3.1. Nonconservative Behavior of Phosphorus

Biological processes are the main reason for the nonconservative behavior of dissolved phosphorus in the mixing zones of river and sea waters. This is shown in the interrelated change in the phosphorus content and the various characteristics of the biological processes: the concentrations of nutrients, oxygen, chlorophyll, and organic detritus, and the pH value. The leading role of production–destruction processes in the transformation of dissolved phosphorus runoff has been established for the mouths of many rivers (Table 16): Neva, the small rivers of the Kandalaksha Bay of the White Sea, Onega, Kyanda, Northern Dvina, Mezen, Pechora, Ob, Yenisei, most of the small rivers in the Black and the Azov Sea catchments, Volga, Ural, Yangtze, the Far Eastern rivers Razdolnaya and Uda, and Peace (USA). Due to the seasonal and interannual dynamics of phytoplankton development, the distribution of nutrients in the mixing zones of river and sea waters is also subject to seasonal and interannual variability. The consumption of dissolved mineral phosphorus by biota occurs in the surface layer or in the vertically mixing water column throughout the entire salinity range and varies from 6 to 100% of its content in river runoff. The most intensive consumption of mineral phosphorus by aquatic organisms is observed during the vegetative season and is most often accompanied by the extraction from the solution of another biogenic element, silicon ^{[71][72][73][74][82][83][91][92]}[71,72,73,74,82,83,91,92]. Along with this, in the mouths of some rivers (Strelna of the Kandalaksha Bay of the White Sea, Chernaya of the Sevastopol Bay of the Black Sea, and Razdolnaya of the Amur Bay of the Sea of Japan), the removal of dissolved mineral phosphorus was found even in winter, with relatively low biological activity. Under stratification conditions, the plots of river and sea water mixing show the influence of two processes that regulate the concentration of dissolved mineral phosphorus at different depths. Either the behavior of phosphorus in surface waters is conservative, or it is removed as a result of biological assimilation, while in the deep layers, phosphorus, on the contrary, enters the solution due to the destruction of the organic matter deposited on the bottom. As a result, the relationships between the concentration of dissolved mineral phosphorus and the chlorinity (salinity) acquire orderliness only when the points are grouped along the horizons ^{[77][78][103][106]}[77,78,103,106]. This distribution is typical for the estuaries of the Ob and Yenisei Rivers, in which aquatic organisms consume 6–57% of the mineral phosphorus supplied via river runoff, and the destruction of organic matter provides an increase in its concentration by 50–150%, relative to the content in the river water. A distinctive feature of the release of mineral phosphorus into the solution during the destruction of organic matter is the simultaneous entry into the water column of the mineral forms of nitrogen ^[77][96][77,96]. The predominance of destruction processes over production processes is also often found throughout the entire water column, periodically occurring in the mouths of the Onega River, some rivers of the Kandalaksha Bay of the White Sea, the Ashamba River of the Black Sea coast of Russia, and the Uda River of the Sea of Okhotsk, and leading to an increase in the flux of dissolved mineral phosphorus by 35–100%. An even greater transformation of river runoff under the influence of destruction processes occurs when pore solutions from stirred up sediments enter the mixing zone. This situation is typical for the mouths of the Mezen and the Ural Rivers and in some periods is observed in the mouths of the Black Sea rivers Anapka and Ashamba, where the additional input of mineral phosphorus into the solution exceeds its removal via river runoff by 90%, 150–300%, 7.0 times, and 9.5 times, respectively. Firstly, the organic matter in the bottom sediments is remineralized with the release of phosphates, and secondly, it can reduce iron (III) phosphates to iron (II) phosphates, which causes the input of part of the phosphorus into a dissolved state. The behavior of dissolved organic phosphorus characterizes the process of the destruction of organic matter in the mixing zone of river and sea waters. Its additional intake indicates the excess of the rate of the release of dissolved organic forms over the rate of complete phosphorus recycling and is observed in the mouths of the Northern Dvina and Salgir Rivers. A decrease in the concentration of dissolved organic phosphorus relative to the line of the conservative mixing of water masses occurs at a higher rate of its mineralization than its release into the solution during destruction and was noted in the mouths of the Kyanda River and rivers of the Black Sea coast of Russia (Anapka, Ashamba, Mezyb, and Hotetsai). Both losses and excesses of dissolved organic phosphorus were recorded at the mouth of the Onega River in different periods and at different salinity intervals. The distribution of dissolved organic phosphorus is close to conservative at the Mezen River estuary and the Serebryanka River mouth (Sikhote Alin Reserve). A significant part of the phosphorus in suspended matter in rivers is represented by reactive mineral and organic compounds, which can be sources of dissolved phosphorus in the mixing zones of river and sea waters. From the experimental data [107], it follows that the amount of phosphorus passing from river suspensions into the dissolved state increases with an increasing salinity. Some curves of the distribution of dissolved phosphorus concentration have a deflection in the salinity range from 0 to 9‰ [103]. In this case, the similar form of such relationships for dissolved iron and aluminum in the absence of clear signs of the biological consumption of phosphorus indicates the physicochemical removal of the latter from the solution as a result of coprecipitation with iron and aluminum hydroxides. It can be expected that the terrigenous hydroxophosphates of iron and aluminum in the marine environment will be unstable due, firstly, to an increase in the pH value, accompanied by the displacement of phosphates by hydroxyl ions, and, secondly, to an increase in the concentration of dissolved calcium, which promotes the formation of apatite phases typical for ocean sediments. This assumption is confirmed by the results of observations [108], which showed that iron–calcium hydroxophosphates in oceanic pelagic sediments decompose during diagenesis into more stable iron oxyhydroxides and apatite. The replacement of the terrigenous hydroxophosphates of iron and aluminum with apatite should begin already in the river mouth areas and cause an increase in the concentration of dissolved phosphorus. Therefore, the input of iron and aluminum phosphates into the salinized portion of the river mouth area as a component of the suspended matter and bed load can lead to a partial release of dissolved phosphorus and, in some cases, be the cause of its nonconservative behavior. From these positions, one can explain the decrease in phosphorus content with the increase in salinity in the bottom sediments of the mouths of the Pamlico and Potomac Rivers ^[109][110][109,110].

