The goal of this study was a harmonization of diatom identification and counting among diatomists from the Scandinavian and Baltic countries to improve the comparison of diatom studies in this geographical area. An analysis of the results of 25 diatomists following the European standard EN 14407 during an intercalibration exercise showed that a high similarity was achieved by harmonization and not because of a long experience with diatoms. Sources of error were wrong calibration scales, overlooking of small taxa, especially small Navicula s.l., misidentifications (Eunotia rhomboidea was mistaken for Eunotia incisa) and unclear separation between certain taxa in the identification literature. The latter was discussed during a workshop with focus on the Achnanthes minutis-M. Kahlert (*) : sima group, the separation of Fragilaria capucina var. gracilis from F. capucina var. rumpens, and Nitzschia palea var. palea from N. palea var. debilis. The exercise showed also that the Swedish standard diatom method tested here worked fine with acceptable error for the indices IPS (Indice de Polluo-sensibilité Spécifique) and ACID (ACidity Index for Diatoms) when diatomists with a low similarity (Bray-Curtis <60%) with the auditor in at least one of the samples are excluded.

19High-latitude lakes usually have a high penetration of light, due to their low productivity 20 and low concentration of dissolved organic matter (DOM), but large variations in lake optical 21 properties can be found within and between regions. We investigated the underwater light shallow lakes that are already illuminated to the bottom even in the darkest systems. 40Keywords: dissolved organic matter, high-latitude lakes, lake optical properties warming (Williamson et al., 1999). They regulate the transmission of both photosynthetically 47 active radiation (PAR; 400-700 nm) and ultraviolet radiation (UVR; 280-400 nm) (Scully & 48 Lean, 1994; Morris et al., 1995; Laurion, Vincent & Lean, 1997; Huovinen, Penttilä & 49 Soimasuo, 2003; Bracchini et al., 2006) and therefore contribute to defining the species 50 composition in lakes (Rautio & Korhola, 2002), the ratio between auto-and heterotrophic 51producers (Jansson et al., 2000, Forsström, Roiha & Rautio, 2013, and the overall benthic and 52 pelagic productivity (Karlsson et al., 2009).The vegetation in the catchment, catchment to lake ratio, and the productivity of the lake have 55 a prominent impact on the concentration and composition of DOM. In small oligotrophic lakes 56 with low chlorophyll-a concentration and large catchment areas a high proportion of carbon is 57 derived from terrestrial and wetland sources dominated by higher terrestrial plant productivity 58 (Bade et al., 2007). The organic carbon leaching from forests and wetlands constitute mainly 59 of slow-degrading and nutrient poor material dominated with humic and fulvic constitutes 60 (McKnight & Aiken, 1998; McKnight, Aiken & Smith, 1991; McKnight et al., 1994) With the exception of Lake Kilpisjärvi (lake area 3710 ha, max depth 57 m), the MBW (Table I). Due to their shallowness, the majority of lakes (11) were isothermal during the sampling. 214Lake water temperature varied between 5.9 and 11.9 ºC being highest at lowest altitudes. 215Conductivity had highest range in ST, between 0.7 and 4.3 mS m -1 . The pH of three lakes, two (inorganic nutrients mainly below the detection limit, total P 3-13 µg l -1 and total N 71-410 µg 219 l -1 ) and low turbidity (< 1.0 FNU), with highest values generally measured from MBW. 220Chlorophyll a concentration varied between 0.2 µg l -1 and 2.4 µg l -1 and phytoplankton biomass 221 was low (less than 0.5 mg l -1 ) in all the study lakes. Only in the deepest lake (Kilpisjärvi) of the 222 three that were sampled from two different water layers, was chlorophyll a markedly lower in 223 the deeper water layer compared to the 1m depth (Table I). Most lakes were dominated by 224 chrysophytes (Chrysophyceae), but in a few lakes the dominating algal group was green algae 225(Chlorophyceae), cryptophytes (Cryptophyceae) or dinoflagellates (Dinophyceae). 226Dinoflagellates were most common in MBW lakes with high DOC and color (Korsajärvi, 227Koddojärvi and Ristijärvi) (L. Forsström, unpublished data). 228ANOSIM identified two groups separating ...

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