Polyhumic dystrophic rivers - an unique habitat for water mites (Acari, Hydrachnidia)?

Polyhumic dystrophic rivers are very rare in Poland and polyhumic rivers (both poly- and dystrophic) are weakly researched in general. The aim of the study was to present a detailed faunistic and ecological analysis of the water mites of polyhumic dystrophic rivers from Janów Forests Landscape Park (central-eastern Poland) and to compare the Hydrachnidia communities of those rivers with the Hydrachnidia of non-polyhumic rivers of this area. In small, fully polyhumic rivers Hydrachnidia fauna was poor in species and individuals, the populations of most species were very small and the characteristic feature of these rivers was very low species diversity. Therefore, the pronounced dystrophy of small polyhumic rivers should be considered a factor restricting the development of larger Hydrachnidia populations. In partially polyhumic rivers the water mite fauna was more diverse than in fully polyhumic rivers. The greater diversity of fauna resulted from the migration of species and individuals from the upper reaches of the river (non-polyhumic) and from greater habitat diversity. As a general conclusion we can say that in the fauna of polyhumic dystrophic rivers it is impossible to indicate species that could be considered characteristic of these habitats and that distinguish the Hydrachnidia communities of polyhumic rivers from those of non-polyhumic rivers. The Hydrachnidia communities of polyhumic rivers differ from those of non-polyhumic rivers in terms of population size. Quantitative impoverishment of fauna is observed in polyhumic rivers (especially small, fully polyhumic ones). More research should be done to determine specificity of water mite fauna of polyhumic rivers.


Introduction
The term 'polyhumic rivers' is new. In 2014, the typology of rivers in Poland was revised so that two previously distinguished types of watercourses, 'stream or brook in an area affected by peat-forming processes' and 'small or medium river in an area affected by peat-forming processes' were combined into one category: 'streams and rivers in valleys with a large share of peatlands' (Updated list of JCWP and SCWP ... 2014). According to Górniak (2016), these types of watercourses should correctly be referred to as polyhumic rivers.
A characteristic feature of polyhumic rivers is the large share of dissolved organic matter (DOM) and partial organic matter (POM), which gives the water a permanent or periodic brown-rust colour (Górniak

Study area and sites
The research was conducted in the Janów Forests Landscape Park (central-eastern Poland). A general description of that area and its riverine system can be found in Stryjecki et al. (2018). Two polyhumic rivers (Czartosowa and Łukawica) and the polyhumic section of the River Bukowa were included in the study. The Czartosowa is a small lowland river, about 10 km long. The river flows through densely forested areas throughout its course. It flows out of a marshy, boggy area. The source of the river (50°38'55.8"N; 22°27'12.5"E) is located at 209.5 m above sea level (Geoportal 2019). The Czartosowa is a right-bank tributary of the park's largest river, the Bukowa. It flows into the Bukowa in its middle course (50°36'56.5"N; 22°19'55.1"E). The estuary of the Czartosowa is located at 175.2 m above sea level (Geoportal 2019). The difference in altitude between the source of the river and its mouth is about 34 m, for an average drop of 3.4‰. Sandy sediments predominate in the river. Aquatic vegetation is very poor. A distinctive feature is the tea-coloured water.
The Łukawica is a typical lowland river, about 34 km in length. From the 4th to the 26th kilometre of its course, it flows through densely forested areas. The source of the river is in the village of Słupie (50°45'09.8""N; 22°19'00.6"E) at an altitude of 211.0 m above sea level (Geoportal 2019). The Łukawica is a right-bank tributary of the River San. It flows into the San near the village of Wola Rzeczycka (50°39'26.7"N; 21°59'52.5"E). The mouth of the Łukawica is located at 146.2 m above sea level (Geoportal 2019). The difference in altitude between the source of the river and its mouth is about 65 m, for an average drop of 1.9‰. The river feeds two large complexes of fish ponds. Water from the ponds is periodically drained into the river.
