**6.2 Reference conditions for Prespa Lake**

Establishing the reference conditions for Prespa Lake (or any other lake) is a far more difficult task to perform. If the only reasonable and justified principle regarding every water ecosystem as a separate entity (the state-changed approach opposite to spatial state classification – Moss et al., 1997) is applied, than Prespa Lake cannot be compared for its reference parameters to any other lake (even with Lake Ohrid to which Prespa Lake is the major water source). This is even more important if the very turbulent and variable past of Prespa Lake is taken into account. Namely, the lake was formed by three rivers (underwater flows which are still detectable in the lake) which were constrained by karstic masses blocking their way to Ohrid Lake. Only then did the Prespa Lake ecosystem start to develop with a very variable surface area and volume in the past; there are numerous human constructions (buildings, roads) recorded at the lake's bottom today. All of these characteristics describe Prespa Lake as a very large water source body, intensively mixed by numerous sub-lacustrine sources of water and with very unstable water mass basically depending on climate, hydrologic regime and human activities. It is also a system in which

Environmental Changes in Lakes Catchments

has also been recorded during the summer months.

deserves much more attention in the future.

1999).

as a Trigger for Rapid Eutrophication – A Prespa Lake Case Study 109

Regarding heavy and toxic metals (Fig. 33) the greatest increase is recorded in concentrations of *zinc* and *manganese*, but *lead* is also showing a steady increase and a sudden peak in present times. These results are also corroborative of the obvious increase of

The results obtained for the total P content in the present day sediments of Prespa Lake (Fig. 34) are quite interesting. It can be concluded that the phosphorus in Prespa Lake has the crucial role in the overall eco-physiology of the system. It is not deposited at a regular pace and it is also not used in a predictive manner; a significant increase of phosphorous input

**Prespa Lake - Total P in sediment**

March July

**mg/kg**

Fig. 34. Total P content measured in recent sediments at the sampling sites of Prespa Lake. Compared to the results obtained from the analyses of the core samples (Fig. 35), the phosphorus in Prespa Lake reveals other important features. Firstly, it has been deposited in the recent sediments in significantly higher quantities (almost three times higher) than recorded in the core samples. Secondly, its predominance over nitrogen has been taking place in the last 500 years. Thirdly, Prespa Lake has never been a nitrogen limiting lake, since the values for total nitrogen are almost constant throughout the analysed period. Therefore, the principal nutrient that is driving the observed changes in the lake's plankton communities (cyanobacterial 'water blooms') is phosphorus. Observed occurrence of the cyanobacterial 'water blooms' at L5 sampling site (village Dolno Dupeni) and the results for the phosphorus deposition at the same area of the lake is more than just a coincidence which

L1 L2 L3 L4 L5

There are very few organisms, or their remains, that are well preserved in lacustrine sediments and can be easily retrieved for observations. Having siliceous cell walls, diatoms are probably the ultimate choice (Krstic et al., 2007) for both monitoring of the recent and paleo environments, since they quickly and constantly change their assemblages according to environmental conditions and their specific autecological preferences (Stoermer & Smoll,

human impact due to waste input in the lake's sediments in the past 500 years.

there is a constant mixing of the water column, either by winds or powerful underwater currents and sources, which also means a constant supply of nutrients in the water column.

In a water body such as this, and combined with the classical lack of continual monitoring (especially regarding biology) data, establishing of reference conditions has been proven a formidable task. Nevertheless, we have succeeded in acquiring core samples dated from 10 ka BP and to briefly analyse for the first time the basic chemical (major cations, heavy metals, total N and P content) and biological (diatom assemblages) parameters in the core layers dated from 0.5, 1, 2, 5 and 10 ka BP respectively.

Regarding the concentrations of major cations and heavy metals obtained from the analyses of Prespa Lake core samples (Fig. 32), Prespa Lake is dominated by *aluminium* and *iron*  throughout the analysed period. On the other hand, *calcium* has increased more than three times in the same period, as well as *sodium* in the last 500 years, while *potassium* by 30%. These are clear signs of human alterations of the natural conditions.

