4.2.1. Species Composition
Species composition of diatom assemblages significantly differed between the sampling points, indicating the effect of various level of land use.
Among the most dominant species, Achnanthidium delmontii and Amphora inariensis was characteristic at the reference point (RN).
Interestingly, these species along with Navicula tripunctata
were not reported in the list of species reaching a relative abundance of at least 5% at a minimum of one site during JDS3 (see Reference [47
]). The explanation for this may be: (1) the abundance of these species has increased since 2013, (2) the difference between the two research studies which performed the identification, then and now.
In previous research, we found Achnanthidium delmontii abundant in a sample, but it was difficult to identify and to distinguish from Achnanthidium pyrenaicum and Achnanthidium rivulare under a light microscope. Therefore we studied the sample under a scanning electron microscope, identified the species as A. delmontii and determined the features (valve shape, central area and stria density) that allowed the identification of the species under a light microscope. In contrast to the two most similar species mentioned above, on the raphe valve of A. delmontii, the central area forms a more or less rectangular fascia. Moreover, A. delmontii has less dense striae than A. rivulare and is wider than A. pyrenaicum, which has narrower apices. Using these features we were able to unambiguously identify A. delmontii in samples presented here. Although the methodology of the Water Framework Directive does not require the use of a scanning electron microscope, based on our previous experiences we recommend its use for small-sized species that can be confused under a light microscope.
was described from the Mediterranean river Cèze in 2012 [50
]. It was characteristic of cobbles (diameter 6–40 cm) in an Argentina river [51
]. Its invasive behavior was reported [50
Lange-Bertalot et al. [52
] wrote about Amphora inariensis
, noting that “according to Krammer and Lange-Bertalot [53
] it colonizes oligotrophic, anthropogenically undisturbed standing freshwater habitats with low to medium electrolyte content but it is found in a variety of environments, sometimes moderately impacted.” Of its traits only a few are known: it is an oligotrophic, micro-sized, low profile species.
Left bank upstream of Budapest was a transition between the reference RN and the heavily impacted downstream points. Here species, like Amphora pediculus, Nitzschia inconspicua
and Navicula antonii
, more tolerant to trophic and/or saprobic conditions [52
], became characteristic, (Achnanthidium delmontii
and Amphora inariensis
were also present but in smaller amounts). Amphora pediculus
is a dominant species in the River Danube [47
]. It was also found in downstream samples in a lower proportion. Besides, the most heavily impacted downstream points were dominated by Cocconeis euglypta, Navicula cryptotenella and N. tripunctata
During this summer period, alkaliphilic, oligohalobous, beta-mesosaprobic, eutrophic, tolerant N-autotrophic, oxybiontic or moderate oxygen requirement, low profile taxa were dominant in the epilithic diatom community of the section of the River Danube near Budapest. On the right bank upstream of Budapest, where the extent of the land-use was the most negligible, there was the lowest ratio of eutrophic and highest ratio of polyoxybiontic, nanosized and LPG species. On the left bank at the same site, there was the highest ratio of facultative and obligatory N-heterotrophs. The species with lower oxygen requirement occurred in higher abundance downstream of Budapest than upstream.
Ecological groups [20
] are among the most frequently studied traits and several studies have investigated their relationship with environmental variables. The low profile group (LPG) includes species of short stature, including prostrate, adnate, erect and slow-moving species that are adapted to high current velocities and to low nutrient concentrations [21
]. It is beneficial for the LPG attached strongly to the substrate when the establishment of a three-dimensional biofilm is prevented; however, it still persists in the case of copious periphytic growth but in lower numbers and suffers resource limitation [20
]. Tapolczai et al. [19
] challenged the idea that because low-profile species are suppressed in thick biofilm this means that they have an advantage under low-resource circumstances. It has the ability to withstand abrasion and physical damage, therefore dominating in unstable habitats, like epipsammon [20
The high profile group (HPG) includes species of tall stature, including erect, filamentous, branched, chain-forming, tube-forming, stalked and colonial centrics and large size taxa [21
]. These taxa represent the canopy layer of the biofilm [19
], having a more direct connection with the ambient water [54
] and preferring high nutrient concentrations (however, the relationship between the HPG and nutrient concentrations was the weakest in Passy [20
]’s study). They are adapted to low current velocities being more abundant in the more stable or more sheltered habitats, like epilithon and the epiphyton [20
The motile group (MG) includes fast-moving species that are superior competitors for nutrients in nutrient-rich environments and can move to resource-rich microhabitats avoiding stress. The MG was significantly higher in the unstable epipelon, where it can escape from being buried [20
]. This ability of motile diatoms was what Bahls [56
] based his siltation index on (the sum of the percentages of Navicula
species and varieties). This index was used during the National Water Quality Assessment (NAWQA) Program in the United States, although the relationships between the silt index and indicators of stream siltation processes (e.g., substrate embeddedness and suspended-sediment concentrations) were relatively weak, presumably because of the influence of nutrient conditions [57
We observed that low profile diatoms were most abundant in almost all samples; the second most abundant group was the motile, which became more dominant mainly in LN samples. High profile species could reach relative abundance above 5% only in one sample (12.75% in 0730 LN). As trophic conditions were eutrophic during the whole study period (as was shown by the environmental variables and the high relative abundance of eutrophic taxa), the proportions of ecological groups can be explained by hydromorphological reasons rather than nutrient concentration. Most of the LPG taxa which occurred in our samples were eutrophic, this observation also contradicts the hypothesis on its adaptation to a too low nutrient concentration. The formation of the thick and complex biofilm may be prevented by the frequent moving and slewing-around of the small cobbles.
