Biology Centre of the Academy of Sciences of the Czech Republic, Institute of Hydrobiology

Important recent research results of the Institute

**Eel attacks-A new tool for assessing European eel (Anguilla anguilla) abundance and distribution patterns with gillnet sampling**

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Because of its complex life cycle, cryptic behavior, body shape, ability to excrete mucus and excellent maneuvering, the European eel (Anguilla anguilla) is difficult to monitor. For example, in gillnets eels represent the most unusual catch, together with water birds, game, frogs and semiaquatic snakes. During many years of gillnetting in Dutch and Czech freshwaters, we registered characteristic involutions of gillnet netting with a partially eaten fish inside. We concluded that these involutions were the result of eels attacking fish caught in gillnets and called the occurrences eel attacks. Eel belong to species that are not able to separate a bite out of bigger prey using teeth, thus they use rotational feeding instead. They grasp prey and spins rapidly around their long axis, thus tearing pieces of the prey that can be swallowed. Rotational feeding has been described in all eel species as well as in other 20 fish species, amphibian with snake-form body and crocodiles.

When we compared the abundance of eel attacks in gillnets with the abundances of eels recorded using active gears (beach seining and trawling), we found a positive and significant correlation (partial Spearman R = 0.640) that can be used for rough eel-abundance assessment. The most frequently attacked fishes were young individuals of abundant species (Perca fluviatilis, Sander lucioperca, Rutilus rutilus and Osmerus eperlanus) and small species (Gymnocephalus cernua) up to 100mm standard length. The eels preferred pikeperch and avoided roach and smelt. The reason for prey species preferences was most likely prey species distribution rather than size selectivity. The eels attacked fishes in gillnets during the night, most likely after midnight. The eel attacks were not distributed homogenously among the studied reservoirs, geographical areas and years. The frequency of eel attacks was higher in benthic than in pelagic habitats. Eel attacks were more frequent in Dutch than in Czech reservoirs, corresponding to a general decline of eel densities with increasing distance from the sea. The number of eel attacks also declined significantly from 1998 to 2008, which agrees with the general decline of eel stocks in Europe. There are likely many reasons for this decline, and the most cited reasons are overexploitation, seaward-migration barriers, changes in oceanographic conditions, habitat loss and contamination, decreased production of the glass eel, infestation with the swim-bladder nematode Anguillicola crassus, predation by cormorants, poor-quality spawners, relocations of glass eel, and reductions of available prey caused by the reoligotrophication of freshwater habitats. Accordingly, eels were classified as a critically endangered species in the IUCN Red List.

The present study showed that eel attacks can be successfully used as a new, simple and nonintrusive tool for monitoring eel abundance using gillnets. Gillnets are widely used in fish monitoring, and hence, gillnet sampling performed throughout Europe has a great potential for eel-abundance assessment.

Research outputs:

Prchalov�, M., Kube�ka, J., ��ha, M., �ech, M., J�za, T., Ketelaars, H.A.M., Kratochv�l, M., Mrkvi�ka, T., Peterka, J., Va�ek, M., Wagenvoort, A.J., 2013. Eel attacks - A new tool for assessing European eel (Anguilla anguilla) abundance and distribution patterns with gillnet sampling. Limnologica 43, 194-202. doi:http://dx.doi.org/10.1016/j.limno.2012.09.003

**Research on the reproduction biology of perch (Perca fluviatilis L.) in reservoirs and newly created lakes**

The distribution of egg strands of perch and factors affecting this distribution were studied in Chaba�ovice Lake (years 2007-2010 and 2012) and in ��mov Reservoir (2007 and 2011) using boat observation and SCUBA divers. In total, 206 dives in duration of c. 60 minutes each were performed during the perch spawning period. In case of Chaba�ovice Lake, factors influencing the depth distribution of egg strands were identified as waves, temperature, duration of the daylight period and light attenuation in the water column (Fig. 1). Factors influencing the selection of spawning sites were identified as wind inducing current, internal seiches and temperature instability of the water column. In case of ��mov Reservoir, factors influencing the depth distribution of egg strands were identified as presence of appropriate spawning substrates and light attenuation in the water column (Fig. 1). Factors influencing the selection of spawning sites were identified as presence of littoral vegetation, especially in sheltered bays.

