Principal threats to the conservation of freshwater habitats in the continental biogeographical region of Central Europe

Freshwater habitat types show significant differences in resilience to threats, pressures and activities as they can occur at a variety of scales and differ greatly in their inherent variability. Both groups of standing and running water bodies within the CBR area in Europe are mainly in danger of habitat loss, which is also the greatest threat to freshwater biodiversity (Čížková et al. 2013; Zorilla-Miras et al. 2014; Hein et al. 2016). A key driver of habitat loss is land-use change (Janssen et al. 2016). Landscapes have changed dramatically in the last 50 years resulting from a combination of factors, including human population growth and rapid technological advancement (Freudenberger et al. 2013). Many studies have highlighted high rates of land-use change since the period after World War II, which has been associated with human population growth and processes such as urban sprawl, mass migration, and agricultural intensification (Van Eetvelde and Antrop 2004; Amici et al. 2015). Similar phenomena, such as urbanization and residential and commercial development, have directly and indirectly affected the catchment areas of the studied freshwater habitats, such as 3110, 3130, 3140, 3150, 3160, and 3260, as well as their structure and functions (Foley et al. 2005, Amici et al. 2015). Biodiversity losses induced by changes in land use are driven not only by urban sprawl and agricultural intensification but also by the abandonment of traditional rural landscapes, which leads to the initiation of natural succession (Agnoletti 2014; Beilin et al. 2014). This trend was observed even in regions with decreasing populations outside of Poland, notably in Italy and eastern Germany (Ustaoglu and Williams 2017). In the case of numerous standing water habitats, another problem is highlighted: whether the ecosystem change is a result of natural succession or human activity (Abell et al. 2019).

Intensive grazing and cultivation in the catchment area was indicated as a cause of the poor condition of the studied habitats (3110, 3130, 3140, 3150, 3160, and 3260). The growing farm and parcel sizes and increasing mechanization have resulted in homogenization of the mosaic-like cultural landscapes and, to a large extent, the synchronized management of activities, resulting in a decrease in land-use diversity (Grzybowski 2014). This result has also led to a reduction in the richness of semi-natural components associated with management practices. In this study, the poor management of forms of nature conservation and absent or improperly directed conservation measures were indicated for habitats 3140 and 3260 (Fig. 4).

We stated that negative impacts on the conservation status of examined habitats 3110, 3130, 3140, 3150, 3160, 3260, and 3270 are caused by transportation and service corridors including parking areas (threats to habitats 3110, 3140, and 3260) and railway lines (threats to habitats 3150, 3130, and 3260). Transportation is associated with an increasingly dense road network, which contributes to the barrier effect leading to biodiversity and ecosystem fragmentation and finally their loss (Hawbaker and Radeloff 2004; Freudenberger et al. 2013). Moreover, an escalation in traffic volume affects biodiversity through noise, artificial lighting, pollution, and other direct variables (Parris and Schneider 2009; Selva et al. 2011). Increased traffic density has been reported (Alkemade et al. 2009; Eigenbrod et al. 2009) as having a negative impact on biodiversity on larger landscape scales within the “road-effect zone”.

Significant threats to freshwater habitats in the CBR have resulted from human-induced changes in hydraulic conditions that have modified entire natural systems. A negative impact on the habitat conservation status has occurred in habitats 3110, 3130, 3140, 3150, 3160, 3260, and 3270 (Figs. 2, 3, 4). Related regulatory issues, such as riverbed incision, lowering of the groundwater level, and changes in catchment land use, are commonly reported as the main causes of biodiversity loss in flood-prone areas (e.g., Hein et al. 2016; Janssen et al. 2016).

A particularly vulnerable group of aquatic habitats to external threats are floodplain water bodies, which are commonly known as biodiversity hotspots. Their high biological diversity is maintained due to mosaics of numerous aquatic habitats (Ward and Stanford 1995; Wilk-Woźniak et al. 2019), and succession is a key natural process in floodplains (Ortmann-Ajkai et al. 2018). The biodiversity of floodplain habitats (mainly 3150) is threatened by numerous factors, such as water regulation, drainage (Tockner and Stanford 2002; Čížková et al. 2013), land-use change (Zorilla-Miras et al. 2014; Hein et al. 2016), overexploitation (Harrison et al. 2010), point source and diffuse chemical pollution from neighbouring agricultural land (Glińska-Lewczuk 2005; Hein et al. 2016), the spread of invasive species (Mölder and Schneider 2011; Hein et al. 2016), and climate change (Tockner and Stanford 2002; Čížková et al. 2013).

