**5.2 Prespa Lake**

96 Studies on Environmental and Applied Geomorphology

decomposition processes, such as: *Melosira varians, Fragilaria capucina, Ulnaria ulna, Achnanthidium lanceolatum, Cocconeis placentula* var*. euglypta, Navicula phyllepta, Navicula cryptotenella, Navicula halophila, Navicula lanceolata, Navicula tripunctata, Navicula cryptocephala, Frustulia vulgaris, Reimeria sinuata, Gomphonema olivaceum, Gomphonema angustatum, Gomphonema micropus, Gomphonema aff. olivaceoides, Encyonema silesiacum, Amphora pediculus, Nitzschia linearis, Nitzschia palea, Surirella minuta*. Usually, the green branched alga *Cladophora glomerata* is also markedly present in these water bodies, bearing a rich epiphytic growth usually by *Cocconeis placentula* var*. euglypta*, but sometimes blue-green cyanobacteria *Heteroleiblenia kossinskajae* or *Pseudoanabaena limnetica* were observed as

Finally, the last detected algal assemblage was found in the most severely polluted river bodies of Prespa Lake watershed, like Golema Reka 5, 6, 7, and Istočka 2, and thus denoting **bad or poor ecological conditions**. The typical example of this algal assemblage was found at Golema Reka 5 sampling site where the diatom flora is reduced to as much as only 3 - 4 taxa, like *Nitzschia palea, Navicula cryptotenella* and *Ulnaria ulna* representing more than 98% of all detected cells. Highly decreased algal biodiversity is replaced by a mass development of two cyanobacterial species *Pseudoanabaena limnetica* and *Phormidium limosum* which

The WFD requires classification, in terms of ecological status, for all European surface waters. The classification should be based on reference conditions, which are intended to represent minimal anthropogenic impact and observed deviation from these conditions (Andersen et al., 2004). For each surface water body type, type-specific biological reference conditions were established, representing the values of the biological quality elements for

Among the biological communities, the macrozoobenthos is by far the most frequently used bioindicator group in standard water management (Hering et al., 2004). Numerous biotic index and score systems have used macrozoobenthos in the assessment of running waters (Rosenberg & Resh, 1993). The most represented biotic index or score methods are: taxa richness, number of EPT taxa, Saprobic Index (SI), Biological Monitoring Working Party (BMWP) Score, Average Score Per Taxon (ASPT), Danish Stream Fauna Index (DSFI). All indices were part of the respective national method planned for biological monitoring in the

Thus, in the frame of the project "Development of Prespa Lake Watershed Management Plan" and according to WFD requirements, categorisation of the delineated water bodies in the Prespa Lake watershed based on macrozoobenthos was done. Two metrics (EPT richness and DSFI) in assessment of the ecological health of the rivers were used. These metrics were selected because statistical power to detect a difference between the nutrient enriched and non-impacted sites was >0.99 for total taxa richness and number of EPT taxa.

In order to assess the ecological conditions of the river water bodies and related macrozoobenthos assemblages, the analyses of collected samples were performed to detect the presence of so called *positive* (like Ephemeroptera, Plecoptera, Trichoptera, Diptera, Gammaridae and even Astacidae) versus *negative* taxa (usually Oligochaeta – Chironomidae or Tubificidae). Pollution sensitive taxa like *Ecdyonurus venosus*, *Baetis alpinus*, *Capnia vidua*,

significant epiphytes.

completely cover the rocks on the bottom.

that surface water body type at high ecological status.

context of the Water Framework Directive (Birk & Hering, 2006).

DSFI also had relatively high power >0.95 (Sandin & Johnson, 2000) (Table 7).

Out of the WFD's biology quality elements, phytoplankton, zoobenthos, macrophytes and fish were in the focus of investigations, supported by a full range of physic-chemical analyses including heavy metals and priority substances. Water for the chemical analyses was sampled as a collective sample from the full water column on the site or as sediment, while the basic physical parameters were measured at every sample depth. Special attention was paid to the sampling of plankton for algae and benthic habitats (littoral, sub-littoral and profundal) for the macrozoobenthos analyses.

In order to determine the *reference conditions* for Prespa Lake, analyses of core samples dated 10 ka (500, 1,000, 2,000, 5,000 and 10,000 years respectively; the deepest analysed core sample from approximately 30 metres of the core depth) were performed for the first time regarding the total phosphorus content and diatom composition.