Figure 1. Comparison of the light (PAR) attenuation in the water column of (a) ��mov Reservoir and of (b) Chaba�ovice Lake measured around noon in mid-May. The original data were recalculated to 500 μmol m^-2 s^-1 of PAR entering the water surface. Grey bars indicate the zone of the water column where PAR reached values of at least 1% of the surface value (i.e. the zone of photosynthetic activity). The vertical bars show the Secchi disk depths. Dashed lines indicate the average depth of egg strands in ��mov Reservoir in 2007 and 2011 (pooled data) and in Chaba�ovice Lake in 2007-2010 and 2012 (pooled data). Dotted lines indicate the depth limit to which 95% of egg strands were spawned.

Research outputs:

�ech, M., Peterka, J., ��ha, M., Vej��k, L., J�za, T., Kratochv�l, M., Dra�t�k, V., Mu�ka, M., Znachor, P., Kube�ka, J. (2012). Extremely shallow spawning of perch (Perca fluviatilis L.): the roles of sheltered bays, dense semi-terrestrial vegetation and low visibility in deeper water. Knowledge and Management of Aquatic Ecosystems 406/9: 1-12.

�ech, M., Vej��k, L., Peterka, J., ��ha, M., Mu�ka, M., J�za, T., Dra�t�k, V., Kratochv�l, M., Kube�ka, J. (2012). The use of artificial spawning substrate in order to understand the factors influencing the spawning site selection, depth of egg strands deposition and hatching time of perch (Perca fluviatilis L.). Journal of Limnology 71/1: 170-179.

�ech, M., Peterka, J., ��ha, M., Mu�ka, M., Hejzlar, J., Kube�ka, J. (2011). Location and timing of the deposition of egg strands by perch (Perca fluviatilis L.): the roles of lake hydrology, spawning substrate and female size. Knowledge and Management of Aquatic Ecosystems 403/8: 1-12.

�ech M., Peterka J., ��ha M., Dra�t�k V., Kratochv�l M., Kube�ka J. (2010). Deep spawning of perch�(Perca fluviatilis, L.) in the newly created Chaba�ovice Lake, Czech Republic. Hydrobiologia 649: 375-378.

�ech, M., Peterka, J., ��ha, M., J�za, T., Kube�ka, J., (2009). Distribution of egg strands of perch (Perca fluviatilis L.) with respect to depth and spawning substrate. Hydrobiologia 630, 105-114.

The utility of predatory fish in biomanipulation of deep reservoirs

Piscivorous fish may drive trophic cascades and maintain good water quality in lakes and reservoirs. Biomanipulation by stocking piscivores has been therefore suggested as a useful tool to regulate phytoplankton biomass in anthropogenically eutrophicated systems. We evaluated the top-down effects of piscivores on lower trophic levels in 13 deep, stratified Czech reservoirs that were built by damming rivers. In addition, the importance of piscivores was assessed in three deep, bank-side Dutch reservoirs that are artificially destratified. All the Czech reservoirs are regularly stocked with piscivores while the Dutch reservoirs have never been stocked. The piscivores had no significant top-down effect on the biomass of planktivorous and benthivorous fish and on phytoplankton biomass (measured as chlorophyll-a concentration) in the Czech reservoirs, although the proportion of piscivores in the fish community was high in half of them. The planktivore and benthivore biomass and the phytoplankton biomass correlated strongly with phosphorus concentration, which is in line with bottom-up control by nutrient supply. On the other hand, piscivores in the Dutch reservoirs dominated the fish community and apparently contributed to good water quality. Nevertheless, the primary cause of relatively low phytoplankton biomass in the Dutch reservoirs was the artificial destratification inducing light limitation on phytoplankton. We conclude that biomanipulation cannot substantially reduce phytoplankton biomass in the eutrophicated systems with high external nutrient input. The input should be reduced to control phytoplankton and if it is not possible, artificial destratification can be applied.

The study was financially supported by the Czech Ministry of Agriculture (QH81046), and the project CEKOPOT (CZ.1.07/2.3.00/20.0204) co-financed by the European Social Fund and the state budget of the Czech Republic.

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Va�ek M., Prchalov� M., Peterka J., Ketelaars H.A.M., Wagenvoort A.J., �ech M., Dra�t�k V., ��ha M., J�za T., Kratochv�l M., Mrkvi�ka T., Blabolil P., Boukal D.S., Duras J., Kube�ka J., 2013: The utility of predatory fish in biomanipulation of deep reservoirs. Ecological Engineering 52: 104-111.

http://dx.doi.org/10.1016/j.ecoleng.2012.12.100

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Pikeperch Sander lucioperca is an important piscivorous fish in many reservoirs.	High nutrient inputs cause nuisance algal blooms.

