Depth is an important parameter which affects the trophic status of lakes (Çevlik and Elibol, 2009). It has a profound effect on water temperatures, water chemistry and the functioning of the ecosystem. The quantity of heat transferred to the lake water based on parameters like solar and atmospheric radiation, air temperature, relative humidity, wind velocity and cloudiness, changes the temperature of surface waters according to seasons. The mixing caused by wind is limited by the buoyancy of water. This situation arises from the different densities of water at various temperatures. Temperature is an important parameter affecting water density (Chapra, 1997). The resulting thermal stratification impedes oxygen to reach the deeper water layers (McKee and Wolf, 1963).
The lake is divided into an epilimnion, metalimnion and hypolimnion layers due to thermal stratification. At times when temperature differences in the layers decrease and wind effects become more prevalent, a vertical mixing takes place which also determines the oxygen profile in the lake. The dissolved oxygen in the hypolimnion (deepest layer) which is disconnected from the atmosphere during stratification decreases in proportion to its organic matter content. During mixing periods the oxygen levels at all depths become more or less equal. Especially in summer, when the stratification is strong, the hypolimnion is easily depleted of oxygen due to bacteria requiring oxygen to decompose organic matter (Bengtsson et al., 2012).
There are several studies in literature concerning the thermal stratification in lakes in relation to different aims and methods. Elci (2008) studied the relationship of thermal stratification with flow and wind velocity in the Tahtalı Lake (Turkey) with field studies and statistical analysis and further investigated the effects of thermal stratification on water quality parameters like dissolved oxygen and suspended solids. Baharim et al. (2011) measured temperature and dissolved oxygen in a tropical lake at four sampling stations in depth intervals of one meter, continuing the weekly sampling schedule for 13 months. They compared the occurrence times of stratification and mixing with the changes in iron and manganese concentrations.
Duration and timing of thermal stratification and the period of vertical mixing are very important for chemical and biological processes occurring in deep temperate lakes. A study about climate change indicated that wind speed has a large effect on temperature and stratification, at times greater than the effects in changes in air temperature, and wind can act to either amplify or mitigate the effect of warmer air temperatures on lake thermal structure depending on the direction of local wind speed changes (Magee and Wu, 2017). Some lakes respond to such changes in terms of thermal regime transitions like from dimictic to monomictic. Climatic trends increased summer epilimnion temperatures and, for deep lakes, frequency and duration of thermoclines (Ficker et al., 2017). These effects induce changes in diatom communities and lead to differentiated patterns between shallow and deep lakes and parks (Edlund et al., 2017). Climate change has the potential for adversely affecting fish and fisheries in lakes. Longer stratification periods, and more frequent and widespread periods of bottom hypoxia in productive areas of the lakes have affected fish habitats (Collingsworth et al., 2017). Besides climate change and global warming, anthropogenic heat discharges into inland waterbodies may influence water temperature and lead to prolonged stratification periods (Vinna et al., 2017).
As the stratification effectively inhibits the mass transfer between the hypolimnetic zone and the upper layers, the oxygen in the lower layer is depleted rapidly based on the organic matter content. The depletion rate is normalized against the hypolimnetic volume or surface area and respectively called VHOD (volumetric hypolimnetic oxygen demand) or AHOD (areal hypolimnetic oxygen demand) in literature (Wetzel et al., 2000). In some studies, the change in VHOD in a sequence or years was determined (Matthews and Effler, 2006, Matzinger et al., 2010). Studies concerning the artificial increase of the hypolimnetic oxygen levels and dealing with the quantitation of the required oxygen supply rely on the determination of the depletion rate (Bryant et al., 2011; Debroux et al., 2012; Moore et al., 2012).
As eutrophication is defined as the acceleration of biological productivity resulting from enhanced nutrient and organic loading, hypolimnetic oxygen consumption rates or deficits can result in a useful tool for analyzing temporal changes in water quality. When long-term dissolved oxygen observations are available, in the case of lack of detailed data on water quality and biological production, the oxygen deficit can indicate the general trophic status and overall changes in water quality (Green, 1996).