Fig. 32. Major cations and heavy metals in core samples of Prespa Lake.

Fig. 33. Heavy and toxic metals in core samples of Prespa Lake.

there is a constant mixing of the water column, either by winds or powerful underwater currents and sources, which also means a constant supply of nutrients in the water column. In a water body such as this, and combined with the classical lack of continual monitoring (especially regarding biology) data, establishing of reference conditions has been proven a formidable task. Nevertheless, we have succeeded in acquiring core samples dated from 10 ka BP and to briefly analyse for the first time the basic chemical (major cations, heavy metals, total N and P content) and biological (diatom assemblages) parameters in the core

Regarding the concentrations of major cations and heavy metals obtained from the analyses of Prespa Lake core samples (Fig. 32), Prespa Lake is dominated by *aluminium* and *iron*  throughout the analysed period. On the other hand, *calcium* has increased more than three times in the same period, as well as *sodium* in the last 500 years, while *potassium* by 30%.

**Major cations and metals in core sediment of Prespa Lake**

0 10 20 30 40 50 60

0 500 1000 1500 2000 2500

0 10 20 30 40 50 60 70 80 90 100

**Heavy and toxic metals in core sediment of Prespa Lake**

**mg/g**

Zn Pb Ni Cu Cr Cd Ag Mn

**Mn**

μ**g/g**

Na Fe Mg K Ca Al

layers dated from 0.5, 1, 2, 5 and 10 ka BP respectively.

10000 BP

Present average

10000 BP 5000 BP 2000 BP 1000 BP 500 BP

5000 BP

2000 BP

1000 BP

500 BP

Present average

These are clear signs of human alterations of the natural conditions.

Fig. 32. Major cations and heavy metals in core samples of Prespa Lake.

Fig. 33. Heavy and toxic metals in core samples of Prespa Lake.

Regarding heavy and toxic metals (Fig. 33) the greatest increase is recorded in concentrations of *zinc* and *manganese*, but *lead* is also showing a steady increase and a sudden peak in present times. These results are also corroborative of the obvious increase of human impact due to waste input in the lake's sediments in the past 500 years.

The results obtained for the total P content in the present day sediments of Prespa Lake (Fig. 34) are quite interesting. It can be concluded that the phosphorus in Prespa Lake has the crucial role in the overall eco-physiology of the system. It is not deposited at a regular pace and it is also not used in a predictive manner; a significant increase of phosphorous input has also been recorded during the summer months.

Fig. 34. Total P content measured in recent sediments at the sampling sites of Prespa Lake.

Compared to the results obtained from the analyses of the core samples (Fig. 35), the phosphorus in Prespa Lake reveals other important features. Firstly, it has been deposited in the recent sediments in significantly higher quantities (almost three times higher) than recorded in the core samples. Secondly, its predominance over nitrogen has been taking place in the last 500 years. Thirdly, Prespa Lake has never been a nitrogen limiting lake, since the values for total nitrogen are almost constant throughout the analysed period. Therefore, the principal nutrient that is driving the observed changes in the lake's plankton communities (cyanobacterial 'water blooms') is phosphorus. Observed occurrence of the cyanobacterial 'water blooms' at L5 sampling site (village Dolno Dupeni) and the results for the phosphorus deposition at the same area of the lake is more than just a coincidence which deserves much more attention in the future.

There are very few organisms, or their remains, that are well preserved in lacustrine sediments and can be easily retrieved for observations. Having siliceous cell walls, diatoms are probably the ultimate choice (Krstic et al., 2007) for both monitoring of the recent and paleo environments, since they quickly and constantly change their assemblages according to environmental conditions and their specific autecological preferences (Stoermer & Smoll, 1999).

