Keyword

1 Introduction

Eutrophication of natural water bodies is understood as the process of growth of the overall productivity of the ecosystem of a water body, including water masses, bottom sediments, organisms inhabiting them and relations between them. The process of eutrophication leads to the increase of the organic matter in the water of the reservoir.

The degree of eutrophication of a reservoir depends on the size of its external and internal biogenous loads (phosphorus and nitrogen) and can be slowed down by reducing them. Ways to decrease the external biogenous load of the Gulf of Taganrog of the Sea of Azov were discussed earlier (Zhidkova et al. 2014, 2018; Zhidkova 2017; Zhidkova and Gusakova 2014a, b). In this work we consider internal biogenous load of this water object and we define it as a flux of biogenes from the Gulf bottom with the subsequent inclusion in biotic circulation; the internal load is determined by the amount of phosphorus and nitrogen accumulated by the bottom sediments, and the rate of their desorption from the bottom sediments into the water.

The characteristic feature of the Sea of Azov, caused by climatic factors, is the high variability of elements of the water balance and the components of the chemical runoff that predetermines the variability of the biogenous elements balance and forces to look for new methodical approaches to its assessment, analysis and forms of representation (Sukhinov 2006). Many researchers (Zhidkova and Gusakova 2014; Sukhinov et al. 2016; Sukhinov 2006) specify that the concentration of the biogenous elements in the waters of the Sea of Azov substantially depends on the short-period weather phenomena when the structure of the internal chemical and biological processes in the reservoir is broken. These phenomena mention all thickness of waters, the sea “lives” from a storm to storm.

A number of modern works questions deal with issues of anthropogenic eutrophication, including the Sea of Azov. For example, Sukhinov et al. (2016) developed the mathematical model of the solution of the eutrophication of waters of a shallow reservoir considering the movement of a water stream, microturbulent diffusion, gravitational subsidence, spatial and uneven distribution of temperature and salinity, and also the polluting biogenous substances, oxygen phyto- and a zooplankton, etc. Such number of the input data in the model differs in complexity, the need of use of a huge data file and the supercomputing system that does it very expensive and imposes certain restrictions on use of the method in environmental practice when monitoring a water body.

In reservoirs it is almost impossible to define the origin of phosphorus or nitrogen in connection with the different sources of their receipt. On the one hand, this is the input of biogens from outside the catchment areas (precipitation, groundwater, etc.). And on the other hand, due to the internal capabilities of the reservoir: phyto- and zooplankton organisms that decompose in the water column, products excretion of planktonic organisms, bottom sediments from which the biogens enter the photic zone, release the dissolved organic compounds by algae and bacteria, which break down to form dissolved biogens. Besides, there is an allocation of biogenes with excrement planktonic, bentosny, invertebrate and fishes. All these factors make internal load of a reservoir. The internal biogenic load indicates the intensity of the nutrient cycle, because the balance of substances reflects the redistribution of individual chemical elements in the process of geochemical and biogeochemical migration. Management of these processes will allow operating ecosystems of reservoirs.

Thus, the aim of this study is to assess the value of internal biogenic loads on the ecosystem of the Gulf of Taganrog of the Sea of Azov from the standpoint of eutrophication, namely, phosphorus and nitrogen loads.

2 Materials and Methods

Mineral phosphorus is present at water mainly in the form of phosphate and hydrophosphate. It is a part of any organic substance, but in water its contents very small and in pure reservoirs is estimated in thousand shares of a milligram per liter.

During seasonal changes phosphate, as well as nitrate, almost disappear in the near-surface zone of the water area in summer. When organisms die, part of the phosphate enters the water, and also settles in the upper layer of the bottom sediments. Favorable conditions for the accumulation of phosphate arise in the anaerobic conditions inherent to the bottom layers of the shallow, well heated area in the absence of wind hashing. In water from the bottom sediments phosphorus comes back together with iron and again comes to biotic circulation. The increased number of compounds of phosphorus serves in water and rainfall as the evidence of accumulation of organic substance. Therefore, it is considered to be The low indicator of phosphorus in a reservoir is considered to be the indicator of pure water.

