Abstract
Razim Lake is the biggest of Romania’s freshwater lakes and along with other basins as Golovita, Zmeica and Sinoie constitutes a system of great ecological significance, playing also an essential role in the supply of water for irrigation, fishery exploitation, farming, flood prevention, recreational navigation and water tourism. Due to their importance, the environmental conditions in the Razim - Sinoie coastal lakes have attracted an increased public attention in contemporary society. To assess the levels, dissemination and potential sources of contamination in the above-mentioned lagoon system, random sampling was used to collect water and sediment samples from every lake and several analytical techniques were performed to investigate their environmental characteristics. The results obtained from this study indicated that, in water, concentrations of various physico-chemical parameters are, mostly, in agreement with correlated environmental standards. Slight variations and/or occasional exceeding of the maximum admissible limits were generally limited to small areas showing levels that would not warrant special concern. In sediments, the mean concentrations of some specific trace metals were below the levels of potential effect. Benthic samples revealed 31 taxa belonging to 16 zoo-benthal subdivisions. The results of this study showed good ecological status despite local several natural and anthropogenic stressors as fishery exploitation, farming, recreational navigation and water tourism.
1 Introduction
Water quality impairment is especially more alarming with anthropogenic interventions and climate change that are increasing environmental risk in many natural water bodies, such as rivers, lakes, coastal lagoons etc. Consequently, the water quality monitoring and surveillance becomes imperative in terms of environmental conservation and preservation and expected future changes in the environment. The Water Framework Directive (WFD) [1] addresses all rivers, lakes, transitional and coastal waters of the European Union to be in good ecological status in the near future; in this sense, improvements in monitoring and evaluation programs need to be developed permanently in order to achieve a better understanding of aquatic ecosystems [2]. Aquatic ecosystems are also likely to have been altered substantially by agricultural activity (e.g., fertilizer usage) and urbanization (e.g., wastewater production) [3]. There is a lot of evidence of water quality degradation, reported in sedimentological, mineralogical, geochemical, etc. studies that reveal the water quality changes [4] of different water systems. Quality assessment of transitional waters is considered an essential extension of such studies related to environmental quality assessment. Transitional waters are defined as ”bodies of surface water in the vicinity of river mouths which are partially saline in character as a result of their proximity to coastal waters, but which are substantially influenced by freshwater flows” [1]. These include estuaries, deltas and coastal lagoons, evolving ecosystems with distinctive ecological boundary conditions [5, 6, 7]. The lagoons are one of the most productive ecosystems in the world [8], being also of great economic importance through fishing exploitation, farming and water tourism [9, 10]. The variability of the environmental conditions is a common characteristic of brackish waters such as coastal lakes, estuaries, and lagoons due to their natural susceptibility to environmental imbalance [11]. Transitional waters and coastal areas are complex aquatic systems that have been widely impaired by increased anthropogenic pressures [12], as well as a result of global climate changes [13, 14]. The environmental conditions in lacustrine ecosystems are governed by several physico-biogeochemical processes [15] which in turn depend on climate conditions and/or extreme climate events, as well as on the flux of exchange between fresh-water and marine water inputs. Several studies based on integrative data [16, 17] focus on assessing the environmental status of lagoons [18, 19] and support their sustainable management requirements. Monitoring of water and ecosystem quality in the coastal zone should be performed at spatial and temporal scales that adequately address the objectives of the specific study. Numerous specialists in different scientific fields have used a variety of approaches in coastal zone environmental quality assessment through developing and applying environmental indicators and indices [20, 21], evaluating mostly (i) the biological effects of sediment contamination [22, 23], (ii) the interactive toxic effects of complex chemical mixtures in the sediment [24, 25], (iii) the effects of urban water discharges [26].
Water pollution caused by industrial, household or agricultural waste, may pose a range of threats to human health and the wider environment. As stated by the European Commissions (EC) information, 20 percent of all surface water in the EU is seriously threatened by pollution [1]. The fragile and complex ecosystem of the Danube Delta and Razim-Sinoie Lagoon Complex is under increasing pressure from development and incoming upstream pollution [27]. The main source of pollution with impact on the deltaic lagoon edifice is represented by the Danube River. At a regional level (Central and Eastern Europe), the Danube River basin passes through a series of significant recent changes in water quality, including physical, chemical and biological water quality [2]. The main factors that affect the water quality of the Danube River basin are represented by organic pollution, nutrient pollution, hazardous substances pollution, microbial pollution and alterations due to the hydro morphological pressure [28]. Moreover, the building of the Iron Gate dams (1972 and 1985) had a negative impact on the sedimentation decreasing by 58.74%, while after 1989 these values grew 2.25 times [29]. The hydrology and water quality characteristics of the lacustrine systems considerably fluctuate, being originated from the seasonal and interannual variability of the river and lake water flow, as a result of the water transport, periodical floodings and water flow under reed beds [30]. The spatio-temporal variations of hydrological status may strongly influence the aquatic ecosystems of the area, i.e., phytoplankton and zooplankton communities, plant, fish etc. [30, 31, 32].
Razim - Sinoie Lagoon Complex stands out as the largest lagoon (1145 km2) in Romania, [33], as well as for its ecological, historical and socioeconomic importance. The lagoon complex and the Danube Delta edifice are of particular concern for breeding and wintering birds [34, 35], even if significant parts of the delta itself were actually irreversibly destroyed or impaired by the end of the 1980s. Over the past 70 years, the environmental quality of this deltaic-lagoon edifice has been strongly influenced by anthropogenic activities (e.g., potential upstream incoming contaminants arriving from the riparian countries of the Danube River, as well as Romanian local sources, the regulation and hydro-technical works in the Danube River, Danube Delta and lagoon complex etc.). To a lesser extent, the lagoon system experienced a variety of negative effects and habitat loss, but the closure of the connection between the lakes and the sea created a decline in water salinity [36]. Additionally, [37], mentions that the main anthropogenic impact has a connection with the hydrological interventions that lead to decreasing salinity and increasing eutrophication. Although several previous studies have investigated the Danube Delta Biosphere Reserve (DDBR) area under several aspects (i.e., the genesis of the units, the hydrology, sedimentology, evolution and morphodynamics, population structure and dynamics of ecological communities) there is far less information available for pre-existing data or coordinated databases regarding the environmental quality assessment in this area. The existence of several interrelated and interdependent factors induces water quality impairment and damage to the functioning of the Danube Delta. In the past, external high nutrient loads of the Danube river upstream of the delta, have been advanced by the engineering impoundments (e.g., levees, dikes, cut-offs, canalization) and other anthropogenic pressures (i.e., intensive agriculture, forestry and fisheries, the Danube River hydraulic regime changes) [38]. Anyway, in the last period, there is in progress the development of comprehensive monitoring programs and implementation of multiple measures aimed to protect the environment.
The aim of this study is to assess the physico-chemical and biological characteristics of the water and sediments from the Razim - Sinoie lagoon complex, an area of great ecological concern. Study sites are areas of high natural relevance, as being an important part of the DDBR, included in international conventions and directives, most of them being subject to the potential impact of anthropogenic pressures. Therefore, in terms of the European Water Framework Directive criteria, integrating measurements, data and results obtained within this study may improve the existing knowledge database, which is useful to identify trends in evolution, further environmental assessments etc.