Environmental Changes in Lakes Catchments

as a Trigger for Rapid Eutrophication – A Prespa Lake Case Study 111

Fig. 36. Comparative presentation of diatom assemblages retrieved from 0.5-10 ka BP core samples of Prespa Lake and some of the most dominant and characteristic taxa in the investigated core samples: *1. Cyclotella ocellata, 2. Stephanodiscus rotula, 3. Aulacoseira granulata, 4. Aulacoseira ambigua, 5. Karayevia clevei var.balcanica* f*.rostrata, 6. Diploneis ostracodarum, 7. Diploneis mauleri, 8. Cavinula scutelloides, 9. Surirella bifrons, 10. Gyrosigma* 

*macedonicum, 11. Camplylodiscus noricus.* 

played the crucial role in increasing of the trophic status of Prespa Lake.

phosphorus and possibly other nutrients not yet analysed in the core samples. The presented time frame corroborates the strong possibility that human activities have

Fig. 35. Total P and total N Prespa Lake core sediments.

By analysing the diatom assemblages in different core layers of Prespa Lake presented on Figure 36, in order to reveal possible changes in dominant planktonic or benthic taxa and thus deduce the corresponding changes of environmental conditions forced by human activities, the following observations can be formulated:


012345678

By analysing the diatom assemblages in different core layers of Prespa Lake presented on Figure 36, in order to reveal possible changes in dominant planktonic or benthic taxa and thus deduce the corresponding changes of environmental conditions forced by human

0 200 400 600 800 1000 1200 1400

Total P Total N

**Total P (mg/kg)**

**Total N (mg/g)**

• Diatom assemblages along the 10 ka core of Prespa Lake are surprisingly uniform. Only very slight changes in dominance of specific taxa can be observed; typically dominant throughout the core are *Cyclotella ocellata, Stephanodiscus rotula, Diploneis mauleri* and

• Diatom flora of Prespa Lake is very rich in taxa as recorded before (Levkov et al., 2006). But, the overall composition of taxa in the communities point to an ecosystem which is naturally rich in nutrients and enables development of diverse microflora which is reflecting the basic **mesotrophic state** (according to present state of knowledge regarding diatom nutrient preferences and autecology) of the environment at least up

• The only observed important occurrence of a diatom form that can be conclusive for a significant increase of nutrients in the ecosystem is the appearance of *Aulacoseira* spp. (especially *Aulacoseira granulata*) in the sediments approximately 1000 BP, and persisting in the communities to the present. This unique, but very subtle, change in diatom taxa dominance can be connected to the recorded high increase of phosphorus concentration in the Prespa Lake sediments presented on Fig. 35. For comparison, the *Aulacoseira* taxa determined in Prespa Lake can be found in co-dominance with various cyanobacterial taxa (which are usually regarded as potentially toxic) in the plankton of highly eutrophic lakes like Dojran Lake in Macedonia (Fig. 37 -Krstic et al., in prep). Since we cannot see the cells (or their remains) of other algae in the core layers, by deduction from the present knowledge we can conclude that Prespa Lake has become eutrophic, at least during the most productive periods, due to an increase of

Fig. 35. Total P and total N Prespa Lake core sediments.

activities, the following observations can be formulated:

*Camplylodiscus noricus*.

10000 BP

5000 BP

2000 BP

1000 BP

500 BP

to 10.000 years BP.

phosphorus and possibly other nutrients not yet analysed in the core samples. The presented time frame corroborates the strong possibility that human activities have played the crucial role in increasing of the trophic status of Prespa Lake.

Fig. 36. Comparative presentation of diatom assemblages retrieved from 0.5-10 ka BP core samples of Prespa Lake and some of the most dominant and characteristic taxa in the investigated core samples: *1. Cyclotella ocellata, 2. Stephanodiscus rotula, 3. Aulacoseira granulata, 4. Aulacoseira ambigua, 5. Karayevia clevei var.balcanica* f*.rostrata, 6. Diploneis ostracodarum, 7. Diploneis mauleri, 8. Cavinula scutelloides, 9. Surirella bifrons, 10. Gyrosigma macedonicum, 11. Camplylodiscus noricus.* 

Environmental Changes in Lakes Catchments

µg/L 53

period.

0

Macrh

April

May

July

August

5

10

15

20

25

good water quality status.

Rockström et al., 2009).