A major cause of freshwater species and habitat losses has been channelization of rivers and their associated floodplain habitats (Clarke 2015). Such interactions were indicated mainly for habitats 3260 and 3270 (Fig. 4). Large, lowland river systems are naturally highly connected across their flood plains with small waterbodies, wetlands, and multiple channels that are all connected during periods of flooding and high water (Amoros and Bornette 2002; Grzybowski 2014). A loss of river system continuity in lake habitats (3110, 3130, 3140, 3150, and 3160) and water abstractions from surface waters (3140, 3150, and 3160) were demonstrated in our study, affecting the status of conservation habitats (Fig. 4). This connectivity can provide potential refuge from high flow and pollution events, which is important for maintaining conditions for a range of species (e.g., Amoros and Bornette 2002).

Flooding modifications often affect a mosaic of habitats differing in sediment properties, humidity, abundance, composition, and succession states of fauna and flora, as well as productivity and diversity (Hefting et al. 2013). Flooding modifications were indicated as a threat to habitats 3130 and 3140 and were shown to be an indirect threat to all habitats except 3220 and 3240 (Fig. 4). Due to reduced hydromorphological dynamics, most floodplains in Europe have been degraded. This result has led to, among other things, a decrease in the habitat types that are an essential part of floodplains (Percic et al. 2009).

Habitat conditions in active floodplain areas have often been substantially altered by human impacts, not only by hydrotechnical structures but also by pollution from fertilizers and chemical contaminants, the introduction of invasive species, or by intensive forestry (e.g., Schnitzler et al. 2005; Mitsch et al. 2012). Surface water pollution, reported as a cause of poor habitat conservation status, was indicated for all lake habitats (3110, 3130, 3140, 3150, and 3160). In habitats 3110 and 3260, an important role of discharge from the agricultural or forest catchment was indicated. In habitats 3140 and 3260, the poor management of watercourse maintenance was indicated (Fig. 4). A negative example of the effect of nutrient input by fertilization of adjacent meadows to ditches on phytodiversity was given, e.g., by Müller et al. (2016). Depending on the heterogeneity of humidity, the succession stage of a ditch, and the intensity and frequency of maintenance, ditches are not only characterized by a distinct species composition but they also provide important habitats for rare species and species relevant for conservation (Garniel 2000; Herzon and Helenius 2008). One should remember, however, that the pivotal role of a reclamation ditch is to drain an area towards a recipient (river or lake). Thus, ditches can also accelerate nutrient transfer from meadows and pastures and increase the pollutant load discharged directly to water bodies. As a result, overfertilisation has become a key factor decreasing the conservation status of floodplain lakes (3150) in rural areas (Glińska-Lewczuk 2005).

In view of the above, irregular ditch cleaning with differences in timing, partial cleaning, or half-site cleaning (Garniel 2000) and a cleaning frequency of 2–3 years (Van Strien et al. 1991) have been shown to maximize phytodiversity on the local scale. As nitrogen accumulates especially in irrigation ditches, the removal of biomass after mowing may be favourable for species that are sensitive to nutrient-rich conditions. The strong impact of mowing time and frequency on species composition has been well studied (Manhoudt et al. 2007; Meier et al. 2017), and the temporal diversity of disturbances has been highlighted as especially relevant for regional species diversity in agricultural landscapes (Meier et al. 2017). Therefore, the scope, frequency, and type of maintenance work on the drainage network affect the condition of the examined aquatic habitats that are important for the EU.

Biological invasions, together with habitat disturbance, are among the main causes of biodiversity decline in inland aquatic habitats (Rodríguez-Merinoa et al. 2018). The number of non-native aquatic plant species in Europe has increased since the early 1990s (Keller et al. 2011) as humans are vectors for these species in a highly globalized world. The lack of barriers allows their numbers to continually increase (Havel et al. 2015; Strayer 2010). Additionally, habitat alterations can create invasion opportunities, which can transform natural habitats and thus create new niches, a process that facilitates the establishment of various non-native aquatic species (Zedler and Kercher 2004).