Phosphorus removal as a result of sorption and coprecipitation processes occurs, as a rule, at the initial stage of river and sea water mixing during the period of low biological activity and is observed in the mouths of the rivers Clyde (Great Britain), Mandovi and Chilka Lake (India), Mekong (Vietnam), Raritan (USA), and Amazon, accounting for 15–100% of the removal of dissolved mineral phosphorus via river runoff (Table 16). The input of dissolved phosphorus due to desorption from river suspensions penetrating into the marine environment is also a common phenomenon established for the mouths of the Black Sea rivers Mezyb and Hotetsai, Mekong in the area of medium salinity, Yangtze, Serebryanka (Sikhote Alin Reserve), Old Mill Creek (USA), Orinoco (Venezuela), Amazon at medium salinity, and Zaire (Table 16). The maximum desorption values (13–300% of the content in river water, or 1–75 μg P/L) are reached at a salinity of 7–15‰, and the mixing curves have a convex shape.

The spatial separation of the processes of phosphorus sorption and desorption (predominance of sorption in the freshwater part of the mixing zone, and desorption in the area of intermediate salinity) confirms the distribution of phosphorus and iron in the bottom sediments of the river mouth areas. Thus, at the Pamlico River mouth, the concentrations of phosphorus and iron in the bottom sediments of the riverine part of the mixing zone closely correlate with each other (r = 0.98), whereas when approaching the sea boundary of the mouth area, this relationship becomes less pronounced (r = 0.86–0.77). The same concentrations of iron correspond to lower phosphorus concentrations in the marine part of the mouth, which indicates the release of the latter from the bottom sediments [111]. This combination of sorption and desorption in the estuaries is called the phosphate buffer mechanism ^{[60][112][113][114]}[60,112,113,114]. Many authors have tried to determine the concentration of dissolved mineral phosphorus at which an equilibrium between the water and bottom sediments is established. According to experiments ^{[99][112][113]}[99,112,113], the equilibrium concentrations of dissolved phosphates are in the range of 22–46 μg P/L. Convincing results of field observations proving the existence of such a concentration limit have not yet been obtained, although in the studied estuaries, with the exception of the Mekong River mouth, the concentrations of desorbed phosphorus do not really exceed the values recorded in the experiments and amount to 1–28 μg/L ^{[82][89][94][102][104][105]}[82,89,94,102,104,105].

3.2. Conservative Behavior of Phosphorus

Despite the active participation of phosphorus in the intrabasin biological and chemical processes, cases of its conservative behavior were established (Table 16), which were observed either under conditions of the severe pollution of the aquatic environment (the mouths of the European rivers Clyde, Scheldt, Rhine–Meuse, Ems, Weser, Elbe, Rhone, and Tiber), or during periods of low biological activity (the mouths of the Mississippi River; Amazon River; the rivers of the Arctic; and the Far Eastern rivers Knyazhaya, Niva, Kolvitsa, Onega, Semzha, Indiga, Pechora, and Usalgin).

The concentration of dissolved mineral phosphorus in the waters of polluted rivers (93–520 μg/L) is an order of magnitude higher than its average content in the rivers of the world. Increased concentrations of dissolved mineral phosphorus were also found at the sea boundary of the mouth areas of these rivers: up to 242 and 280 μg/L on the near-shore zone of the Clyde and Tiber River mouths. The conservative behavior of phosphorus in the mouths of these European rivers can be explained by the fact that the absolute values of the fluctuations in its concentration in the mixing zones at such a high content in river or sea waters are comparable to the amount of phosphorus involved in the intrabasin processes. In addition, most of the observations in which the conservative behavior of phosphorus was recorded were carried out in the autumn–winter period, when the intensity of production processes in the temperate zone decreases with the intensification of the biological processes, and the conservative behavior of phosphorus can turn into nonconservative behavior within several weeks, which was noted, for example, for the Rhone River delta [115]. The conservative distribution of dissolved mineral phosphorus in the mouths of the Onega, Mississippi, and Amazon Rivers that appeared in the winter–spring period is also, apparently, caused by the low activity of aquatic organisms.

Separately, itwe should consider the conservative behavior of dissolved phosphorus during the vegetative season in the mouths of the small Arctic and Far Eastern rivers that are not subject to strong anthropogenic impact. The concentration of suspended matter in the mouths of these rivers in spring and early summer is small due to the slow thawing of soils in the catchments and the low water temperature preventing phytoplankton development, which limits the participation of phosphorus in physicochemical and biological processes.

Thus, the conservative behavior of dissolved phosphorus in the mixing zone of river and sea waters is an atypical phenomenon and occurs in special conditions when the biological and chemical processes at river mouths are suppressed as a result of an unfavorable combination of natural and anthropogenic factors.

3.3. Phosphorus Balance in the Mixing Zones of River and Sea Waters

An analysis of the mixing curves indicates the complex nature of the dynamics of dissolved phosphorus fluxes in river mouth areas, with the combination of conservative and nonconservative distribution and the spatiotemporal variability of the latter, including a change in the direction of transformation. Therefore, the calculations of the values of the removal or input of phosphorus in the mixing zones of river and sea waters based on data for relatively short time intervals turn out to be insufficiently representative for balance estimates.