Freshwater sport fish management and enhancement - Threadfin shad prey production in tropical reservoirs

Results description: Our team FishEcU (Institute of Hydrobiology, BC AS CR, v.v.i.) had a three-year subcontract (2009-2011) within a five-year project of the Mississippi State University (applicants: J.W. Neal, R. Kroger, C.G. Lilyestrom, financial support: Puerto Rico department of Natural and Environmental Resource). The aim of the subcontract was to describe populations of prey species threadfin shad Dorosoma petenense in reservoirs of Puerto Rico. Threadfin shad is small and short-lived species from family of clupeids, which feed on plankton. In Puerto Rico, threadfin shad is principally the only prey species for large-mouth bass Micropterus salmoides, which is the most desired species of sport fishing in USA and Puerto Rico as well. It has been showed that, contrary to predictions, bass do not grow to trophy sizes in Puerto Rican reservoirs. Possible explanation is lack of prey, i.e. low abundance of threadfin shad. Within the subcontract, we selected the best gear (gillnets or trawl) for sampling shad firstly. We found out that the assessment of shad abundance in conditions of tropical reservoirs of Puerto Rico can be best derived from trawl catches. Threadfin shad abundance varied through the year due to high densities of freshly hatched shad that were most numerous in winter and spring catches. Further, spatial pattern of shad distribution was studied, which was important for description of ecology of this species as well as for proper sampling gear. Threadfin shad preferred upper parts of the water column down to 3 m of depth and their catches were higher in the open water in comparison to inshore habitats. Abundance of threadfin shad was related to trophy of a reservoir being higher with higher concentration of nutrients.

Research outputs:

Prchalov�, M., Neal, J. W., Munoz-Hincapi� M., J�za, T., ��ha, M., Kube�ka, J., 2012. Comparison of gillnets versus fixed-frame trawl for sampling threadfin shad in tropical reservoirs. Transactions of the American Fisheries Society 141, 1151-1160. doi: 10.1080/00028487.2012.675922.

Neal, J.W.,� Prchalov�, M., 2012. Spatiotemporal distributions of threadfin shad in tropical reservoirs. North American Journal of Fisheries Management 32, 929-940. doi:10.1080/02755947.2012.700903.

The influence of the trawl mouth opening size and net colour on catch efficiency during sampling of early stages of perch (Perca fluviatilis) and pikeperch (Sander lucioperca) in the bathypelagic layer of a canyon-shaped reservoir

The long-term effort of the Fish Ecology Unit is obtaining of the "true picture" of the fish stock in lakes and reservoirs. Absolutely essential for its finding is the knowledge of the efficiency of our sampling techniques. In continuity on the former attempts to observe the efficiency of fish sampling tools we focused on the avoidance behavior of the earlier stages of percid fry, which often utilize deep (bathypelagic) layers of the water column during the day. The efficiency of different sized (0,5m x 2m, 1m x 2 m, 2m x 2m, 4m x 2m) and coloured (black, white) ichthyoplankton trawl nets was investigated during daytime in the bathypelagic layer of the ��mov Reservoir (Czech Republic) in late May 2008. Eight to 16 mm long perch (Perca fluviatilis) and pikeperch (Sander lucioperca) fry were the dominant species in fry catches in this layer. We did not find any significant differences between the densities of either species when black and white nets were compared or when different sized nets were compared. Neither were there any significant differences in body lengths of the fish caught in the different trawls. Our results indicate that both perch and pikeperch fry in the bathypelagic layer are quite passive, without observable avoidance behaviour to the ichthyoplankton trawl nets. This is in contrast to active avoidance of the same species and sizes previously observed in the epipelagic layer.

J�za, T., �ech, M., Kube�ka, J., Va�ek, M., Peterka, J., Mat�na, J. 2010. The influence of the trawl mouth opening size and net colour on catch efficiency during sampling of early fish stages. Fisheries Research 105: 125-133.

J�za, T., �ech, M., Kube�ka, J., Va�ek, M., Peterka, J., Kratochv�l, M., Frouzov�, J., Mat�na, J. 2012. The influence of the trawl mouth opening size and net colour on catch efficiency during sampling of early stages of perch (Perca fluviatilis) and pikeperch (Sander lucioperca) in the bathypelagic layer of a canyon-shaped reservoir. Fisheries Research 123-124: 21-25.