2 Materials and methods
2.1 Study area
The vast unity, known as the Danube Delta Biosphere Reserve (DDBR) is located in Romania, in the central southeastern part of Europe, in the lower course of the Danube River [39]. DDBR (5800 km2) consisting of the Danube Delta and Razim - Sinoie Lagoon Complex compartments, has a triple international protective status, being a Biosphere Reserve, a Ramsar Site and a World Cultural and Natural Heritage [40]; it occupies the 3rd place in the world in terms of the largest biodiversity, being the environment for a multitude of rare species of plants, animals [33], huge colonies of aquatic birds and fish, many of them being considered endangered species. The Razim-Sinoie Lagoon Complex is situated between the southern part of the coastal Danube Delta and the north-western coast of the Black Sea, where the Danube freshwater merges with saltwater from the sea. The complex holds a total surface area of about 1145 km2, of which the lagoons and limans spread out over 863 km2. The main lakes of the lagoon complex are Razim (Razelm) with an area of 415 km2 and a maximum depth of 3.5 m [41], Goloviţa (118.7 km2), Zmeica (54.6 km2) and Sinoie (171.5 km2), (Figure 1). The complex is supplied with fresh water from the Sf. Gheorghe branch through the Dunavăt and Dranov canals, and communicates with the sea via Gura Portitei, Periboina and Edighiol inlets. Actually, the Razim-Sinoie lacustrine complex incorporates a suite of former lagoons changed into lakes (e.g., Razim, Golovita, and Zmeica) and contemporaneous lagoons with man-made inlets (e.g., Sinoie Lagoon), along with other lakes, wetlands, barrier beaches, and sandbars. Several studies have been carried out on the evolution of the deltaic-lagoon edifice and its prevailing processes. A series of events on the geomorphological and geological evolution has been increasingly approached in the literature, starting with [42, 43, 44] and up to the most recent contributions elaborated by [45, 46, 47]. Furthermore, significant environmental studies have been reported by [33] and [48, 49]. The coastal lagoon complex, initially, a marine bay (e.g., the former Halmyris Gulf) became gradually isolated from the sea by the sand accretion and redistribution of these sandbars [50]. The evolution of the coastal lagoon complex in response to human interference under the impact of hydro-technical works has been lately considered. During the 70s, the interruption of the water exchanges between the Razim and Golovita lakes led to increased residence time, eutrophication and salinity [51]. Then, in the latter half of the twentieth century, the man-made system of dams and dikes and water drainage work has had some serious ecological repercussions and in some cases, the effects of dams located in the sector south of the former inlet (e.g., Gura Portitei), have been mentioned as an influential factor being responsible for increasing coastal erosion [52, 53].
Configurations and control of the main inlets, canal deepening and dredging and engineering works led to the obstruction of the connections between the different sectors of the Razim - Sinoie Lagoon System, as well as those between the two inlets (Periboina and Edighiol) of the Sinoie Lake with the sea. These circumstances converted the ecological conditions of a brackish lagoon to a freshwater lake [54], having major implications on the existing flora and fauna. Present paper includes a broad limnological investigation of coastal lakes as Razim, Golovita, Zmeica and Sinoie, along with some river or canal control points (e.g., Sf. Gheorghe Branch, Mustaca, Cocos, and Dunavăt canals).
2.2 Field collection and laboratory analysis
Sampling was conducted during two campaigns, Campaign A, carried out in May 2016 (the wet season) in Razim and Golovita Lakes and Campaign B held in August 2016 (the dry season) in Golovita, Zmeica, and Sinoie Lakes. The sampling station network is shown in Figure 1. Information collected included location, description, GPS coordinates, water depth, the main climatic parameters and the real-time water level[1]. The Danube River is the most important freshwater and sediment supplier to the Danube Delta, respectively in the coastal environment area. Water level changes in the hydrological regime of the Danube River have been noticed during sampling periods. Accordingly, a higher mean value of the river water level (278.17 cm) was recorded during Campaign A (May), in comparison to a lower mean value of the river water level (141.92 cm) registered in Campaign B (August), (Figure 2). Additionally, changes in climate conditions have been observed, and as a consequence, Campaign A (May) was characterized by normal temperatures and rain episodes, contrary to Campaign B (August) when it was drought, high temperatures and lack of precipitation.
Water quality was investigated by measuring in situ parameters; surface water samples (1 m) were collected using non-metallic water sampler. Sample collection and handling were done according to national standards. The sediment sampling (sediment-water interface, 0-20 cm) were obtained using a Van-Veen grab sampler. For laboratory analyses, the water and sediment samples were kept in standard conditions until reaching laboratories. Water quality parameters were measured using a WTW Multiline P4 Multiparameter (e.g., temperature, pH, dissolved oxygen, conductivity, oxido-reduction potential), HACH 2100Q (e.g., turbidity), HACH 5000 - UV-Vis – Spectrophotometer (e.g., nitrites, nitrates, phosphates, sulphates) and HI 83200 Multiparameter Photometer (e.g., total chlorine, copper, iron, nickel, zinc, chromium, manganese).
The lithology of the sediment samples was generally determined using observations based on visual (macroscopic) description. To determine the lithological composition of the sediments (content of the total organic matter, carbonates and siliciclastic fraction), the samples were processed using standard Loss on Ignition (LOI) method [55, 56, 57]. In this regard a high-temperature electric furnace SNOL 8.2/1100°C was used. The total organic matter content and the minerogenic matter/mineral residue (the inorganic non-carbonate fraction) were obtained by calcination at 550°C [55], respectively at 950-1000°C [58]. The assessment of the carbonate content was done according to Loss On Ignition (LOI) Protocol[2]. Based on the weight percentage of the carbonate content [59] the sediment samples were grouped in: non-carbonate sediments (less than 0.5% of the total weight of dry sediment) low carbonated sediments (0.5 - 1%), and carbonated sediments (more than 1%).
Major, minor and trace components in surficial sediment samples (Fe2O3, TiO2, Rb, Sr, Zr and V) were analyzed by X-ray fluorescence spectrometry on a VRA - 30 XRF sequential spectrometer, fitted with an X-ray tube with chromium anode, directly on pressed pellets. An analyzer crystal LiF 200 was employed to select the characteristic wavelengths, measurements being done with a Na (Tl) J scintillation detector. Titration methods were used for analyzing CaCO3 [60], and TOC (total organic carbon), [61]. MnO, Cr, Zn, Ni, Co, Cu and Pb were analyzed by flame atomic absorption spectrometry and Cd by electrothermal atomic absorption spectrometry on a Pye Unicam SOLAAR 939E double beam absorption spectrophotometer with deuterium lamp background correction. Accuracy and precision of AAS and XRF analyses were checked with several SRMs from US Geological Survey, NISTI and IAEA. Compared with the certified concentrations, recovery for AAS varied from 93.2% - Co to 99.4% - Pb, while for XRF the recovery range was 90.3% (Zr) -104.4% (Sr). Precision, expressed as the coefficient of variation for 6 replicate determinations varied for FAAS between 0.8% (Zn) and 4.5% (Mn), for XRF from 0.2% (Fe2O3) to 6.2% (V), the highest variability has been recorded for Cd -12.5%, GFAAS determination.