To obtain more reliable estimates, Savenko and Zakharova [116] summarized the results of balance studies carried out in river mouths and bays for a year or more (Table 17). As follows from the data presented, on average, a significant part of the phosphorus is removed per year. The maximum removal (80–94%) is characteristic of the total phosphorus, including the suspended fraction (P_dissol + P_susp), and only a third of this value is associated with physical sedimentation, while the rest of the phosphorus is removed as a result of biosedimentation ^[117][118][117,118]. Biological processes also play a major role in the extraction of mineral phosphorus from the solution (40–80%) due to its transfer to the composition of suspended organic matter, which is subsequently deposited at the bottom. Total dissolved phosphorus (P_total) is retained in river mouths in much smaller amounts (7–38%). This is corresponds to the observational data presented in Table 16, according to which the losses of the dissolved forms of mineral and total phosphorus during biological consumption in the mixing zones of river and sea waters are on average 46 and 25%.

Table 17. Balance estimates of phosphorus losses in the river mouth areas and bays.

Object	Phosphorus Form	Observation Period	[P], μg/L		Phosphorus Balance, %	Supposed Cause of Transformation	Reference
			River Boundary	Sea Boundary
Accounted	Full	Accounted	Full
Europe	2365	80	52	92.3	116	160	310
Asia	10,152	53	–	484	913	–	1094
North America	19.1
7840	12	24	9.8	82	237	988
South America	11,700	84	22	124	148	286	1300
Australia and Oceania	2370	4	1	1.9	48	1.2	205
Africa	4110	49	34	58	120	85	249
Whole world	38,537 (41,700)	52	19	770	1481 (1603)	769	4154 (4495)

¹ Values in parentheses are calculated for water runoff of 41,700 km³/yr.

2.4.3. Dissolved Phosphorus in Groundwater Runoff

Groundwater phosphorus runoff is difficult to estimate due to the limited amount of available information. According to calculations [62], the proportions of the dissolved forms of mineral and total phosphorus in the groundwater and river runoff into the seas of the Russian Arctic are approximately the same, at 11–13% (Table 15). A similar proportion of dissolved mineral phosphorus in groundwater and river runoff also follows from the global estimates [63]. With an average concentration of phosphorus in the groundwater of the Critical Zone of 58 μg/L [57] and a groundwater runoff value of 2200 km³/yr, the phosphorus removal into the ocean is 0.13 Mt/yr, or ~8% of the river runoff of dissolved mineral phosphorus.

Table 15. The proportion of dissolved phosphorus in groundwater and river runoff into the marginal seas of the Russian Arctic [62].

Receiving Water Body	Proportion of Groundwater Runoff from River Runoff, %
	Water	P	min	P	total
White and Barents Seas	14.6	15.3	15.5
Kara Sea	10.3	12.3	–
Laptev Sea	7.6	6.9	5.1
Northern taiga landscapes	21.7
East Siberian Sea	6.7	5.8	6.6
Average for the Arctic seas of Russia	10.0	12.8	1	10.7	1

¹ Weighted mean for water runoff.

3. Phosphorus in the Mixing Zone of River and Sea Waters

The final stage of the transformation of the river runoff of dissolved and suspended matter is carried out in the mouth area of rivers, as a result of which the ratio of the dissolved and suspended forms of chemical elements entering the ocean changes. The dissolved components with conservative behavior are involved in intrabasin chemical and biological processes to an insignificant extent, and their content linearly depends on the ratio of the proportions of the sea and river water masses in the mixing zone. The components with nonconservative behavior are also added into the solution or are removed from it as a result of their involvement in different processes occurring in the water column or at the water–air and water–bottom boundaries. In this case, the linearity of the relationship between the component concentration and the ratio of the water mass proportions is violated. The best indicator of the ratio of the sea and river water mass proportions is the isotopic composition of water; however, the concentration of chemically inert chlorides, which is more accessible for measurements, is used more often. The conservative behavior of the dissolved component i in the mixing zone of river and sea waters is described by the linear relationship between its concentration [i]_mix and chloride content [Cl]_mix:

[i]mix = a + b[Cl]mix,

where a ≈ [i]rw is a constant parameter; b is the slope ratio taking positive or negative values at a higher or lower concentration of the component i in seawater in comparison with river runoff; and the subscript “rw” denote the concentrations in river water. If the componenti is removed from the solution or, on the contrary, its internal source is present, the line showing the actual distribution of the concentrations of the relevant component is located, respectively, below or above the calculated line of conservative behavior (Figure 6). This equation is widely used to determine the type of behavior of chemical components in the mixing zones of river and sea waters.