Reproduction biology of perch (Perca fluviatilis L.) in model Chaba�ovice Lake (North Bohemia, Czech Republic)

Using SCUBA divers we finished the most complex and most detail research on perch reproduction biology ever. We found that in Chaba�ovice Lake perch used at least seven different spawning substrates, particularly curly pondweed Potamogeton crispus, common reed Phragmites communis and worm weed Artemisia sp. However, while living submerged vegetation, although more abundant, was generally avoided, dead submerged vegetation was strongly preferred. It appears that dead vegetation is an ideal spawning substrate for perch since placement of the egg strands over those hard, complex three-dimensional structures ensures that the eggs remain well oxygenated 24 hours a day. The depths at which perch egg strands were deposited increased significantly during the spawning period (late April - early June) and basically corresponded to the position of 10-12�C water layers. Surprisingly, the depth of perch spawning was controlled also by the duration of the daylight period and finally by fish inner clock. At the beginning of perch spawning period also wind and waves influenced the depth distribution of egg strands. Factors influencing the selection of spawning sites were identified as wind inducing current, internal seiches, temperature instability of the water column and extreme displacement of water mass. Ignoring deeper strata while searching for perch egg strands (>3 m; as it was very common in previous works of both European and North American authors) may cause drastic underestimation of perch spawning success in lakes, since this fish is able to deposit its eggs to a depth of over 20 m.

Examples of typical spawning substrates of perch in Chaba�ovice Lake. (a) Common reed (Phragmites communis). (b) Black elder (Sambucus nigra). (c) Worm weed (Artemisia sp.) (d) Curly pondweed (Potamogeton crispus). (e) Eurasian water milfoil (Myriophyllum spicatum). (f) Common rush (Juncus effusus). (g) Dead trees and branches. (a), (b), (c), (f), (g) are dead plants, bushes and trees drowned during the filling of the lake in 2001. Actual depth of each finding is stated in the photograph.

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The relationship between the week of sampling and depth (mean � S.D.) of newly spawned egg strands of perch in Chaba�ovice Lake in the areas with artificial spawning substrate (A.S.S.) and areas outside the A.S.S. (3 May - 3 June 2010). The mean temperature of the egg strands incubation is given for the A.S.S. areas and individual weeks.

�ech, M. - Peterka, J. - ��ha, M. - J�za, T. - Kube�ka, J.: Distribution of egg strands of perch (Perca fluviatilis L.) with respect to depth and spawning substrate. Hydrobiologia. �. 630 (2009), s. 105-114.
�ech, M. - Peterka, J. - ��ha, M. - Dra�t�k, V. - Kratochv�l, M. - Kube�ka, J.: Deep spawning of perch (Perca fluviatilis L.) in the newly created Chaba�ovice Lake, Czech Republic. Hydrobiologia. �. 649 (2010), s. 375-378.
�ech, M. - Peterka, J. - ��ha, M. - Mu�ka, M. - Hejzlar, J. - Kube�ka, J.: Location and timing of the deposition of egg strands by perch (Perca fluviatilis L.): the roles of lake hydrology, spawning substrate and female size. Knowledge and Management of Aquatic Ecosystems. �. 403/08, s. 1-12.
�ech, M. - Vej��k, L. - Peterka, J. - ��ha, M. - Mu�ka, M. - J�za, T. - Dra�t�k, V. - Kratochv�l, M. - Kube�ka, J.: The use of artificial spawning substrates in order to understand the factors influencing the spawning site selection, depth of egg strands deposition and hatching time of perch (Perca fluviatilis L.). Journal of Limnology. �. 71(1) (2012), v tisku.

Factors responsible for distribution and diversity of two major groups of Betaproteobacteria

Two key groups of microbes affiliated with the Betaproteobacteria received our immense attention during the last year. Those globally distributed groups, represented here by the genus Limnohabitans and species-like cluster of Polynucleobacter, differ fundamentally in their habitat preferences, distribution, metabolical activities and abundance. Just recently, a successful isolation and description of the first strains of the genus Limnohabitans has made it possible to conduct a comprehensive study focused on niche separation between their coexisting, closely related members. Interactions between these strains, predators and viruses have been recorded, suggesting contrasting ecophysiological capabilities between the closely related ecotypes. Moreover, ecological differentiation within the most important subspecies of Polynucleobacter was identified and described. Different ecotypes of freshwater Polynucleobacter were found to occupy contrasting habitats.