The analyses of the benthos samples and data analysis were done according to the usual methodologies used in freshwater benthology, i.e., the harvesting multihabitat technique which is a modified version of AQEM Method (Assessment system for the ecological Quality of streams and rivers throughout Europe using benthic Macroinvertebrates), a procedure used in the monitoring of the ecological status of surface water bodies in Romania. The sediment samples were washed through 1.000 mm, 0.500 mm, 0.250 mm and 0.125 mm mesh sieves, and preserved in 4% neutralized formaldehyde solution. Subsequently, a Carl Zeiss SteREO Discovery V8 stereo microscope (magnification: 10X) and an Axio Star microscope (magnification: 100X) were used to identify individual organisms to the lowest taxonomic level (e.g., species, genus, family).
3 Results
3.1 Spatial distribution of physico-chemical and water quality parameters
The measurement results of the physicochemical parameters at four sampling sites are shown in Table 1.
RAZIM Lake (May 2016) | Value | T | pH | O2 | O2 | N- | N- | P- | Chla | EC | TDS | Turb | TSS | ORP | |
(°C) | (units) | (mg·L−1) | (%) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μg·L−1) | (μs/cm) | (mg·L−1) | (mg·L−1) | (NTU) | (mg·L−1) | (mV) | ||
min | 22.8 | 8.49 | 9.29 | 110 | 0.006 | 0.01 | 0.06 | 2.54 | 423 | 211.5 | 38 | 2.12 | 7 | –29 | |
max | 24.50 | 8.83 | 13.33 | 157.10 | 0.016 | 0.03 | 2.19 | 21.42 | 510.00 | 255.00 | 47 | 24.8 | 43 | 16 | |
mean | 23.75 | 8.64 | 10.70 | 126.58 | 0.009 | 0.02 | 0.84 | 8.81 | 466.40 | 233.20 | 42.11 | 13.46 | 20.9 | –5.4 | |
GOLOVITA Lake (May 2016) | Value | T | pH | O2 | O2 | N- | N- | P- | Chla | EC | TDS | Turb | TSS | ORP | |
(°C) | (units) | (mg·L−1) | (%) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μg·L−1) | (μs/cm) | (mg·L−1) | (mg·L−1) | (NTU) | (mg·L−1) | (mV) | ||
min | 23.8 | 8.53 | 9.05 | 106.2 | 0.012 | 0.02 | 0.08 | 5.54 | 496 | 248 | 40 | 7.03 | 14 | –3 | |
max | 25.3 | 8.62 | 9.63 | 115.6 | 0.024 | 0.03 | 1.66 | 11.34 | 527 | 263.5 | 45 | 27.2 | 32 | 11 | |
mean | 24.48 | 8.57 | 9.33 | 111.3 | 0.016 | 0.02 | 0.85 | 8.27 | 513.8 | 256.9 | 42.75 | 14.85 | 21.75 | 7 | |
GOLOVITA Lake (August 2016) | Value | T | pH | O2 | O2 | N-NH4 | N- | N- | P- | P | EC | TDS | Cltotal | Ca | |
(°C) | (units) | (mg·L−1) | (%) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μs/cm) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | ||
min | 24.3 | 8.88 | 10.86 | 130.2 | 0.3 | 0.005 | 0.01 | 0.04 | 0 | 389 | 194.5 | 0.01 | 33 | 60 | |
max | 29.1 | 9.07 | 12.2 | 148.4 | 0.45 | 0.007 | 0.01 | 0.11 | 0 | 492 | 246 | 0.03 | 39 | 120 | |
mean | 25.49 | 9.00 | 11.71 | 141.92 | 0.38 | 0.006 | 0.01 | 0.08 | 0.00 | 447.44 | 223.72 | 0.02 | 36.33 | 83.33 | |
Value | Mg | Turb | TSS | ORP | Sio2 | cr | Cu | Zn | Fe | Mn | Ni | ||||
(mg·L−1) | (NTU) | (mg·L−1) | (mV) | (mg·L−1) | (μg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μg·L−1) | |||||
min | 10 | 14.2 | 36 | 3 | 0.15 | 3 | 0 | 0 | 0.01 | 0.3 | 0.07 | ||||
max | 20 | 40.2 | 55 | 20 | 0.40 | 37 | 0.09 | 0 | 0.01 | 0.3 | 0.1 | ||||
mean | 15.00 | 24.76 | 43.38 | 11.56 | 0.28 | 18.67 | 0.04 | 0.00 | 0.01 | 0.30 | 0.09 | ||||
ZMEICA Lake (August 2016) | Value | T | pH | O2 | O2 | N-NH4 | N- | N- | P- | P | EC | TDS | Cltotal | Ca | |
(°C) | (units) | (mg·L−1) | (%) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μs/cm) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | ||
min | 24.70 | 9.02 | 9.47 | 113.30 | 0.06 | 0.005 | 0.01 | 0.07 | 0 | 410.00 | 205.00 | 0.01 | 28.00 | 50 | |
max | 26.80 | 9.73 | 19.02 | 236.00 | 0.2 | 0.019 | 0.02 | 0.16 | 0.2 | 790.00 | 395.00 | 0.05 | 59.00 | 100 | |
mean | 25.92 | 9.29 | 12.46 | 158.66 | 0.12 | 0.011 | 0.02 | 0.10 | 0.07 | 520.14 | 260.07 | 0.03 | 40.83 | 78.33 | |
Value | Mg | Turb | TSS | ORP | Sio2 | Cr | Cu | Zn | Fe | Mn | Ni | ||||
(mg·L−1) | (NTU) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μg·L−1) | |||||
min | 10 | 2.54 | 7.00 | 3.00 | 0.09 | 2 | 0 | 0 | 0.01 | 0.2 | 0.04 | ||||
max | 20 | 40.00 | 60.00 | 40.00 | 1.64 | 45 | 0.07 | 0 | 0.04 | 0.3 | 0.09 | ||||
mean | 17.50 | 13.81 | 26.50 | 18.33 | 0.62 | 21.67 | 0.01 | 0.00 | 0.02 | 0.25 | 0.07 | ||||
SINOIE Lake (August 2016) | Value | T | pH | O2 | O2 | N-NH4 | N- | N- | P- | P | EC | TDS | Cltotal | Ca | |
(°C) | (units) | (mg·L−1) | (%) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μs/cm) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | ||
min | 25.20 | 8.69 | 9.79 | 118.70 | 0.28 | 0.005 | 0.01 | 0.06 | 0.1 | 389.00 | 194.50 | 0.01 | 29 | 40 | |
max | 27.70 | 9.41 | 15.72 | 195.50 | 1.67 | 0.007 | 0.01 | 1.85 | 0.8 | 9820.00 | 4910.00 | 0.09 | 69 | 110 | |
mean | 26.41 | 9.13 | 13.31 | 165.03 | 0.80 | 0.006 | 0.01 | 0.70 | 0.32 | 1813.94 | 906.97 | 0.04 | 45 | 75.00 | |
Value | Mg | Turb | TSS | ORP | SiO2 | Cr | Cu | Zn | Fe | Mn | Ni | ||||
(mg·L−1) | (NTU) | (mg·L−1) | (mV) | (mg·L−1) | (μg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (mg·L−1) | (μg·L−1) | |||||
min | 5 | 3.10 | 8.00 | 31.00 | 0.10 | 0 | 0 | 0 | 0.01 | 0.2 | 0.04 | ||||
max | 20 | 50.30 | 71.00 | 112.00 | 1.98 | 78 | 0.13 | 0 | 0.01 | 0.5 | 0.41 | ||||
mean | 12.50 | 27.37 | 43.33 | 61.50 | 0.54 | 13.67 | 0.06 | 0.00 | 0.01 | 0.33 | 0.24 |
The results of the main physico-chemical parameter measurements (Table 1) were related to national environmental standards and regulation enforcement[3]. Other investigated indicators (not included in the national standard) were compared to ensure compliance with the relevant environmental standards, such as total dissolved organic and inorganic substances[4], turbidity[5], total suspended solids[6], oxidation-reduction potential [62] and the silica content[7].
