Rush vegetation plays an important role in the process of inhibiting nutrient inflow, depending on the effectiveness of surface effluents from the drainage basin. It is a natural ecotonic zone [26], with buffer properties against inflowing nutrients. Recreational use of lakes contributes to fragmentation and, in consequence, to the lack of continuity of the zone. This results in easier nutrient inflow to a water body and in water pollution [27]. The most common plants in this zone include Phragmites communis, Ceratophyllum demersum and Polygonum amphibium. Aquatic vegetation in the lacustrine littoral zone is often inhabited by different microorganisms, including microfungi [12, 28]. Yeast present in the phyllosphere often compete with phytopathogens for nutrients, and they also inhibit their growth and development by taking in organic matter from the leaf surface. This is of special importance when the fungi isolated from the phyllosphere can pose a hazard to human health and when the submerged vegetation zone is in close vicinity to the places of recreation [1,2,3]. About 200 of the many yeast species occurring in the aquatic environment are pathogens [23, 25].
Rush vegetation has been used for years in bioindication of aquatic ecosystems. Its characteristic structure with an extensive absorption system enables absorption of various microelements from the environment, both harmful and necessary for the plant [29]. Therefore, species of rush plants in assemblies of plants, as well as aesthetic values, also play an important ecological role. The health condition of coastal and aquatic vegetation has an indirect impact on the life of the aquatic fauna, for which it is a shelter and breeding place [30].
As a reservoir of fungi potentially hazardous to humans, the phyllosphere serves as a filter in a water body which provides the first significant barrier for microfungi from the soil environment. When water ripples and washes the leaf surface, fungi cells may be washed down into surface water, posing a serious sanitary and epidemiological hazard. Korniłłowicz et al. [31] and Simi et al. [32] report that various species of fungi in the aquatic environment may derive from birds nesting nearby. However, no bird nests were found near the sampling sites in our study. The lakes which the surveys were conducted in are located within the administrative boundaries of the city, which results in reduced nesting in the coastal area. Fragments of the phyllosphere were collected during a period of intense exploitation of recreational and bathing sites located in the immediate vicinity of designated research areas.
Yeast and bacteria and/or moulds occur on leaf surface, forming aggregates or multicellular biofilms [33]. However, they are not the first group of microorganisms which settle the phyllosphere. This place is first settled by bacteria, which change the structure of cuticle and the lamina of the leaf. They are followed by yeast, with the filamentous fungi, which include numerous pathogens, being the last [28]. The moment of depositing and adhering of yeast on the lamina of the leaf usually occurs in summer, when the lamina surface is well developed. Fungi development is sometimes initiated by an increase in organic matter amount between spring and autumn, which is correlated with the plant growing season. The process of macrophyte decomposition indirectly increases the amount of biogenic elements, including nitrogen and phosphorus, thereby accelerating water eutrophication. Yeast floating in the pelagic zone in water under anthropopressure have higher enzymatic activity, increasing with the degree of eutrophication, which is directly associated with strain pathogenicity [3]. The presence of yeast, including potentially pathogenic species, has been confirmed by the findings of this study. Obtaining as many as 36 fungi species from the phyllosphere of Lake Skanda—which is highly eutrophicated—is worrying because up to eight of them belong to the genus Candida, which are important agents in aetiology of mycosis caused by Saccharomycetes. This can be demonstrated by the fact that the same species, such as Candida albicans, C. krusei and C. tropicalis, were found in clinical materials [1, 34] and at bathing sites in urban lakes [1,2,3, 35].
Selected water reservoirs differ from each other in the nature of the shoreline and the catchment area, or in the development of adjacent areas. Tyrsko lake, located in the north-west of Olsztyn, is characterised by the greatest landscape values. It is an area slightly transformed by man. Its shores are high and steep in places [36], and the state of water transparency is included in the second class of cleanliness, making it a great place for diving enthusiasts. An additional advantage of the lake is the lack of surface inflows and outflows. The second lake, Kortowskie, is located in the southwestern part of the city. It is a flow-through reservoir fed by five watercourses: the Starodworski, Parkowy and Leśny Stream, drainage line, and the Kortówka river, which is also its surface outflow [37]. It is a lake with an advanced level of eutrophication, intensively used for tourism and recreation. Its catchment has a forest-agricultural character. From the south-eastern side, it is surrounded by the gardens of the University of Warmia and Mazury and the infrastructure of the academic town with its recreation part (park, marina, water equipment rental, guarded swimming pool) [38]. The established Kortowski experiment has been in operation since 1959, consisting in removing pollutants to the Kortówka river with the help of hypolimnion waters [39]. According to research conducted in 2014 by Smoter et al. [40], the ecological status of the lake was assessed as poor (class IV water quality). The third lake—Skanda, is located on the south-eastern outskirts of Olsztyn [40]. Like Kortowskie lake, it belongs to reservoirs with an advanced degree of eutrophication, intensively used for recreational and tourist purposes [38]. Its shoreline is well developed, and its shores are quite varied, from flat through gently raised to steep [40], with visible traces of anthropopressure [38]. An important role is played by the runoff from agricultural areas, as well as pollution from two non-canalised farms located in the close proximity to the lake. The ecological state of the lake is classified as poor (class V of water quality) [40].
Considering the position of the isolated microfungi in the biosafety classification, it should be noted that the dominant group includes species of the BSL-2 group, i.e. potential human and other vertebrate pathogens [22, 23]. On the other hand, finding such a large number of yeast species inhabiting the littoral zone phyllosphere confirms that the zone acts as a filter for the lake catchment area [41].