Acta Botanica 2-2015.indd ACTA BOT. CROAT. 74 (2), 2015 287 Acta Bot. Croat. 74 (2), 287–302, 2015 CODEN: ABCRA 25 ISSN 0365-0588 eISSN 1847-8476 DOI: 10.1515/botcro-2015-0025 Halophilic diatom taxa are sensitive indicators of even short term changes in lowland lotic systems ZSUZSANNA KÓKAI1, ISTVÁN BÁCSI2, PÉTER TÖRÖK3, KRISZTINA BUCZKÓ4, ENIKŐ T-KRASZNAI1, CSABA BALOGH1, BÉLA TÓTHMÉRÉSZ3, VIKTÓRIA B-BÉRES1* 1 Environmental Protection and Nature Conservation Authority, Trans-Tisza Region, Debrecen, Hatvan u. 16, H-4025 Hungary 2 University of Debrecen, Department of Hydrobiology, Debrecen, Egyetem tér 1, H-4032 Hungary 3 MTA-DE Biodiversity and Ecosystem Services Research Group, Debrecen, Egyetem tér 1, H-4032 Hungary 4 Hungarian Natural History Museum, Budapest, Könyves Kálmán krt. 40. H-1476 Hungary Abstract – The occurrence and spread of halophilic diatom taxa in freshwater lotic eco- systems are infl uenced both by natural processes and anthropogenic pollution. Diatom as- semblages were regularly monitored in lowland lotic systems in Hungary (Central Eu- rope) during the unusually dry year of 2012. Highly pronounced changes in diatom composition were observed from spring to autumn. Halophilic taxa (especially Nitzschia sensu lato species) appeared in the dry autumn. In addition, the total relative abundances of halophilic species also increased up to autumn. Abundance of Nitzschia cf. lorenziana and Nitzschia tryblionella showed a positive correlation with chloride and phosphate con- centration, while that of other taxa like Tryblionella apiculata or Tryblionella calida showed a positive correlation with the concentration of nitrate. Our fi ndings clearly dem- onstrated that these halophilic and mesohalophilic diatom taxa were sensitive indicators of even short-term changes in lowland lotic ecosystems, such as the increasing salt concen- tration from spring to autumn caused by the lack of rainfall and/or environmental loads. Keywords: cell size, dry year, freshwater river, nutrient load, salt concentration, taxa number, temporal dynamics Introduction Diversity, expressed often as the number of taxa, has an essential role in ecosystem processes and functioning, food chains, and ecosystem integrity (COTTINGHAM et al. 2001, * Corresponding author, e-mail: beres.viktoria@gmail.com KÓKAI Z., BÁCSI I., TÖRÖK P., BUCZKÓ K., T-KRASZNAI E., et al. 288 ACTA BOT. CROAT. 74 (2), 2015 FRANCE and DUFFY 2006, SMUCKER and VIS 2011, BORICS et al. 2014). The structure and functioning of ecosystems can be altered by changes in species diversity (PORTER et al. 2013). Biodiversity is infl uenced by biotic- (VASAS et al. 2013), or abiotic- (Ács et al. 2006), natural- (LOTTER et al. 1997, BUCZKÓ et al. 2013, CATALAN et al. 2013) or human induced processes (BÁCSI et al. 2013). Numerous anthropogenic factors cause short- or long-term changes in biodiversity (e.g. eutrophication, changes in some land-use processes, mining, and industry – TILMAN 1999, HOOPER et al. 2005, SMUCKER and VIS 2011). It is important to stress that aquatic assemblages respond much more sensitively to natural or anthropogenic disturbances than terrestrial communities (to changes in water fl ow conditions, precipita- tion, to the increase of nutrient concentrations or salinity – SMUCKER and VIS 2011, PORTER et al. 2013). Benthic diatoms have a decisive structural and functional role in freshwater ecosystems (BOLLA et al. 2010, SMUCKER and VIS 2011). They are often the dominant photosynthetic al- gal group in the biofi lm (PASSY 2002, ANTONIADES et al. 2004, KIRETA et al. 2012) and they have a crucial role in primary production (e.g. KIRETA et al. 2012). They are