In this study two man-made water bodies in Inner Anatolia (Turkey) are investigated. The Porsuk Reservoir is prone to intense pressure from anthropogenic activities. The Borabey Pond on the other hand is more protected and only slightly affected by agricultural activities. The hypolimnetic oxygen demand of these two lakes were determined. The thermal stratification pattern in the water bodies was determined by monitoring the temperature profile along the depth. In tandem, the volumetric structure was investigated with bathymetric mapping. From these two data sets, the hypolimnetic zone thickness and volume were inferred. Further, the oxygen depletion dynamics was determined by following changes in dissolved oxygen concentrations. Consequently the effects of anthropogenic pressures on the lakes were assessed.2 STUDY AREA 2.1 The Borabey Pond
The Borabey Pond is situated in the foothills of Bozdağ Mountain to the north of the city of Eskişehir at an elevation of 920 m. The pond was constructed to serve irrigation purposes in 1991–1992 (Anonymous, 2011). The geographical location is given in Fig. 1.
The pond is 200 m wide, 620 m long, on the average. The average depths is 8.5 m and the deepest point is 18.8 m at maximum water level. The minimum talveg elevation is 906 m and the maximum level is at 924.80 m. The water course feeding the lake is a small stream which dries up in summer. Water is released for irrigation via a pipe located at the dam at an elevation of 906.62 m. The total storage capacity of the pond is 1.6 million m3. The watershed of the pond is agriculturally dominated with almost no industry and mining. Therefore the pond is open only to agricultural pollution from its watershed with an area of 8.5 km2. Principally the pond receives nutrients from the surrounding agricultural areas which has increased its trophic state to mesotrophic since the building of the dam (Kaya, 2013).2.2 The Porsuk Reservoir
The Porsuk reservoir is a hypertrophic water body located in Inner Anatolia, Turkey (Chaplot et al., 2005). The reservoir sits on the Porsuk stream, an important 460 km long stream upon which a large watershed relies for water supply to be used for irrigation, and industrial and domestic purposes.
The reservoir is 1 000 m wide, 23 km long on the average. The average depth is 10 m and the deepest point is 49.70 m at maximum water level. The minimum talveg elevation is 855 m and the maximum level is at 890 m. The water course feeding the lake is the Porsuk stream which continuously flows all year. Water is released for irrigation and Eskisehir city water needs via an outlet located at the dam at an elevation of 844.65 m. The total storage capacity of the reservoir is 525 million cubic m and the reservoir has a 1 325-km2 total watershed area at the entrance of the reservoir. The Porsuk stream is a highly polluted stream carrying industrial domestic, agricultural and mining wastes from sources located in its watershed (Albek, 2003). These pollutants carried by the stream lower the water quality of the Porsuk Reservoir considerately and it is currently a hypertophic body (Muhammetoglu et al., 2005).3 MATERIAL AND METHOD 3.1 Mapping the bathymetry of the Borabey Pond
To produce the bathymetric map of the pond, the Sontek M9 Acoustic Deppler Current Profilier (ADCP) equipment was utilized. The equipment, besides providing fast and reliable current and discharge data for streams, can be also used for obtaining depth measurements after conducting required modifications on output data. The equipment can obtain depth profiles in the range 0.2 to 80 m (Göncü et al., 2014a).
Standing water bodies can show variations in water temperature and salinity in the vertical profile due to stratification. This leads to wrong depth soundings when the changes are not accounted for. As a remedy, a Castaway-CTD equipment was utilized to measure temperature and salinity values along the depth in order to modify the depth measurements of ADCP accordingly (Göncü et al., 2014b).
The ADCP equipment was used at many points along the width and length of the Borabey Pond to make depth measurements. The ArcGIS software was utilized to transfer the data to a GIS in the WGS84 coordinate system. Figure 3 shows the excursions made with ADCP while making the depth sounding. The bathymetric map is shown in Fig. 4 (Göncü et al., 2014b).
