Changes in The VegeTaTion of a small lowland RiVeR Valley (Krąpiel, NW polaNd) after dredgiNg

The paper presented the impact of dredging on the vegetation of a regulated section of a lowland river based on the example of the Krąpiel River (NW Poland). The field research with pre-investment monitoring was conducted over three vegetation seasons in the years 2008–2010. Mechanical dredging and alignment of the river bed caused changes in vegetation structure. Reducing the diversity of habitat contributed to observed impoverishment of plant communities and species that build them. In the first year after dredging, communities from class Bidentetea tripartitae (chenopodietum rubri) were dominant along the flood terrace covered with a layer of dredging material. In the second year after dredging, * University of Szczecin, Faculty of Biology, Department of Plant Taxonomy and Phytogeography, ul. Wąska 13, 71-415 Szczecin, Poland, e-mail: ditta@univ.szczecin.pl ** University of Szczecin, Faculty of Biology, Department of Invertebrate Zoology and Limnology, ul. Wąska 13, 71-415 Szczecin, Poland *** Maria Curie-Skłodowska University, Department of Zoology, Akademicka 19, 20-033 Lublin, Poland **** University of Life Sciences in Lublin, Animal Ecology and Wildlife Management, Department of Zoology, Akademicka 13, 20-033 Lublin, Poland 168 E. Stępień, A. Zawal, P. Buczyński, E. Buczyńska terophytes were again replaced by rush communities, dominated by phalaridetum, phragmitetum and locally riparian tall herb communities.


study area
The Krąpiel River, located in north-west Poland, is the largest right tributary of the Ina River.With a length of 65 km, it is classified as a small river.The area of the Krąpiel River catchment, mainly located in the Stargardzki District (Zachodnio pomorskie Province), amounts to 596 km 2 .From its sources to the Changes in The Vegetation of a Small Lowland River Valley… Kania, it has the character of a lowland loess or loamy stream.From the Kania to its mouth, it is classified as a lowland gravel river.The river mostly flows through agricultural areas.Along its substantial section, it flows in a postglacial channel.From Pęzino, it transports water through the moraine plateau area in a deep gorge valley.Almost throughout its length, the Krąpiel River valley is overgrown by deciduous forests.From the sources to the Krzywnica, it flows through a Natura 2000 area -Ostoja Ińska.The mouth section of the river is under protection as Natura 2000 area called Dolina Krąpieli (Raport... 2010).
Along the analysed section, the river flows through a complex of large fish ponds, adjacent to extensive areas overgrown with rush vegetation, regularly flooded in spring.Water inflow in the ponds is regulated by means of a dam located north of the Krzywnica village.The river here has a character of a slowly-flowing canal (current velocity from 0.002 to 0.16 m/s) with a depth of approximately 0.8 m, and mean width of 5 m.Before the investment, the river had muddy bottom, locally entirely overgrown by vegetation.The flood terraces with a width of approximately 5 m and relatively steep slopes were occupied by rush communities, and convolvuletalia sepium communities in the upper parts.Bushes and tree stands dominated by willow developed locally at the water.The dredging of the riverbed combined with clearing of trees and bushes occupying its banks was performed in 2008.The resulting dredging material was mostly deposited on the right bank of the river.

material and methods
The field research was conducted over three vegetation seasons in the years 2008-2010.Pre-investment monitoring was conducted during the first year of the research.Over the two following years, changes in the vegetation after dredging were observed.The research concerned the section of the Krąpiel River below the dam in Krzywnica, from the former railway bridge to the fork of the Krąpiel and Stara Krąpiel Rivers (N: 53.422909°; E: 15.193214° to N: 53.408067°; E: 15.198897°) (Fig. 1).Phytosociological relevés were performed along the studied river section by means of the Braun-Blanquet method (1964) with additional categories (Barkman et al. 1964).The species nomenclature was adopted following Mirek et al. (2002).The classification of plant communities was adopted following Brzeg and Wojterska (2001).A database was created with the help of the software Turboveg for Windows (Hennekens, Schaminee 2001).Herbarium collections were deposited in the herbarium of the Department of Plant Taxonomy and Phytogeography of the University of Szczecin.

