Theses

1. In our studies on environmental parameters, significant differences were identified between values of Secchi transparency and turbulence among the basins of different depths of Lake Balaton. The transparency of the water was found to decrease from East to West in the basins of the lake. The highest values of RMS-turbulence were measured in the Szigliget and the Keszthely basins. RMS-turbulence was around 1.1–

1.5 cm s^-1 in the absence of wind and waves, and 1.5–3 cm s^-1 at a wind velocity of 100–200 cm s^-1 and in the presence of the resulting gentle wave activity.

At a wind velocity of 400–600 cm s^-1 and in the presence of the resulting surface wave activity, an RMS-turbulence of 4–13 cm s^-1 was recorded. Examination of the turbulence distribution of water columns of different depths revealed that the water in front of the reed belt as well as water measuring about 2 m in depth are readily stirred up, and there exists no layer with relatively low turbulence among the layers with different depths that would be favourable for the zooplankton assemblage to inhabit.

The agitating effect of the wind is less pronounced in the case of water columns of 3–

4.4 m depth, and highly different turbulence values are established in the individual water layers, allowing stratification of zooplankton assemblages. Based on the two methods of assessment, values of energy dissipation rate varied in the range of 1.70 x 10^-7 – 3.60 x 10^-1 m² s^-3 and 2.4 x 10^-4 – 2.5 x 10^-3 m² s^-3, respectively, and proved to be significantly higher in shallow water than in the deeper regions of the lake. The outstanding values observed at the Szigliget and Keszthely sampling sites attest that these regions of Lake Balaton are especially exposed to the agitating effect of turbulence. In Lake Balaton, a lake with a mean water depth of 3.25 m and a large surface area, there is little space available for the dissipation of wind-generated energy.

Kolmogorov length scales based on the energy dissipation rates calculated using the two different methods fell into the range of 0.25–0.42 mm and 0.15–0.27 mm, respectively. The diameter of the smallest eddies is much smaller and the value of turbulent shear forces much larger than in deeper lakes or in the sea. Since the average size of zooplankton in Lake Balaton is in the range of 0.25–2.5 mm, the data reveal that the Kolmogorov length scale in Lake Balaton, while constantly varying as a function of water depth and wind velocity, often coincides with the size of plankton organisms.

Based on data in the literature, the size of plankton organisms in a given water body may not exceed the diameter of the smallest turbulent currents spreading downward from the surface, because shear tension would otherwise damage sensitive plankton organisms. According to the Kolmogorov length scales resulting from our calculations, turbulence in Lake Balaton often creates an unfavourable environment for the zooplankton assemblage.

2. Zooplankton density decreased along the lake from the Western towards the Eastern basins. The distribution of planktonic crustaceans as a function of water depth showed individual characteristics specific for each species and developmental stage.

The ratio of nauplius larvae was nearly twofold in the littoral zone as compared to that in open water. The opposite tendency was observed in the case of Cladocera species:

their ratio in open water was about threefold relative to that recorded in the littoral zone. No uniform tendency was observed for copepods – the case of each species was different. The abundance of Eudiaptomus gracilis was lower in the littoral zone than in open water. There was no significant difference between the percentage ratios of cyclopoids recorded at the individual sampling sites.

3. According to our analysis, in addition to the seasonal and spatial effect, secondary factors influencing the structure and density of the zooplankton assemblage were turbulence, water depth, the time of day, Secchi transparency, water temperature and the presence of predatory zooplankton species. The significance of the factors enumerated may be highly different for each of the individual planktonic groups. The seasonal pattern of water temperature data showed a close correlation with changes in zooplankton density. In the case some of Cladocera species (e.g. Alona affinis, Diaphanosoma brachyurum), different developmental stages and egg-laying females, the determining factors were month and time of day. The effect of the presence of the predatory species Cyclops vicinus and Leptodora kindtii was negligible as compared to that of the other environmental parameters. The group affected the most by Secchi transparency was cladocerans. Water temperature affected two crustacean groups, namely Cladocera species and Mesocyclops leukarti.

