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 May to Map of the study area Razim, Golovita, Zmeica and Sinoie lakes. The physico-chemical parameters and their spatial variation of distribution in water from a to g and sediment from h to j are indicated color scale bar.
The distribution maps were realized with the Surfer software - Golden Software, Inc. 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. May to a maximum of 9. 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.
Both their averages and variation ranges in surface waters were similar for the two sampling campaigns. It has to be noted that occasional concentrations of orthophosphates tested in some water samples were considerably increased in the period of measurements. Accordingly, it is about water samples collected in May from Razim L. Its average concentrations in both lakes were almost similar in value. The silica content SiO 2 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. All the lakes were characterized by low levels of SO 4 2.
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.
The same is valid for the investigated lakes August that present similar high levels of turbidity in Golovita, Zmeica and Sinoie lakes. 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.
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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. The physical characteristics of the bed-sediments collected from Razim, Golovita, Zmeica and Sinoie, along with some river or canal control points e. Table 2 Summary of the physical properties of bed-sediments samples. To conclude, these investigated sections of controls are characterized by rather high siliciclastic content.
To conclude, the sediment samples of Razim and Golovita May are characterized by rather high siliciclastic content. 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. The CaCO 3 and TOC contents of the bed-sediments have varied in a relatively narrow range among investigated locations. Table 3 Summary of the major chemical components and heavy metals of the bed-sediments collected from Golovita, Zmeica and Sinoie August. The spatial distributions of major and minor components in bed-sediments collected from Golovita, Zmeica, and Sinoie are plotted in Figure 5.
Map of the study area Golovita, Zmeica and Sinoie lakes. The major and minor components and their spatial variation of distribution in bed-sediments are indicated color scale bar.
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Tested samples also presented variable and low levels for Fe 2 O 3 content, and, respectively, MnO contents fluctuating in a relatively narrow range among investigated sites Figure 5. The results showed that the mean concentrations of heavy metals in bed-sediment samples were low compared with the quality standards. They fluctuated in a relatively narrow range among investigated sites, having low values.
The predominant benthic organisms identified in Golovita, Zmeica and Sinoie were found to belong to 31 taxa, being affiliated to 16 groups i. The benthic organisms are characterized by eudominant and euconstant species i. 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, , a marine species with a very high abundance in Sinoie Lake. Two species of Cumaceans were identified, Pterocuma rostrata Sars, , encountered in samples collected from Sinoie Lake, and Pseudocuma cercarioides Sars, , present in samples gathered from Zmeica and Sinoie.
Abundance of the benthic populations expressed as an average in Golovita, Zmeica and Sinoie lakes. In addition, the submerged aquatic plant species as Myriophyllum spicatum Linnaeus, , Azolla filiculoides Lamarck, , Elodea nutalii H. John, , Potamogeton perfoliatus Linnaeus, , that grow in standing or in slow-moving water were identified. 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. 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.
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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. 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. 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.
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Surface concentrations ranged between a maximum on August in Zmeica 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.
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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. 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. 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 CO 2 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 CO 2 in the colder seasons, when the photosynthesis diminishes or even disappears.
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