�imek, K. - Kasalick�, V - Jezbera J. - Jezberov� J. - Hejzlar, J. - Hahn, M. W.: Broad habitat range of the phylogenetically narrow R-BT065 cluster representing a core group of the betaproteobacterial genus Limnohabitans. Applied Environmental Microbiology (2010): Ro�. 76, �. 3 (2010) s. 631- 639.
Hahn, M. W. - Kasalick�, V. - Jezbera J. - Brandt U. - �imek, K. : Limnohabitans australis sp. nov., isolated from a freshwater pond. International Journal of Systematic and Evolutionary Microbiology. Ro�. 60, (2010) s. 2946 - 2950.
Kasalick� V. - Jezbera, J. - �imek, K. - Hahn M. W.: Limnohabitans planktonicus sp. nov., and Limnohabitans parvus sp. nov., two novel planktonic Betaproteobacteria isolated from a freshwater reservoir. International Journal of Systematic and Evolutionary Microbiology. Ro�. 60, (2010) s. 2710 - 2714.
Hahn, M.W. - Kasalick�, V. - Jezbera J. - Brandt U. - Jezberov� J. - �imek, K.: Limnohabitans curvus gen. nov., sp. nov., a planktonic bacterium isolated from a freshwater lake. International Journal of Systematic and Evolutionary Microbiology. Ro�. 60, (2010) s 1358-1365.
�imek, K. - Kasalick�, V. - Hor��k, K. - Hahn, M. W. - Weinbauer M. G.: Assessing niche separation in coexisting Limnohabitans strains through interactions with a competitor, viruses, and a bacterivore. Applied Environmental Microbiology Ro�. 76, �. 5 (2010) s. 1406-1416
Jezberov�, J. - Jezbera, J. - Brandt, U. - Lindstroem, E.S. - Langenheder, S. - Hahn, M.W.: Ubiquity of Polynucleobacter necessarius ssp asymbioticus in lentic freshwater habitats of a heterogenous 2000 km2 area. Environental Microbiology Ro�. 12, �. 3 (2010) s. 658-669.
Jezbera, J. - Jezberov�, J. - Brandt, U. - Hahn, M.W.: Ubiquity of Polynucleobacter necessarius subspecies asymbioticus results from ecological diversification. Environmental Microbiology (2010) .

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Polyphasic Characterisation and Taxonomic Revision of Planktonic Cyanobacteria Anabaena spp. (Nostocaceae)

Morphological diversity of planctonic Anabaena populations (Nostocaceae, cyanobacteria) was studied in fishponds and reservoirs of the Czech Republic in 2004-2009. Clonal strains of various morphospecies were isolated and maintained under constant culture conditions. The plasticity of morphological features used for single morphospecies identification was studied in eight selected strains under varied experimental conditions (temperature, light intensity, nitrogen, phosphorus). Phosphorus concentration was the main driving factor of morphological variability. Morphologies of the Anabaena strains studied were compared with the field morphologies of 61 Anabaena populations observed in the Czech Republic. The range of morphological variability of single strains in laboratory spanned the total variability of the populations of relevant morphospecies or morphospecies complex observed in the field. Delimitations and proper descriptions of single Anabaena morphospecies were discussed in the light of partial 16S rRNA gene sequences of the studied strains. A new generic entity Sphaerospermum was defined for morphospecies with spherical akinetes adjacent to heterocytes that also displayed 16S rRNA gene sequence markedly different from other planktonic Anabaena morphospecies (approx. 92% sequence similarity). The type species of this genus is S. reniforme and the genus also comprises species S. aphanizomenoides and S. kisselevianum.

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Zapom�lov�, E., �eh�kov�, K., Znachor, P., Kom�rkov�, J. (2007): Morphological diversity of coiled planktonic types of the genus Anabaena (cyanobacteria) in natural populations � taxonomic consequences. Cryptogamie/Algologie 28: 353-371.

Zapom�lov�, E., Hisem, D., �eh�kov�, K., Hrouzek, P., Jezberov�, J., Kom�rkov�, J., Korelusov�, J., Znachor, P. (2008): Experimental comparison of phenotypical plasticity and growth demands of two strains from the Anabaena circinalis/A. crassa complex (cyanobacteria). Journal of Plankton Research 30: 1257-1269.

Zapom�lov�, E., Hrouzek, P., �eh�kov�, K., �aback�, M., Stibal, M., Caisov�, L., Kom�rkov�, J., Luke�ov�, A. (2008): Morphological variability in selected heterocytous cyanobacterial strains as a response to varied temperature, light intensity and medium composition. Folia Microbiologica 53: 333-341.

Zapom�lov�, E., Jezberov�, J., Hrouzek, P., Hisem, D., �eh�kov�, K., Kom�rkov�, J. (2009): Polyphasic characterization of three strains of Anabaena reniformis and Aphanizomenon aphanizomenoides (cyanobacteria) and their re-classification to Sphaerospermum gen. nov. Journal of Phycology, 45 (6): 1363-1373.