Figure 6. Relationships between the concentration of dissolved component i and chloride content by the conservative behavior (1) and availability of processes of additional input (2) or removal (3) of this component in the mixing zone of river and sea waters: (a,b) are cases when the concentration of component i in the river water is accordingly below or above that in the seawater.

Active participation in biological processes brings about the nonconservative behavior of phosphorus, which is observed in most river mouths of the world. The consumption of dissolved mineral phosphorus by phytoplankton leads to a decrease in its content in the water down to “analytical zero”. The mineralization of the precipitated organic detritus causes the input of phosphorus into a solution in the lower layers of the water column and at the water–bottom boundary. Approximately half of the phosphorus entering the bottom as part of the organic detritus, after the mineralization of organic matter, can return back to the water with circulating currents or during the stirring up of the bottom sediments [64]. In addition to the production–destruction processes, an important role in the transformation of dissolved phosphorus runoff in the mixing zone of river and sea waters is played by the transformation processes of phosphorus-containing solid phases, sorption–desorption, and coprecipitation, as well as the diagenetic processes that control the phosphorus fluxes at the water–bottom boundary [6]. Table 16 summarizes the data on the distribution of the concentrations of the dissolved forms of mineral, organic, and total phosphorus in the mouth areas of large and small rivers of the world. The behavior of phosphorus in river mouths can be both nonconservative and conservative. In some cases, a complex type of phosphorus distribution was established exhibiting different behavior in various parts of the mixing zone.

Table 16. The behavior of dissolved phosphorus in the mouth areas of world rivers.

River	Receiving Sea Area	Phosphorus Form	Observation Period	Salinity, ‰		[P], μg/L		Phosphorus Runoff Transformation, %	Supposed Cause of nonconservative Behavior	Refe