routinely used in biomonitoring assessments (e.g. BERTHON et al. 2011, VÁRBÍRÓ et al. 2012), because they respond sensitively to the changes in the physical and chemical parameters of freshwater habitats (fl ow conditions, conductivity, ion composition, and nutrient conditions – ÁCS et al. 2003, BERTHON et al. 2011, SMUCKER and VIS 2011, RUSANOV et al. 2012, STENGER-KO- VÁCS et al. 2013, B-BÉRES et al. 2014). Increased nutrient load or conductivity induce changes of species composition in diatom assemblages (e.g. ZIEMANN et al. 2001, STENGER- KOVÁCS et al. 2013, B-BÉRES et al. 2014) via the frequent appearance of new taxa (e.g. some halophilic, mesohalophilic eutraphenic taxa) while the abund ance of some others decreases (e.g. ZIEMANN et al. 2001). Structural parameters of diatom assemblages (i.e. the composi- tion of size classes – BERTHON et al. 2011) can also change considerably, because of the in- creased interspecifi c competition caused by the immigrants. The occurrence and spread of halophilic and mesohalophilic diatom taxa in freshwater lotic ecosystems are caused both by natural processes (e.g. by the increase of nutrient con- centrations caused by a decreased water discharge in lack of rainfall) and/or by anthropo- genic pollution (e.g. fertiliser run off, atmospheric sedimentation, mining). In the spring of 2012 there was high precipitation, while the summer and autumn period was unusually dry in Hungary, as revealed by the data of the Hungarian Meteorological Service and the General Directorate of Water Management. Decreased water fl ow condi- tions occurred mainly because of the low precipitation input in the dry months in lowland rivers and channels (B-BÉRES et al. 2014, KÓKAI et al. 2014). Diatom assemblages of 15 Hungarian lowland small to large rivers and channels were studied during the humid spring and dry autumn period of 2012. In the present study we have focused on the changes in taxa composition, taxa number, cell size classes and the evenness of diatom assemblages with a special emphasis on halophilic/mesohalophilic taxa. We hypothesised the following: (i) taxa number and abundance of halophilic and mes- ohalophilic taxa increase from spring to autumn with the increase of conductivity and nutri- ent content; (ii) the ratio of small disturbance-tolerant taxa and large halophilic/mesohalo- philic taxa increases from spring to autumn according to augmented conductivity and nutrient content and (iii) due to the increasing conductivity, the evenness of species diver- sity decreases from spring to autumn. HALOPHILIC DIATOM TAXA IN LOWLAND LOTIC SYSTEM ACTA BOT. CROAT. 74 (2), 2015 289 Materials and methods Sampling setup Samples were collected from 15 channels and small to large rivers of the Trans-Tisza Re- gion of Hungary in 2012 (Fig.1). Altogether, 10 environmental factors were measured. Con- ductivity (COND), concentration of dissolved oxygen (DO), pH and water temperature (T) Fig. 1. The study area on Trans-Tisza region of Hungary: (a) localization of the study area (marked with grey borders); (b) sampling sites on the rivers and channels with EOV coordinates, grey lines: borders of the study area, black lines: the rivers and channels. The abbreviations of the watercourses are the following: Dióéri-főcsatorna (DIO), Dögös-Kákafoki-főcsatorna (DKA), Hamvas-csatorna (HAM), Holt-Sebes-Körös (HSK), Hortobágy (HOR), Hortobágy- Berettyó (HBE), Keleti-főcsatorna (KFC), Kígyósi-főcsatorna (KIG), Kis-Körös-főcsatorna (KKO), Kösely (KOS), Kutas-főcsatorna (KUT), Nagy-ér (NAG), Nagytóti-Toprongyos- csatorna (NTO), Sárréti-csatorna (SAR), Sebes-Körös (SKL). Arrows indicate three repre- sentative sampling sites shown on Fig. 2. a b KÓKAI Z., BÁCSI I., TÖRÖK P., BUCZKÓ K., T-KRASZNAI E., et al. 290 ACTA BOT. CROAT. 