The average water temperature distribution was consistent with the expected seasonal variations for continental-temperate climate regions. During the both surveys no extreme values were recorded (Table 1).
All the lakes were generally well-aerated with a high amount of oxygen dissolved in water (Figure 3a). Levels of dissolved oxygen throughout all sampling sites were systematically higher than the corresponding values for atmospheric-water equilibrium at the measured temperatures, with saturations varying from 106.2% (Golovita L.) up to 236% (Zmeica L.), the mean concentrations ranging from 9.33 mg·L−1in Golovita L. (May) to 12.46 mg·L−1in Zmeica L. (August).
The pH value of the lakes varied slightly in the four lacustrine areas. The vast majority of the pH values measured in the surface waters was found in the range of slightly alkaline waters. The pH average values varied from 8.57 - e.g., Golovita L. (May) to a maximum of 9.29 in Zmeica L., where the highest oxygen concentrations were identified too. The highest pH values recorded at Zmeica and Sinoie lakes (Figure 3b), located near the coastal area were significantly higher than those values recorded for Razim and Golovita sites.
The concentrations of N-
Chlorophyll ”a” (Chla) was determined only in samples collected in May, in Razim and Golovita lakes. Its average concentrations in both lakes were almost similar in value.
The silica content (SiO2) was measured only in the driest period (August), and its values were relatively uniform in the investigated lakes as Golovita, Zmeica, and Sinoie.
The concentrations of salinity parameters (e.g., EC, TDS,
The concentrations of calcium (Ca) tested in water samples (August) were relatively higher in Golovita compared to Zmeica and Sinoie. Instead, the magnesium (Mg) concentration, measured in the same locations was very low.
The concentration of turbidity and TSS (total suspended solids) showed a high variability. All the investigated lakes were characterized by high turbidity. The turbidity (Figure 3e) varied at measured points, ranging in a relatively wide interval, with high values registered in Razim and Golovita (May). Also, the content of the TSS measured in Razim and Golovita (May) noticed significantly higher values. The same is valid for the investigated lakes (August) that present similar high levels of turbidity in Golovita, Zmeica and Sinoie lakes.
Regarding the distribution of the TSS measured in Golovita, Zmeica and Sinoie (August), notable fluctuations were encountered (Figure 3f). Both turbidity and TSS content were higher in Sinoie Lake in contrast to Golovita and Zmeica, a spatial differentiation being noticed in this sense.
The oxidation-reduction potential (ORP) showed little variation at all sampling points (Figure 3g). Between the wettest and driest periods were no major differences, in the samples measured in Razim and Golovita (May), in comparison to measurements performed on Zmeica, Golovita, and Sinoie (August). The obtained results were within the normal range of variation for natural waters.
Some samples of surface water collected (August) from Golovita, Zmeica and Sinoie lakes reveals lower levels of total chlorine, copper, iron, nickel, and zinc. Instead, higher levels of chromium were noticed in few sampling stations gathered from Golovita, Zmeica and Sinoie lakes.
3.2 Sediment Characteristics
3.2.1 Bulk lithological parameters
The physical characteristics of the bed-sediments collected from Razim, Golovita, Zmeica and Sinoie, along with some river or canal control points (e.g., Sf. Gheorghe Branch, Mustaca, Cocos, and Dunavăt canals) are shown in Table 2. The main lithological components of the lacustrine sediments, as the total organic matter (TOM%), total carbonates (CAR%) and siliciclastic/minerogenic (SIL%) showed significant variations among investigated sites. As shown in Table 2, the average (%) TOM content in the samples gathered from the sections of controls (Sf. Gheorghe and connections canals) reached generally a low level, with values below 50% of the total weight of dry residue, as in Cocos Canal, Sf. Gheorghe branch, and, subsequently, in Mustaca, Cocos and Dunavăt canals. The average (%) CAR content presented values greater than 1% of the total weight of dry sediment and ranged between the investigated sampling sites as in Cocos C., Sf. Gheorghe branch (May) and, subsequently, in Mustaca, Cocos and Dunavăt canals (August).
Location | Physical properties of lacustrine sediments | |||||
---|---|---|---|---|---|---|
Value | Water content (WC%) | Dry matter residue (DM%) | Total organic matter (TOM%) | Carbonates (CAR%) | Siliciclastic fraction (SIL%) | |
Cocos C. (May) | - | 25.13 | 74.87 | 2.28 | 11.69 | 86.03 |
Razim L. (May) | min | 21.91 | 53.00 | 1.20 | 12.31 | 42.77 |
max | 47.00 | 78.09 | 44.86 | 28.89 | 82.53 | |
mean | 36.58 | 63.42 | 22.54 | 16.14 | 61.32 | |
Golovita L. (May) | min | 29.45 | 54.05 | 10.48 | 11.81 | 37.09 |
max | 45.95 | 70.55 | 51.10 | 15.40 | 75.01 | |
mean | 38.16 | 61.84 | 26.12 | 13.63 | 60.25 | |
Sf. Gheorghe (May) | - | 32.52 | 67.48 | 6.19 | 15.21 | 78.59 |
Sf. Gheorghe (May) | - | 38.14 | 61.86 | 26.26 | 10.79 | 62.95 |
Sf. Gheorghe (May) | - | 21.12 | 78.88 | 0.51 | 8.17 | 91.32 |
Golovita L. (August) | min | 12.58 | 75.21 | 33.54 | 7.18 | 18.84 |
max | 24.79 | 87.42 | 73.47 | 10.72 | 56.72 | |
mean | 16.1 | 83.81 | 54.55 | 9.27 | 36.18 | |
Zmeica L. (August) | min | 9.69 | 79.04 | 16.83 | 9.93 | 24.63 |
max | 20.96 | 90.31 | 65.44 | 22.57 | 66.04 | |
mean | 14.58 | 85.42 | 35.07 | 15.94 | 48.99 | |
Sinoie L. (August) | min | 7.53 | 73.92 | 12.13 | 8.64 | 27.76 |
max | 26.08 | 92.47 | 61.35 | 24.14 | 72.09 | |
mean | 18.16 | 81.84 | 39.44 | 14.25 | 46.31 | |
Mustaca C. (August) | - | 12.16 | 87.84 | 29.45 | 22.35 | 48.20 |
Cocos. C. (August) | - | 13.91 | 86.09 | 15.02 | 10.56 | 74.41 |
Dunavăt C. (August) | - | 9.50 | 90.50 | 13.37 | 9.96 | 76.67 |
The average (%) SIL content presented higher values, over 50% of the total weight of dry residue, for Cocos, Sf. Gheorghe branch (May) and, subsequently, in Mustaca, Cocos and Dunavăt canals (August). To conclude, these investigated sections of controls are characterized by rather high siliciclastic content. Also, in the investigated lakes, the average (%) TOM content had values below 50% of the total weight of dry residue as in Razim and Golovita (May), Zmeica and Sinoie (August), or that slightly exceeds this limit-Golovita (August). The average (%) CAR content revealed varying levels with values more than 1% of the total weight of dry sediment as in all investigated bed-sediment samples. The average (%) SIL content fluctuated and presented higher values, over 50% of the total weight of dry residue, as in Razim and Golovita (May) or exhibits levels below the limit of 50%, as in Golovita Zmeica and Sinoie L. (August). To conclude, the sediment samples of Razim and Golovita (May) are characterized by rather high siliciclastic content. The areal distribution and concentration (%) of the lithological components (i.e., total organic matter, carbonates and siliciclastic material) in surficial sediments from Razim-Sinoie Lagoon System are plotted in Figure 3 (h-j). The results obtained in this study estimated that the majority of the tested sediments cannot be affiliated to a single type of sediment (Figure 4).