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Sphaerospermum reniforme from the P��k fishpond (South Bohemia, Czech Republic) in the original phytoplankton sample (July 2006). White arrow indicates spherical akinetes adjacent to a heterocyte, which is a morphological feature characteristic for the genus Sphaerospermum (i.e. autapomorphic feature).

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History of Aluminium Runoff from Catchment into Ple�n� Lake

HBI ASCR in co-operation with University of Maine (US) reconstructed the history of terrestrial export of aluminium (Al) to Ple�n� Lake (Czech Republic) since the lake origin 12,600 BC, and predicted Al export for 2010�2050 on the basis of mass budget studies, palaeolimnological data, and MAGIC modelling. We focused on three major Al forms; ionic Al (Ali), organically-bound Al (Alo), and particulate Al hydroxide (Alp) and their effect on P cycle in the lake. In early post-glacial time, Ple�n� Lake received high terrestrial export of Al, but with a minor proportion of Alp (4�25 �M), and concentrations of Ali and Alo were negligible, as well as Al effect on P release from sediments. Since the forest and soil development (9900�9000 BC), erosion has declined and soil organic acids increased export of Alo from soils. Photochemical liberation of Ali from Alo and resulted in P immobilization in lake sediments and prevented P release to the water column. The terrestrial Alo leaching (7.5 �M) persisted throughout the Holocene until the industrial period. Then, Ali concentrations continuously increased (up to 28 �M in the mid-1980s) due to atmospheric acidification, and the Al effect on the in-lake P cycling further progressed. Ple�n� Lake rapidly recoveries from acidification due to a decline in acidic deposition and Alo (5.5 �M) is predicted to dominate Al concentrations around 2050.

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Kop��ek, J. � Mare�ov�, M. - Hejzlar, J. � Norton, S. A.: Natural inactivation of phosphorus by aluminum in pre-industrial lake sediments. Limnol. Oceanogr. 52 (2007), s. 1147�1155.

Kop��ek, J. - Hejzlar, J. - Norton, S. A.: Proton production by transformations of aluminium and iron in lakes. Water Research 42 (2008), s. 1220�1228.

Kop��ek, J. - Hejzlar, J. - Ka�a, J. - Norton, S.A. - Porcal, P. � Turek, J.: Trends in aluminium export from a mountainous area to surface waters, from deglaciation to the recent: Effects of vegetation and soil development, atmospheric acidification, and nitrogen-saturation. Journal of Inorganic Biochemistry, 103 (2009), s. 1439�1448.

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Diel vertical migrations, distribution and ontogeny of bathypelagic layer of European perch (Perca fluviatilis L.) fry in reservoirs

We have found that in the pelagic zone of reservoirs, European perch (Perca flu viatilis L.) fry create simultaneously two different communities � epipelagic and bathypelagic. The epipelagic perch fry spent the whole 24 hours in the upper 4 m of the water column (i.e. above the thermocline) whereas the bathypelagic perch fry performed diel vertical migrations with an amplitude exceeding 10 m, being in relatively warm epilimnion during the night and migrating into the cold hypolimnetic layers during the day. At this time the bathypelagic perch fry community was present as a thick scattering layer, which could be followed by the echosounder. The epipelagic perch fry remain pelagic until June or July, and then move inshore to the littoral area. On the other hand, the bathypelagic perch fry remain in the open water up to autumn. Moreover, unlike bathypelagic perch fry no epipelagic perch fry could be found in the reservoir where thermal stratification had been destroyed by flooding and windy weather. The phenomenon of the bathypelagic perch fry was overlooked for decades and was restricted to the role of an interfering noise on the screen of commercial fishermans echosounders. Using scientific echosounders and complementary net catches the bathypelagic perch fry layer was recently recorded along most of the longitudinal profile of several canyon-shaped reservoirs (up to 55 km long). The size, abundance and shoaling activity of bathypelagic perch fry increased significantly in direction from the dam towards the tributary following the reservoirs trophic gradient. The presence of the bathypelagic perch fry layer in the riverine part of those reservoirs is restricted by local floods or cold hypolimnetic water released from the reservoir situated upstream in the cascade.