74 (2), 2015 were measured in the fi eld with a portable multiparameter digital meter (Multi 350i-WTW, Germany). Water samples were kept at 4 °C in cooler bags during transportation to the labo- ratory for the measurement of the concentrations of hydrogen carbonate (HCO3– – titrimetric method; MSZ 1987), chloride (Cl– – argentometric method; MSZ 2009a), ammonium (NH4+ – spectrophotometric method; MSZ ISO 1992), nitrite (NO2–), nitrate (NO3– – spectrophoto- metric method; MSZ 2009b), and phosphate (PO43– – spectrophotometric method; MSZ EN ISO 2004). The year of 2012, especially the autumn period, was very dry, according to the data of the Hungarian Meteorological Service. The amount of precipitation was only 427 mm in this year, which was 25% less than the hundred-year average (569 mm). Sample collection and preparation Benthic diatom samples were collected from macrophytes, samples were fi xed in Lu- gol’s solution in the fi eld. Diatom valves were prepared using the hot hydrogen-peroxide method (MSZ EN 2003). Naphrax synthetic resin was used for embedding (MSZ EN 2003). A Leica DMRB research microscope with Leica PL FLUOTAR objective with 100× magnifi cation and 1.30–0.60 aperture was used for the identifi cation of diatom taxa. At least 400 valves were counted (MSZ EN 2004). KRAMMER and LANGE-BERTALOT (1986, 1988, 1997a, b), KRAMMER and LANGE-BERTALOT (1991a, b, 2004a, b), and POTAPOVA and HAMILTON (2007) were used during diatom identifi cation. The OMNIDIA version 5.2 soft- ware package (LECOINTE et al. 1993) and Algaebase (GUIRY and GUIRY 2014) were used for naming the diatom taxa. In each site three independent samples were collected. Data processing and analyses Diatoms were classifi ed into halophilic/mesohalophilic and non-halophilic taxa groups according to ZIEMANN et al. 2001. Diatom taxa were assigned to fi ve size classes (Tab. 1) according to BERTHON et al. 2011. The biovolume of diatom taxa was calculated by using the OMNIDIA version 5.2 software package (LECOINTE et al. 1993). To compare the taxa number of each size class with the total taxa number, we calculated the size class ratio (based on taxa numbers) using the following simple equation: size class ratio (based on taxa numbers) = = taxa number in a size class × total taxa number–1 Furthermore, comparing halophilic taxa number of each size classes to the total halo- philic taxa number, we calculated the size class ratio (based on halophilic taxa numbers) using the following simple equation: Tab. 1. Diatom size classes according to BERTHON et al. 2011. Diatom size classes Biovolume (μm3) S1 5 – 99 S2 100 – 299 S3 300 – 599 S4 600 – 1499 S5 ≥ 1500 HALOPHILIC DIATOM TAXA IN LOWLAND LOTIC SYSTEM ACTA BOT. CROAT. 74 (2), 2015 291 size class ratio (based on halophilic taxa numbers) = = halophilic taxa number in a size class × total halophilic taxa number–1 Principal component analysis (PCA) was used to validate the importance of environ- mental factors according to LEPŠ and ŠMILAUER (2003) and STENGER-KOVÁCS et al. (2013). To analyse the relationship between diatom taxa composition and validated environmental factors a detrended correspondence analysis (DCA) was used, in which environmental fac- tors were added by weighted averaging (LEPŠ and ŠMILAUER 2003, B-BÉRES et al. 2014). Results Changes in environmental variables All of the 10 measured environmental variables were included into the PCA (DO, COND, pH, T and concentrations of HCO3–, Cl–, NH4+, PO43–, NO2–, NO3–). The fi rst three axes of PCA explained 93% of the total of species