3.2.2 Chemical components of bed-sediments
Based on their concentrations it was found that CaCO3, TOC, Fe2O3, TiO2, and MnO constitute major components of the tested bed-sediments of the Golovita, Zmeica and Sinoie lakes (Table 3). The CaCO3 and TOC contents of the bed-sediments have varied in a relatively narrow range among investigated locations.
GOLOVITA Lake (August 2016) | Value | CaCO3 | TOC | Fe2O3 | TiO2 | MnO | Zr | Sr | Rb |
% | % | % | % | % | μg/g | μg/g | μg/g | ||
min | 14.01 | 0.71 | 4.28 | 0.66 | 0.049 | 166 | 230 | 87 | |
max | 28.83 | 2.69 | 6.24 | 0.85 | 0.101 | 353 | 384 | 119 | |
mean | 20.97 | 1.58 | 5.46 | 0.72 | 0.08 | 232.56 | 291.22 | 104.89 | |
Value | Zn | Ni | Cr | V | Co | Pb | Cu | Cd | |
μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | ||
min | 49.4 | 28.2 | 53.8 | 75 | 9.26 | 14.4 | 14.79 | 0.208 | |
max | 96.8 | 51.6 | 89.4 | 112 | 14.06 | 29.56 | 39.39 | 0.545 | |
mean | 76.64 | 41.80 | 70.76 | 87.11 | 11.47 | 22.90 | 28.28 | 0.37 | |
ZMEICA Lake (August 2016) | Value | CaCO3 | TOC | Fe2O3 | TiO2 | MnO | Zr | Sr | Rb |
% | % | % | % | % | μg/g | μg/g | μg/g | ||
min | 13.05 | 0.25 | 1.8 | 0.32 | 0.014 | 149 | 249 | 43 | |
max | 23.81 | 1.66 | 4.66 | 0.89 | 0.08 | 388 | 397 | 97 | |
mean | 19.88 | 0.77 | 2.98 | 0.54 | 0.04 | 245 | 313.5 | 67.71 | |
Value | Zn | Ni | Cr | V | Co | Pb | Cu | Cd | |
μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | ||
min | 10.7 | 3.5 | 15.4 | 20 | 1.53 | 1.72 | 2.1 | 0.071 | |
max | 68.6 | 32.6 | 62.7 | 88 | 11.53 | 22.26 | 27.17 | 0.591 | |
mean | 32.90 | 17.03 | 32.04 | 62.79 | 5.20 | 8.51 | 10.79 | 0.29 | |
SINOIE Lake (August 2016) | Value | CaCO3 | TOC | Fe2O3 | TiO2 | MnO | Zr | Sr | Rb |
% | % | % | % | % | μg/g | μg/g | μg/g | ||
min | 12.49 | 0.95 | 1.73 | 0.32 | 0.019 | 115 | 232 | 46 | |
max | 30.7 | 4.19 | 5.23 | 0.8 | 0.082 | 425 | 511 | 103 | |
mean | 20.41 | 2.55 | 3.72 | 0.59 | 0.05 | 240.88 | 312.82 | 79.12 | |
Value | Zn | Ni | Cr | V | Co | Pb | Cu | Cd | |
μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | μg/g | ||
min | 10.9 | 8.2 | 30 | 35 | 2.05 | 1.69 | 1.64 | 0.085 | |
max | 77.2 | 41.2 | 81.6 | 101 | 11.98 | 22.57 | 32.77 | 0.547 | |
mean | 46.83 | 26.53 | 54.44 | 72.06 | 7.59 | 12.12 | 17.63 | 0.32 | |
Value | CaCO3 | TOC | Fe2O3 | TiO2 | MnO | Zr | Sr | Rb | |
% | % | % | % | % | μg/g | μg/g | μg/g | ||
Mustaca C. | 15.62 | 3.26 | 4.41 | 0.72 | 0.065 | 225 | 233 | 92 | |
Cocos C. | 10.23 | 2.81 | 3.36 | 0.65 | 0.041 | 166 | 200 | 74 | |
Dunavăt C. | 10.19 | 2.57 | 2.60 | 0.66 | 0.031 | 223 | 203 | 60 | |
Value | Zn | Ni | Cr | V | Co | Pb | Cu | Cd | |
μg/g 75.4 | μg/g 37.2 | μg/g 66.7 | μg/g 95 | μg/g 9.63 | μg/g 13.46 | μg/g 30.65 | μg/g 0.585 | ||
Mustaca C. | 75.4 | 37.2 | 66.7 | 95 | 9.63 | 13.46 | 30.65 | 0.585 | |
Cocos C. | 42.8 | 29.3 | 46.3 | 84 | 7.44 | 5.90 | 11.13 | 0.291 | |
Dunavăt C. | 22.6 | 20.2 | 37.3 | 79 | 4.27 | 3.19 | 4.71 | 0.137 |
The spatial distributions of major and minor components in bed-sediments collected from Golovita, Zmeica, and Sinoie are plotted in Figure 5.
Tested samples also presented variable and low levels for Fe2O3 content, and, respectively, MnO contents fluctuating in a relatively narrow range among investigated sites (Figure 5). The level of heavy metal (Rb, Co, Ni, Sr, Cu, Pb, Zn, Cd, Cr, V, and Zr) concentrations of the investigated sites (Golovita, Zmeica and Sinoie) varied significantly between types of heavy metals and sampling stations. The results showed that the mean concentrations of heavy metals in bed-sediment samples were low compared with the quality standards. The concentrations of the technophillic metals (Zn, Ni, Cr, V, Co, Pb, Cu, and Cd) followed similar patterns. They fluctuated in a relatively narrow range among investigated sites, having low values. However, it was observed that Zn, Ni, Cr, Co, and Pb exhibited the highest concentration in Golovita, and respectively, V, Cu, and Cd, in Mustaca Canal while the lowest concentrations of these metals were noticed in Zmeica and Dunavăt C.