The increase in the flow of phosphorus from the bottom of the reservoir (an increase in ecosystem productivity) indicates the increase of the eutrophication of the ecosystem. According to Martynova (2008), at production of the phytoplankton of 200 g/cm2 per year, the internal phosphorus load (a phosphorus stream from the bottom of the water body) sharply increases and becomes comparable with external phosphorus load of the water area. As a result, the eutrophication of the reservoir is accelerated. The most moveable form of phosphorus in the solid phase of the bottom sediments is considered its sorbate. The main reason of the increase of the internal phosphorus load in the eutrophic water object is the increase in the area of the bottom sediments with anaerobic conditions. The formed zones with the deficiency of oxygen in the benthonic layer accelerate the release of the phosphate sorbed in the aerobic conditions by the iron compounds. Alexandrova, Matishov and coauthors (Alexandrova et al. 2013; Matishov et al. 2015), describes such zones in the Sea of Azov. For example, in 2013, the distribution over the water area, as well as in the water column, was studied for the content of dissolved oxygen, pH, nutrients (nitrogen, phosphorus, silicic acid). The development of thr powerful clogging phenomena in the sea was noted because of the density stratification of the water column, the formation of a restoration situation in the surface layer of bottom sediments, which contributed to the enrichment of the water column by the biogenic elements.

Phosphates are well sorbed by the hydroxides of aluminum, manganese and clay materials, but iron is the main sorbent of phosphates. Even in those bottom sediments where the part of phosphate is connected with the aluminum hydroxide, other part them is connected with the iron hydroxide, so, there is a dependence of size of a stream of phosphate on redox-conditions.

The destruction of the organic matter also plays a significant role in formation of the stream of phosphorus from the bottom. The higher eutrophic index of the water body, the more powerful the flow of phosphorus if other is equal. The bond between the internal load of phosphorus and the organic matter in the bottom sediments is linear and depends on the eutrophication of the water body (the eutrophic index) (Martynova 2008).

The compounds of nitrogen and phosphorus coming to the reservoir in a solid phase are partially buried in the bottom sediments and partially returned to water.

The difference in the storage capacity of the bottom in relation to nitrogen and phosphorus is defined by the size of the particles. Therefor, the considerable part of the phosphorus entering the bottom is connected with the mineral particles, which reach the bottom almost without destruction. Nitrogen is precipitated almost exclusively only with the organic matter, most of which is mineralized in water. Therefore, nitrogen is removed from the bottom sediments more actively: 45–70% from accumulated at the bottom whereas phosphorus: 5–25%.

By the size of the particles of the bottom of the Sea of Azov, the following types of the bottom sediments are distinguished: pelitic (clay) and fine aleuritic silts; large silts; fine sand; seashell. Pelitic silts prevail in the Gulf of Taganrog, occupying the entire eastern and central parts of the Gulf in the interval of depths of 4.5–6.5 m. The underwater slope of the Gulf in the range of depths of 2.5–4.5 m is made up of large siltstone and fine aleurite silt. Fine-grained sands are deposited along the perimeter of the Gulf from the edge to depths of 1.5–2.5 m.

The increase of the eutrophication of the water body leads to the increase in the flow of the organic matter to the bottom. As a result, the processes of biochemical oxidation in the bottom sediments are intensified, redox conditions change, and the content of mobile forms of nitrogen and phosphorus increases; namely, their flow from the bottom to the water intensifies. In the eutrophic water bodies, this flow can become a source of the secondary nutrient intake. Thus, the eutrophic level of the reservoir is one of the main factors that determines the internal load of reservoirs.

3 Results

Let us carry out an assessment of the size of the flux of biogenous matter from the bottom of the Gulf of Taganrog of the Sea of Azov in water, based on the ideas of its mechanisms as concentration diffusion and convective transport.

Two methodological techniques are proposed for calculating nitrogen fluxes from the bottom sediments into the water of the Gulf of Taganrog of the Sea of Azov. The first of them involves the use of the Fick equation, the second is developed by Neverova-Dziopak (2003), who constructed a mathematical dependence of the nitrogen flux from the bottom sediments into water on the weight ratio of the concentration of organic carbon to the concentration of total nitrogen (C/N). The lower the ratio, the greater the percentage in the decomposition products of organic substances is ammonia nitrogen (NH4+) (Kuznetsov 1970). The nitrogen flux from the bottom sediments into water increases with decreasing C/N ratio. The experimental data were approximated by Neverova-Dziopak E. as the linear equation:

$$ J_{N} = 122.5 - 7.82 \cdot x $$
(1)

where

JN:

the nitrogen flux from the bottom sediments, mg N/m2 day;

x:

the C/N relation.

The diffusive movement of the dissolved matter in the bottom sediments obeys the first law of Fick:

(2)

where

J:

the size of the diffusive flux, mg/m2 day;

:

gradient of the matter concentration in the pore solution of the bottom sediments and the bottom water;

D:

coefficient of diffusion, m2/day.

