| Wetlands is an important component of the natural ecology system with a complex chemical-physic characters of the water, there are a lot of endemic species distributed in the wetlands. It is very significant for both the taxonomical and ecological research to study the diatom in this area. Great Xing'an Mountains located in the northeast of China. This area is characterized by high humidity and low temperature, and has the highest latitude swamps and the widest distribution of bogs in China, but the study on the diversity of algae in this area is rare. From the year 2004 to 2006, samples were collected from this mountain. This paper study on the both taxonomical and ecological characters of the diatoms distributed here.1. Totally 540 samples were observed and photographed.323 species,45 varieties and 1 form were identified belong to 3 classes 15 orders 30 families and 74 genera. Among them 122 taxa are newly recorded in China. Simple descriptions of the morphological and ecological characters are given for each species. LM images are given for all the species, SEM images are given for some species.150 plates are attached at the end of this paper. In this study,3 new species were observed, they are:Pinnularia subaldenii Q. X. Wang & Y. Liu, Pinnularia paludosa Y. Liu & Q. X. Wang, Diatoma rupestris Y. Liu & Q. X. Wang; 7 prepared new species were observed (6 in Gomphonema,1 in Eunotia). Both LM and SEM images are given for these new species.2. Taxonomical work are followed with Round et al. (1990)'s system. Many genera name are used which were never or less used in China before. Detailed descriptions for these genera are given.3. Polymorphism is a variety of the valves in diatom, include several kinds of ways in both vegetative and sexual production. Two kinds of polymorphism were observed in our study, one is observed in the sexual production in the genus Meridion. We found that in one frustule both valves of Meridion circulare are acostated, one rimoportule appeared at each end of the valve. In girdle view, some frustules have one valve costated and another valve acostated. This is the first report for these valves in the world. Another kind of polymorphism which was observed in our samples is Janus cell in Gomphonema in vegetation production. Different kinds of striae present in two valves in one frustule were observed as one valve with coastly striae and another one with finely striae. Some studies were reported in the world about this kind of frustules, this is the first report in China.4. The diatom community and distribution were compared within different sites and habitats. Diatoms in the wetlands of Great Xing'an Mt. with a high diversity at the genera level, most of them are found in Bacillariophyceae,59 genera were observed in this class. In Coscinodiscophyceae, only 6 species belong to 4 genera were observed, In Fragilariophyceae,37 species belong to 13 genera were observed. Pinnularia is the most abundant genus,61 taxa were observed, following with Eunotia 29 taxa, Gomphonema 23 taxa, Navicula 18 taxa, Cymbella and Surirella 16 taxa repciectively, Cymbopleura and Nitzschia 14 taxa respectively. Dominant species are rare. Taxa which were widely distributed in all kinds of sites and habitats were rare too. The difference of diatom community is obviously among these habitats, some taxa with limited distribution were treated as endemic taxa in some habitats. In our study, samples were collected from 9 habitats, among them swamps have the most abundant species,263 taxa were observed, include 65 endemic taxa; 220 taxa were observed in lake shore; water seepage has the least species, only 23 taxa were observed. No endemic species were observed in ponds, water seepage, streams and springs.5. Biogeography of diatoms from the wetlands in Great Xing'an Mt. was studied preliminarily. Most of diatoms in Great Xing'an Mt. were widely distributed in north hemisphere and north alpine. Compared with other research, the diatom flora are similar with European and North American, different with South American, African and other tropic area.
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