With the help of the area-volume calculation (3D analyst) module of the ArcGIS software, the surface area and volume relationships with elevation were established between the maximum and minimum water elevations of 906 and 924 m above mean sea level.3.2 Mapping the bathymetry of the Porsuk Reservoir
For the bathymetry of the Porsuk Reservoir, data belonging to a mapping study conducted between the years 2000–2002 by General Directorate of State Hydraulic Works in Turkey (DSI) were utilized as a base. Figure 5 shows the bathymetric contour plot of the reservoir created from the data mentioned. The figure also shows the excursions of the ADCP to measure up to date depths to compare with the previous depth soundings. During the excursions, temperature and salinity profiles were also obtained to modify depth measurements accordingly. The 3D bathymetric map of the Porsuk Reservoir obtained from the combined data (DSI study+ADCP soundings) is displayed in Fig. 5. ArcGIS was then utilized to find surface areas and volumes depending on the depth.3.3 Temperature and dissolved oxygen profiles
During the excursions, the temperature profile was obtained with the Castaway-CDT equipment and the dissolved oxygen profile with a Hach HQ40d equipment. Castaway-CDT can measure water temperature, conductivity and measurement depth (based on pressure) simultaneously with 3 sensors. Also the measurement location can be precisely pinpointed using GPS. The dissolved oxygen measurements were conducted with HACH HQ40d at 1 m depth intervals. Between February 2013 and November 2014, 46 temperature/salinity and 41 dissolved oxygen profiles were obtained from the Borabey Pond. The location of the measurements are shown in Fig. 6.
For the Porsuk Reservoir, the profile locations were chosen at the widest and deepest parts of the water body to be able to represent the whole water volume. The locations are shown in Fig. 7. On May 21, 2013 profiles of temperature and dissolved oxygen concentration were conducted at 5 locations along the length of the lake to determine whether the profiles show district patterns based on geographical location (Fig. 7). Though there are spatial variations, it was concluded that the thermal stratification pattern is the same everywhere and further profile measurements were concentrated at the widest part of the reservoir which comprises the most voluminous portion. Profiles were obtained between February 2013 and November 2014 at fairly regular intervals and 30 profiles ware taken.3.4 The determination of the thermal stratification depths based on relative thermal resistance to mixing (RTRM) values
The intensity of stratification in water bodies is generally obtained from RTRM values which represent relative thermal mixing resistances at consecutive depths. The first studies about RTRM were conducted by Birge (1910). Though many studies call the index as RTRM, Becker et al. (2009), Branco et al. (2009) and Alpaslan et al. (2012) called it the column stability index (RWCS-Relative Water Column Stability).
The index is calculated as follows (Chimney et al., 2006):
where ψ, is the dimensionless RTRM index. ρz2 and ρz1 are water densities at depths z1 and z2 and ρ4 and ρ5 are water densities at 4 and 5℃. The index for the Borabey Pond and Porsuk Reservoir were calculated from the measured water densities as obtained from the Castaway-CTD equipment during the observation studies.
RTRM values are obtained at 1 m depth intervals and those values that are larger than 20 point to the upper and lower limits of the metalimnion. Also the RTRM values, when summed up, represent the total resistance of the water body to mixing. Use of the index is displayed in Fig. 8.
Thus, the RTRM index is a simple and informative tool for the determination of stratification of lakes. It can pinpoint the location of the thermocline, show the depth of the metalimnion and the stability of the stratification (Kortmann, 2011).3.5 The determination of AHOD
The rate of loss of the mass of dissolved oxygen, normalized to the surface area of the hypolimnion is called the areal hypolimnetic oxygen deficit (AHOD, g/(m2·d)) and it is considered to be a quantitative representation of oxygen depletion in waterbodies (Wetzel and Likens, 2000; Matthews and Effler, 2006). The AHOD representation is widely used as an index of the productivity of stratified lakes. It is very useful to compare lakes with each other in this respect (Lasenby, 1975; Walker, 1979; Beutel, 2003; Matthews and Effler, 2006) AHOD (Areal Hypolimnetic Oxygen Demand) is obtained by dividing the slope of the linear region in the hypolimnetic dissolved oxygen versus time profile with the upper boundary area of the hypolimnetic region (Matthews and Effler, 2006).