variance. The PCA revealed that the fac- tors with the highest correlation were with the fi rst axis conductivity (0.8627), concentra- tions of HCO3– (0.7718), Cl– (0.8469), and PO43– (0.9569), with the second axis DO (0.6450), and with the third axis the concentration of NO3– (0.5022). Conductivity showed a very high correlation with the concentration of HCO3– (0.9033), Cl– (0.9419), and PO43– (0.9870), and HCO3– and Cl– both with PO43– (0.8036 and 0.8744, respectively). Thus, in the DCA only the following validated factors were included: concentrations of NO3–, HCO3–, Cl–, and DO. The gradient lengths for the fi rst and second axis in DCA were 4.186 and 3.026, while the cumulative percentage variances of species data were 21.6 and 28.9 for the fi rst and second axis, respectively. Changes in total and halophilic taxa number – size classes and size class ratio (taxa number) Highly pronounced changes in diatom composition were observed from the humid spring to the dry autumn. For better clarity, changes are plotted in fi gure 2 only for three representative sites: Dögös-Kákafoki-főcsatorna (DKA), Kis-Körös-főcsatorna (KKO) and Kösely (KOS) (marked with arrows on Fig. 1B). Taxa number increased up to autumn almost in all cases (Fig. 2A), a lot of taxa appeared only in the dry autumn period (Tab. 2). When size class ratios in spring were compared to those in autumn, there were clear in- creases only in the case of large taxa (S5 size class – 80% of all sampling sites; each repre- sentative site in Fig. 2A) and small taxa (S1 or/and S2 size classes – 73% of all sampling sites; KOS on Fig. 2A). Similar changes occurred also in halophilic taxa numbers. They increased from spring to autumn in two thirds of the sampling sites (each representative site in Fig. 2B). From the identifi ed 31 halophilic and mesohalophilic diatom taxa, 13 taxa appeared only in the sam- ples collected in autumn. The most important difference between the changes of total taxa number and the changes of halophilic taxa number was the distribution of changes in the size classes. While total taxa number increased in all size classes almost in all sites, a clear increase was detectable in halophilic taxa number only in cases of S2 (60%), S4 (80%) and S5 (53.3%) size classes. Moreover, the increase of taxa numbers in the different size classes did not defi nitely mean the increase of the size class ratio. It increased only in the cases of KÓKAI Z., BÁCSI I., TÖRÖK P., BUCZKÓ K., T-KRASZNAI E., et al. 292 ACTA BOT. CROAT. 74 (2), 2015 S2 and S4 and/or S5 size classes (73% and 53% of all sampling sites, respectively; S2: each representative site, S4 and S5: DKA and KOS on Fig. 2B). Changes in total and halophilic taxon abundance – size classes Increasing relative abundances of small taxa (S1 and/or S2 size classes) were observed from spring to autumn in almost every site (DKA and KKO on Fig. 2C; Tab. 3). Neverthe- less, not only the abundances of small taxa, but also the abundances of large taxa (S5 or/and S4 size classes) were increased (S4: DKA and KOS, S5: KKO and KOS on Fig. 2C). How- ever, the contribution of halophilic taxa to the increasing abundance of S4 size classes was Fig. 2. (a) The total taxa number of different cell size classes and size class ratios based on total taxa number; (b) number of halophilic taxa of different cell size classes and size class ratios based on number of halophilic taxa; (c) relative abundances of different cell size classes and rela- tive abundances of halophilic taxa of different cell size classes in spring and in autumn at three representative sampling sites. DKA – Dögös-Kákafoki-főcsatorna, KKO – Kis-Körös- főcsatorna, KOS – Kösely; TTN – total taxa number, HTN – halophilic taxa number, SCR – size class ratio and RA – relative abundances. HALOPHILIC DIATOM TAXA IN LOWLAND LOTIC SYSTEM ACTA BOT. CROAT. 