The following four polyhumic sites were selected for the research. Site 1 -on the River Czartosowa in the village of Szklarnia (50°38'30.4"N;22°23'46.4"E) A site located about 5 km from the source, in a dense pine forest; the water table is shaded. The river bed lies fairly deep below the ground surface, meandering slightly, with a width of 2.5 m to 3.5 m at the bends. The depth varies from 0.1 m at the banks to 0.4 m in the central part of the river. The water has a high content of humic compounds and is brown in colour. During the study period there were significant fluctuations in the water level and associated fluctuations in current velocity. Sediments were sandy and slightly silty by the banks. Vegetation was very sparse, with sporadic Sparganium emersum Rehmann, 1871 and Fontinalis sp. Site 2 -in the River Łukawica in the village of Bania (50°41'54.6"N; 22°10'18.7"E) A site located about 14 km from the source in a mixed forest, with a shaded water table. The bed was about 2.5 m wide and 0.3-0.5 m deep. Sediments were silty and thick, composed of sand and silt in places. Small amounts of Potamogeton pusillus L., 1753, Hydrocharis morsus-ranae L., 1753, Elodea canadensis Michaux, 1803 and Sagittaria sagittifolia L., 1753 grew by the banks. In the central part of the river there were no plants at all, but there were numerous branches and rotting tree trunks. The water flow was slow, and periodically the water was nearly stagnant.
Sites 3 and 4situated in a polyhumic stretch of the Bukowa River. For a description of those sites see Stryjecki et al. (2018, sites 4 and 5 in the paper).

Field research
The field research was conducted from April to November 1996 and from March to October 1997. Samples were taken once a month. Sampling was done with a hand net. The net had a round frame 0.25 m in diameter and 250 µm mesh size. A single sample was taken over a distance of about 10 m. A total of 128 samples were collected from polyhumic rivers. The material comprised a total of 1,163 individuals. The material collected in the field was transported to the laboratory and segregated on white cuvettes. The keys used to identify water mites were Gerecke (2003Gerecke ( , 2009, Davids et al. (2007), Di Sabatino et al. (2010), and Gerecke et al. (2016). Species nomenclature and systematics were adopted according to Davids et al. (2007), Di Sabatino et al. (2010) and Gerecke et al. (2016). Species were classified as very rare and rare according to Biesiadka (2008).
During collection of hydrobiological samples, the basic physical and chemical indicators of the water were measured: temperature (°C), pH, electrolytic conductivity (μS/cm), dissolved oxygen (mg O2/l), and water saturation with oxygen (%). The measurements were made using a Slandi kit (TM204 thermometer, PH204 pH meter and CM204 conductivity meter) and an Elmetron CX401 multifunction meter. The water current was determined by the floating object method, by measuring the object's flow time over a distance of 10 m.
Data from three non-polyhumic rivers were used for comparative purposes. The following nonpolyhumic rivers/sites were included: the River Bukowa (Stryjecki et al. 2018, sites 1-3), the River Branew (Stryjecki 2019 in press, 2 sites), and the River Rakowa (unpublished data, 2 sites). Non-polyhumic rivers were researched at the same time and in the same area as the polyhumic rivers in this study. The comparative material comprised a total of 224 samples and 6,940 individuals.

Statistical analyses
Descriptive statistics (sums, means, range, and standard deviation) were calculated using PAST ver. 3.16/2019 software (Hammer et al. 2001). The software was also used to calculate the Shannon-Wiener index (H'). Analyses of quantitative faunistic similarities based on the Bray-Curtis formula were carried out using BioDiveristy Pro ver. 2 software (McAleece et al. 1997). Similarity dendrograms were generated using BioDiveristy Pro software. Group Average was used to create clusters.
The normality of the data distribution was checked by the Shapiro-Wilk test. The data were tested for homogeneity of variance using Levene's test. The t test was used to compare two independent samples when data had normal distribution and the variance was homogenous. The Mann-Whitney U test (Z) was used to compare two independent samples when data did not have normal distribution. All tests were carried out in Statistica 13.1 software. The statistical significance level was set at p = 0.05.
The highest mean number of individuals per sample in the polyhumic rivers was recorded in the River Bukowa (12.9 ind./sample), and the lowest in the Czartosowa (4.4 ind./sample). In the non-polyhumic rivers, the mean numbers of specimens were much higher (Table 3). When all rivers of each category are considered together, the number of individuals per sample in the polyhumic rivers is much lower than in the non-polyhumic rivers (9.1 and 31.0, respectively) - Table 3.  Table 3.