4. Zooplankton organisms exhibited diverse sensitivities towards the impact of turbulence. Among filter feeders, turbulence had a significant effect on the ontogenesis, mortality and localization of Cladocera species and, to a smaller extent, of the copepod Eudiaptomus gracilis. Based on the data in the pertinent literature, the reasons underlying this phenomenon may be the negative effect of sediment resuspended by increasing turbulence on the efficiency of feeding on the one hand and destruction by shear tension on the other.

5. 24-hour field observations reveal that Daphnia cucullata x galeata and Eudiaptomus gracilis perform daily vertical migration in calm weather, in the course of which they withdraw into the deeper water layers during the day and are distributed evenly along the water layers at night.

According to our results, disintegration of the vertical stratification of zooplankton in Lake Balaton starts in the RMS-turbulence range of 3–3.5 cm s^-1 and, as suggested by the ratio of windy days in the area, is a common phenomenon in the lake. The distribution of zooplankton in the various water layers differed at sampling sites of different depths: in relatively deep water the animals were distributed relatively evenly, whereas in shallow water the bulk of zooplankton was localized in the middle 100–150 cm water layer.

6. Our laboratory experiments attest that turbulence has a strong impact on the species composition and the mortality of zooplankton in Lake Balaton even at a low suspended material content. Comparison of the zooplankton populations of the turbulent and control aquariums revealed that the crustacean group most sensitive to the unfavourable effects of turbulence are cladocerans.

1 7 F ü g g el é k

1 (Siófok) 2 (Tihany közép) (Tih part Bosmina coregoni Intercept 950,7921950,792131,55066<0,00130,218426**** "Var2" 1131,72565,849918,77691<0,00121,11203**** Error 4068,2813530,135416,71041**** Bosmina petés nőstény

Intercept 9,8452819,84527513,93228<0,00130**** "Var2" 9,8550924,9275436,973080,001320,171569**** Error 95,398061350,70665210,646659**** Calanoid copepoditIntercept 535,53841535,5384129,106<0,00131,046713**** "Var2" 55,8391227,91956,73080,001622,327328**** Error 559,98691354,148112,660654**** Calanoida össz + cop,

Intercept 44082,51144082,51134,9603<0,00137,05724**** "Var2" 7561,9223780,9611,5756<0,001221,78454**** Error 44095,49135326,63125,90937**** Cladocera diverzitásIntercept 48,17429148,17429319,0911<0,00130,515869**** "Var2" 1,2890620,644534,26920,01610,558471******** Error 20,381421350,1509720,735614****

105 Függelék 1. táblázat/2: A Siófoki-medence egyes zooplankton csoportjai mennyiségi különbözőségének elemzése egyutas ANOVA-val. SSDedr. of FreedomMSFpVar2zooplankton változó Mean

1 (Siófok) 2 (Tihany közép) (Tih part Cyclopoida copepoditIntercept 2008,49712008,49771,51161<0,00132,016436**** "Var2" 595,9252297,96310,60882<0,00112,9302**** Error 3791,65213528,08626,740165**** Cyclops összIntercept 9690,4619690,45885,38182<0,00133,92719**** "Var2" 1687,422843,717,43386<0,00118,88142******** Error 15321,9135113,496212,86176**** Cyclops össz + cop, Intercept 69026,01169026,01115,6774<0,001312,5457**** "Var2" 14801,6727400,8312,4027<0,001119,0452**** Error 80556,04135596,71236,92106**** Daphnia cucullata x galeata

Intercept 6705,1816705,18247,8532<0,00130,52624**** "Var2" 4524,6222262,3116,14554<0,00125,85799**** Error 18916,18135140,12114,96909**** Daphnia petés nőstényIntercept 9,8452819,84527513,93228<0,00130**** "Var2" 9,8550924,9275436,973080,001320,171569**** Error 95,398061350,70665210,646659**** Diaphanosoma brachyurumIntercept 6753,2416753,24340,70829032,63567**** "Var2" 1871,482935,7425,640620,00426,70107******** Error 22395,63135165,894112,09297****

