PHYTOPLANKTON AS AN INDICATOR OF TROPHIC STATUS OF LAKE OHRID
Keywords:
Lake Ohrid, phytoplankton, chlorophyll a, trophic statusAbstract
Lake Ohrid is the oldest extant lake in Europe with more than 200 endemic species. It is a transboundary lake located on the Balkan Peninsula with two thirds of its area belonging to Macedonia and one third to Albania. This research aimed to analyze the abundance and diversity of phytoplankton, its spatial and seasonal distribution and chlorophyll a concentration to assessment the trophic status of Lake Ohrid. The study was conducted in five sampling campaign, seasonally from September 2025 to August 2026 at a sample station in the pelagic zone at eight depths and at six sampling stations in the littoral zone. Water samples were collected from each target depth using Niskin bottles and subsequently stored in accordance with standard protocols. Phytoplankton identification and enumeration were performed using an inverted microscope and the Utermöhl method. Chlorophyll a concentration was determined spectrophotometrically after extraction with 90 % ethanol. The trophic status of the lake was estimated using the values of Carlson’s Trophic State Index according to chlorophyll a concentration. Phytoplankton community included six divisions as Cyanophyta, Bacillariophyta, Chlorophyta, Chrysophyta, Charophyta and Pyrrophyta. Bacillariophyta,was the most important group in terms of species number, followed by Charophyta and Chlorophyta. Algal diversity in the littoral zone was higher than in the pelagic zone, particularly regarding diatoms. In Lake Ohrid, there was a significant difference in the composition of the phytoplankton community at different depths in the same season, as well as at the same depths in different seasons. The phytoplankton abundance in Lake Ohrid was the highest in spring period, and then significantly decreased in the summer when the values in the upper layer were especially low. In the autumn period, the values were lower compared to the winter period. The concentration of chlorophyll a, as an indicator of phytoplankton production, the highest value, corresponding to the phytoplankton density, was recorded in winter at 20 meters depth with 1.62 μg l-1 and at 10 meters depth with 1.54 μg l-1 and the lowest values were observed at 150 meters depth. In summer the highest chlorophyll a concentration was observed at 50 m depth with value of 1.02 μg l-1. The distribution of chlorophyll a in Lake Ohrid completely coincides with the general distribution of chlorophyll a in oligotrophic lakes from the temperate zone. According to the Trophic State Index, the pelagic and all littoral zones of Lake Ohrid are characterized as oligotrophic. In accordance with the Water Framework Directive, we can define the status of the pelagic and littoral zones of Lake Ohrid as high. In conclusion, phytoplankton are effective bioindicators for assessing trophic status and lake primary productivity. To preserve the oligotrophic status of Lake Ohrid in future, it is essential to reduce anthropogenic pressures and nutrient input in order to prevent the process of eutrophication.
References
Carlson, R. E. (1977). A trophic state index for lakes. Limnologyand Oceanography 22(2), 361-369.
Dutta, S., Dhan, S.A., Najim Uddin, S., Saha, B. (2023). Phytoplankton as an indicator of Eutrophication. Eco. Env. & Cons. 29(3), 1042-1052.
Evayani, N., Fitriah, L., Ihwan, K., Husain, P., & Nirwana, A. R. (2026). The role of phytoplankton as an indicator of trophic status and primary productivity in marine waters. Journal of Natural Resource Education and Conservation. 1(2), 48–53.
ISO 10260 (1992). Water quality – Measurement of biochemical parameters – Spectrometric determination of the chlorophyll-a concentration. ISO Geneva, 1-6.
Kasprzak, P., Padisák, J., Koschel, R., Krienitz, L, Gervais, F. (2008). Chlorophyll a concentration across a trophic gradient of lakes: an estimator of phytoplankton biomass? Limnologica. 38, 327–338.
Marshall, C. T. & Peters, R. H. (1989). General patterns in the seasonal development of chlorophyll a for temperate lakes. Limnol. Oceanogr. 34, 856-867.
Mineeva, N. (2024). Production characteristics of phytoplankton as indicators of the trophic state of artificial reservoirs. Inland Water Biology, 16(3), 333–342.
Mitic, V. (1993). Seasonal and spatial distribution of the phytoplankton in the pelagic zone of lake Ohrid., Ecology and environmental protection. 1, 7-10.
