ASSESSMENT OF TROPHIC STATUS AND WATER QUALITY IN THE LITTORAL AND PELAGIC ZONES OF LAKE OHRID BASED ON PHYSICO-CHEMICAL PARAMETERS
Keywords:
anthropogenic pressure, water quality, eutrophication, trophic status, littoral zoneAbstract
Lake ecosystems are highly dependent on the characteristics and processes occurring within their surrounding catchment areas, which directly influence the physical, chemical, and biological properties of the receiving water bodies. Anthropogenic pressures and natural processes within the catchment can substantially affect nutrient inputs and consequently influence water quality and trophic conditions. Increased nutrient availability may induce changes in the trophic status of the littoral zone, which may subsequently affect the ecological conditions of the pelagic zone. The trophic status of an aquatic ecosystem reflects its level of biological productivity and provides an important indication of its ecological condition. Lake Ohrid is among the most important natural freshwater ecosystems in North Macedonia and is globally recognized as one of the oldest lakes, supporting exceptionally diverse and endemic biota. Despite its exceptional ecological value and predominantly oligotrophic character, Lake Ohrid is not immune to the ongoing pressures associated with anthropogenic activities and the progressive process of eutrophication. Anthropogenic influence is particularly evident in certain parts of the shoreline, where localized areas of increased pressure or so-called “hot spots” may occur and potentially affect water quality and trophic conditions.
The aim of this study was to assess water quality and trophic status in the littoral zone of Lake Ohrid, with particular emphasis on areas affected by anthropogenic activities, and to provide a comparative assessment with a selected site in the pelagic zone. Seasonal sampling was conducted during 2025 and 2026 at six littoral sites distributed along the Lake Ohrid shoreline and at one pelagic sampling site, where a vertical profile comprising eight depths was investigated. Water samples were collected, stored and handled according to standard procedures prior to laboratory analyses. The investigated physico-chemical parameters included concentration of dissolved oxygen, biochemical oxygen demand (BOD₅), oxidizable organic matter expressed as KMnO₄ consumption, concentration of total phosphorus (TP), and total nitrogen (TN). Standard limnological methods were applied for the determination of the selected parameters. The trophic status of the investigated sites was evaluated using Carlson’s Trophic State Index (TSI), the OECD classification, and the national Directive for Classification of Surface Waters. The results revealed pronounced spatial differences in the degree of anthropogenic influence among the investigated littoral sites. The concentrations of biodegradable organic matter ranged from 4.578 mg L⁻¹ KMnO₄ at the Desaret site to 12.597 mg L⁻¹ KMnO₄ at the Grashnica littoral and 12,929597 mg L⁻¹ KMnO₄ at Euro Hotel. The highest concentrations of total phosphorus were recorded at the Grashnica littoral (18.29 μg L⁻¹ TP) and the Sateska River littoral (20.15 μg L⁻¹ TP), indicating a pronounced influence of inflowing waters. Regarding the Trophic State Index based on total phosphorus, the lowest value was recorded in the pelagic zone (TSI-TP = 28), whereas the highest values were recorded at the Grashnica (TSI-TP = 46) and Sateska River littoral (TSI-TP = 47) sites. The application of Carlson’s method, based on total phosphorus (TP) concentration and Secchi disk transparency (SD), provided numerical values of the trophic state index and enabled the classification of the investigated sites according to their trophic conditions. The obtained values indicate that the littoral zone of Lake Ohrid ranged from oligotrophic to oligo-mesotrophic conditions, whereas the pelagic zone corresponded to an oligotrophic state (average TSI-SD=21).The observed spatial variability indicates that localized anthropogenic pressures can result in measurable differences in physical and chemical conditions and trophic status within different parts of the lake. Overall, the findings demonstrate that river mouths, agricultural areas, and intensively used tourist zones represent important localized sources of anthropogenic pressure on the littoral zone of Lake Ohrid. The results confirm the importance of continued and systematic monitoring of both littoral and pelagic zones, integrating physical, chemical and biological parameters, together with targeted management measures aimed at reducing anthropogenic pressures, preventing further deterioration of water quality, and safeguarding the ecological integrity and long-term sustainability of this unique lake ecosystem.
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