Trophic state of the Limensky Bay water area (Southern coast of Crimea, Black Sea)

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Introduction

The Southern Coast of Crimea is a territory with a unique recreational potential. However, tourism activities influence the environmental situation in the region. In Crimea, especially on its southern coast, environmental risks once arose due to the haphazard planning of cities, which did not consider issues most important for the ecological state of the territories, including optimisation of the drainage system, primarily storm water drainage, transport infrastructure [1]. The main source of marine waters pollution of the Southern Coast of Crimea is wastewater discharge from sewage treatment facilities and recreational complexes such as health resorts and recreation centers [2]. At the height of tourist seasons, the risk of the coast overpopulation increases, which inevitably results in pollution of the beach zone and adjacent sea areas.

Recreational resources of the coastal sea zone rely mainly on the natural environment quality. Thus, the water area of the Crimean southern coast is monitored to determine the state of ecosystems and trends in their variability and to develop recommendations for the rational use of natural resources [2, 3].

One of the adverse consequences of anthropogenic impact on the ecological state of the marine environment may be an increase in the trophic state of the water area. Eutrophication of water bodies is often caused by excessive input of nutrients and easily oxidised organic matter, the main source of which is river runoff and wastewater, whose influence is local in nature. It is impossible to find a general method to assess the trophic state of different marine areas. Each study chooses an approach driven by the choice of indicators and their number when calculating various ecological indices, taking into account a limited set of measured parameters and indicators of the marine environment. The E-TRIX ecosystem trophic state index is based on the concentration of the main nutrients (nitrogen and phosphorus), the degree of water oxygenation and chlorophyll a concentration. The advantage of E-TRIX is the use of standard monitoring characteristics, which allows for a comparative analysis of trophic state in different marine areas, while not only qualifying but also quantifying the water body state.

In various studies (e. g. [4]), the E-TRIX index is calculated from monitoring data. However, a sufficient number of observations at different points in space is not always available. Mathematical modelling makes it possible to fill data gaps and assess the ecosystem state, while considering the variability of its components. In addition, mathematical modelling allows predicting the evolution of an ecosystem under the influence of natural, climatic and anthropogenic factors.

As study object, we have chosen Limensky Bay near the settlement of Katsiveli. This water area is under the least anthropogenic impact and distant from large-scale industrial sewage.

Limensky (Goluboy) Bay is located on the Southern Coast of Crimea between Cape Kikineiz and Mount Koshka. In the southwestern part of the bay, there is a stationary oceanographic platform (Fig. 1). The main pollution sources in the bay are wastewater from Katsiveli and the discharge pipe of used water from the fun water park Goluboy Zaliv.

 

Fig. 1. Study area

 

The hydrological structure of the Limensky Bay waters mainly depends on coastal currents and their variability. An analysis of in situ data shows that

a quasi-stationary west-south-west current is observed along the coast near Cape Kikineiz. The average annual modulus of the current velocity vector was maximum in the near-surface layer and varied within 5.8–9.4 cm/s during the monitoring period [5].

Upwelling occurs in this area under longshore winds with a westerly component (southwesterly, westerly and northwesterly). Such a rise of cold nutrient-rich deep waters to the surface has the greatest impact on the content of hydrochemical components in spring and summer periods. As a result, partial pollution of the coastal zone is possible.

The chemical composition of Limensky Bay waters is unique. The content of dissolved oxygen directly depends on the temperature indicators, and its concentration increases sharply (up to 115%) when surging phenomena occur [6]. Hydrochemical studies showed the presence of two unfavourable areas with reduced dissolved oxygen content (97%) [7].

Coastal waters are generally characterised by moderate concentrations of nutrients. The absence of phosphate deficiency (~ 0.1 µM) is typical of this area even during the phytoplankton growth and development [8], which, in turn, indicates active dynamic processes contributing to phosphate input from the underlying sea layers. The concentration of nitrogen forms was relatively low (0.2 μM) according to the data of the studies carried out in the recreational areas of the Southern Coast of Crimea.