3.2.3 Biological characteristics of the sediments
The predominant benthic organisms identified in Golovita, Zmeica and Sinoie were found to belong to 31 taxa, being affiliated to 16 groups (i.e., Halacarides, Hydridae, Nematodes, Oligochaetes, Hirudinea, Gasteropods, Bivalves, larvae of Chironomids, Ostracods, Corofidae Gammaridae, Cumacea, Heteroptera, Trichoptera, Lepidoptera and Ephemeroptera). The benthic organisms are characterized by eudominant and euconstant species (i.e., larvae of Chironomids, Oligochaetes, Amphipoda represented by genera Gammarus and Corophium, as well as Polychaeta by Hypania invalida (Grube, 1860) species); their total individuals represent 75.1-100% of the specimens collected. Then, these are followed by the constant species (50-75%), as well as by accessory species or accidental forms (<50%), (Figure 6). A higher proportion of insects, characterized by a large abundance of Chironomids larvae, were found in permanent waters of the lake system compared to temporary ponds. Among the Nematodes, it was identified the species Prooncholaimus eberthi, (Filipjev, 1918), a marine species with a very high abundance in Sinoie Lake. Two species of Cumaceans were identified, Pterocuma rostrata (Sars, 1894), encountered in samples collected from Sinoie Lake, and Pseudocuma cercarioides (Sars, 1894), present in samples gathered from Zmeica and Sinoie.
In addition, the submerged aquatic plant species as Myriophyllum spicatum (Linnaeus, 1753), Azolla filiculoides (Lamarck, 1974), Elodea nutalii (H. St. John, 1848), Potamogeton perfoliatus (Linnaeus, 1753), that grow in standing or in slow-moving water were identified.
4 Discussion
4.1 Spatial distribution of physico-chemical and water quality parameters
Razim-Sinoie is a large brackish water lagoon system located in the southern part of the Danube Delta to which is geologically and ecologically closely linked. Its dynamic environments are mainly controlled by the changes of fluvial, marine, climatic and anthropogenic influences. Additionally, the morphology of the coastal lagoon induces spatial and temporal fluctuations in the physico-chemical water parameters. Both, the fresh-and saltwater influxes influence the vertical and horizontal fluctuations as a result of the water that they supply. To a large extent, the water quality in the lagoon complex is triggered by the quality of the Danube River as a result of the interconnections between them. Practically, if pollution alters the Danube water quality, it will evidently affect the lagoon system.
The results indicated that the analyses of water samples exposed good environmental conditions in connection with different physico-chemical parameters. Data gathered during both surveys showed that the investigated lakes were generally in very good and/or good status (Class 1 and 2, respectively, according to [1]), in terms of physical-chemical parameters. The results also indicated that some parameters has undergone various modifications, and slightly exceed the standard of a certain category. Further discussions on individual parameters are presented in the following sections.
Even if the Danube water level oscillations registered during sampling periods were relatively significant, namely a higher river level (May) in comparison to a lower river level (August), they did not have a major influence, because the stream flow into the lakes was probably relatively regular.
Profiles of temperature, dissolved oxygen and pH showed little variation throughout the both surveys (May and August) in the lagoon system indicating no significant variation to little variation. Temperature, dissolved oxygen and pH were measured on each sampling campaign with data recorded for every water sample. It is known that, too low or too high temperatures pose a serious stress to aquatic organisms (e.g., fish) and adversely affect their growth [63]. Temperature and dissolved oxygen concentration play an important role in the ecology of a lake, affecting aquatic biota, as well as influencing the algal blooms [64]. In both surveys, water temperatures were relatively uniform throughout the water surfaces, with weak fluctuations. Given that Razim-Sinoie is shallow, has a large extended and a constant supply of waters, the water surface is usually mixed by winds. Depending on the climatic conditions, there may be higher or lower thermal turbulences. The water temperatures were relatively uniform throughout the investigation and relatively constant values were appropriately on the entire surface of the lakes. In the studied area, the water temperature values did not show significant overruns, being consistent with the expected seasonal climate variations during both sampling campaigns (wet and dry seasons). Surface water temperature show constant values, being 2–3°C lower in May compared to August data.
The dissolved oxygen content is the best-known indicator of water quality [65], being in interconnection with a series of other water indicators (temperature, photosynthetic activity of algae and plants, color, odor, transparency etc.). During the investigation, dissolved oxygen concentrations were within the limits imposed by the national guidelines. Surface concentrations ranged between a maximum on August in Zmeica (19.02 mg·L−1) to a minimum on May in Golovita (9.05 mg·L−1). In August there was a notable increase in dissolved oxygen concentration at the surface in comparison to May period. Dissolved oxygen maintained slightly similar values over much of the May and relatively increasing in August. Anyways, higher dissolved concentrations were recorded in both intervals. The surface water samples investigated in this study are well-oxygenated. The obtained results could be related to several factors as water depth, wind regime, temperature, and a high local rate of dissolved oxygen or as a result of re-oxygenation. Higher concentrations of dissolved oxygen may also indicate the occurrence of a significant algal bloom and may be the result of photosynthetic oxygen production. The lake variations are most likely determined by local conditions influencing the phytoplankton and green algae development and, in a lesser measure, by the superposition of the sampling time on the diurnal metabolic cycle of the oxygen producers. The increase of dissolved oxygen concentrations from May to August remarked in Golovita L., is probably due to the phytoplankton and green algae development. The obtained values for dissolved oxygen content are in close agreement on the estimated threshold values for Class I. Former studies showed that in this research area were no significant overruns regarding the dissolved oxygen content and the water quality was considered good [66]. Consequently, the obtained results from this study are consistent with those identified on Razim, Golovita, and Sinoie by previous investigations [67]. Summarizing, the results obtained during this study are similar to the other outcomes previously accomplished in some DDBR lakes [68, 69], in the last years.
The pH value of all water samples of the study area slightly fluctuated, and it was noticed that the area appears slightly alkaline in nature. The pH controls the acidity or alkalinity concentration in water lakes [65], regulating the in-lake biogeochemical reactions and processes (biologic activity, solubility and chemical compound processing). Measured pH profiles and their spatial variation in the lagoon system are shown in Figure 3b. At the surface, the pH oscillates approximately from 8.61 in May to 9.15 (pH) during August. During May the pH of the water was relatively consistent within the investigated lakes. However, in the August period, the pH tends to increase. In this area, the pH of the water depends on the nature and properties of the fresh-and saltwater influxes that impacts the drainage basins, as well as by the geological background. Clear differences in pH occurred in May related to August. The values of pH show a slightly alkaline character passing to an alkaline one. In this study, a seasonality of pH was observed between the driest and wettest period, probably linked to changing primary producer (algae and macrophytes) biomass and corresponding photosynthetic rates. Dissolved CO2 consumption in the photosynthesis shifted pH to more alkaline values, most of them exceeding the upper limit of the normal variation range in natural waters. The pH values are probably temporary and will revert to values closer to the equilibrium with the atmospheric CO2 in the colder seasons, when the photosynthesis diminishes or even disappears. The pH is greatly affected by photosynthetic activity of aquatic flora, temperature and the amount of organic constituents [70]. The obtained values for pH levels are in close agreement on the estimated threshold values established by the reference [A3]. These results are similar to those reported from Razim, Golovita, and Sinoie lakes [67]. There is no sign of acidification of the investigated water samples as the pH>7. The relative normality showed by pH values did not indicate the existence of any significant sources of acidic or alkaline compounds in the investigated lacustrine areas, or in the immediate vicinity. Both parameters as dissolved oxygen and pH showed seasonality, probably in response to changing primary producer (algae and macrophytes) biomass and corresponding photosynthetic rates.
