Images from the Field Exploring Bulgaria’s Freshwater Life

Field observations reveal only part of the story of a freshwater ecosystem. Samples collected from Lake Okoto, Lake Bezbog, and Zlatna Panega were examined alongside observations of aquatic vegetation, zooplankton, and macroinvertebrates.

At Zlatna Panega, the remarkable turquoise colour reflects the interaction of light with clear, mineral-rich karst water and fine particles originating from the surrounding limestone.

Among the notable findings was an odonate larva in Lake Okoto, where the water temperature reached almost 20°C during sampling. At this elevation, the observation raises interesting questions about how changing weather patterns and lake conditions may affect mountain aquatic life. A single measurement cannot demonstrate a long-term climate trend, but it provides a valuable signal for continued monitoring.

At Bansko’s city lake, the intense green colour and floating algal accumulations were signs of a phytoplankton bloom. Laboratory measurements recorded 20 ± 0.7 mg/L nitrates and 0.29 ± 0.03 mg/L phosphates. Together with the lake’s shallow depth, strong sunlight and limited circulation, this nutrient availability supports rapid algal growth during summer. The assessment classified the condition as pronounced eutrophication and recommended monitoring phytoplankton, chlorophyll-a, oxygen and nutrients. 

At Lake Bezbog, dense stands of Sparganium angustifolium, with their long, narrow leaves, occupied parts of the littoral zone.

Lake Okoto presents a more complex picture. Its dissolved nitrate, nitrite and phosphate concentrations were below the analytical quantification limits, but this alone does not exclude biological productivity because nutrients may already be incorporated into algal and periphyton biomass. During sampling, the surface water reached 19.6°C, with pH 8.48 and oxygen saturation of 138%. This combination is consistent with elevated photosynthetic activity in the surface layer.
Understanding these ecosystems requires combining field surveys and laboratory analysis with long-term measurements, sensors, satellite observations, ecological expertise, and regulatory data. Each method contributes a different part of the picture.