Results
Over the three years of the study, the occurrence of 25 associations and communities of plants was recorded along the studied section of the Krąpiel River, belonging to 11 classes, 12 orders, and 15 alliances.Small fragments of shrubs dominated by willow developed on the river banks, particularly in the lower course of the analysed section.Patches of Salicetum triandro-viminalis containing Salix viminalis and young Salix fragilis showed high coverage in the herbaceous layer of species from class artemisietea, particularly from order convolvuletalia sepium, such as urtica dioica, aegopodium podagraria, Glechoma hederacea, calystegia sepium, anthriscus sylvestris, and Galium aparine, similarly as small patches of Salicetum cinereae.The willow shrubs are systematically removed from the banks of the studied river section in order to facilitate water flow.The shrubs occurring around a seep with a character of a small limnocrene were unaffected by dredging.

Communities from class Lemnetea minoris
In 2009, only sinusions of lemna minor were observed, occupying small areas between helophytes, rarely developing patches with an area of more than 0.5 m 2 .They only developed in August.This was related to the clearing of aquatic vegetation in the river channel and an increase in the water flow velocity.In 2010, already in July, along sections of the river with a lower discharge, pleustophytes occupied even 70% of the water surface.Phytocoenoses of lemno-Spirodeletum polyrhizae anchored among patches of rooted vegetation were dominant.

Communities from class Potametea
The river channel, not shaded by trees and bushes in many places, was overgrown by patches of vegetation from class potametea.The largest areas of the muddy and muddy-sandy bottom, particularly in shallow parts of the river not shaded by vegetation, were covered by submerged dense patches of Elodeetum canadensis, often developing almost single-species aggregations.Similar phytocoenoses were recorded in the Cybina River Valley (Gołdyn et al. 2005).Due to the frequent contact of patches of the association with Sagittario-Sparganietum emersi, arrowhead and European bur-reed occurred in the patches with various coverage.Species from genus callitriche were sometimes encountered.
In particularly shallow places, on sandy or sandy-muddy bottom, phytocoenoses of species-poor association of ranunculo-callitrichetum polymorphae developed.They formed small, poor in species, convex patches with low contribution of accompanying species, particularly Elodea canadensis, Sparganium emersum, and Sagittaria sagitifolia, similarly as recorded in the Kwacza River (Osadowski 2009).On substrate with contribution of gravel and stones, at faster river current (x m/s) and depth of 0.2 m, larger patches with a high contribution or even prevalence of Fontinalis antipyretica developed.
No Elodeetum canadensis or ranunculo-callitrichetum polymorphae were observed along the studied section of the Krąpiel River after dredging.Elodea canadensis only occurred in low coverage in patches of other communities from class potametea and in the rush of arrowhead and European bur-reed.Scarce patches of potametum lucentis were recorded, containing only one typical species, namely potamogeton crispus.
In faster current, on sandy-gravel bottom, phytocoenoses from nuphar lutea fo.submersa developed, accompanied by species typical of the alliance of ranunculion fluitantis -Sagittaria sagittifolia var.vallisneriifolia and Sparganium emersum fo.fluitans.Patches of the community were only observed at one site behind the bridge.It was not destroyed during dredging.Small patches of nymphaeo albae-nupharetum luteae were only recorded at two sites.They consisted of nuphar lutea with a high contribution of species from class phragmitetea.In 2008, they were observed at two sites.After dredging, the area of the patches was reduced.Changes in The Vegetation of a Small Lowland River Valley…

Rush communities from class Phragmitetea
Rush communities were the most important component of the vegetation of the analysed river section.Phytocoenoses of Sagittario-Sparganietum emersi developed throughout the river current's cross-section.They were quite poor in floristic terms.Low rushes consisted of both of the species typical of the association, accompanied by other species typical of class phragmitetea in low coverage, such as Butomus umbellatus and phalaris arundinacea.On the water surface, pleuston species were anchored among the plants.Dredging only reduced the coverage of the association for a short time.Already in August 2009, its patches developed throughout the river current's cross-section in many places.
Scarce small patches of Oenantho aquaticae-roripetum amphibiae developed on the shallow river banks before and after dredging.They were dominated by roripa amphibia, accompanied by scarce rush species in low coverage.
In the bank zone, in places with stagnant water, Glycerietum maximae rush was encountered.Great manna grass, dominant in the patches, was particularly accompanied by other rush species.The rush was not observed in the first year after dredging due to the steep slopes of the river channel.After dredging, no patches of associations of Butometum umbellati or acoretum calami were recorded either.The phytocoenoses developed in the form of small patches on the edge of the channel, in contact with other rush communities.