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�ech, M., Kratochv�l, M., Kube�ka, J., Dra�t�k, V., Mat�na, J.: Diel vertical migrations of bathypelagic perch fry. - Journal of Fish Biology 66: 685-702 (2005).
�ech, M., Kube�ka, J.: Ontogenetic changes in the bathypelagic distribution of European perch fry Perca fluviatilis monitored by hydroacoustic methods. - Biologia, Bratislava 61, 2: 211-219 (2006).
�ech, M., Kube�ka, J., Frouzov�, J., Dra�t�k, V., Kratochv�l, M., Mat�na, J., Hejzlar, J.: Distribution of the bathypelagic perch fry layer along the longitudinal profile of two large canyon-shaped reservoirs. - Journal of Fish Biology 70: 141-154 (2007).
�ech, M., Kube�ka, J., Frouzov�, J., Dra�t�k, V., Kratochv�l, M., Jaro��k, J.: Impact of flood on the distribution of bathypelagic perch fry layer along the longitudinal profile of large canyon-shaped reservoir. - Journal of Fish Biology 70: 1109-1119 (2007).

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Genetic and ecological differentiation of Daphnia longispina species complex in deep canyon-shaped reservoirs

Species complex of Daphnia longispina comprise the most common taxons of Daphnia living in the vast majority of standing waters in the whole Europe. It consists of species D. galeata, D. longispina and D. cucullata which do hybridize and their microscopic identification is often obscure. We routinely use the allozyme markers or other sophisticated DNA analyses for reliable identification of parental taxons or their potential hybrids. The analysis of spatial heterogeneity and genetic differentiation of Daphnia genus in 11 deep canyon-shaped reservoirs has shown that the most common species is Daphnia galeata. If is D. cucullata also occurring, eventually the hybrids D. galeata x cucullata, they are most often in in upstream end of the reservoir. The opposite pattern was found for D. longispina and its hybrids, which seemed to prefer me talimnetic or hypolimnetic strata in the downstream locations during the day. This spatial differentiation of Daphnia taxa might be related with species �specific adaptations against fist predation.

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Se�a, J. � Petrusek, A. � Mach��ek, J. � �milauer, P. (2007): Spatial distribution of the Daphnia longispina specie complex and other planktonic crustaceans in the heterogenous environment of canyon-shaped reservoirs. J. Plankton Res. 29: 619-628.

Petrusek, A. � Se�a, J. � Mach��ek, J. � Ruthova, S. � �milauer, P. (2008): Daphnia hybridization along ecological gradients in pelagic environments: the potential for the presence of hybrid zones in plankton. Phil. Trans. R. Soc. B 363: 2931-2941.

Dlouh�, S. � Thielsch, A. � Kraus, R.H.S. � Se�a, J. � Schwenk, K. � Petrusek, A. (2010): Identifying hybridizing taxa within the Daphnia longispina species complex: a comparison of genetic methods and phenotypic approaches. Hydrobiologia, in press.

Van��kov�, I. � Se�a, J. � Petrusek, A. (2010): The stabilizing effect of resting eggs banks of the Daphnia longispina species complex for longitudinal taxon heterogeneity in long and narrow reservoirs. Hydrobiologia, in press.

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Application of the PDMPO technique in studying silica deposition in natural populations of Fragilaria crotonensis (Bacillariophyceae) at different depths in a eutrophic reservoir

In July � October 2006 at weekly intervals, we measured silica deposition in the summer diatom assemblage at various depths in a eutrophic ��mov Reservoir using PDMPO labelling technique. Diatom assemblage was exclusively dominated by Fragilaria crotonensis, which formed pronounced subsurface maxima (2 � 6.5 m). Concentrations of the main nutrients (Si and P) were low over the whole season, however, at depth the nutrient availability was higher than at the surface. Fragilaria silica deposition rates were eight times higher at the surface than at depth. A half of the population was involved in silica deposition at the surface while only 20 % active cells were found at depth. At the surface, silica deposition was limited by phosphorus deficiency; the effect of dissolved Si was not statistically significant. Silica deposition at depth was significantly constrained by low light availability despite the 1 % average light attenuation at depth, which is supposed sufficient for photosynthesis. This study represents the first attempt to employ the PDMPO technique coupled with quantitative image analysis of PDMPO fluorescence in freshwater limnology. Based on our results, PDMPO probe appears to be an appropriate proxy for resource limitation studies of natural diatom populations.

Fragillaria colony� in fluorescence microscope.

Green = PDMPO labelled sites of silica deposition,

Red = chlorophyll autosluorescence

Znachor, P. - Nedoma J. (2008): Application of the PDMPO technique in studying silica deposition in natural populations of Fragilaria crotonensis (Bacillariophyceae) at different depths in a eutrophic reservoir. J. Phycol. 44: 518�525.