Nitrogen and phosphorus are important nutrients which can contribute to the growth of algae in DDBR lakes. The DDBR lakes represent an enhanced capacity for nutrient storage, especially during the aquatic plant growth season [71]. High concentrations of nutrients can affect water quality such as acidification, eutrophication and impairing the aquatic organisms [72]. The Danube Delta’s capacity to retain nutrients assessed by the difference between nutrient input and sum output show that Danube Delta acts as a bypass for the nutrients in the main branches and as a filter in the aquatic complexes, retaining the nitrogen in all hydrological conditions, but releasing the phosphorus [73]. From 1970 until 1990, emissions of nitrogen compounds and related human activities along the Danube River strongly influence the amount of nutrients deposited at Danube Delta’s ecosystems. Moreover, the hydro-morphological changes within the deltaic area, transposed the inflow average of water input from approximately 260 m3/s around 1951 to 620m3/s in the 1960-1990 period, inducing to the intensification of the eutrophication process [31, 33]. Besides, by comparing the phosphorus dynamics and retention in Danube and its delta, have been shown, that even if there is a descending trend, in the Danube River, the effect of this trend is delayed inside the delta [74, 75]. In this study, measured nitrogen parameters included N-
All samples of surface water collected from both surveys revealed a high variability of water conductivity (EC), total dissolved solids (TDS) and sulphates
(
Turbidity and total suspended solids (TSS). Some values above the maximum content level established as a threshold for turbidity [A5] and total suspended solids [A6] were detected in many water samples from Razim and Golovita (May), as well as in Golovita, Zmeica, and Sinoie (August). Both turbidity and TSS content were higher in Sinoie Lake in contrast to Golovita and Zmeica, a spatial differentiation being noticed in this sense. Probably, marine influences induce variation in turbidity and TSS content. A link between these values and interchange of salty and fresh water may be assumed.
Generally, the analyses of water and sediment samples from the Razim-Sinoie Lagoon Complex revealed that the coastal lake system is in near good ecological condition. This picture of the Razim-Sinoie water quality conditions is based on the measurement performed at the time of sampling. Even supposing that the natural lagoon ecosystem is in accordance with natural water quality, if any, significant changes in water quality may alter the ecosystem. The surface water quality is changeable and may be impaired by a wide range of natural and anthropic influences. For instance, in one day the water quality may be perfect, and on another day it may suddenly deteriorate, because waters are continually dynamic and accidental spills is unpredictable. The dynamics of this lagoon system are mostly influenced by the Danube freshwater flows on the one hand, and saltwater influx on the other hand. Most likely, the Danube River can change its quality, state, due to its significant flow rates increasing dilution capacity (high volume of water that can be more heavily influenced by potential pollutants) and self-purification capacity. Therefore, from upstream to downstream may take place a concentration decrease of potential pollutants under the action of the physico-chemical and biological agents interacting at the level of the water. The effects of the Danube inflows diminish as the transport distance increases.
4.2 Sediment Characteristics
Sediment quality assessment is considered an essential component of research programs designed to evaluate the environmental quality. Several integrated, sedimentological, mineralogical, paleontological, geochemical, geophysical and biological research studied addresses to the DDBR. The first important sedimentological map based on grain size characteristics was elaborated by [76], for the Razim - Sinoie lacustrine complex, bringing also several new data regarding the dynamics of modern sedimentation processes. With reference to the Danube Delta, there are some previous data reported by [77, 78, 79] approaching the main physico-chemical characteristics of lacustrine sediments. Additionally, some sediment quality data focused on lithology, chemical composition and heavy metal contents within the Danube Delta are provided by [69], aimed to evaluate the status and quality trends of evolution with respect to sediment and water contamination in the Danube Delta. Newly-released papers comprise integrated magnetic susceptibility and lithological data on recent lacustrine sediments from the delta [80, 81], and lagoon system [82, 83, 84].
4.2.1 Bulk lithological parameters
The Razim-Sinoie coastal lake ecosystem is affected by sediment transportation (loads of suspended materials) and autochthonous sedimentation. A general estimation of the sediment composition of this lagoon system was done according to the percentages by weight, based on the organic matter, carbonates and minerogenic fraction content, calculated from the total weight of the dry residue [85], (Table 2). Our lithological analysis pointed out a spatial differentiation of the tested sediment samples conforming to percent proportions of mineral or organic matter. Overall, the Razim - Sinoie Lagoon Complex is characterized by accumulations of sediments with relatively high levels of siliclastic content, typical in this area, due to high loads of suspended materials of the Danube. In this case, the accumulation of sediments showed a clear dependence on local factors, such as the environmental characteristics of lacustrine and marine environment i.e., inconstant amount of water and sediment input through Sf. Gheorghe Branch via Dunavăţ and Dranov canals, parental material originating from the shores of lakes, coastal erosion etc. The obtained results indicated that the majority of the tested sediments could not be affiliated by a single type of sediment (Figure 4). These results allowed the partitioning of the investigated catchments into several spot areas representing probably distinct hydrogeomorphological regimes. Hence, areas with mineral sediment type (high minerogenic material load) were identified in Razim and Golovita (May), respectively, Zmeica and Sinoie (August), which transit into subordinate mineral-organic sediment or organic sediment as in Golovita (August). As the carbonate contents formed a significant percentage of many investigated sediment samples had been categorized as carbonate sediments. Most probably, the main source of the total carbonate content is the autochthonous detritus i.e., fragments of shells, skeleton structural parts, and internal waste products that can originate within the investigated area.