106 Függelék 1. táblázat/3: A Siófoki-medence egyes zooplankton csoportjai mennyiségi különbözőségének elemzése egyutas ANOVA-val. SSDedr. of FreedomMSFpVar2zooplankton változó Mean

1 (Siófok) 2 (Tihany közép) (Tih part Diaphanosoma petés nőstényIntercept 9,8452819,84527513,93228<0,00130**** "Var2" 9,8550924,9275436,973080,001320,171569**** Error 95,398061350,70665210,646659**** Eudiaptomus gracilisIntercept 19494,99119494,9992,47927033,90787**** "Var2" 4213,4322106,729,99373<0,001214,33872**** Error 28458,53135210,8118,16348**** Eudiaptomus petés nőstényIntercept 9,8452819,84527513,93228<0,00130**** "Var2" 9,8550924,9275436,973080,001320,171569**** Error 95,398061350,70665210,646659**** Mesocyclops leuckarti adIntercept 8624,4918624,4977,84994<0,00133,51773**** "Var2" 1625,462812,7327,33622<0,00118,41023******** Error 14955,77135110,784212,28939**** Mesocyclops petés nőstényIntercept 9,8452819,84527513,93228<0,00130**** "Var2" 9,8550924,9275436,973080,001320,171569**** Error 95,398061350,70665210,646659**** naupliusIntercept 378950,21378950,2276,5705<0,001136,51819**** "Var2" 27897,7213948,810,1803<0,001354,69089******** Error 184973,71351370,2269,31903**** teljes zooplanktonIntercept 227514512275145228,7092<0,001351,4902**** "Var2" 476543223827223,9523<0,0011139,8633**** Error 134294813599482201,9833**

107 Függelék 2. táblázat: Siófoki zooplankton csoportok mennyiségi adatainak folytonos környezeti változókkal való kapcsolatának elemzése többszörös lineáris regresszióval RMSmax RMS %hőmérsékl.vízmélységsecchi Leptodora k. Cyclops v. df R2p Bosmina coregoni - 0,77 (<0,001) 0,33 (0,04) - 0,42 (0,02) - 0,48 (0,009) 0,31 (0,05) 5,430,320,0 calanoida copepodit- 0,39 (0,005) nsnsnsnsnsns1,470,1570,0 Calanoida össz + cop. - 0,44 (<0,001) nsnsnsnsns0,33 (0,0098) 2,460,290<0 Cladocera diverzitásns0,33 (0,02) nsnsnsnsns1,470,1080,0 Cladocera fajszám0,42 (<0,001) nsnsns0,26 (0,045) nsns2,460,244<0 Cladocera össz- 0,34 (<0,001) ns0,25 (0,01) nsnsns0,612 (<0,001) 3,450,683<0 Copepoda diverzitásnsnsnsns0,35 (0,01) nsns1,470,1230,0 cyclopoida copepoditns- 0,30 (0,02) nsnsnsns0,39 (0,004) 2,460,2180,0 Cyclops össz0,30 (0,05) - 0,45 (0,02) nsns0,77 (<0,001) 0,27 (0,036) 4,440,349<0 Cyclops össz + cop. 0,36 (0,015) - 0,74 (<0,001) nsns0,87 (<0,001) 0,26 (0,03) 4,440,410<0 Cyclops vicinus ns0,76 (0,000) ns0,40 (<0,001) nsns2,460,467<0 Daphnia cucullata x galeatansnsnsnsnsns0,57 (<0,001) 1,470,323<0 Diaphanosoma brachyurumnsns0,57 (<0,001) 0,32 (0,011) nsnsns2,460,355<0 Mesocyclops leuckarti 0,40 (0,011) ns0,57 (<0,001) 0,47 (0,003) nsnsns3,450,316<0 teljes zooplankton- 0,23 (0,007) nsnsnsnsns0,81 (<0,001) 2,460,685<0

