Additional information on the distribution and ecology of the recently described diatom species Geissleria gereckei

The Raška River and the Mlava River belong to the Black Sea drainage basin. The Raška River is a tributary of the Ibar River in the southwestern part of Serbia. The Mlava River is a tributary of the Danube in eastern Serbia (Marković 1980). The rivers have a carbonate bedrock and run through trout ponds. Samples were collected at the sites located downstream and upstream of the trout ponds. Some of the sites are exposed to seasonal desiccation when the discharge drops.

Diatoms were sampled from 5 localities in the Raška and Mlava rivers in April, June, August, and November 2011, and March and May 2012. Epilithon and epibryon were considered in the Raška River, while only epilithon was sampled in the Mlava River. Samples were immediately fixed with formaldehyde to a final concentration of 4%. Conductivity, oxygen, pH, and water temperature were measured in the field. Hydrochemical analyses were carried out at the Institute of General and Physical Chemistry, University of Belgrade, following standard methods (APHA 1998). Diatom samples were treated according to standard methods to obtain permanent slides (Krammer & Lange-Bertalot 1986). Permanent slides, prepared material, and aliquots of the samples were deposited in the diatom collection of the University of Belgrade, Faculty of Biology.

Slides from the Raška (14 samples) and Mlava (8 samples) rivers were used for microscope observations. Light microscope observations and micrographs were made using a Zeiss AxioImagerM.1 microscope with DIC optics and AxioVision 4.8 software. SEM observations were made at the Institute of Physics, University of Belgrade, using a TESCAN MIRA 3 scanning electron microscope with maximum accelerating voltage of 30 kV. Sample surfaces were sputtered with gold using QUORUM TECHNOLOGIES MINI SPUTTERCOATER SC7620 for enhanced conductivity.

Trying to gather as much data on the distribution of Geissleria gereckei as possible, the following databases were carefully checked: Pesio and Tanaro Valleys Nature Park (Ligurian Alps, the south-western extreme of the Alps; Mogna et al. 2015), central to eastern Switzerland and the Jura (the north-western/ central Alps; Taxböck, L., Karger, D.N., Kessler, M. & Cantonati, M. 2016 submitted paper), CRENODAT (Trentino, the south-eastern Alps; Cantonati et al. 2012), CESSPA (Province of Verona, Veneto Region, Italy; Cantonati et al. 2016), Berchtesgaden National Park (Bavaria, Germany; Cantonati & Lange-Bertalot 2010), Gesäuse National Park (Styria, Austria; MC unpublished data), Julian Pre-Alps Nature Park (Friuli Venezia Giulia Autonomous Region, Italy) (the south-eastern Alps; Cantonati 2003). In the case of the database established before the description of G. gereckei, also direct inspection of the permanent mounts was carried out to check for possible occurrence of the species. Finally, repeated literature searches were carried out (“*Geissleria*gereckei*”) in the Web of Science (WOS) and Google Scholar.

Main morphological characteristics. Valve outline linear-elliptic to elliptic. Ends subcapitate and protracted. Length 9.9-18.5 μm, breadth 5.0-7.1 μm; length/width ratio 2.0-2.8. Raphe filiform; axial area narrow, linear; central area characterized by longer and shorter striae. Single stigma placed in the center of the valve. Striae 13-17/10 μm, radiate throughout (Fig. 1a-g).

Figure 1

SEM: external view (Fig. 2b): Proximal ends of the raphe expanded and moderately deflected whilst distal ends are hooked. Striae uniseriate throughout. Areolae apertures as short slits (55-60 in 10 μm). Annulus evident as three rows of areolae larger than in stria areolae, apically elongated. In each row, from 3 to 6 apically elongated areolae are present.

Figure 2

SEM: internal view (Fig. 2 a and c): Distal raphe ends slightly deflected in a small, aureole-shaped helictoglossa, central raphe endings straight. Annulus structure with warty outgrowths.

Distribution. The Raška and Mlava River. G. gereckei was found both in the epilithon and in the epibryon. Relative abundance in epilithic diatom communities of the Raška River ranged from 0.5 to 3%. Only one specimen of G. gereckei was recorded in epiphytic diatom assemblages of the Raška River and in epilithic diatom assemblages of the Mlava River.

Associated diatoms. The most frequent diatoms which were found together with G. gereckei (Raška River epilithon) were: Achnanthidium affine (1.2-18%), A. minutissimum (9.2-48%), Amphora pediculus (11-32%), Cocconeis lineata (0.5-32%), Diatoma vulgaris (0.2-20%), Gomphonema parvulum (0.2-2.8%), G. tergestinum (0.3-39%), Navicula cryptotenella (0.2-4.2%), and Nitzschia dissipata (0.5-5%).

Ecology. The Raška and Mlava rivers are small mountain rivers, running at altitudes between 296 and 557 m a.s.l. Samples were collected at the sites located downstream and upstream of the trout ponds, which explains the high nutrient values. Conductivity values are consistent with the carbonate bedrock (Table 1). The highest abundance of G. gereckei was observed at microhabitats (river wetted surface and bryophytes) exposed to seasonal desiccation.

Table 1

Value ranges of physical and chemical parameters of the sites with Geissleria gereckei

G. gereckei was also found in one spring in the south-eastern Alps (Ziola Bassa, AT0756, Vigolana Upland), in springs Alpine foothills (Verona Province, Lessinia, ML0622) and in eight springs of the south-western Alps (Table 1). These springs are small, with low discharge (probably <0.5 l s^-1 in most cases), and likely to be affected by periodic desiccation. All of these springs are fed by carbonate aquifers, and have medium conductivities (200-400 µS cm^-1).