1,2] or total phosphorus content [3], or the content of both forms [4,5].Also, the variation of the content of some parameters that characterize the level of eutrophication, the content of nitrogen compounds, chlorophyll and total phosphorus were researched [6][7][8][9].A few studies are focused on research of various phosphorus forms in water and particulate materials [10][11][12].
Phosphorus concentrations in rivers result from both external inputs and internal loading from the bottom sediments.Its chemistry in sediments is greatly infl uenced by the oxidation-reduction status (redox potential).Under oxidized conditions, ferric and manganese oxides and hydroxides are important adsorption sites for phosphorus; however, under reducing conditions these minerals are unstable.Mobilization-immobilization processes on the particle surface of sediments occur through interstitial water and its participation.During the desorption process from sediments, phosphorus compounds are accumulated in interstitial water, then according to environmental factors (pH, oxidationreduction potential Eh, ionic strength, or water mineralization etc.) can be immobilized in the water horizon overlying the bottom sediments.
The objectives of this work were (i) to evaluate peculiarities of spatial and seasonal dynamics of phosphorus forms for the entirely system water -particulate materials -bottom sediments along the Prut River, (ii) to systematize the results obtained during of 2009-2012 years and (iii) to estimate the level of trophicity and quality of the Prut River.

Material and methods
During spring and summer of 2009-2012 years the samples of water, particulate materials and bottom sediments were collected along the Prut River (sampling sites: Costesti, Sculeni, Ungheni, Valea Mare, Frasinesti, Poganesti, Leova, Stoianovca, Cahul, Brinza, Caslita-Prut, Giurgiulesti).The bottom sediments with thickness of 0-5 cm were collected using all-plastic dredge sampler, fresh material being wet-sieved through 2 mm all-plastic sieve [24].Then the samples were placed in plastic containers and stored in a portable refrigerator at 4°C during transportation to the laboratory.
The scheme for determination of phosphorus forms in water and particulate materials (Fig. 1) according to classifi cation of World Health Organization (WHO) was applied [25].According to WHO, phosphorus compounds occurred in natural waters are classifi ed into 12 phosphorus forms, by chemical type -(i) orthophosphates, (ii) acid hydrolysable phosphates (poly-and pyrophosphates), (iii) organic phosphorus, (iv) total content, and by physical state -(i) dissolved (fi lterable), (ii) particulate, (iii) total content.
Additionally, the scheme was tested for estimation of phosphorus content in the bottom sediments [26][27][28].The amount of total phosphorus was determined according to U.S. Geological Agency [24].The content of inorganic phosphorus was determined according to WHO methods for inorganic particulate phosphorus [25].The content of phosphorus forms in interstitial water was determined after centrifugation of fresh (wet) sediments.There were established amount of orthophosphates, condensed and organic phosphorus according to World Health Organization recommendations [25].

Peculiarities of phosphorus content dynamics in water and particulate materials
The spatial dynamics of total phosphorus forms (namely, total P1, total dissolved P5 and total particulate P9) during of 2009-2012 years, in general, had indicated the increase of their amounts along the Prut River (Fig. 2).Seasonal dynamics of the content of phosphorus forms was the same, being established a decrease of total content of phosphorus forms from spring to summer.
The dynamics of dissolved phosphorus forms was less uniform along the Prut River (Fig. 3).During the spring of 2010 and 2011 years the spatial dynamics, in general, had the same trend, being recorded the maximal content of the orthophosphates (P6) and condensed forms (P7) at the middle sector of the river (stations Ungheni, Valea Mare).
For particulate forms of phosphorus (namely, orthophosphates P10 and organic phosphorus P12) an increase of their amounts was recorded along the river, recording the highest values at the lower sector of the river (stations Cahul, Caslita-Prut, Fig. 4).Seasonally, a decrease of particulate phosphorus from spring to summer was established during of 2009-2012 years.

Peculiarities of phosphorus dynamics in bottom sediments and their interstitial water
The spatial dynamics of phosphorus forms in the bottom sediments had, in general, the same trend during the spring of 2009 -2011 years (Fig. 5).The higher contents of inorganic form (P14) were registered on the middle course of the Prut River.For organic phosphorus (P15) the highest contents during 2010 and 2011 years were recorded in the bottom sediments on lower course of the river.Multi-annual dynamics showed that the content of phosphorus forms in the bottom sediments was not changed signifi cantly.
Chemistry Journal of Moldova. General, Industrial and Ecological Chemistry. 2013, 8(2), 57-66    During the summer the spatial dynamics of phosphorus forms in bottom sediments was less uniform along the Prut River (Fig. 6).Seasonal dynamics of phosphorus forms in bottom sediments was more pronounced during 2009 and 2010, being registered the tendency of decreasing of inorganic phosphorus and the tendency of increasing of organic phosphorus from spring to summer.
Chemistry Journal of Moldova. General, Industrial and Ecological Chemistry. 2013, 8(2), 57-66 Ratio of inorganic: organic phosphorus (P14 : P15) in bottom sediments was similar during 2004, 2009, 2010 and 2011 years (Fig. 7), being recorded the predominance of the inorganic phosphorus which constituted about 65-95% of total amount.Also, the increasing tendency of the percentage of organic phosphorus from spring to summer was registered.Phosphorus content in interstitial water was computed per volume of interstitial water (IW) extracted from 1 kg of bottom sediments (Fig. 8).Seasonal dynamics of phosphorus forms in interstitial water was more pronounced during 2009 and 2010, being registered the tendency of increasing of inorganic phosphorus and the tendency of decreasing of organic phosphorus from spring to summer.
During the spring of 2009 year the predominance of the organic phosphorus in interstitial water of bottom sediments was recorded which constituted about 45-78% of total phosphorus amount, while during the summer the proportion of organic phosphorus was lower (1.7-13% of total amount, Fig. 9).