108 Függelék 3. táblázat: Tihany nyíltvízi mérőpont zooplankton csoportok mennyiségi adatainak folytonos környezeti változókkal való kapcsolatának elemzése többszörös lineáris regresszióval RMSmax RMS %hőmérsékl.vízmélységsecchi Leptodora k. Cyclops v. df R2p Cladocera diverzitás0,45 (<0,001) - 0,49 (<0,001) 0,27 (0,03) nsnsnsns3,510,305<0 Cladocera fajszám0,44 (<0,001) - 0,40 (0,003) nsns0,29 (0,02) nsns3,510,349<0 Cladocera össznsns0,62 (<0,001) 0,32 (0,009) -0,52 (<0,001) nsns3,510,32<0 Copepoda diverzitás0,39 (0,003) - 0,39 (0,005) nsns0,27 (0,04) nsns3,510,306<0 Copepoda fajszámnsnsnsns0,29 (0,03) nsns1,530,085<0 cyclopoida copepoditnsnsnsns0,37 (<0,001) ns0,7 (<0,001) 2,520,625 <0 Cyclops összns0,29 (0,02) 0,33 (0,01) nsnsns2,520,19<0 Cyclops össz + cop. nsns0,47 (<0,001) nsnsns1,530,217<0 Cyclops vicinus nsns- 0,30 (0,01) nsns0,42 (<0,001) 2,520,29<0 Daphnia cucullata x galeatans-0,33 (0,015) 0,70 (<0,001) ns-0,82 (<0,001) nsns3,510,34<0 Mesocyclops leuckarti nsns0,02 (0,02) nsns- 0,03 (<0,001) 0,99 (<0,001) 3,510,996<0 naupliusnsnsns-0,278 (0,007) 0,6 (<0,001) nsns2,520,495<0 teljes zooplanktonnsns0,31 (0,02) ns- 0,36 (0,005) ns0,6 (<0,001) 3,510,583<0

109 Függelék 4. táblázat: Tihany parti mérőpont zooplankton csoportok mennyiségi adatainak folytonos környezeti változókkal való kapcsolatának elemzése többszörös lineáris regresszióval RMSmax RMS %Hőmérsékl.vízmélységsecchi Leptodora k. Cyclops v. df R2p Bosmina coregoni ns0,36 (0,03) nsnsns0,58 (<0,001) ns2,310,3120,0 calanoida copepodit- 0,40 (0,007) nsnsnsnsns0,49 (<0,001) 2,310,390<0 Calanoida össz + cop. - 0,27 (0,02) nsnsnsns0,28 (0,019) 0,71 (<0,001) 3,300,650<0 Cladocera diverzitásnsns0,98 (<0,001) ns0,60 (0,002) - 0,29 (0,035) ns3,300,531<0 Cladocera fajszámnsns0,52 (<0,001) nsnsns0,31 (0,04) 2,310,3310,0 Cladocera össz- 0,33 (0,029) nsnsnsnsns0,52 (<0,001) 2,310,360<0 Copepoda diverzitásnsns0,37 (0,03) nsnsnsns1,320,134<0 Copepoda fajszámnsns0,55 (<0,001) nsnsnsns1,320,304<0 cyclopoida copepodit- 0,47 (0,004) 0,35 (0,04) ns0,47 (<0,001) nsns0,52 (<0,001) 4,290,663<0 Cyclops össz + cop. nsnsns0,46 (0,006) nsns1,320,211<0 Daphnia cucullata x galeatans- 0,39 (0,021) nsnsnsnsns1,320,155<0 Diaphanosoma brachyurum- 0,29 (0,04) nsnsnsnsns0,6 (<0,001) 2,310,426<0 Eudiaptomus gracilisnsnsnsnsns0,35 (0,002) 0,77 (<0,001) 2,310,669<0 nauplius- 0,31 (0,037) ns0,74 (<0,001) ns0,42 (0,03) nsns3,300,494<0