Concentration of nitrogen and phosphorus in the pore solution of the bottom sediments are defined by the mineralization speed of the organic substances in the bottom sediments. When concentration of matter in natural waters is 1–2 and more orders lower, than in steam solution, the concentration gradient can be accepted equal in size of concentration of substance in the pore solution.

The coefficient of diffusion depends on the diameter and length of the pores, temperature, charge and the mass of the ion, etc. For the diffusion of ammonium ion and phosphate the following data on its value are presented:

  • 2.4 × 10−6cm2/s and 0.93 × 10−6cm2/s (T = 24 ℃) (Kuznetsov 1970).

In view of mean annual values of the content of carbon, nitrogen and phosphorus in the bottom sediments of the Gulf of Taganrog of the Sea of Azov, calculate the biogenous internal load of the studied water area.

The mean annual concentration of carbon and nitrogen in the water of the Gulf of Taganrog:

  • C = 0.348 mg/l;

  • N = 0.062 mg/l.

The water area square is:

  • S = 5300 km2.

Then the internal load of nitrogen of the studied water area is:

  • JN = 75.58 mg N/m2 day.

The average volume weight of the bottom sediments is about 1.5 t/m3. Then the mass of the active layer of the bottom sediments (Ma) in the studied water area is:

  • Ma = 159 × 106 t.

At average humidity of the bottom sediments of 38%, the mass of the air solid (the mass of the active layer of the bottom sediments of the Gulf of Taganrog) in the bottom sediments is:

  • Ms = 98.6 × 106 t.

At concentration of nitrogen in the top layer of the studied bottom sediments of 0.062 g/g, the mass of the general nitrogen in the active layer of the bottom sediments of the Gulf of Taganrog of the Sea of Azov is:

  • MN = 6.11 × 106 t.

At calculation of the intake of nitrogen for the Fick equation, accept that the mass of nitrogen in the 2 cm layer of the bottom sediments is 6.11 × 106 t.

The process of ammonification is described by exponential dependence with the constant 0.03 days−1. Then the receipt of ammonium ion is:

  • MNH4+  = 183 × 103 t/day.

The concentration of nitrogen in the pore solution is 1700 mg/l.

The coefficient of diffusion is 2.4 × 10−6.

The flux quantity of the bottom sediments in the water is:

  • JN = 35 mg N/m2 day.

Thus, at average size 1700 mg N/m2 day the internal load of nitrogen for the Gulf of Taganrog is 67.7 × 103 t/year.

As for the influence of the secondary intake of nitrogen on the processes of the eutrophication in the Gulf of Taganrog of the Sea of Azov, the daily gain of the concentration of nitrogen in the water is:

where the capacity of the water area is 25 × 109 l.

Phosphorus of the bottom sediments. The accumulation of phosphorus in the bottom sediments happens both in the organic, and in the mineral forms, and from 40 to 80% of the organic phosphorus getting to the reservoir is mineralized.

The dissolved mineral phosphorus is present as the orthophosphate, which concentration is limited by the solubility and depends on the redox conditions, pH and water salinity. The dissolved phosphates form compounds with iron, aluminum and calcium. In aerobic conditions, the speed of the removal of phosphate from silts to water is 5–10 times lower, than in the anaerobic.

In the water area of the Sea of Azov, the huge zone of the anaerobic contamination of the ground (about 1000 km) is fixed from year to year. It is characterized by almost the total absence of the oxygen in the benthonic layer at calms or close to them wind situations. Several such zones were also found in the waters of the Gulf of Taganrog (Sukhinov et al. 2016); however, their lifetime is calculated in days due to the shallow water and the agitated waters.

Sizes of sorption and desorption of phosphorus in the aerobic and anaerobic zones of the Gulf of Taganrog of the Sea of Azov have to differ considerably owing to the distinctions of the redox potential in the benthonic sheets of water, pH and salinity.

In the aerobic zones, the phosphorus flux from the bottom sediments in the water is be defined, mainly, by the ratio of speed of the mineralization of the organic phosphorus and the speed of the chemisorption of the mineral phosphorus. Thus the speed of the chemisorption is commensurable or even higher the speed of the mineralization.

In the anaerobic zones the phosphorus flux, is be defined generally both the desorption speed of the mineral phosphorus and the speed of the mineralization of the organic phosphorus. Whereupon Vdesorb ≥ Vsorb.

For the fresh-water silts the following values of the coefficient of diffusion of hydro phosphate is given in literature: 0.31 × 10−6–0.93 × 10–6 cm2/s (Kuznetsov 1970).

Let us calculate a phosphorus flux from the bottom sediments of the waters of the Gulf of Taganrog when the value of the coefficient of diffusion is maximum.