Gulf of Riga, Baltic Sea	P	min	1989	–
A.S.
–	−57	Biological consumption	[	119	]
“
P	total	“	–	–	−7	As above	“
Vigo Bay, Spain	P	min	1986	394	12	−40	Biological consumption and sedimentation	[64]
	P	total	“	–	–	−38	As above	“
Mikawa River mouth, Japan	P	dissol	+ P	susp	July 1983	930	–	−80	“	[118]
Delaware River mouth, USA	P	min	April 1986–July 1988	–	–	−61	Biological consumption	[120]
	“	As above, winter	–	–	−75	As above	“
	“	As above, spring	–	–	−135	“	“
	“	As above, summer	–	–	−70	“	“
	“	As above, autumn	–	–	+33	Destruction of organic matter	“
	P	total	April 1986–July 1988	–	–	−23	Biological consumption	“
	“	As above, winter	–	–	−53	As above	“
	“	As above, spring	–	–	−56	“	“
	“	As above, summer	–	–	−37	“	“
	“	As above, autumn	–	–	+52	Destruction of organic matter	“
Potomac River mouth, USA	P	min	1979–1981	–	–	−81
August 2017
0–18
0–10
3.0	5.0	+56	Destruction of organic matter	“
Biological consumption	[
P	org	June 2011	0–18	0–10	3.9	5.6	+51	As above	[73]
121	]
Huizache-Caimanero Lagoon, Mexico	P	dissol	+ P	susp	1969–1981	93	170	−94	Biological consumption and sedimentation	[117]
“	January 2017	0–6	0–4	43	7.0	+100	“	A.S.
“	“	0–6	4–6	43	7.0	−30	Mineralization	“
“	August 2017	0–18	0–6	16	6.0	−21	As above	“
P	total	June 2011	0–18	0–10	5.3	6.1	+26	Biological consumption and destruction of organic matter	[73]
“	January 2017	0–6	0–4	50	19	+44	Destruction of organic matter	A.S.
“	“	0–6	4–6	50	19	−40	Mineralization	“
“	August 2017	0–18	0–4	19	11	−10	As above	“
Kyanda	“	P	min	August 2016	0–21	0–18	16	5.3	−8	Biological consumption	“
		P	org	“	0–21	0–4	25	6.5	−15	Mineralization	“
		P	total	“	0–21	0–14	41	12	−18	Biological consumption and mineralization	“
Northern Dvina	“	P	min	April 2003	0–25	0–18	32	9.0	−17	Biological consumption	[74]
		“	June 2003	0–23	0–6	18	7.3	−22	As above	“
		“	July 2016	0–14	0–2	3.1	2.3	−22	“	A.S.
		P	org	“	0–14	0–7	11	5.3	+110	Destruction of organic matter	“
		P	total	“	0–14	0–7	14	7.6	+93	As above	“
Mezen	“	P	min	July 2009	0–21	0–1	27	10	+88	Input from bottom sediments	[73]
		“	August 2015	0–21	0–1	25	6.0	+93	As above	[75]
		P	org	July 2009	0–21	–	2.5	0.5	~0	–	[73]
		P	total	“	0–21	0–1	30	11	+82	Input from bottom sediments	“
Semzha	“	P	min	August 2018	0–17	–	8.7	6.6	~0	–	A.S.
Indiga	Barents Sea	P	min	April 1981	0–30	–	2.0	18	~0	–	[76]
		“	September 1981	0–30	–	6.0	4.0	~0	–	“
Pechora	Pechora Sea	P	min	July 1984	0–30	0–7	16	20	−75	Biological consumption	“
Ob	Kara Sea	P	min	September 1993	0–34	0–20	43	50	−28	As above	[77]
		“	“	0–34	20–34	43	50	+120	Destruction of organic matter	“
		“	August 1999	0–33	0–15	45	90	−33	Biological consumption	“
