An integrated approach to assess the ecological and chemical status of lakes with HOD/AHOD: a case study of two lakes

The thermal stratification of the lakes impedes the transfer of atmospheric oxygen into the lower layers of the lake. In lakes which are affected by diverse anthropogenic influences, the increasing organic matter amounts lead to a sharp decrease in hypolimnetic oxygen amounts, aided by thermal stratification, and anaerobic conditions arise. The determination of hypolimnetic oxygen demand (HOD) and areal hypolimnetic oxygen demand (AHOD) and their monitoring represent an integrated approach to investigate the oxygenation of lakes, the nutrient conditions, and the physicochemical dynamics. In this study, two lakes differing by size and affected by different anthropogenic sources, are investigated in this respect. At first, bathymetric studies were conducted to determine the depth, surface area, and volume relationships. Then, based on monitoring studies conducted in 2013 and 2014, the thermal stratification dynamics and layer properties were established using the relative thermal resistance to mixing (RTRM) index based on temperature and density profiles. Following this, the oxygen depletion rates were determined by oxygen and temperature profiling in the hypolimnion. For the years of 2013 and 2014, the AHOD values for the Borabey Pond which is far from anthropogenic influences, were found to be 0.848 and 0.569 g O₂/(m·d), respectively. The AHOD values for the Porsuk Reservoir which was overburdened for years by industrial and domestic pollution were found to be 4.263 and 5.099 g O₂/(m·d), larger than its counterpart by almost sevenfold. The HOD and AHOD monitoring can be considered to a valuable tool for assessing the ecological and chemical status of lakes within Annex 5 of the Water Framework Directive and as an integrated approach to assess and monitor the status of lakes.