Biological denitrification is a generally economically viable and reliable technology for nitrate (NO₃⁻-N) and nitrite (NO₂⁻-N) removal from wastewater. Nitrate/nitrite reduction requires an electron donor, which can be supplied from influent wastewater organic compounds, added as an external carbon source, or by endogenous respiration. Decisions regarding the selection of an external carbon source should take into consideration its cost, sludge generation quantity, the rate and efficiency of denitrification, and chemical storage safety. The expense of waste management and external carbon sources accounts for up to 50% of the overall cost of wastewater treatment. The carbon source characteristics required for biological denitrification affect the microbial community structure, denitrification rate, and intermediate products. This review is based on a bibliometric analysis and a systematic literature review providing information and insight into the topic of the denitrification process using different carbon sources. In this review, the main focus was on discussing alternative carbon sources vs. traditional carbon sources in terms of the carbon source price, C/N ratio, denitrification efficiency and rate, operational parameters, and advantages and disadvantages, as well as the limitations in the denitrification process. Future perspectives may include the operating parameters influencing the stability of the removal performance; the maintenance and improvement of nitrate removal; a study of the diversity of the microbial community; research on the application of new alternative carbon sources in denitrification; and N₂O emission detection and minimisation.

The Plitvice Lakes National Park is inscribed on UNESCO’s World Heritage List. The lake system is composed of 16 cascading lakes of different sizes separated by tufa barriers, which are the park’s key phenomenon. The lakes are characterized by highly diverse trends of the characteristic hydrological indicators—mean annual water levels, discharges, and tufa barrier growth. The analyses carried out in this paper identified that in the period before the early 1990s, Kozjak Lake had a trend of decreasing discharges, together with a trend of increasing water levels and growing tufa barriers. In contrast to this, in the period after 2001, a trend of increasing discharges was recorded, as well as a trend of decreasing water levels and decreasing tufa barriers. A potential cause of the barriers decreasing in size were the extremely high discharges during the last decade, which resulted in increased erosion of the tufa barriers. Losses of water due to the sinking from the lake system as well as the upper Korana course were confirmed, and it was identified that during the analyzed period the losses had not changed significantly. It was determined that the losses of water from Kozjak Lake occurred during low-water periods; however, they depended not only on the quantity of water flowing through the lakes but also on the hydrological conditions underground. The analyses carried out and the methodological procedures used in the analyzed area of the Plitvice Lakes are useful examples for the performance of analyses at similar lakes in karst formed by tufa deposition processes.

Plitvice Lakes National Park is the oldest protected area in the Republic of Croatia and the biggest by its surface. The park is designated as the UNESCO World Heritage Site. Outstanding universal value is recognized within significant natural and geological processes, habitats and biodiversity. Only 1% of the park’s large surface is the main focal point for visitors and active tourism. We evaluated management of the park through Strengths, Weaknesses, Opportunities and Threats (SWOT) analysis interpreting internal and external factors. High conservation of biodiversity, investments in different projects and high number of employees are considered as strengths. High number of visitors in small area and lack of visitor management plan, educational programs and marketing strategy are weaknesses. Threats are present through the inadequate management of drinking water supplies, lack of wastewater treatment plant and excessive construction in villages. Our opportunity definitely lies in protected nature and biodiversity. Considering very good status of the park’s finances, there aren’t any significant barriers for sustainable tourism approach, development of educational programs, various investments and adoption of new management plans. However, these activities must be beyond any potential political influence and they should have continuity in order for the park to be an example of quality management in the years to come.

Invasive fish species pose a major threat to freshwater ecosystems by outcompeting native species and altering habitat conditions, leading to biodiversity loss. The Plitvice Lakes National Park, a UNESCO World Heritage site in Croatia, harbors a unique but species-poor native fish community, including Danube trout (Salmo labrax), Italian minnow (Phoxinus lumaireul), Italian golden loach (Sabanejewia larvata), and Italian spined loach (Cobitis bilineata). However, human-introduced species such as rainbow trout (Oncorhynchus mykiss), rudd (Scardinius erythrophthalmus), chub (Squalius cephalus), and northern pike (Esox lucius) have established thriving populations, with rudd emerging as the dominant invasive species. To mitigate their ecological impact, targeted removal efforts were implemented using casting nets and electrofishing, aiming to reduce invasive populations while minimizing harm to native species. Monitoring activities, performed during the course of the removal project, revealed significant declines in invasive fish abundance, demonstrating the effectiveness of these management interventions. Rudd abundance in Prošćansko Lake decreased by 56.9% and biomass by 69.7% over three years, while rainbow trout abundance in the B-C-M system declined by 92.7%, and its distribution shrank to a single locality, indicating strong population reduction. These findings highlight the importance of implementing active conservation measures to control invasive fish populations, thereby improving the status of native fish communities and preserving ecological balance.