Macrophytes and fish samples from the selected sampling sites on Prespa Lake were collected during June 2010 and according to WFD sampling guides.

#### **Basic physical parameters**

The basic physical parameters detected in the waters of Prespa Lake (Fig.24) revealed some interesting features of this unique ecosystem. For example, recorded temperatures show a normal and gradual increase towards warmer months, but there was no sharp and rapid decrease in one water layer (thermocline) during the warmest month (July 2010) although the water temperatures between the deepest and the shallowest parts differ by more than 10ºC. This may be a result of a very turbulent climate in the sampling period with constant mixing of the water layers or as a consequence of intensive discharge of the deep water sources (sub-lacustrine water sources) again related to the rainy season. High deep water temperatures of 14-15ºC also indicate the full intensity of thermal insulation and possible full scale mixing of the entire water column during storms, which results in a constant nutrient supply of the epilimnion layer.

These results are sharply opposite to frequent statements of oxygen depletion in the deep water layers of Prespa Lake; the lowest recorded


Environmental Changes in Lakes Catchments

Prespa Lake watershed (Fig. 13).

activity or human input) in this period.

**Nutrient status** 

PO

EU legislation.

0.51

March

April

1.52

2.53

3.54

which shows conductivity values at around 800 – Krsti

as a Nitrate Vulnerable Zone as stated in EU legislation5.

Fig. 24. Detected nutrient levels in Prespa Lake.

April

May

July

March

April

May

L1 - v.Stenje L2 - v.Asamati L3 - v.Krani L4 - v.Nakolec L5 - v.Dolno Dupeni

July

March

April

May

July

March

April

May

Total sulphates

July

5 Nitrates Directive (91/676/EEC). 0

May

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March

as a Trigger for Rapid Eutrophication – A Prespa Lake Case Study 99

Another peculiarity recorded in these investigations was a very low pH reaction of the deep waters (beyond 10 m on L1 and L4) in July 2010, ranging as low as 3.7! This condition can arise if decomposition (bacterial) of organic material releases carbon dioxide and thus increases the amount of dissolved carbon dioxide; an increase in carbon dioxide decreases pH. Organic acids, often expressed as dissolved organic carbon (DOC), also decrease pH (Strumm & Morgan, 1981). Both of these possibilities point to the increased pressure on the lake in the spring-early summer period, which was also recorded in the data for the rivers in

Conductivity and TDS results obtained for the Prespa Lake (Fig. 20) sampling sites are in the realm of natural conditions for this type of lake (for example, comparison to Dojran Lake

increase of conductivity and corresponding TDS values in the deep water layers in July 2010, thus supporting the above statements of release of charged ions (either by microbial

Detected nutrient levels in Prespa Lake sampling sites (Fig. 24) fully reflect the overall conditions already established for the watershed. The lake is dominated by sulphates, the same as the rivers in the watershed (Fig.14), but there is also a marked presence of total N basically due to elevated concentrations of nitrates and ammonia. Regarding ammonia, the whole investigated area was found to be in the III-IV category class as stated in the domestic legislation, while with regard to the total presence of nitrates Prespa Lake has to be declared

With respect to *phosphorus* content in the waters of Prespa Lake the situation is even worse;

4-P) place the lake in realm of hyper-eutrophic conditions, both regarding domestic and

detected *total phosphorus concentrations* (based on the sum of detected values for P

ć, 2011). Nevertheless, there is a slight

2 O

NO3-N (mg/l) NO2-N (mg/l) NH3-N (mg/l) Total N (mg/l) P2O5-P (mg/l) PO4 -P (mg/l) Total P Total sulphates (mg/l)

5-P and

values during our investigations were 5-5.3 mg

 L\* Another peculiarity recorded in these investigations was a very low pH reaction of the deep waters (beyond 10 m on L1 and L4) in July 2010, ranging as low as 3.7! This condition can arise if decomposition (bacterial) of organic material releases carbon dioxide and thus increases the amount of dissolved carbon dioxide; an increase in carbon dioxide decreases pH. Organic acids, often expressed as dissolved organic carbon (DOC), also decrease pH (Strumm & Morgan, 1981). Both of these possibilities point to the increased pressure on the lake in the spring-early summer period, which was also recorded in the data for the rivers in Prespa Lake watershed (Fig. 13).