Both for finding RTRM values and determining AHOD, the measurements obtained in the two water bodies (Borabey Pond and Porsuk Reservoir) during the 2013 and 2014 surveys were utilized. Within the scope of this study, pond/reservoir volumes and surface areas as obtained from the bathymetric survey were matched with the depth, and measured dissolved oxygen concentrations at the measurement dates. The oxygen mass present at every depth interval of 1 m were obtained from the dissolved oxygen concentrations and volume. The total oxygen content in the hypolimnetic zone was calculated after the hypolimnetic layer volume was determined by the use of RTRM indices. The oxygen mass in the hypolimnetic zone in the period after the date the mixing period ends and the stratification commences was plotted versus time. The mass decrease rate (HOD) was determined by linear regression in the region where the oxygen mass decreases linearly. This value was then normalized with respect to the upper surface area of the hypolimnetic layer which gave the areal hypolimnetic oxygen demand (AHOD).4 RESULT AND DISCUSSION
The RTRM index values versus time at the deepest location in the Borabey Pond with the temperature and dissolved oxygen profiles are displayed in Fig. 9.
When the profiles and RTRM index values are assessed together, it can be observed that in 2013 the thermal stratification in the pond commenced at the end of April and continued till the end of August, while in 2014 the commencement occurred towards the middle of April and the stratification ended at the end of August. The total RTRM values indicated that the total resistance to mixing was higher in 2013 compared to 2014. It was observed that the lake stratified once a year and can be considered as monomictic.
During periods of stratification the hypolimnetic dissolved oxygen levels drop and as observed in the figure, anaerobic conditions prevail at times.
Using the RTRM indices the depths and consequently volumes of the hypolimnion, epilimnion and metalimnion were calculated for all observation times and the results are displayed in Fig. 10.
The same procedures were also applied to the Porsuk Reservoir. Here, the stratification onset times were end of April beginning of May for 2013 and middle of April for 2014. The cumulative RTRM index values gave a higher mixing resistance in 2013 (Fig. 11). The stratification loses its significance in autumn. In winter month a negligible stratification pattern is observed in the upper layers but this is not reflected in the RTRM indices in a significant manner. So, the Porsuk Reservoir is also a monomictic water body. The volumes of the respective stratified layers are represented in Fig. 12.
As observed in Fig. 11 the amount of water devoid of oxygen is great in the Porsuk Reservoir. Especially in the summer months, the water below 5–10 m is depleted of oxygen and anaerobic conditions prevail throughout the season.
In the determination of HOD for the Borabey Pond, the dissolved oxygen concentrations were multiplied with the hypolimnion volumes to get the amount of dissolved oxygen in the hypolimnion. The resulting changes in the hypolimnetic dissolved oxygen amounts (in kg O2) with time are presented for 2013 and 2014 in Fig. 13. The linear portions of the profile (corresponding to the stratification periods) gave -83.473 kg O2/d for 2013 and -45.236 kg O2/d for 2014 as the trend slopes respectively. The changes in the hypolimnetic zone average dissolved oxygen concentrations are given in Fig. 14.
Table 1 presents the oxygen amounts in the measurements points and the corresponding upper hypolimnion surface areas.
The average hypolimnion surface area for 2013 is 98 493 m2 and 79 499 m2 for 2014, respectively. When the slopes found from Fig. 15 are divided by the values above, the AHOD value for 2013 comes out to be 0.848 g O2/(m2·d) and 0.569 g O2/(m2·d) for 2014. The HOD values are 0.114 mg/(L·d) for 2013 and 0.073 mg/(L·d) for 2014, respectively.
The hypolimnion upper boundary surface areas are 21 163 214 m2 for 2013 and 14 224 997 m2 for 2014. The AHOD for the Porsuk Reservoir in 2013 is 4.263 g O2/(m2·d) and the AHOD for 2014 is 5.099 g O2/(m2·d). The drops in dissolved oxygen concentration per day (HOD values) are 0.237 mg/ (L·d) and 0.236 mg/(L·d) for 2013 and 2014, respectively.