		“	“	0–33	15–33	45	90	+150	Destruction of organic matter	“
Yenisei	As above	P	min	September 1993	0–34	0–20	5.0	50	~0	–	“
		“	“	0–34	20–34	5.0	50	+50	Destruction of organic matter	“
		“	September 2009	0–30	0–12	20	33	−30	Biological consumption	[78]
		“	“	0–30	12–30	20	33	+100	Destruction of organic matter	“
		“	September 2010	0–17	0–2	7.1	1.5	−57	Biological consumption	“
		“	April 2016	0–26	0–10	19	22	−6	As above	“
		“	“	0–26	10–26	19	22	+47	Destruction of organic matter	“
Rhone	Mediterra- nean Sea	P	min	1983– 1984	1–37	–	170	6	~0	–	[79]
Tiber	Tyrrhenian Sea	P	min	July 1984	0–37	–	93	280	~0	–	[80]
		“	November 1984	0–37	–	93	22	~0	–	“
		P	total	July 1984	0–37	–	93	310	~0	–	“
		“	November 1984	0–37	–	155	22	~0	–	“
Salgir	Azov Sea	P	min	February 2016	0–15	0–8	540	116	−29	Biological consumption	A.S.
		P	org	“	0–15	0–4	70	90	+54	Destruction of organic matter	“
		P	total	“	0–15	0–4	610	206	−19	Biological consumption and destruction of organic matter	“
Chernaya (Crimea)	Black Sea	P	min	February 2004	0–17	0–2	7.1	1.8	−65	Biological consumption	[81]
Anapka	As above	P	min	May 2014	0–16	0–3	18	13	+700	Input from bottom sediments	[82]
		P	org	“	0–16	0–7	8.8	0.7	−58	Mineralization	“
		P	total	“	0–16	0–3	27	14	+450	Input from bottom sediments	“
Ashamba	“	P	min	July 2010	0–15	0–11	14	11	+35	Destruction of organic matter	“
		“	August 2010	2–13	2–9	82	25	−11	Biological consumption	“
		“	January 2011	0–10	0–3	2.2	14	+950	Input from bottom sediments	“
		P	org	July 2010	0–15	0–11	1.0	5.0	−320	Mineralization	“
		“	August 2010	2–13	2–9	21	6.4	−8	As above	“
		“	January 2011	0–10	0–1	1.7	1.4	−80	“	“
		P	total	July 2010	0–15	0–11	15	16	~0	–	“
		“	August 2010	2–13	2–9	103	31	−8	Biological consumption	“
		“	January 2011	0–10	0–3	3.9	15	+430	Input from bottom sediments	“
Mezyb	“	P	min	September 2010	0–15	2–9	5.5	7.7	+180	Desorption from suspended matter	“
		P	org	“	0–15	0–11	4.0	5.3	−37	Mineralization	“
		P	total	“	0–15	2–5	9.5	13	+100	Desorption from suspended matter	“
Hotetsai	“	P	min	September 2010	0–12	2–9	4.7	7.1	+170	As above	“
		P	org	“	0–12	–	0.3	0.3	~0	–	“
		P	total	“	0–12	2–9	5.0	7.4	+170	Desorption from suspended matter	“
Vulan	“	P	min	July 2006	0–16	0–2	8.0	1.9	−32	Biological consumption	“
Volga	Caspian Sea	P	min	August 2003	0–11	0–1	37	10	−60	As above	[83]
		“	August 2004	0–10	0–1	12	4.0	−63	“	“
		“	August 2005	0–5	0–1	41	2.6	−91	“	“
		“	September 2006	0–9	0–1	14	3.4	−64	“	“
Ural	As above	P	min	April 2016	0–8	0–5	16	29	+150	Input from bottom sediments	[84]
		“	April 2017	0–5	0–3	1.6	1.1	+300	As above	“