The top 5 mm of the silt bottom sediments are oxidized when the concentration of the dissolved oxygen is more than 8 mg/l in natural waters. Iron and manganese are present mainly in the form of the hydroxides. The mineral phosphorus is occluded on hydroxides or forms insoluble compounds. Therefore, the intake of phosphate from the bottom sediments to water strongly decreases in the aerobic conditions.

The bottom sediments become “a trap” for phosphorus. At the aerobic exchange the bulk of the phosphate, deleted from the bottom sediments, is the flux, formed owing to the mineralization of the organic substance.

The calculation of the phosphorus flux from the bottom sediments of the waters of the Gulf of Taganrog is made based on the following assumptions:

  • the processes of the chemisorption of the mineralized phosphorus are not considered;

  • the source of phosphate is the organic phosphorus;

  • the process of the phosphatification is described by the exponential dependence;

  • the constant of speed of the phosphatification is 0.018 days−1 (Neverova-Dziopak 2003);

  • the volume of pore solution is equated to the volume of the active layer of the bottom sedimints (2 cm thickness)—10.6 × 106 m3;

  • the gradient of the concentration of phosphate is equal to the concentration in the pore solution;

  • the coefficient of diffusion is 0.93 × 10–6 cm2/s.

Then the mass of the organic phosphorus in the bottom sediments of the Gulf of Taganrog is:

  • Mp = 344.5 × 103 tons,

where the average annual concentration of phosphorus in the bottom sediments is 0.065 × 10−5 t/t.

The maximum size of the intake of the mineralized phosphorus from the bottom sediments to the water is 337.6 tons in the first 24 h.

The concentration of phosphorus in the pore solution is calculated by analogy with the concentration of nitrogen (3180 mg/l).

At the accepted coefficient of diffusion (0.93 × 10−6 cm2/s), the size of the diffusion flux of phosphorus in the waters of the Gulf of Taganrog is:

  • JPм = 25.5 mg P/m2 day.

Thus, the internal load of phosphorus for the waters of the Gulf of Taganrog is:

  • Jp = 4.93 t/year.

The maximum daily gain of the mineral phosphorus in the water is 0.54 × 10− 3 mg/l.

4 Discussion

Calculation of the intake of nitrogen and phosphorus made oversized. In fact, the phosphorus flux from the bottom sediments to the waters of the the Gulf of Taganrog is less.

We do not give the similar calculation for the anaerobic zones in the water area because of the small amount of the time of the existence of such zones in the Gulf of Taganrog. Nevertheless, the general approaches to the calculations are as follows.

It is known that when the value of the redox potential of the bottom water decreases to 0.24 V, the active diffusion of iron, manganese and phosphorus begins from the bottom sediments due to the reduction of hydroxides and the destruction of the complex compounds. Under these conditions, the bottom sediments can be a significant source of the secondary intake of phosphate.

Besides, solubility of phosphate increases when the salinity does. With a high content of organic elements in the silts, a significant fraction may also be the flux of phosphorus formed during the mineralization of the organic compounds.

Therefore, in calculating the flux of phosphorus from the bottom sediments in the anaerobic zones, where there is a deficit of oxygen in the bottom layers, both the organic and mineral phosphorus should be taken into account.

With the 5300 km2 area of water body, average annual concentrations of phosphorus (0.348 mg/l) and nitrogen ( 0.062 mg/l):

  • the flux quantity of the bottom sediments in the water is 35 mg N/m2 day;

  • the internal load of nitrogen for the Gulf of Taganrog is 67.7 × 103 t/year;

  • the size of the diffusion flux of phosphorus in the waters of the Gulf of Taganrog is 25.5 mg P/m2 day;

  • the internal load of phosphorus for the waters of the Gulf of Taganrog is 4.93 t/year;

  • the external load of the Gulf of Taganrog is defined by the Don River runoff and the sewage;

  • the external load of phosphorus is 4580 tons per year,

  • the external load of nitrogen is 25,559 tons per year;

  • the size of the internal nitrogen load is 67,700 tons per year,

  • the size of the internal phosphorus load is 4930 tons per year;

  • the overall nitrogen load is 93,259 tons per year;

  • the overall phosphorus load is 9510 tons per year.

Thus, in spite of the fact that the calculation of the intake of nitrogen and phosphorus to the water area of the Gulf of Taganrog is made with a reserve, the size of the internal nitrogen and phosphorus loads of the water is represented rather high and has to be considered at calculations of the eutrophication processes.

The factors of the overall biogenous load that can change the current state of the waters of the Gulf of Taganrog are absent at present. No measures to reduce phosphorus and nitrogen loads are carried out.