Level of trophicity and quality of the water
The large amounts of phosphorus compounds in aquatic ecosystems have the fertilizing effect that affects eutrophication degree.The European Community Directive 91/676/EEC recommends that the level of eutrophication in rivers should be determined on the basis of quality parameters, such as nitrate, phosphorus compounds, chlorophyll, oxygen etc.In accordance with the recommendations [29] eutrophication level is estimated according to the total phosphorus content (Tab.1), while according to further recommendations [30] the level of eutrophication for rivers is estimated on the basis of soluble reactive phosphorus (Tab.2).Chemistry Journal of Moldova. General, Industrial and Ecological Chemistry. 2013, 8(2), 57-66  On the basis of the total phosphorus content (Tab. 1) and orthophosphates content (Tab.2) the eutrophication level of the Prut River should be attributed to oligotrophic-mesotrophic level of eutrophication.

Summer
The quality of surface waters in Romania is regulated by Order 1146/2002 which includes fi ve-quality classes [31].The values specifi ed for each class correspond to the maximum allowable for respective quality class (Tab.3).On the basis of presented data in this work, the water of the Prut River should be attributed to the class I of quality according to phosphorus-orthophosphate content (P6), and to classes I-II of quality according to the content of total phosphorus (P1)., 2003, [32]), the tolerated threshold (the arithmetic means calculated over a three-year period) for soluble reactive phosphorus SRP (mainly orthophosphates) is 0.20 mg/L (Tab.4).The orthophosphate concentrations (the form P6, Fig. 3) registered for the Prut River during of 2009-2012 years didn't exceed this ecological threshold for eutrophication, the value of arithmetic mean being about 0.13 mg/L.L. Postolachi et al. / Chem. J. Mold. 2013, 8(2), 57-66 Actually, there aren't yet established any quality criteria for phosphorus content in the bottom sediments.Also, the criteria for phosphorus content in the interstitial water aren't standardized.In the study [33], the preliminary quality parameters of phosphorus-orthophosphates in the interstitial water of bottom sediments were proposed (Tab.5).
According to the presented data for the content of phosphorus-orthophosphates in the interstitial water of sediments (Fig. 8), the level of eutrophication of the Prut River could be attributed to the oligotrophic level (P6 <0.1mgP/L), episodically (during the summer of 2009 year) to the mesotrophic level of eutrophication.This level of eutrophication is similar to those established on the basis of the content of phosphorus-orthophosphates in water.

Conclusions
The scheme for determination of phosphorus forms in water and particulate materials according to World Health Organization classifi cation was evaluated.Additionally, this scheme was tested for estimation of phosphorus content in bottom sediments.The supplemented scheme allows the analysis of the phosphorus forms for the entirely system "water -particulate materials -bottom sediments", extending possibilities for interpretation of phosphorus dynamics in natural waters.
The peculiarities of the spatial and seasonal dynamics of phosphorus in the water-particulate materials-sediments system during of 2009-2012 years along the Prut River were established.It was found an increasing trend of total dissolved phosphorus and in particulate materials along the Prut River.The maximum of dissolved phosphorus forms recorded on middle sector of the river (Valea Mare-Stoianovca) indicates the presence of pollution sources.
On the basis of obtained data, the water of the Prut River should be attributed to classes I-II of quality according to Romanian standards.The eutrophication level of the Prut River should be attributed to oligotrophic-mesotrophic level of eutrophication according to UE directives.According to the European Commission, the orthophosphate concentrations registered for the Prut River during of 2009-2012 years didn't exceed this ecological threshold for eutrophication.Actually, there are not yet established any quality criteria for phosphorus content in the bottom sediments.

Figure 1 .
Figure 1.The scheme for analysis of the phosphorus forms in water and particulate materials according to WHO classifi cation (forms P1-P12) [25].Supplemented scheme for analysis of the phosphorus forms in sediments (forms P13-P15) [26-28].

Figure 2 .
Figure 2. Dynamics of total phosphorus forms along the Prut River (P1, P9 and P5 -total phosphorus, total phosphorus in particulate materials and total dissolved phosphorus, respectively).

Figure 8 .
Figure 8. Dynamics of phosphorus forms (mg/V IW ) in interstitial water (IW) of bottom sediments along the Prut River during the spring and summer of 2009 and 2010 years.Data are computed per volume of interstitial water (IW) extracted from 1 kg of bottom sediments.Phosphorus forms in interstitial water: P6 -orthophosphates, P7 -condensed forms (poly-and pyrophosphates), P8 -organic phosphorus.

Figure 9 .
Figure 9. Proportions of inorganic (P6+P7) and organic (P8) phosphorus in interstitial water of bottom sediments along the Prut River during the spring and summer of 2009 year.