Mandovi	Arabian Sea	P	min	February 1981	0–36	0–12	3.0	6.0	−100	Sorption on suspended matter	[85]
Cauvery	Bay of Bengal	P	min	July 1986	0–15	0–2	267	108	−98	Unspecified	[86]
Chilka Lake	As above	P	min	November 1988	0–21	0–6	22	14	−24	Sorption on suspended matter	[87]
Mekong	South China Sea	P	min	November 1987	0–26	0–2	25	9.0	−84	As above	[88]
		“	“	0–26	2–15	25	9.0	+300	Desorption from suspended matter	“
Yangtze	East China Sea	P	min	June 1980	0–27	18–22	15	<3	−100	Biological consumption	[89]
		“	November 1981	0–33	0–24	7.0	11	+63	Desorption from suspended matter	“
		“	August 1981	0–32	25–28	23	<1	−44	Biological consumption	[90]
Razdolnaya	Sea of Japan	P	min	July 2001	0–32	0–25	60	<5	−27	Biological consumption	[91]
		“	“	0–32	0–9	80	<3	−70	As above	“
		“	August 2005	0–32	0–24	25	<3	−52	“	[92]
		“	February 2008	0–34	0–7	75	15	−28	“	[93]
Serebryanka	As above	P	min	July 2009	1–18	1–15	8.4	8.4	+13	Desorption from suspended matter	[94]
		P	org	“	1–18	–	0.3	5.2	~0	–	“
P	total	“	1–18	1–15	8.7	14	+12	Desorption from suspended matter	“
Usalgin	Sea of Okhotsk	P	min	July 2016	0–25	–	54	9.0	~0	–	[95]
		P	total	“	0–25	–	70	25	~0	–	“
Uda	As above	P	min	“	0–27	0–24	9.0	29	+100	Destruction of organic matter	“
		P	total	“	0–27	0–24	10	30	+70	As above	“
Sacramento	Pacific Ocean	P	min	May 1976	0–29	0–20	84	43	+74	Unspecified	[96]
		“	July 1976	0–30	0–30	77	43	+56	As above	“
		“	September 1976	0–30	0–28	81	15	+108	“	“
		“	November 1976, March 1977	0–29	0–18	62	62	+125	“	“
Mississippi	Gulf of Mexico	P	min	March 1971	10–33	–	71	15	~0	–	[97]
		“	January 1973	6–33	–	81	5.0	~0	–	“
		“	May 1973	0–20	8–20	90	5.0	−68	Biological consumption	“
		“	October 1983	0–34	0–3	248	3.0	+24	Dissolution of calcium phosphates	[98]
Peace	As above	P	min	October 1981	0–30	0–25	2790	496	−80	Biological consumption	[99]
Raritan	Atlantic Ocean	P	min	June 1982	0–22	0–3	12	50	−100	Sorption on suspended matter	[100]
		“	October 1982	0–27	0–9	600	31	−50	As above	“
Hudson	As above	P	min	March 1974	0–29	0–4	25	46	+156	Wastewater inflow	[101]
		“	August 1974	0–29	0–15	50	56	+250	As above	“
		“	August 1988	0–29	0–12	62	68	+90	“	“
Old Mill Creek	“	P	min	December 1978	0–27	0–9	6.0	12	+195	Desorption from suspended matter	[102]
Orinoco	“	P	min	November 1979	0–35	0–6	7.0	<3	+42	As above	[89]
		“	“	0–35	6–20	7.0	<3	−100	Biological consumption	“
Amazon	“	P	min	May 1976	0–35	8–15	15	3.0	−80	Biological consumption	[103]
		“	December 1982	0–36	0–2	20	6.0	−15	Sorption on suspended matter	[104]
		“	“	0–36	2–20	20	6.0	+118	Desorption from suspended matter	“
		“	May 1983	0–32	–	28	12	~0	–	“
Zaire (Congo)	“	P	min	November 1976	0–35	0–20	28	6.0	+100