74 (2), 2015 293 Tab. 2. Diatom taxa appearing only in the dry autumn period. For explanation of size class types see Tab. 1. Taxa name Type of size class Fistulifera saprophila S1 Nitzschia microcephala S1 Sellaphora seminulum S1 Nitzschia angustatula S2 Cocconeis neodiminuta S2 Encyonema minutum S2 Navicula cf. kotschyi S2 Nitzschia clausii S2 Denticula tenuis S3 Hippodonta S3 Karayevia clevei S3 Kolbesia ploenensis S3 Encyonema caespitosum S4 Luticola goeppertiana S4 Nitzschia cf. lorenziana S4 Nitzschia prolongata S4 Surirella suecica S4 Anomoeoneis sphaerophora S5 Caloneis amphisbaena S5 Cymbella lanceolata S5 Craticula cuspidata S5 Ctenophora pulchella S5 Eunotia formica S5 Gomphonema spp. S5 Gyrosigma scalproides S5 Nitzschia angustata S5 Tryblionella calida S5 Tab. 3. Relative abundance of those small sized taxa the abundance of which increased from spring to autumn. For explanation of size class types see Tab. 1. Taxa name Type of size class Achnanthidium eutrophilum S1 Eolimna minima S1 Mayamaea permitis S1 Eolimna subminuscula S2 Mayamaea atomus S2 Nitzschia paleacea S2 Nitzschia pusilla S2 Planothidium frequentissimum S2 KÓKAI Z., BÁCSI I., TÖRÖK P., BUCZKÓ K., T-KRASZNAI E., et al. 294 ACTA BOT. CROAT. 74 (2), 2015 much more pronounced (maximum 84%; each representative site in Fig. 2C), while halo- philic taxa only slightly (maximum 17%) contributed to the increasing abundance of small taxa in autumn. Overall, relative abundances of halophilic taxa increased at 53% of all sam- pling sites from spring to autumn (each representative site on Fig. 2C). While abundances of certain halophilic and mesohalophilic taxa (Tab. 4) showed posi- tive correlations with chloride and hydrogen carbonate concentrations, other taxa (Tab. 5) correlated positively with nitrate (Fig. 3). Changes in evenness Evenness of diatom assemblages also changed from spring to autumn, but inversely to the taxa number: the latter increased almost in every site, while in contrast, evenness de- creased from spring to autumn (Fig. 4A). The number and/or ratio of those taxa the relative abundances of which were under 0.01 clearly increased in autumn relative to total taxa number (Fig. 4A). This was also was observed in the case of halophilic taxa (Fig. 4B). Evenness decreased from spring to autumn at 73.3% of the sampling sites (Fig. 4B). The taxa number and the ratio of those halophilic taxa the abundances of which were under 0.01 Tab. 4. Halophilic diatom taxa correlating positively with chloride and hydrogen carbonate concen- tration. For explanation of size class types see Tab. 1. Taxa name Type of size class Diatoma tenuis S4 Nitzschia cf. lorenziana S4 Craticula buderi S5 Craticula halophila S5 Nitzschia tryblionella S5 Tab. 5. Halophilic diatom taxa correlating positively with nitrate. For explanation of size class types see Tab. 1. Taxa name Type of size class Nitzschia microcephala S1 Nitzschia angustatula S2 Nitzschia clausii S2 Nitzschia frustulum S2 Navicula schroeteri S4 Nitzschia fi liformis S4 Nitzschia prolongata S4 Surirella suecica S4 Anomoeoneis sphaerophora S5 Caloneis amphisbaena f. subsalina S5 Nitzschia angustata S5 HALOPHILIC DIATOM TAXA IN LOWLAND LOTIC SYSTEM ACTA BOT. CROAT. 