Similarly, Trábert et al. [55
] found that ecological groups can reflect the effect of substrate and differences between flow regimes rather than nutrient loads. In their study conducted in the River Danube, the HPG also had a low relative abundance in the River Danube compared to the LPG and the MG, due to its inability to withstand the disturbance of high current velocity and water-level fluctuations.
The HPG could not grow under decreased light intensity [58
] which can be caused by high suspended matter content and turbidity [55
]. In contrast, the LPG is more common in shaded environments [58
The planktic group was introduced by Rimet and Bouchez [21
] (as “planktic guild”) who applied this term to taxa adapted to lentic environments with a morphological adaptation that enables them to resist sedimentation and to filamentous taxa that were included in the HPG in Passy [20
]’s system (with the exception of filamentous taxa which are benthic). These diatoms could settle down for hydromorphological reasons rather than because of changes in water quality [59
]. Planktic taxa can reach even a relatively high proportion in benthic assemblages in sections close to reservoirs, in late-successional stages of matured biofilm or when settling down due to a reduced flow rate [60
]. In our study, these species (mainly Discostella pseudostelligera
, Stephanodiscus minutulus
(Kützing) Cleve and Möller, Cyclostephanos invisitatus
(M.H. Hohn and Hellermann) E.C.Theriot, Stoermer and Håkasson and Cyclotella atomus
Genkal and Kiss), reached a significantly higher proportion in the downstream samples.
Beside ecological groups, size is an also commonly used trait. It is a master trait that significantly influences physiological activities (e.g., growth, metabolism) [62
] and determines the ability of a species to recover after disturbance [58
]. In our study, nano-sized diatoms prevailed at the points upstream of Budapest, while their relative abundance decreased downstream in favor of other size classes, particularly the very large taxa. Lange et al. [58
] also found that the proportion of small cell sizes decreased at a higher level of farming intensity, supporting their hypothesis that increased nutrient supply supported the growth of larger cells. Small sizes are advantageous under nutrient-limiting conditions because of the high surface to volume ratio and are more beneficial in competition for nutrients under nutrient-limited conditions, while large cell sizes may be advantageous under conditions of fluctuating nutrients because of their increased nutrient storage capacity in vacuoles that may be enormous, relative to cell volume [62
Applying the same size classes that we used, Berthon et al. [17
] found that the c2 and c5 size class (micro and very large in our study) significantly discriminated between the trophic and the organic pollution classes and the c3 (meso) between the trophic classes.
There are traits linked to physiological properties (e.g., nitrogen uptake strategy) which are more difficult to measure and are characteristic of species and not of genus; however, these can be more clearly linked to environmental variables and can be applied in ecological status assessments of waters (e.g., the percentage of nitrogen heterotrophic diatoms was used for the organic sources of nitrogen by Peterson and Porter [48
]). Such metrics (e.g., the percentage of eutrophic, nitrogen-autotrophic, nitrogen-heterotrophic, halophilic diatoms) were used in the evaluation of stream-water quality during the NAWQA Program in the United States [57
In our study, the proportion of tolerant nitrogen autotrophs increased downstream of Budapest. These diatoms can use an inorganic nitrogen source but they can tolerate an elevated concentration of organically bound nitrogen [22
] and their increased relative abundance may indicate a higher nutrient load. In Gore Creek (Colorado, USA) the increasing biovolume of nitrogen autotrophic diatoms was consistent with the increasing inorganic nitrogen (nitrate) concentration that was affected by the discharge from a wastewater treatment plant [63
]. In LN samples, the relative abundance of nitrogen heterotrophs increased (tolerant nitrogen autotrophs remained dominant). Facultative nitrogen heterotrophs can process organic nitrogen to supply cellular energy requirements (in addition to photosynthesis), as well as acquiring sufficient nutrients required for nitrogen metabolism [22
]. Thus, the abundance of nitrogen heterotrophs often increases in turbid rivers (or shaded streams) that are organically enriched with nitrogen [48
Among the trophity adaptation strategies, the eutrophic one was dominant but its proportion was significantly lower in RN samples than in the other samples. In the case of the Yellowstone River basin eutrophic and nitrogen indicators appeared to better indicate trophic conditions than measured nutrient concentrations [48
]. Nutrient concentrations generally were low throughout the length of the Yellowstone River (total nitrogen ranged between 0.3 and 0.4 mg·L−1
and total phosphorous varied from 0.016 to 0.038 mg·L−1
); however, indicators of algal biomass and the percentage of eutrophic and nitrogen indicator diatoms were relatively high in the middle segments of the river and near the mouths of major tributaries [48
]. Instantaneous dissolved nitrate loads were high, which was reflected in the relatively high percentage of nitrogen autotrophs and this corresponded closely with the abundance of eutrophic diatoms [48
Polyoxybiontic taxa and diatoms with moderate oxygen requirement showed opposite changes, indicating the effect of the land-use gradient. Saprobity was strongly associated with oxygen requirement. In cases in which both traits were known, oligosaprobic species were all polyoxybiontic, β-mesosapric ones were mainly oxybiontic and most of the α-mesosaprobic taxa had moderate oxygen requirements. This is due to the fact that the saprobity trait combines indicator properties for the presence of biodegradable organic matter and the oxygen concentrations [22
]. Distribution of many taxa are not limited by decreasing saprobity but only by increasing pollution levels; therefore the presence in a sample of taxa which are not resistant to heavy loads of organic pollution can be used to differentiate this sample from samples which are taken from heavily polluted water bodies; the presence of taxa which are very tolerant to organic pollution does not always indicate the presence of organic pollution [22