Lake Prespa and Lake Ohrid, located in south-eastern Europe, are two lakes of extraordinary ecological value. Although the upstream Lake Prespa has no surface outflow, its waters reach the 160 m lower Lake Ohrid through underground hydraulic connections. Substantial conservation efforts concentrate on oligotrophic downstream Lake Ohrid, which is famous for its large number of endemic and relict species. In this paper, we present a system analytical approach to assess the role of the mesotrophic upstream Lake Prespa in the ongoing eutrophication of Lake Ohrid. Almost the entire outflow from Lake Prespa is found to flow into Lake Ohrid through karst channels. However, 65% of the transported phosphorus is retained within the aquifer. Thanks to this natural filter, Lake Prespa does not pose an immediate threat to Lake Ohrid. However, a potential future four-fold increase of the current phosphorus load from Lake Prespa would lead to a 20% increase (+0.9 mg P m )3 ) in the current phosphorus content of Lake Ohrid, which could jeopardize its fragile ecosystem. While being a potential future danger to Lake Ohrid, Lake Prespa itself is substantially endangered by water losses to irrigation, which have been shown to amplify its eutrophication.

Organochlorine pesticides were determined in water and sediment samples collected from the littoral zone of Lake Prespa, as well as from its three main tributaries (the rivers Golema, Brajcinska and Kranska), during the period 2004 to 2006. In addition, muscle tissue samples of barbus fish (Barbus prespensis Karaman, 1928) collected from the littoral zone of Lake Prespa were also analysed. The obtained results give an overview of the contamination levels of these problematic compounds at their potential sources in the river mouths, in the potentially affected, species-rich littoral section of the lake and in the muscle tissue of one selected fish species, collected near the rivers' deltas. Special attention was paid to the presence of some DDT metabolites (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (p,p'-DDE); (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (p,p'-DDD) and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (p,p'-DDT). The extraction of pesticides from water samples was done by liquid-liquid partition in dichloromethane. For the sediment and fish tissue we used solid-liquid extraction. The extracted residues were analyzed on a gas chromatograph equipped with an electron capture detector (GC-ECD). The results of the respective studies indicated the presence of DDT metabolic forms in the samples of the three analysed matrixes. The highest levels of presence for these pollutants were found in the muscle tissue of the fish samples. The total DDTs content in the analysed muscle tissue samples range from 11.67 to 13.58 μg kg⁻¹ of fresh tissue. The average total DDTs content for the sediment samples were within the range of 2.32 to 4.17 μg kg⁻¹ of dry sediment. Higher DDT metabolites content were found in the sediments collected from the rivers than in the samples from the littoral zone. The lowest average total concentrations of DDTs, on the other hand, were recorded in the water samples and ranged between 0.036 and 0.057 μg L⁻¹. The obtained results indicated that the dominant metabolic form in the samples of the three investigated matrixes (water, sediment and fish tissue) from Lake Prespa was p,p'-DDE. There was a very good linear correlation in this study between the content of DDT's (total DDT metabolites) detected and the percentage of total organic material in the sediment. The detected concentrations are clearly below the toxicity thresholds; consequently, severe effects on the endemic species of Lake Prespa are not very likely.

A systematic study has been performed for determination of a range of organochlorine pesticides in the water, sediments and fish samples in the Macedonian part of the Lake Ohrid and its larger tributaries. The obtained results give an overview of the contamination levels of these problematic compounds (i) at their potential sources in the river mouths, (ii) in the potentially affected, species-rich littoral section of the lake, and (iii) in the muscle tissue of one selected fish species Barbus peloponnesius, Valenciennes, collected near the rivers’ deltas. The organochlorine pesticides measured in all three matrixes were: γ-HCH, ΣHCH (sum of α-isomer, β-isomer and δ-isomer), endosulfan (total of α and β endosulfan), DDT metabolites (p,p’-DDE, p,p’-DDD and p,p’-DDT). The range of the observed concentrations for p,p’-DDT was between 0.006 μg l–1 in the water samples from Daljan and St. Naum and 0.036 μg l–1 in the water from the Koselska River. The values for p,p’-DDT measured in dry sediment ranged from 0.121 μg kg–1 in the sample from St. Naum to 1.8 μg kg–1 in the dry sediment from the Velgoška River and between 0.553 μg kg–1 for total endosulfan and 5.982 μg kg–1 for p,p’-DDE of wet fish biomass. The most abundant of the detected organochlorine pesticides was the sum of DDT metabolic forms, i.e. p,p’-DDT, p,p’-DDE and p,p’-DDD. The detected concentrations are clearly below toxic thresholds and consequently severe effects on the endemic species of Lake Ohrid are not very likely.