Communities of terophytes from classes Isoëto durieui-Juncetea bufonii and Bidentetea tripartitae
In the first year after dredging, communities from class Bidentetea tripartitaepatches of chenopodietum rubri were dominant along the flood terrace covered with a layer of dredging material.In physiognomic terms, they were distinguished by the dominance of polygonum lapathifolium subsp.lapathifolium, frequently developing dense patches.Among typical species from class Bidentetea, only rorippa palustris showed higher coverage.The patches contained numerous species from class artemisietea, although few of them sometimes showed higher coverage, such as Elymus repens, Myosoton aquaticum, and urtica dioica.Relatively high coverage of rush species was observed, namely phragmites australis or phalaris arundinacea.
Patches of the community, developed before dredging on the fragment of the flood terrace cleared from bushes, and observed in the second year after dredging on the bank of the analysed section, were distinguished by a high contribution of numerous species from class Bidentetea tripartitae, and contribution of species from class isoëto durieui-juncetea bufonii -juncus bufonius, cyperus fuscus, and Gnaphalium uliginosum.Species from classes artemisietea were very scarce, except for species in allince with calystegion sepium.
Patches of juncetum bufonii developed right behind the ponds in the first year after dredging.They were observed on a low fragment of the flood terrace cleared from vegetation, covered with a thin layer of dredging material on which water stagnated for a certain time.They were dominated by juncus bufonius.The abundant accompanying species included those typical of class Bidentetea tripartiae.Changes in The Vegetation of a Small Lowland River Valley…
Spaces amongst concrete grid paving units along the dirt road were occupied by patches of the carpet community (lolio-plantaginetum association) dominated by two typical species -lolium perenne and plantago major.They developed on a relatively low levee on humid levelled substrate in contact with communities from class artemisietea and Epilobietea angustifolii, but sometimes also phragmitetea (phalaridetum arundinaceae).The phytocoenoses were particularly represented by patches of associations from alliance calystegion sepium.They developed on the boundary of rushes near the top of the flood terrace, under single trees Salix and alnus, and in fringes of fragmentarily developed thickets of Salicetum triandro-viminalis.The most common association was urtico-convolvuletum sepium, consisting of shoots of nettle urtica dioica, with abundant calystegia sepium and Galium aparine.They were accompanied by scarce other species from class artemisietea, such as cirsium arvense, anthriscus sylvestris, Glechoma hederacea, Elymus repens, and species from rush contact phytocoenoses, namely phragmites australis and phalaris arundinacea.Scarce patches of Eupatorietum cannabinii were observed of edges of scarps, with numerous representation of species from alliance calystegion sepium.Phytocoenoses of Soncho palustris-archangelicetum litoralis were distinguished in physiognomic terms due to the abundant aspect of angelica archangelica subsp.litoralis.They were characterised by a considerable contribution of species from alliance calystegion sepium, namely calystegia sepium, carduus crispus, Symphytum officinale, Stachys palustris, and Epilobium hirsutum.Patches of communities from alliance calystegion sepium were abundant before dredging.In the first year after dredging, they only fully developed in places not subject to melioration works.In the second year, they were frequently recorded.

Communities from class Artemisietea vulgaris
In the second year after dredging, in the upper parts of the levee, scarce patches of an community from class artemisietea were observed, dominated by cirsium arvense.They developed in contact with phytocoenoses from alliance calysteg-ion sepium and canary grass rush on disturbed substrate covered with a layer of dredging material.The patches showed high contribution of species from alliance calystegion sepium, but also phalaris arundinacea from contact communities.