74 (2), 2015 295 (e.g. Anomoeoneis sphaerophora, Ctenophora pulchella, Caloneis amphisbaena f. subsali- na, Tryblionella calida), increased (87% and 73.3%, respectively) in autumn relative to to- tal halophilic taxa number. Discussion Changes in total and halophilic taxa number In spring there was a high amount of precipitation, while the summer and autumn was unusually dry in 2012 in Hungary. Decreasing water fl ow conditions were thus caused in these dry months in Hungarian lowland rivers and channels (B-BÉRES et al. 2014, KÓKAI et al. 2014). We assumed that taxa number was primarily infl uenced by increase of conductiv- ity (strongly connected with decreasing water fl ow conditions and increasing nutrient con- centration and/or Cl– content), because it has been found that there is a correlation between taxa number and certain ecological parameters (e.g. Cl–, total nitrogen, chemical oxygen demand, conductivity) in small Hungarian rivers (STENGER-KOVÁCS et al. 2013). We hypoth- Fig. 3. The relationship among halophilic diatom taxa composition and environmental factors dis- played by a detrended correspondence analysis (DCA). Cumulative percentage variance of species data are 21.6 and 28.9, while gradient lengths are 4.186 and 3.026 for the fi rst and sec- ond axis, respectively. The concentrations of Cl–, NO3– and HCO3–, and dissolved oxygen (DO) are added as environmental variables using weighted averaging. Halophilic species abbrevia- tions are the followings: Anomoeoneis sphaerophora – ASPH, Caloneis amphisbaena fo. am- phisbaena – CAMP, Caloneis amphisbaena f. subsalina – CASS, Craticula buderi – CRBU, Craticula halophila – CHAL, Diatoma moniliformis – DMON, Diatoma tenuis – DITE, Ento- moneis paludosa – EPAL, Fragilaria pulchella – FPUL, Navicula recens – NRCS, Navicula salinarum – NSAL, Navicula schroeteri – NSHR, Nitzschia angustata – NIAN, Nitzschia an- gustatula – NZAG, Nitzschia capitellata – NCPL, Nitzschia clausii – NCLA, Nitzschia fi lifor- mis – NFIL, Nitzschia frustulum – NIFR, Nitzschia lorenziana – NLOR, Nitzschia microceph- ala – NMIC, Nitzschia prolongata – NPRL, Nitzschia pusilla – NIPU, Nitzschia thermaloides – NTHE, Nitzschia tryblionella – NTRY, Nitzschia umbonata – NUMB, Surirella ovalis – SOVI, Surirella suecica – SSUE, Tryblionella apiculata – TAPI, Tryblionella calida – TCAL, Tryblionella hungarica – THUN, Tryblionella levidensis – TLEV. KÓKAI Z., BÁCSI I., TÖRÖK P., BUCZKÓ K., T-KRASZNAI E., et al. 296 ACTA BOT. CROAT. 74 (2), 2015 esised that the number of those taxa that prefer higher nutrient concentration and/or con- ductivity would be higher in autumn than in spring. This fi rst hypothesis was confi rmed: in the samples collected in autumn just such new taxa appeared, those that prefer high nutrient concentrations or high levels of conductivity (e.g. Anomoeoneis sphaerophora, Caloneis amphisbaena f. subsalina, Craticula spp., Encyonema caespitosum, Eolimna minima, Fis- tulifera saprophila, Gyrosigma scalproides, Nitzschia clausii, Nitzschia lorenziana, Sellaphora seminulum – ZIEMANN et al. 2001, ZALAT 2002, BOLLA et al. 2010, RIMET 2012). However, increased ratio of taxa number in different size classes was pronounced only in the cases of small taxa (S1 and S2 classes) and large taxa (S5 class). New taxa appeared in autumn in small size classes (e.g. Eolimna minima, Fistulifera saprophila), are frequent in polluted water (BELTRAMI et al. 2012, KELLY and ECTOR 2012, RIMET 2012). In contrast, a high number of newly-appeared taxa of the S5 size class were halophilic or mesohalophilic (e.g. Anomoeoneis sphaerophora, Caloneis amphisbaena f. subsalina, Nitzschia angustata, Tryblionella calida – ZIEMANN et al. 2001, ZALAT 2002). The number of mesohalophilic and halophilic taxa also showed a considerable increase in S4 size class (e. g. Nitzschia cf. lorenziana, Nitzschia prolongata – ZALAT 2002, CARTER and BELCHER 2010). Fig. 4. (a) Taxa