The dendrogram of faunistic similarities shows a clear grouping of the non-polyhumic rivers Branew and Rakowa (59.8% similarity) and another grouping of the polyhumic section of the Bukowa River with the non-polyhumic stretch of that river (49.5% similarity; Figure 1). The fauna of the polyhumic River Czartosowa was most similar to the Hydrachnidia communities of the polyhumic section of the Bukowa (22.9% similarity). The fauna of the polyhumic Łukawica was quite distinct: it was only 12.3% similar to the Hydrachnidia communities of the non-polyhumic section of the Bukowa, 11.5% similar to the fauna of the polyhumic Czartosowa, and 11.0% similar to the fauna of the polyhumic section of the Bukowa (Figure 1). In the River Łukawica, as many as six species were caught that were not caught in any other river in the park, either polyhumic or non-polyhumic. These were Eylais extendens, Pseudohydryphantes parvulus, Brachypoda versicolor, Piona obturbans, Arrenurus cuspidator and A. neumani. Table 3. Average number of individuals per sample in polyhumic and non-polyhumic rivers. Bu-Ppolyhumic section of the Bukowa River, Cza -the Czartosowa River, Łuk -the Łukawica River, P-total -polyhumic rivers together, Bu-NPnon-polyhumic section of the Bukowa River, Brathe Branew River, Rakthe Rakowa River, NP-totalnonpolyhumic rivers together Higher species diversity was found in the polyhumic rivers (H'= 2.96) than in the non-polyhumic rivers (H' = 2.79). However, in some polyhumic rivers (Czartosowa and Łukawica) species diversity was very lowmuch lower than in the non-polyhumic rivers (Figure 2).  Table 3 Discussion Analysis of the Hydrachnidia fauna of the polyhumic rivers of the Janów Forests Landscape Park indicates that the rivers should be divided into two subcategories: fully polyhumic rivers (the Czartosowa and Łukawica) and partly polyhumic rivers, i.e. those in which only certain parts of the course of the river are polyhumic (the Bukowa). In fully polyhumic rivers, the water mite fauna was both quantitatively and qualitatively poor; many more individuals and species were caught in the non-polyhumic rivers of the park (Stryjecki et al. 2018;Stryjecki 2019). Many more individuals and species have also been found in other lowland rivers of Poland than in the fully polyhumic rivers in the present study (Kowalik 1981;Kowalik & Biesiadka 1981;Cichocka 1996Cichocka , 2006Zawal & Kowalik 2013;Zawal et al. 2017). The fully polyhumic rivers had significantly lower, slightly acidic pH and lower electrolytic conductivity than the non-polyhumic rivers. According to Górniak (2016), low values for pH and electrolytic conductivity in polyhumic dystrophic rivers, as well as low concentrations of calcium and bicarbonates, contribute to the depletion of important elements of food webs, i.e. algae and macrophytes, as well as to the formation of distinctive invertebrate communities in which certain groups are absent (e.g. molluscs). The impoverishment of producers in dystrophic rivers results in the impoverishment of subsequent links in the food chain, including predators such as water mites. The pronounced dystrophy of the park's small polyhumic rivers should be considered a factor limiting the development of larger Hydrachnidia populations. The Czartosowa and Łukawica are small watercourses running through forests and are much less diversified in terms of habitat than the non-polyhumic rivers of the park. Both rivers were shallow, with low abundance of aquatic vegetation, bottom sediments with little variation, and practically no differentiation into lotic and lentic habitats in the transverse profile. The low habitat diversity of small polyhumic rivers can be considered another reason for the impoverishment of their fauna. The fauna of the lower Bukowa was much richer in species and individuals than that of the small, fully polyhumic rivers, but poorer than in the upper, non-polyhumic section of the Bukowa and the nonpolyhumic rivers of the park (Stryjecki et al. 2018;Stryjecki 2019). The markedly greater diversity of the Hydrachnidia communities of the polyhumic section of the Bukowa was due to the migration of many species in the longitudinal profile of the river (Stryjecki et al. 2018). Differences in the physicochemical parameters of the water between the upper and lower course of the Bukowa were not significant (Stryjecki et al. 2018), so species inhabiting the non-polyhumic section of the river could also live in the polyhumic part. The Bukowa is a much larger river than the Czartosowa or the Łukawica, with much greater habitat diversity. The presence of numerous meso-habitats, including well-developed lotic and lentic zones, has translated into in a greater diversity of fauna than in small polyhumic rivers.
Four very rare species and six rare species for Poland were found in the polyhumic rivers. Two of the four very rare species (Pseudohydryphantes parvulus and Atractides samsoni) were found only in the small, fully polyhumic rivers, and a third (Nilotonia borneri) was found only in the polyhumic section of the Bukowa. The fourth species (Parabrachypoda modesta) was also present in non-polyhumic rivers. Of the six rare species for Polish fauna, one (Albia stationis) was found only in polyhumic rivers, and the population of another (Thyopsis cancellata) was larger in the polyhumic rivers than in the non-polyhumic rivers. This analysis indicates that polyhumic dystrophic rivers, including small, fully polyhumic rivers, are habitats whose fauna raises the faunistic value of the area.
