**1.1 Characteristics of the aquatic habitats influencing fungal life**

Aquatic habitats are characterised by a unique balance of allochthonous (external) and autochthonous (internal) organic matter supply, which is controlled largely by watershed characteristics, surface area and location. For example, headwater streams and small ponds receive most of their organic matter from terrestrial riparian vegetation, whereas large lakes are mainly supplied with organic matter internally from algal primary producers. Organic carbon derived from terrestrial vegetation varies substantially from that of algae. Plant remains contain large fractions of lignin, hemicelluloses and cellulose, which prolong microbial decomposition to several months, whereas algae contain much fewer recalcitrant polymers and thus are rapidly mineralised, usually within a few days. In small or shallow lentic systems submerged and emergent aquatic macrophytes often dominate the primary production, representing the most productive non-marine ecosystem worldwide. Aquatic fungi, being heterotrophs, are reliant upon photosynthetically produced organic matter. In order of decreasing biodegradability, the fungal community consumes microscopic algae, macroscopic aquatic macrophytes and terrestrial plant litter (including wood). On localised spatial scales or short-term temporal scales, carbon and nutrients from other sources may gain high importance. Resources derived from animals include fish, fish eggs, carcasses, excuviae, living zooplankton, insects, feathers and hair, while other plant-derived resources include pollen, spores, seeds and fruits (Cole et al., 1990). Interestingly, it seems to be nearly impossible to find a natural organic source that cannot be utilized by aquatic fungi (Sparrow, 1960). This notion points to either a high functional redundancy of a limited set of fungal species or to a high biodiversity of fungal specialists. Most likely, in natural systems both cases occur at the same time. Another interesting feature of aquatic habitats is the coupling of aquatic systems to terrestrial environments via animals, mainly insects, which are able to export nutrients from the aquatic ecosystem (Vander Zanden & Gratton, 2011). It will be shown later, that fungi are often closely associated with insects, which can be key organisms in aquatic freshwater systems. Although often overlooked, fungi represent a common and important component of almost every trophic level of any aquatic ecosystem.