number and evenness of whole diatom assemblages in spring and autumn; (b) taxa number and evenness of halophilic taxa in spring and autumn. The abbreviations of the sam- ling sites are the following Dióéri-főcsatorna (DIO), Dögös-Kákafoki-főcsatorna (DKA), Hamvas-csatorna (HAM), Holt-Sebes-Körös (HSK), Hortobágy (HOR), Hortobágy-Beret- tyó (HBE), Keleti-főcsatorna (KFC), Kígyósi-főcsatorna (KIG), Kis-Körös-főcsatorna (KKO), Kösely (KOS), Kutas-főcsatorna (KUT), Nagy-ér (NAG), Nagytóti-Toprongyos- csatorna (NTO), Sárréti-csatorna (SAR), Sebes-Körös (SKL). HALOPHILIC DIATOM TAXA IN LOWLAND LOTIC SYSTEM ACTA BOT. CROAT. 74 (2), 2015 297 Changes in total and halophilic taxon abundance We hypothesised that the abundance of diatom taxa preferring or tolerating high nu- trient contents would increase from spring to autumn, since nutrient concentration has a key role in shaping diatom assemblages (e.g. STEVENSON et al. 2008, STENGER-KOVÁCS et al. 2013). We observed that the abundances of mobile and/or stalked taxa like Planothid- ium frequentissimum, Cocconeis placentula ssp., Mayamaea permitis, Cymbella lanceo- lata, Encyonema caespitosum, Eolimna minima, Eolimna subminuscula, Gomphonema spp., Nitzschia paleacea highly increased from spring to autumn. These taxa can be fre- quent in water with high nutrient load and conductivity (RIMET 2012, B-BÉRES et al. 2014). The results supported our assumptions; an increase of conductivity is followed by the increase of the abundance of mesohalophilic and halophilic taxa (ZIEMANN et al. 2001). Most of the studied channels and rivers belonged to freshwater type (Cl– < 100 mg L–1), ac- cording to the classifi cation of waters on the basis of Cl– concentration (VAN DAM et al. 2003). Exceptions were the sampling sites Hamvas-csatorna in spring and autumn (HAM: fresh-brackish water, Cl– concentration 274 mg L–1 and 223 mg L–1 respectively) and Nagy- ér in autumn (NAG: brackish-fresh water, Cl– concentration 557 mg L–1). In these sampling sites diatom species of the kind known as halophilic taxa, preferring or tolerating high con- ductivity and nutrient contents were abundant (Nitzschia frustulum, Nitzschia fi liformis – Hamvas-csatorna (HAM) in spring; and Craticula buderi –Nagy-ér (NAG) in autumn, re- spectively; BLIN and BAILEY 2001, ZIEMANN et al. 2001, ZALAT 2002, BONA et al. 2007, RIMET 2012). In other sampling sites, belonging to freshwater types, increasing abundance of oth- er halophilic taxa (e.g. Tryblionella species, Nitzschia cf. lorenziana, Nitzschia pusilla and Navicula schroeteri) occurred with increasing conductivity/Cl– concentration and/or in- creasing nutrient content, confi rming the observations of KAŠTOVSKÝ et al. (2010) and STENGER-KOVÁCS et al. (2013). The composition of cell size classes in diatom assemblages includes useful information about the structure of the given assemblages and also about the pollution level and thereby about the physical and chemical composition of the water (BERTHON et al. 2011). We hy- pothesised that increasing nutrient content and/or conductivity cause conspicuous changes in composition of size classes; namely, the ratio of small disturbance-tolerant taxa and large halophilic/mesohalophilic taxa increases from spring to autumn. Our results confi rmed this second hypothesis, the distribution of small size (S1 and/or S2 classes) and large size (S5 class) classes clearly changed. In accordance with BERTHON et al. (2011), an increasing abundance of S5 class was observed in relation to increasing nitrate and/or phosphate con- centration and decreasing ammonium content. However, a clear increase in S1 size class was also observed in