Andrzej GÓRNIAK *, Adam WIĘCKO, Adam CUDOWSKI University of Białystok, Department of Hydrobiology, Świerkowa St. 20B 15–950 Białystok, Poland, *e-mail: hydra@uwb.edu.pl (corresponding author) FUNGI BIOMASS IN LOWLAND RIVERS OF NORTH-EASTERN POLAND: EFFECTS OF HABITAT CONDITIONS AND NUTRIENT CONCENTRATIONS POLISH JOURNAL OF ECOLOGY (Pol. J. Ecol.) 61 4 749–758 2013 Regular research paper ABSTRACT: Fungi occur in almost all types of waters in the world and play an important role in many processes in the environment, particu- larly in organic matter decomposition. During the last several decades, extensive knowledge was obtained on the ecology of aquatic fungi, but on fungal biomass in the water column less. The study was aimed at simultaneous determination of seasonal and spatial variability of aquatic fungi biomass in the river waters in relation to selected physicochemical water parameters and trophic state in the 27 lowland rivers of north-eastern Poland with length between 17 and 308 km and mean annual discharge 1 – 100 m 3 s –1 . The chemi- cal and mycological evaluation was provided on the basis of 64 water samples taken in the July and October 2010. The chromatographic method of determination of seston ergosterol as a spe- cific organic compound typical for the majority of fungi, was applied. Fungi biomass was higher in summer than in autumn. The particulate or- ganic carbon (POC) and dissolved organic carbon (DOC) concentrations had no effect on fungal biomass in the studied river, as well as forest cover of the catchment and river discharge. Statistically significant correlations of fungal biomass in the river water with concentrations of ammonium (N–NH 4 + ), Kjeldahl (N Kjel ), total organic (TON) and total nitrogen (TN) and dissolved (DP) and dissolved organic phosphorus (DOP) were found. The principal component analysis indicates rela- tions between (TIN) and phosphorus (SRP) and particulate phosphorus (PP) and freshwater fungi, specially in summer when aquatic fungi are able to modify the structure of the nutrients contained in the water column. KEY WORDS: aquatic fungi, ergosterol, low- land rivers, nutrients 1. INTRODUCTION Fungi occur in almost all types of waters in the world, from thermal waters to glaciers and snow (Shearer et al. 2007). They play an important role in many processes in the environment, particularly on organic matter decomposition (Jobard et al. 2010). During the last several decades, extensive knowledge was obtained on the ecology of aquatic fun- gi, especially aquatic group Hyphomycetes (Barlocher 1992, Wong et al. 1998, Krauss et al. 2011). Also taxonomic research on this group of organisms is rapidly developing (Graca et al. 2007, Czeczuga et al. 2010). Data on the biomass of this component of the river biota, however, are still scarce. The main reason is the difficulty to separate me- chanically or optically fungi from the sub- strate inhabited by them (Ne w e l l 1992, Gessner and Newell 2002). Mycological studies on river waters in the temperate zone journal 36.indb 749 2013-12-30 21:06:12