4.2.2 Chemical components of bed-sediments
The Razim-Sinoie Lagoon System, a unique environment with distinct characteristics, is under the threat of pollution, as many other lagoon systems in the world. The chemical investigation aimed to assess the sediment quality in a lagoon system under continental-temperate conditions with strong influences from the Black Sea along with multiple stressors such as natural and anthropogenic factors. Their negative effects on the environment may lead to an important loss of ecosystem with repercussions for habitats and for ecosystem functioning. A special attention should be given to heavy metals coming from various industrial wastewaters, and their subsequent accumulation in sediments. In order to advance a relative chemical characterization of bed-sediments, several chemical components i.e., CaCO3, TOC, Fe2O3, TiO2, MnO, Rb, Co, Ni, Sr, Cu, Pb, Zn, Cd, Cr, V, and Zr (Table 3) were investigated in random sampling stations by different methods. The spatial distributions of major and minor components in bed-sediments collected from Golovita, Zmeica, and Sinoie are plotted in Figure 5. The analytical data reflected a great variability that exists in the chemical components of investigated bed-sediment samples. The variation ranges of the majority of components, as well as their average concentration, are placed generally, to lower values. The major components of the investigated bed-sediments from Golovita, Zmeica, and Sinoie (August), included CaCO3, TOC, Fe2O3, TiO2 and MnO. Relatively variations of CaCO3 and TOC (total organic carbon stored in sediment organic matter) content were encountered among investigated locations. The same trend of variation in a range with low values was also valid for constituents as Fe2O3 and, respectively, MnO. The level of heavy metal concentration of the Golovita, Zmeica, and Sinoie lakes varied significantly between types of heavy metals and sampling stations. The study showed that heavy metal concentrations, including Rb, Co, Ni, Sr, Cu, Pb, Zn, Cd, Cr, V, and Zr, seldom exceeded the maximum content level recommended by standard reference [A3], or, in comparison to normal reference values for trace elements in soil[8], as applicable to sediments. The results showed that the average concentration of heavy metals in bed-sediment samples was a comparatively low contrast to above-mentioned reference standards. All the concentrations of the tested heavy metals (Zn, Ni, Cr, V, Co, Pb, Cu, and Cd) followed similar patterns, i.e., fluctuating in a relatively narrow range among investigated sites (Table 3). From these results, it can be noticed that the concentrations of heavy metals in the bed-sediment are very distinct in different sectors of the investigated lakes. Concentrations of elements as Ni, Cu, Pb, Zn and Cd (Table 3), considered technophillic metals, were below the limits of the quality criteria. The results showed that the enrichment state of heavy metals in the investigated lakes is more probably due to natural geochemical background concentration levels and which, in turn, rely on the influence on natural (abundance of 8 Order no. 756/1997 for the approval of the Regulation on the assessment of environmental pollution source areas, biochemical alterations) and anthropic environmental variables. Regarding the spatial distribution pattern of heavy metals in bed-sediment a special trend of the heavy metal accumulations was not remarked; the occurrence of distribution areas of the above mentioned heavy metals with both lower and relatively higher concentrations (but not exceeding the established thresholds, yet, and not posing levels of potential effect, too), may be observed (Figure 5). The ecological impact of heavy metals in water, sediment, as well as in bioindicator organism for heavy metal pollution (plankton, insect, mollusks, fish, and plant) has been studied in several lacustrine areas of the Danube Delta. Unfortunately, similar studies dealing with heavy metals in the lagoon complex are rather scarce in this area. Most of the studies are concerned with restricted areas of the Danube Delta [86], or, are focused on certain types of analysis carried out in water, sediment [87, 88, 89] or, investigating the ecotoxicological effect of heavy metals on ecosystems at different levels of biological organization [90]. In this context is difficult to assert certainty that there is a positive correlation, or, a similarity, between acquired results obtained in this study compared with other national/international outcomes. The acquired results of the environmental indicators found in these lakes would be desirable to be compared with similar research goal and objectives, fields, types of research methods, or, with aquatic systems that belong to regions with similar environmental characteristics (climatic, geomorphological, geological, ecological factors). Other studies [66] previously performed in the area show that the geochemical results indicated a good quality of the lacustrine superficial sediments. However, were reported higher values of the Cr and Zn concentrations identified in front of the major discharging canal mouths and near the villages located on the western bank of the lacustrine complex but, all other obtained heavy metals concentrations were low. In addition, higher levels of TiO2, MnO and several heavy metals found in few locations, have been correlated with the local presence of the fossil littoral bars.
4.2.3 Biological characteristics of the sediments
The biological analysis aimed to present the quantitative distribution of the benthic organisms in the Golovita, Zmeica and Sinoie. The composition of the benthic macro invertebrate community is a useful tool for assessing ecological status of aquatic ecosystems [91]. During this biological assay an insect richness in permanent waters of the lake system has been noticed compared to temporary ponds [92]. The large abundance of larvae of Chironomids suggests a significant organic load and the existence of an oxygen deficiency in the substrate. The occurrence of the marine species, i.e., Prooncholaimus eberthi, (Filipjev, 1918) in Sinoie Lake can be explained by high salinity. Our results showed that the decrease in taxon richness is related to anthropogenic influences that took effect on the qualitative and quantitative structures of the benthic populations. Before 1956, almost over 70% of the total density and biomass zoobenthos was represented by Ponto-Caspian relict species and only 30% of some freshwater and brackish species [93], then, from 2000, the ratio between them changed completely. Today, the dominant forms are represented by freshwater species. It was noticed that Euryhaline taxa of bivalves, gastropods, crustaceans, i.e., Cardiidae, Syndesmia (Abra) sp. (Lamarck, 1818), Hydrobia sp. (Hartmann, 1821), Briozoa, Balanus sp. (Costa, 1778), marine ostracodes and sensitive freshwater Theodoxus spp. (Montfort, 1810) species disappeared [94]. They have been gradually replaced by newly emerged stenobiont (i.e., more specialized) freshwater species, more resistant forms, such as Anodonta cygnea (Linnaeus, 1758), Corbicula fluminea (Müller, 1774) and Unio pictorum (Linnaeus, 1758). The biological assessment in terms of composition, relative abundance, and diversity of the benthic macro invertebrate communities, concluded that within the investigated area developed mainly freshwater species, and subordinately brackish ones. Therefore, the ecological succession of the lagoon complex showed a gradual transition from the marine to a freshwater environment.
5 Conclusions
Despite the large regional and local pressures in terms of natural and anthropogenic influences, the present study evidenced that the Razim-Sinoie lagoon system is in near good ecological condition.
Both physico-chemical and biological indicators affirmed good ecological status. Investigated lakes are well-oxygenated with alkaline character. The levels of the physico-chemical parameters agreed generally with related environmental standards. In this sense, the results did not identify levels that would warrant special concern. The exception was represented by some unsatisfactory levels for a few indicators encountered in some sectors of investigated lakes, where, individual values only incidentally exceeded the acceptable threshold, but with levels and areal distribution that would not warrant special concern.
With reference to surficial sediments, the lithological analyses have allowed the separation of the two types of recent accumulations, sediments with high levels of siliciclastic content and transitional areas characterized by sediments rich in organic matter, that are consistent with the geomorphological features of the lagoon system. The levels of heavy metals under study were below the recommended allowable limits. In this sense, there has currently no clear evidence of heavy metal contamination in investigated lakes.
The results of the biological assessment suggested that the anthropic disturbances gradually impair the functioning of the lagoon system. In general, lagoon ecosystems are directly related to the complex physical, chemical and biological processes taking place within its environment. A proper functioning of a lagoon system is mainly characterized by their degree of mixing between freshwater and saltwater and the level of dissolved oxygen present in water. Even if, over time it was noticed an intense development of freshwater species to the detriment of marine organisms, it can be appreciated that the natural balance (freshwater and saltwater influences) of the ecosystems is quite well maintained.
In this context, the obtained results are of particular significance since the majority of the investigated physicochemical indicators are key parameters in providing a sustainable habitat that withstand environmental conditions, being as well the binding factors for the survival of aquatic organisms, especially flora and fauna. Even if parts of the obtained data were rather uniform, lacking significant variability, they can provide a basis to assess the environmental conditions. It is important to take actions such as to collect, analyze and regularly disseminate data in order to evaluate and/or identify some potential areas of environmental concern.
Finally, this study improved and increased the database on the environmental conditions of the lagoon system, an area of great ecological importance to the preservation of ecosystems, taking into consideration the great variability and heterogeneity in the numerous variables that control and characterize transitional water systems. Some inaccuracies (slight deviations of the results compared to the admissible limit) seem to be rather related to natural stressful conditions of transitional waters, or, due to the particularities of the local environmental conditions in the study area. Such information may be of significant importance in understanding the functioning of the Razim-Sinoie Lagoon System in order to preserve and protect these vulnerable particular wetlands.
Acknowledgement
The research leading to these results was financed from the Romanian National Authority for Scientific Research and Innovation - ANCSI -”Program Nucleu 37N/2016 - Proiect PN16 45 01 04”. We would like to express our gratitude to the Managing Editor - Jan Barabach, especially the Editor - István Gábor Hatvani, as well as the anonymous reviewers for their constructive comments and recommendations, which helped us to improve the quality of the manuscript. Likewise, we would like to show our appreciation and thanks to Dr. M.-C. Melinte-Dobrinescu and Dr. S. Rădan for reading the manuscript and providing important insights.
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© 2018 I. Catianis et al., published by De Gruyter
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.