autumn, related to the increased nutrient contents and/or conductivity. We suggest that the reason for this was the composition of this size class, namely, 71% of the taxa were mobile (Navicula and Nitzschia spp.) or ruderal species (Staurosira and Pseu- dostaurosira spp.). Generally, mobile taxa prefer enriched habitats, and their abundance and taxa number increase with increasing nutrient contents (FAIRCHILD et al. 1985, VAN DER GRINTEN et al. 2004, BERTHON et al. 2011). Moreover, ruderal strategists prefer habitats with high amounts of nutrients (PADISÁK 2001, PASSY 2007) KÓKAI Z., BÁCSI I., TÖRÖK P., BUCZKÓ K., T-KRASZNAI E., et al. 298 ACTA BOT. CROAT. 74 (2), 2015 Changes in evenness The studied rivers and channels were exposed to both natural and anthropogenic im- pacts (lack of rainfall, agricultural pollution in the form of nutrient loads). These events caused an increased conductivity, and/or Cl– and/or nutrient concentration in almost all sampling sites. The results supported our third hypothesis, that evenness of diatom assem- blages decreases with increasing conductivity and/or nutrient content (NH4+, PO43–), ac- cording to ZALAT (2002), NDIRITU et al. (2006) and STENGER-KOVÁCS et al. (2013). Confi rm- ing our fi ndings, they observed a decreasing diversity in diatom assemblages parallel to increasing salinity/conductivity, and/or nutrient (especially phosphate) contents. Number of taxa with low abundances (under 0.01%) increased remarkably from spring to autumn. This was primarily due to the increasing number of halophilic taxa with low abundances and decreasing abundances of other taxa, which were sensitive to high conductivity or nu- trient content (e.g. Achnanthidium minutissimum, Encyonopsis microcephala – DE FABRI- CIUS et al. 2003, ÁCS et al. 2006, RUSANOV et al. 2009, RIMET 2012, STENGER-KOVÁCS et al. 2013). Conclusions Our results showed that even a one-year study can demonstrate such important changes in benthic diatom assemblages as decreasing diversity, loss of taxa, and increasing appear- ance of halophilic and mesohalophilic species caused by increasing conductivity and/or nutrient content. Our results pointed out those diatom assemblages sensitively indicate short-term changes in lowland lotic ecosystem; we found that studying changes in the ratio of halophilic and mesohalophilic taxa is especially useful for detection of environmental changes caused by either anthropogenic or natural effects. Knowledge based on such re- sults provides an opportunity e.g. for rapid intervention for environmental protection (e.g. water supply in the case of smaller rivers or channels). Our results could form a basis for a forthcoming comprehensive study on the primary or secondary salinisation in Hungarian lowland medium or small rivers and channels. Acknowledgements The authors are thankful for the support of the Bolyai János Research Scholarship of the Hungarian Academy of Sciences, the Internal Research Project of the University of Debrecen (Bácsi I.) and for the kind support of the Hungarian Scientifi c Research Found PD 100192 (Török P.) during manuscript preparation. The work was supported by TÁ- MOP 4.2.1./B-09/1/KONV-2010-0007, TÁMOP-4.2.2_B-10/1-2010-0024, TÁMOP 4.2.2.C-11/ 1/KONV-2012-0010 and TÁMOP 4.2.4.A/2-11/1-2012-0001 »National Excel- lence Program – Elaborating and operating an inland student and researcher personal sup- port system«, projects (Török P., T-Krasznai E., B-Béres V.). The TÁMOP projects are implemented through the New Hungary Development Plan, co-fi nanced by the European Social Fund and the European Regional Development Fund. 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