From March 2010 to March 2012, comprehensive ecological studies were performed in Limensky Bay near the mussel and oyster farm located on the traverse of Kikineiz Bay (Fig. 1). An analysis of the obtained data showed [9–11] that during the study period, the hydrochemical regime around the farm was characterised by good aeration of the water column, relatively low nutrient content and insignificant anthropogenic pressure. Extremely high water temperature (over 26 °C) in July–August 2010 determined low quantitative indicators of phytoplankton (20 mg/L).

No water bloom, typical of coastal waters and caused by the development of certain phytoplankton species, was observed during the study period. High content of ammonium nitrogen (up to 30.3 µg/L) in the warm period of the year, as compared to other mineral forms of nitrogen, was due to organic matter degradation.

The main objective of this work is to determine seasonal changes in the trophic state of waters in the Limensky Bay area using the E-TRIX index based on numerical modelling data.

Materials and methods

The trophic state of Limensky Bay was assessed using the E-TRIX index. It is a function containing the following parameters: dissolved oxygen, mineral nitrogen, total phosphorus and chlorophyll a. According to [12], the trophic state index is determined by the formula

E-TRIX = (lg[Ch·D%O·N·P] + 1.5) / 1.2,

where Ch – concentration of chlorophyll a, µg/L; D%O – absolute deviation of oxygen saturation from 100%; N – concentration of dissolved form of mineral nitrogen, µg/L; P – concentration of total phosphorus, µg/L.

E-TRIX index values can range from 0 to 10. Depending on these values, there are four trophic states: oligotrophic (< 4), mesotrophic (4–5), eutrophic (5–6) and hypereutrophic (6–10).

If the trophic state index exceeds 6, the investigated water area contains high concentrations of nutrients and has low transparency, which can lead to hypoxia in the bottom layers of its waters. If the index does not exceed 4, there are insignificant concentrations of nutrients, good air exchange throughout the entire water column and high transparency [13].

The data on the concentration of chlorophyll a, dissolved oxygen, mineral nitrogen, and total phosphorus, necessary to calculate the E-TRIX index, were derived using a one-dimensional version of the water quality model and its eutrophication module [14]. Before calculating, the model was calibrated using data for 2010–2012 on phytoplankton biomass concentration from [9, 10] and concentrations of nutrients and oxygen from the oceanographic database of the Marine Hydrophysical Institute.

Meteorological data were used as model input parameters: wind speed and direction at 4 h intervals, air temperature at 3 h intervals, photosynthetic active radiation per day, humidity and cloud amount at 6 h intervals. We also used annual variations in transparency, values of seawater temperature, salinity and concentrations of phytoplankton, nutrients, oxygen, organic phosphorus and organic nitrogen. They were set starting from 1 January of the model year.

Results

During the model year, the E-TRIX index varied from 3.82 to 4.39 (average 4.09), which is a transitional trophic state of the study water area from the oligotrophic to mesotrophic one. It also indicates good water quality. The highest value on the 247^th model day (5 September) coincided with the autumn peak of phytoplankton blooming, and the lowest value was recorded on the 365^th day (31 December). From the 105^th to 355^th model day, the index exceeded 4, while on other days it was under that value. In terms of seasons, the average index was the highest in autumn (4.22) while in winter it was the lowest (3.96) (Table 1).

 

Table 1. Change in E-TRIX index depending on the season

Season	Value range	Average
Winter	3.82…4.17	3.96
Spring	3.82…4.20	3.98
Summer	3.91…4.36	4.19
Autumn	4.07…4.39	4.22

 

In paper [15], the trophic state index was calculated from in situ data in the Limensky Bay area using a modified formula. The authors used total nitrogen instead of mineral nitrogen and added silicon concentration as a multiplier under the common logarithm sign. Total nitrogen was taken as recommended in paper [16]. Silicon concentration was included in the general formula by the authors of study [15] for a more accurate assessment of water quality, since silicon is an important nutrient. Therefore, the average E-TRIX value in the above work is slightly higher (4.42) than in our results. Due to the absence of silicon in the chemical-biological module of the water quality model we used, it is not possible to numerically verify the result obtained in paper [15] for Limensky Bay.