4. Conclusions

The formation of the chemical composition of surface waters begins already in the atmosphere during the interaction of aerosols with the condensates of water vapor: cloudy water and the water of atmospheric precipitation. The average median concentrations of mineral and total phosphorus in atmospheric precipitation are 15 and 33 μg/L, respectively; the values of the input of these forms into the earth’s surface are equal to 0.11 and 0.25 kg/ha yr. The content of the soluble forms of total phosphorus in atmospheric precipitation is in the range of 20–80%, with an average value of 55%. The average median concentrations of dissolved mineral and total phosphorus in the waters of the primary hydrographic network are 31 and 95 μg/L, respectively. The concentrations of both forms increase with an increasing anthropogenic load: natural (forest) catchments < agricultural–forest catchments with land use less than 50% < agricultural catchments with land use over 50% < urban catchments. The concentration of dissolved mineral phosphorus, all other conditions being equal, increases with an increase of the phosphorus content in the catchment rocks. The average median concentrations of dissolved mineral and total phosphorus in world rivers are 28 and 85 μg/L, respectively. The minimum values are observed in the rivers of the Arctic and subarctic zone; the maximum values are found in the most densely populated temperate zone and the zone of dry tropics and subtropics. The anthropogenic load is a dominant factor for riverine export, which is confirmed by the presence of a direct relationship between the concentrations of mineral and total phosphorus, on the one hand, and the population density, on the other hand. The distribution of dissolved mineral and total phosphorus in the mixing zones of river and sea waters in the overwhelming majority of cases corresponds to nonconservative behavior. The conservative type of distribution is rarely observed and is found in the mouths of polluted rivers with high phosphorus concentrations, which significantly exceed the possible changes that occur as a result of biological and chemical processes. The decreases in the fluxes of dissolved mineral and total dissolved phosphorus at the river–sea geochemical barrier are 40–80% and 7–38%, respectively.