Conductivity and TDS results obtained for the Prespa Lake (Fig. 20) sampling sites are in the realm of natural conditions for this type of lake (for example, comparison to Dojran Lake which shows conductivity values at around 800 – Krstić, 2011). Nevertheless, there is a slight increase of conductivity and corresponding TDS values in the deep water layers in July 2010, thus supporting the above statements of release of charged ions (either by microbial activity or human input) in this period.

#### **Nutrient status**

98 Studies on Environmental and Applied Geomorphology

30m

25m

20m

15m

12m

11m

10m

9m

7m

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0

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pH Т

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(оC)

March

300

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100

0

Conduc.(µS)

March

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> L1

 July

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 April

 May

> L2

 July

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 April

 May

> L3

Fig. 24. Basic physical parameters detected at Prespa Lake sampling sites. The above findings are additionally corroborated by *dissolved* 

*oxygen* results. Deep water layers had showed relatively high DO values in the summer months, their values even being higher than upper

water layers. Maximal DO values were recorded at around 3-4 m depth layers where the phytoplankton was in maximum development.

These results are sharply opposite to frequent statements of oxygen depletion in the deep water layers of Prespa Lake; the lowest recorded


values during our investigations were 5-5.3 mg

L\*

 July

 March

 April

 May

> L4

 July

 March

 April

 May

L5

 July

TDS (ppm)

Conduc.(µS)

TDS (ppm)

Conduc.(µS)

TDS (ppm)

Conduc.(µS)

TDS (ppm)

Conduc.(µS)

TDS (ppm)

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50

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> L1

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> L2

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> L3

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> L4

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L5

July

(оC)

mg/l

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mg/l

Detected nutrient levels in Prespa Lake sampling sites (Fig. 24) fully reflect the overall conditions already established for the watershed. The lake is dominated by sulphates, the same as the rivers in the watershed (Fig.14), but there is also a marked presence of total N basically due to elevated concentrations of nitrates and ammonia. Regarding ammonia, the whole investigated area was found to be in the III-IV category class as stated in the domestic legislation, while with regard to the total presence of nitrates Prespa Lake has to be declared as a Nitrate Vulnerable Zone as stated in EU legislation5.

With respect to *phosphorus* content in the waters of Prespa Lake the situation is even worse; detected *total phosphorus concentrations* (based on the sum of detected values for P2O5-P and PO4-P) place the lake in realm of hyper-eutrophic conditions, both regarding domestic and EU legislation.

Fig. 24. Detected nutrient levels in Prespa Lake.

 5 Nitrates Directive (91/676/EEC).

Environmental Changes in Lakes Catchments

as a Trigger for Rapid Eutrophication – A Prespa Lake Case Study 101

is usually in concentrations high above the permissible 1 ng\*L-1. It is interesting to notice that the L2 sampling site in the vicinity of the mouth waters of Golema River was found with the lowest number of detected priority substances, contrary to the expected and detected pressures coming from this water body. The other sampling sites along the North-East coast of the lake (L3-L5) and the deepest part on L1 sampling site had a much higher number of detected priority substances and maximal values of separate chemicals. These findings corroborate the proposed intensive mixing of the Prespa Lake waters with significant

**Prespa Lake - Priority substances in ppm**

Fig. 26. Priority substances (in ppm) detected in waters of selected Prespa Lake sampling sites.

March July March July March July March July March July L1 L2 L3 L4 L5

Benzo (a )anthracene Benzo (a) pyrene Naphthalene Dibutilphthalate Bis(2-Ethylhexyl)phthalate

Heptachlor and DDT (V class) > 1 ng/l

Priority substances detected in Prespa Lake pose a significant hazard to biota and humans.

**Prespa Lake - Priority substances in ppb**

Fig. 27. Priority substances (in ppb) detected in the waters of selected Prespa Lake sampling

4,4'-DDT alpha-Endosulfan alpha-HCH beta-HCH gamma-HCH (Lindan)

PCB-52 2,4'-DDD 4,4'-DDD 2,4'-DDE 4,4'-DDE

delta-HCH Heptachlor cis-Heptachlor epoxide

PCB (III-IV class) 1-10 ng/l

March July March July March July March July March July L1 L2 L3 L4 L5

sites.

ng/l

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

μg/l

underwater currents that are spreading the pollution impact to a much wider area.