Znachor, P. - Zapom�lov�, E. - �eh�kov�, K. - Nedoma, J. - �imek, K. (2008): The effect of extreme rainfall on summer succession and vertical distribution of phytoplankton in a lacustrine part of a eutrophic reservoir. Aquatic Sci. 70: 77�86

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Antagonistic and synergistic effects of protistan grazing and viral lysis on production and community composition bacterioplankton

Protistan bacterivory and lysis of bacterial cells by specific viruses, bacteriophages, are two most important factors responsible for shaping the dynamics and community composition of bacterioplankton in aquatic ecosystems. Because phages and flagellates consume or attack the same prey - bacterial cells, antagonistic interactions between these two mortality sources may be expected. However, our current research activities have brought surprising new findings. Selective grazing of protists on particular groups of bacteria induced significant shifts in the taxonomic composition of bacterioplankton towards the dominance of only a few phylotypes, either capable of very fast growth or of filament formation. Since phages do not typically trespass genus boundaries and since phage infection is density dependant, the grazing�induced bacterioplankton community shift could stimulate activity of the host-specific bacteriophages whose proper hosts overgrow in the system. Thus, instead of the assumed antagonism between these two mortality sources, their partial synergy through the stimulation of certain groups of bacteriophages has been found. Analysis of a series of in situ experiments conducted in the ��mov reservoir revealed evidence that under certain circumstances protistan bacterivory and virus lysis indeed can act synergistically. The interplay of both mortality factors caused a rapid increase in the abundance of filament forming bacteria from the genus Flectobacillus, which were apparently both grazing- an d virus-resistant. In contrast, some bacterial groups vulnerable to virus infection were significantly suppressed that led to decreased competition for limiting substrates, which accelerating growth of the Flectobacillus filaments.

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(A) Bacterioplankton and heterotrophic flagelates in epifluorescence microscope: green fluorescnce = RBT bacteria specifically labelled with fluorescent genetic probe (both in plankton and in food vacuoles of flagelates).

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(B + C) Microautoradiography of filamentous bacterium Flectobacillus. Combination of brightfield microscopy and fluorescence microscopy. Density of black silver grains is proporcional to metabolic activity of filaments (uptake of radioactively labelled substrate).

Presence of viruses (C) increases Flectobacillus activity.

�imek, K., Hor��k, K., Jezbera, J., Ma��n, M., Nedoma, J., Gasol, J. M. Schauer. M.: Influence of top-down and bottom-up manipulations on the R-BT065 subcluster of b-proteobacteria, an abundant group in bacterioplankton of a freshwater reservoir. Appl. Environ. Microbiol. 71: 2381-2390 (2005)

Jezbera, J., Hor��k, K., �imek, K.: Food selection by bacterivorous protists: insight from the analysis of the food vacuole content by means of Fluorescence In Situ Hybridization. FEMS Microbiol. Ecol. 52: 351-363 (2005)

Hor��k, K., Ma��n, M., Jezbera, J., Bettarel, Y., Nedoma, J., Sime-Ngando, T., �imek, K.: Effects of decreased resource availability, protozoan grazing and viral impact on a structure of bacterioplankton assemblage in a canyon-shaped reservoir. FEMS Microb. Ecol. 52: 315-327. (2005)

Jezbera, J., Hor��k, K., �imek K.: Prey selectivity of bacterivorous protists in different size fractions of reservoir water amended with nutrients. Environ. Microbiol. 8: 1330-1339 (2006)

�imek K., Horn�k K., Jezbera J., Nedoma,J., Vrba, J., Stra�kr�bov�, V., Macek, M., Dolan, J. R., Hahn M.W.: Maximum growth rates and possible life strategies of different bacterioplankton groups in relation to phosphorus availability in a freshwater reservoir. Environ. Microbiol. 8: 1613�1624 (2006)

Hor��k, K., Jezbera, J., Nedoma,J., Gasol, J.M., �imek,K.: Bacterial leucine incorporation under different levels of resource availability and bacterivory in a freshwater reservoir. Aquat. Microb. Ecol. 45: 277-289 (2006)

Weinbauer, M.G., Hor��k K., Jezbera J., Nedoma,J., Dolan, J.R., �imek K.: Synergistic and antagonistic effects of viral lysis and protistan grazing on bacterial biomass, production and diversity. Environmental Microbiology, 9 (3): 777-788. (2007)

�imek K., Weinbauer, M.G., Hor��k, K., Jezbera J., Nedoma,J., Dolan J.R. Grazer and virus-induced mortality of bacterioplankton accelerates development of Flectobacillus populations in a freshwater community. Environmental Microbiology, 9 (3): 789-800. (2007)

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