There were 14 species found in the polyhumic rivers that were not caught in non-polyhumic rivers. Of these, as many as 8 were found only in small, fully polyhumic rivers. Despite this large number of exclusive species, it is difficult to identify taxa preferring polyhumic rivers. Most of the species exclusive to the polyhumic rivers were caught in small numbers, often as single specimens, so they cannot be considered characteristic of these environments. The species exclusive to the polyhumic rivers included stagnobiontic species that should be considered accidental elements rather than characteristic of polyhumic rivers. A particularly high share of stagnobiontic and stagnophilic species was found in the River Łukawica. They were caught there in very small numbers, but are common in other types of water bodies of the park: fish ponds (Stryjecki 2004a;Stryjecki et al. 2015), small permanent water pools and astatic pools (Stryjecki 2004b) and transitional water bodies (Stryjecki 2003). Stagnobionts and stagnophiles found in the Łukawica may have been an indigenous component of the fauna (slow water flow in the river), but may also have been an exogenous element. Water from the fish ponds was periodically drained into the river, so that species from fish ponds could make their way into the river together with the pond water.
Limnochares aquatica was the only species whose presence in the Łukawica can be considered a response to environmental conditions (stagnant, slightly acidic water). The Łukawica had the largest population of Limnochares aquatica of all rivers of the park. The presence of a large population of this species in a river with lower water pH, a dark silty bottom and numerous decaying tree trunks confirms literatures reports of this species' preference for riparian sediments of permanent stagnant waters or pools in streams, often in decaying wood, roots and leaf litter (Biesiadka 1972;Davids et al. 2007). Limnochares aquatica is sometimes referred to as an acidophilic species (Biesiadka 2008). The fact that many Limnochares aquatica individuals were caught in the River Łukawica confirms this species' preference for water with a lower pH (Davids et al. 2007;Biesiadka 2008). Apart from the Łukawica, Limnochares aquatica has been found in large numbers in the dystrophic pond Imielty Ług (Stryjecki 2004a), in transitional waters at sites with a lower pH (Stryjecki 2003), and in the River Bukowa only at a site with a lower pH (Stryjecki et al. 2018). Limnochares aquatica is found in large numbers in dystrophic lakes (Cichocka 2005;Zawal 2007). Analysis of the habitat distribution of Limnochares aquatica in various waters of the Janów Forests Landscape Park and literature data indicate that this species is characteristic not for dystrophic rivers in particular but for dystrophic waters in general.
In the dendrogram of faunistic similarities, we can see a grouping of non-polyhumic rivers (the Branew and Rakowa) and a grouping of the polyhumic section of the Bukowa with its non-polyhumic section. Together, the Branew, Rakowa and Bukowa form a cluster comprising the larger rivers of the park, with diversified habitats, flowing through open areas or in certain sections through wooded areas. Small, fully polyhumic rivers are not grouped into a separate cluster, which indicates the absence of a group of species that could be identified as the core of the fauna characteristic of polyhumic dystrophic rivers. Apart from species caught in single numbers, mainly characteristic of standing water bodies, the taxa caught in somewhat higher numbers, characteristic of flowing water bodies, indicate that generally the same species that were found in non-polyhumic rivers were found in small, polyhumic dystrophic rivers. The basic difference between the two types of river was that much smaller populations of individual species were found in polyhumic rivers than in non-polyhumic rivers.
Hydrachnidia fauna is poor in species and individuals. The populations of most species are very small. The characteristic feature of these rivers is very low species diversity. 2.
The pronounced dystrophy of small polyhumic rivers should be considered a factor restricting the development of larger Hydrachnidia populations. In partially polyhumic rivers: 1.
The water mite fauna is more diverse than in fully polyhumic rivers. The greater diversity of fauna results from the migration of species and individuals from the upper reaches of the river (non-polyhumic) and from greater habitat diversity. General conclusionverification of the research hypothesis: 1.
In the fauna of polyhumic rivers it is impossible to indicate species that could be considered characteristic of these habitats and that distinguish the Hydrachnidia communities of polyhumic rivers from those of non-polyhumic rivers.

2.
The Hydrachnidia communities of polyhumic rivers differ from those of non-polyhumic rivers in terms of population size. Quantitative impoverishment of fauna is observed in polyhumic rivers (especially small, fully polyhumic ones).