Hutchinson(1938, 1957) set ranges of oxygen deficit for categories of unproductive to productive lakes as: oligotrophic (less than 0.17 g O2/(m2·d)) and eutrophic (greater than 0.33 g O2/(m2·d)). Limits for eutrophy of 0.55 and 0.33 g O2/(m2·d) are proposed by Mortimer (1942) and Hutchinson (1957) respectively (Matthews and Effler, 2006). The 2013 and 2014 values for the Borabey Pond are 0.848 and 0.569 g O2/(m2·d) and place the pond into the eutrophic category. The pond was found as mesotrophic in a recent study based on nitrogen and phosphorus concentrations (Kaya, 2013). But in the literature, AHOD is poorly correlated with seston biomass and/or phosphorus concentrations and it is also poorly related to the TSI values based on phosphorus content (Borowiak et al., 2011).
The AHOD values for the Porsuk Reservoir are much higher, almost sevenfold than the Borabey Pond values. This result is expected as the Porsuk Reservoir is polluted by domestic and industrial sources and its sediment layers are also an oxygen depleting source due to yearlong storage of organic matter in the bottom mud.5 CONCLUSION
The study described found AHOD values of 4.263 (2013), 5.099 (2014) g O2/(m2·d) and HOD values of 0.237 (2013), 0.236 (2014) mg/(L·d) for the Porsuk Reservoir which is a highly polluted water body. The relatively unpolluted Borabey Pond displayed much lower values (AHOD values of 0.848 (2013), 0.569 (2014) g O2/(m2·d) and HOD values of 0.114 (2013), 0.073 (2014) mg/(L·d)). These values, when compared, reflect the important fact that they can be used as indicators of the levels of pollution by anthropogenic sources. This approach also takes into account the oxygen depletion by the sediment layer in addition to the oxygen depletion in the water column. The Porsuk Reservoir is, as mentioned before, a hypertrophic lake and still receives pollutants from its watershed. Oxygen depleting organic matter is primarily supplied to the reservoir by the discharge of domestic wastewaters. The high AHOD values reflect the presence of organic pollution with respect to the Borabey Pond which is relatively poor in incoming organic matter. The highness of Porsuk Reservoir values also is a consequence of long term (around 50 years) accumu-lations of organic matter in the reservoir sediments.
Water Framework Directive 2000/60/EC of the European Parliament and of the Council considers water to be trade commodity but also a heritage to be conserved. The countries bound by the directive are required to monitor the water bodies in relation to the pollution they receive. Several parameters should be monitored to assess the chemical and ecological status of lakes and reservoirs. In this respect, the hypolimnetic oxygen demand is a good indicator which can be used to determine the organic matter dynamics. The monitoring of this parameter is valuable to observe trends in the eutrophic status of the water bodies. So, this fact and the integration of organic matter amounts and oxygen depletion rates makes the parameter important within the directive.
In studies for the determination of AHOD/HOD, the accurate reproduction of the bathymetry of the water body and the exact pinpointing of the onset time of stratification and its duration is of prime importance. Frequent profiling of temperature and dissolved oxygen at the transition period (mixing-stratification) ensure an accurate determination of the onset and duration. Establishing a station in the lake performing continuous monitoring of temperature and dissolved oxygen profile at daily or weekly intervals will provide convenience and unbiased results.6 DATA AVAILABILTY STATEMENT
The data in the study come from two sources:
The batyhmetric data for the Porsuk Reservoir were obtained from the General Directorate of State Hydraulic Works of Turkey with web site http://www2.dsi.gov.tr/duyuru/bedel.htm.
The bathymetric map of the Borabey pond was generated with data collected by the authors with the methods explained in the manuscript. Temperature, density and dissolved oxygen data from both water bodies were also collected by the authors. Data thus obtained and generated in this study in this manner are available from the corresponding author on reasonable request.7 ACKNOWLEDGEMENT
The authors also would like to express their gratitude to the personnel of the Anadolu University Research Institute of Earth and Space Sciences and graduate student Enis Hasanoğlu for assisting field studies.
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