Threats of invasive and other problematic species were not frequently reported in the studied habitats except for habitats 3130, 3240, and 3260 (Fig. 4). The data collected for the needs of our survey do not confirm the occurrence of the following species that have been indicated as threatening the local diversity of freshwater habitats in the CBR in Poland: Cabomba caroliniana, Eichhornia crassipes, Elodea nuttallii, Hydrocotyle ranunculoides, and Myriophyllum heterophyllum (Rodríguez-Merinoa et al. 2018). However, the following species were identified in the littoral zone of the habitats studied, which, in Poland, the Czech Republic (Vardarman et al. 2018), and in central European protected areas (Braun et al. 2016) pose a significant threat to the conservation of biological diversity: Fallopia spp., Heracleum mantegazzianum, Impatiens glandulifera, and Solidago spp. Interestingly, some researchers (Rodríguez-Merinoa et al. 2018) have reported that non-native species are more common in anthropogenic environments than native species. Such environments can be suitable for the formation of unoccupied niches that can be filled by non-native species (Catford and Downes 2010). Therefore, in the case of interference in water habitats, these ecosystems must be protected against the expansion of alien species.

Although the present invasion level is relatively low, the early detection of their localities is essential to eliminate non-native species as sources of reproduction. The rate of spread of indigenous species and the invasion of non-indigenous species are affected by many factors that differ along spatial and temporal scales. Such factors include climate and local weather patterns, vegetation structure, resource availability, the number of species present in secondary regions, propagule pressure, and associated ecosystem processes such as competition, disease, and adaptation (Foxcroft et al. 2007). A positive relationship between Invasive Alien Species (IAS) occurrence and proximity to streams has also been confirmed (Foxcroft et al. 2007; Catford et al. 2011), especially for I. glandulifera (Čuda et al. 2017) and Fallopia spp. (Mandák et al. 2004). These species show a strong preference for such habitats, and streams and rivers subsequently act as spread vectors.

To address the problem of the increasing number of alien neophytes that occur in and around human-made habitats (Lososová et al. 2006; Lambdon et al. 2008), the efforts in terms of prevention, early detection and rapid response, and eradication and management of invasions in Special Areas of Conservation need to be strengthened. A solution can be found in habitat suitability models that have been applied to define the locations most threatened by invasive alien species and to select those areas for regular monitoring (Vardarman et al. 2018).

There is a consensus among many scientists that the earth’s climate is changing and will continue to change at an increasingly rapid pace (Wuebbles et al. 2017). Although the consequences of climate change for freshwater ecosystems are difficult to assess, some of the damaging effects are already clear and likely to increase in both direct and indirect ways (Janssen et al. 2016; Molina-Navarro et al. 2018). Among the indirect effects of climate variability are extreme hydrological phenomena as floods and droughts, which are depicted in Fig. 4.

We only found a direct relationship between poor habitat conservation status and climate change for habitats 3110, 3130, 3140, 3160, and 3260 (Figs. 2, 3, 4). Climate change leads to modifications in the composition, intensity, and frequency of particular environmental elements, which, in turn, lead to increasing threats to extremely rare or even non-existent phenomena in specific areas (Morelli et al. 2016).

The effects of climate change can be also predicted to further impact ecosystems by causing changes in species, phenology, range, and community composition (Chen et al. 2011). According to Pallemearts et al. (2005), assuming even the most optimistic projections of the level of mitigation that can be achieved, freshwater aquatic ecosystems will still be significantly exposed to the impacts of climate change. The European Union published a white paper on climate change adaptation (EC 2009a, b), in which a framework was set out to enhance the EU’s resilience to the impacts of climate change. Although ecosystems are threatened by climate change, they are also part of the adaptation solutions as they perform important services for society such as climate regulation, carbon sequestration, flood protection, and soil erosion prevention. To safeguard these services for society, resilient ecosystems are needed that are able to cope with the impacts of climate change, such as the increased hydrological dynamics caused by weather extremes and the shifting of suitable climate zones.

An increasing problem of water-dependent habitats in the CBR in Europe is the reduction in groundwater levels resulting from natural- and human-induced hydrological modifications and climate change. These same factors are also responsible for the degradation of floodplain systems, particularly riverbed incision and floodplain aggradation (Pataki et al. 2013). Despite the diversity of solutions that can be implemented to replenish water resources or slow down or even reset the degradation of aquatic ecosystems, other co-existing factors may still post threats.

Our results show that even well-protected conservation areas can become focal points for tourism and recreational activities that may reduce habitat quality and biodiversity (Dudgeon et al. 2005). Thus, for most of the global land surface, trade-offs between conservation of freshwater biodiversity and human use of ecosystem goods and services are necessary.