**catchments** 

as a Trigger for Rapid Eutrophication – A Prespa Lake Case Study 113

**Microcystins in Prespa Lake waters**

Fig. 39. Cyanotoxins-microcystins in Prespa Lake waters during the 12 month investigation

September

October

November

March

June

L1 L2 L3 L4 L5

After all presented analyses and elaborations, the reference conditions for the Macro Prespa Lake ecosystem as one water body are presented in Table 10. Presented values for the most important parameters are targeted on the boundary between good and moderate water quality status for Prespa Lake which was surpassed at least a century ago. Having in mind the very high pressure of a variety of pollutants and human influences elaborated in this report, the targeted reference conditions may seem out of reach. But, if the ongoing situation continues, a total turnover of Prespa Lake towards highly eutrophic ecosystem should be expected in a very near future. In that case, the overall status of the Prespa-Ohrid-Crni Drim River system will be jeopardised and much more difficult to control, let alone brought to a

**7. Discussion on the global significance of environmental alterations in lakes** 

The people and their societies are integrated parts of the biosphere. They are dependent on its function and support, but at the same time they are shaping the biosphere globally with marked geological consequences (Steffen et al., 2011). This issue is much broader than the climate change *per se* (Folke et al., 2011). A key challenge for humanity in this new situation is to understand its role in the 'Earth System', start accounting for and governing natural capital and actively shape development in tune with the biosphere (Jansson et al., 1994;

It is usually emphasised that during the last couple of generations we have witnessed an amazing expansion of human activities into a converging globalised society, enhancing the material standard of living for a large proportion of people on Earth (Rosling, 2010). This expansion has been quite pronounced since the 1950s, which predominantly benefitted the industrialised world, has pushed humanity into a new geological era, the *Anthropocene*, and

Fig. 37. Plankton sample from Lake Dojran (August 2010) dominated by *Aulacoseira granulata* and at least three *Microcystis* taxa*;* circular filaments belong to *Lynbya contorta.* 

The final support for the overall conclusion that the Prespa Lake has completed the turnover to a highly eutrophic system came from the analyses of plankton communities during the summer months (Fig. 38). Only two cyanobacteria forms have produced a typical 'water bloom' from May until September, *Anabaena affinis* and *Anabaena contorta*, which have fully replaced the usual plankton dominance of diatoms belonging to genus *Cyclotella.* Consequently, ELISA tests for cyanotoxins (*microcystins*) in the lake's waters have revealed significant presence of these toxins in the summer months (Fig. 39).

Fig. 38. 'Water bloom' caused by *Anabaena affinis* and *Anabaena contorta* in Prespa Lake waters.

Fig. 37. Plankton sample from Lake Dojran (August 2010) dominated by *Aulacoseira granulata* and at least three *Microcystis* taxa*;* circular filaments belong to *Lynbya contorta.* 

Fig. 38. 'Water bloom' caused by *Anabaena affinis* and *Anabaena contorta* in Prespa Lake

waters.

significant presence of these toxins in the summer months (Fig. 39).

The final support for the overall conclusion that the Prespa Lake has completed the turnover to a highly eutrophic system came from the analyses of plankton communities during the summer months (Fig. 38). Only two cyanobacteria forms have produced a typical 'water bloom' from May until September, *Anabaena affinis* and *Anabaena contorta*, which have fully replaced the usual plankton dominance of diatoms belonging to genus *Cyclotella.* Consequently, ELISA tests for cyanotoxins (*microcystins*) in the lake's waters have revealed

Fig. 39. Cyanotoxins-microcystins in Prespa Lake waters during the 12 month investigation period.

After all presented analyses and elaborations, the reference conditions for the Macro Prespa Lake ecosystem as one water body are presented in Table 10. Presented values for the most important parameters are targeted on the boundary between good and moderate water quality status for Prespa Lake which was surpassed at least a century ago. Having in mind the very high pressure of a variety of pollutants and human influences elaborated in this report, the targeted reference conditions may seem out of reach. But, if the ongoing situation continues, a total turnover of Prespa Lake towards highly eutrophic ecosystem should be expected in a very near future. In that case, the overall status of the Prespa-Ohrid-Crni Drim River system will be jeopardised and much more difficult to control, let alone brought to a good water quality status.