Oliveira, S. A., Ferragut, C., & Bicudo, C. E. M. (2024). Phytoplankton functional groups associated with the trophic state of tropical reservoirs. Acta Limnologica Brasiliensia, 36, e45. https://doi.org/10.1590/S2179-975X0524
Paerl, H. W., Valdes-Weaver, L. M., Joyner, A. R., & Winkelmann, V. (2007). Phytoplankton indicators of ecological change in the eutrophying Pamlico Sound system, North Carolina. Ecological Applications, 17(5), 370 88–101. https://doi.org/10.1890/05-0840.1
Patceva, S. (2001). The phytoplankton and the chlorophyll-a as indicators of the trophic state of the water in the littoral and pelagic region of Lake Ohrid (in Macedonian). Master thesis. University “St. Kiril and Methodius” Skopje, Macedonia.
Patceva, S. (2005) - Comparative analysis of the phytoplankton community and the trophic state of Lakes Ohrid and Prespa (in Macedonian). Ph. D. thesis, University “St. Kiril and Methodius” Skopje, Macedonia.
Patceva, S., Mitic, V., Jordanoski, E., Veljanoska-Sarafiloska, E. (2009). Trophic state of Lake Ohrid. J. Int. Environmental Application & Science. 4 (3): 310-316.
Patceva, S., Mitic, V., Jordanoski, M., Veljanoska-Sarafiloska, E. (2004). Influence of the main tributaries on the Trophic state of Lake Ohrid: Conference on water observation and information system for decision support "BALWOIS". Ohrid, Macedonia. http:// www. balwois.com.
Patceva, S., Veljanoska-Sarafiloska, E., & Leshoski, J. (2024). Phytoplankton community as bioindicator of water trophic state in Lake Prespa. In S. Šerbula (Ed.), Proceedings of the 31st International Conference Ecological Truth and Environmental Research (EcoTER’24) (pp. 176–181). University of Belgrade, Technical Faculty in Bor.
Poikane, S., Portielje, R, Van den Berg, M., Phillips, G., Brucet, S., Carvalho, L., Mischke, U., Ott, I., Soszka, H., Van Wichelen, J. (2014). Defining ecologically relevant water quality targets for lakes in Europe. J Appl Ecol. 51.592–602.
Reynolds, C.S. (1980). Phytoplankton assemblages and their periodicity in stratifying lake systems, Holarctic. Ecol., 3, 141–159.
Schneider, C. S., Biberdžić, V., Braho, V., Budzakoska Gjoreska, B., Cara, M., Dana, Z., Đurašković, P., Eriksen, T. E., Hjermann, D., Imeri, A., Jovanović, K., Krizmanić, J., Kupe, L., Loshkoska, T., Lynn Kemp, J., Marković, M., Patceva, S., Rakočević, J., Stojanović, K., Talevska, M., Trajanovska, S., Trajanovski, S., Veljanoska Sarafiloska, E., Vidaković, D., Zdraveski, K., Živić, I., Vermaat, E. J. (2020). Littoral eutrophication indicators are more closely related to nearshore land use than to water nutrient concentrations: A critical evaluation of stressor-response relationships. Science of the Total Environment. 748,141193 DOI: 10.1016/j.scitotenv.2020.141193.
Utermöhl, H. (1958). Zur Vervollkommnung der quantitativen Phytoplankton–Methodik. International Association of Theoretical and Applied Limnology, Communications 9, 1-38.
Vermaat, E. J., Biberdžić, V., Braho, Budzakoska Gjoreska, B., Cara, M., Dana, Z., Đurašković, P., Eriksen, T. E., Hjermann, D., Imeri, A., Jovanović, K., Krizmanić, J., Kupe, L., Loshkoska, ., Lynn Kemp, J., Marković, M., Patceva, S., Rakočević, J., Stojanović, K., Talevska, M., Trajanovska, S., Trajanovski, S., Veljanoska Sarafiloska, E., Vidaković, D., Zdraveski, K., Živić, I., Schneider, C. S. (2022). Relating environmental pressures to littoral biological water quality indicators in Western Balkan lakes: Can we fill the largest gaps? Science of the Total Environment 804 (3):150160. DOI: 10.1016/j.scitotenv.2021.150160
Wellborn, G. A., Skelly, D. K., & Werner, E. E. (1996). Mechanisms creating community structure across a freshwater habitat gradient. Annual Review of Ecology and Systematics, 27(1), 337–363.