Fig. 2 shows the annual variability of the eutrophication indicator with chlorophyll a, total phosphorus, mineral nitrogen and absolute deviation of oxygen saturation from 100%. Of note, phosphates, organic phosphorus and ammonium did not have a pronounced seasonal variability. Nitrates had maximum concentrations in the cold period from December to March and minimum concentrations in the warm period of the year, which is shown in Fig. 2, c. A similar result was described by the authors of paper [9]. The winter nitrate input is caused by convective mixing of waters, while the summer one is due to upwelling. Nitrites have maximum values during the increased dynamic activity of waters from December to March. In the water area of Limensky Bay, the river runoff has no direct influence on the hydrochemical structure, which was noted in [10].

The relative contribution of the components included in the E-TRIX calculation formula was calculated. The highest relative percentage contribution to the calculation formula was made by mineral nitrogen (44.48...51.88%, average 48.17%), followed by the chlorophyll a module (−38.71...−24.02%, average −31.19%), total phosphorus (26.59...30.12%, average 28.32%). The smallest contribution was made by absolute deviation of oxygen saturation from 100% (19.44...28.49%, average 24.09%).

 

Fig. 2. Annual variations of E-TRIX index (solid curve) and concentrations (dashed-dotted curve) of chlorophyll a (a), total phosphorus (b), mineral nitrogen (c), oxygen saturation (d) in the Limensky Bay waters

 

Table 2. Relative contribution (%) of the components to the E-TRIX calculation formula

E-TRIX	D%O	Ch	N	P
Max	24.27	−24.66	44.63	27.18
Min	23.86	−38.36	51.88	29.86

 

Table 2 shows the relative contribution in percent of the components resulting from the formula at the minimum and maximum values of the eutrophication index. At the maximum value of the index, the relative contributions of mineral nitrogen and chlorophyll a modulus are minimal, and at the minimum value of E-TRIX, the contributions of these components are maximal. A similar fact was described in paper [17] for Sevastopol and Yuzhnaya Bays.

If a hydrochemical parameter in the original formula is replaced by 1 (i. e. excluded from the formula), the trophic state index can either increase (if chlorophyll a concentration is excluded) or decrease (if total phosphorus concentration, mineral nitrogen concentration, absolute deviation of oxygen saturation from 100% are excluded) (Fig. 3). The figure shows that the E-TRIX value decreases most of all (almost twice) when mineral nitrogen is excluded, which once again shows that its contribution to the calculation formula in this region is maximum.

 

Fig. 3. Contribution of individual hydrochemical characteristics to E-TRIX quantity

 

We calculated correlation coefficients between E-TRIX index and absolute deviation of oxygen saturation from 100%, concentration of total phosphorus, mineral nitrogen and chlorophyll a. The highest values of the correlation coefficient were obtained with the concentration of chlorophyll a (r = 0.84), mineral nitrogen (r = 0.80) and total phosphorus (r = 0.78). The correlation with absolute deviation of oxygen saturation from 100% is weak (r = 0.47).

Conclusion

Modelling of biogeochemical indicators of Limensky Bay and further calculation of the trophic state index showed that the seawater quality of the water area is good with a transitional (from oligotrophic to mesotrophic) trophic state. During the cold period from the 1^st to 104^th and from 356^th to 365^th model days, E-TRIX was below 4, which corresponds to an oligotrophic trophic state, while on other model days it was higher. The maximum value of the index was on the 247^th model day (4.39), and the minimum value was on the 365^th day (3.82).

The highest correlation of the trophic state index is observed with the concentration of chlorophyll a (r = 0.84), mineral nitrogen (r = 0.80) and total phosphorus (r = 0.78). Calculation of the relative contribution of the components included in the E-TRIX calculation formula showed that the main factor determining the level of eutrophication of Limensky Bay waters is the concentration of mineral forms of nitrogen. Thus, ecosystem modelling and further calculation of E-TRIX may help to assess the ecological status of other water bodies, where in situ sampling is difficult.

Sobre autores

Kira Slepchuk

Marine Hydrophysical Institute of RAS

Autor responsável pela correspondência
Email: skira@mhi-ras.ru
ORCID ID: 0000-0001-5437-4866
Researcher ID: H-9366-2017

Junior Research Associate

Rússia, Sevastopol

Tatiyana Khmara

Marine Hydrophysical Institute of RAS

Email: xmara@mhi-ras.ru
Scopus Author ID: 6506060413
Researcher ID: C-2358-2016

Research Associate

Rússia, Sevastopol

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