The structure of vegetation before dredging
Before dredging, the largest areas in the middle of the river channel were occupied by the association Sagittario-Sparganietum emersi, accompanied by patches of Elodeetum canadensis.At two sites, the occurrence of yellow water-lily was recorded.It occurred in the submerged form, developing an community with nuphar lutea fo.submersa.It also developed small patches of nupharo-nymphaeetum association.On stones near the former railway crossing and near the small bridge, a ranunculo-callitrichetum polymorphae association developed.The presence of numerous scroll ridges permitted the development of diverse rush communities on their surface (phragmitetum communis, phalaridetum arundinaceae) and in stagnant waters between them, at the foot of the scarp (Butometum umbellati, acoretum calami, Glycerietum maximae).On the boundary of the rush communities at the bank of the channel, patches of Oenantho-rorippetum association were recorded.lemnetum minoris and lemno-Spirodeletum polyrhizae phytocoenoses developed between the emerged plants.Locally on the surface of the flood terrace, in the close vicinity of the river, on the aggradate muds deposited by high water levels, scarce patches of summer terophyte communities developed, namely chenopodietum rubri.The phytocoenoses were recorded at sites where bushes were cleared in the previous year for the purpose of unblocking the river channel.
Common reed and canary grass rushes developed on the surface of the flood terrace.In the upper parts of the scarp, they were in contact with riparian tall herb communities (Soncho palustris-archangelicetum litoralis, urtico-convolvuletum sepium).The riparian tall herb communities sometimes developed on the edge of the flood terrace (Eupatorietum cannabini and Soncho palustris-archangelicetum litoralis, and more seldom urtico-convolvuletum sepium).On the top of the scarp, riparian tall herb communities or rush communities were in contact with phytocoenoses of carpet community lolio-plantaginetum developed on the dirt road.calamagrostietum epigeji community only developed on the deposited material near the bridge and road.Changes in The Vegetation of a Small Lowland River Valley… On the flood terrace, in certain fragments of the river, willow shrubs developed (Salicetum cinereae, Salicetum triandro-viminalis).Amongst the shrubs at one site in the embankment, cardamino-chrysosplenietum association was recorded.The river banks were locally overgrown by trees -willows and alder, but their low density permitted abundant growth of vegetation in the river current and at the banks.
Such a distribution of vegetation was determined by the location of the analysed section between extensive fish ponds, and by its regulation involving maintenance measures (removal of selected trees and bushes, mowing of scarps).

The structure of vegetation after dredging
Conducting maintenance works in winter (mechanical dredging and desludging of the bottom, profiling the flood terrace and deposition of the dredging material on it, and considerable clearing of trees and bushes) resulted in a substantial change in the vegetation cover of the river valley.
The implemented investment caused changes in the structure of the river channel and the character of the occurring vegetation.The width of the river channel remained unchanged, but its bottom and slopes were levelled, similarly as the surface of the flood terrace, where the obtained dredging material was additionally deposited.The modification of the structure of the river channel involved the destruction of its herbaceous vegetation and clearing of the belts of willow shrubs obstructing the water flow.Only single trees remained.Before the investment, the river bottom showed quite varied character due to numerous scroll ridges and diverse vegetation.It was largely covered by a thick layer of mud and loam.In some fragments, the bottom was sandy or gravel-sandy.After the investment, the levelled bottom was mostly sandy or sandy-loamy with a slight contribution of mud (Zawal et al. 2014;Szlauer-Łukaszewska, Zawal 2014).The transformations of the structure of the river channel and floor terrace, involving a reduction in the diversity of the habitats of the river, affected the character of the occurring vegetation.
The levelled surface within the channel was dominated by the Sagittario-Sparganietum emersi association.The first plants were observed in May.They covered almost the entire river channel in August.The recolonisation occurred from the banks towards the middle of the channel (Henry, Amoros 1996).Patches of nymphaeo albae-nupharetum lutea, and community with nuphar lutea fo.submersa developed at sites with weaker current, smaller than in the preceding year, were only observed at sites unaffected by dredging.Duckweed communities in the form of small sinusions of lemna minor were weakly represented.They only appeared in high abundance in the second year.Small patches of Oenantho aquaticae-roripetum amphibiae were observed on the boundary of the channel.After dredging, scarce and not recorded before patches of potametum lucentis appeared.They consisted of potamogeton crispus, preferring habitats with gravel substrate and faster water flow, as observed in the Danube River in Hungary (Engloner et al. 2013).The relevant habitat conditions in the Krąpiel River channel were recorded in the vicinity of bridges.
No patches of ranunculo-callitrichetum polymorphae were encountered after dredging.No specimens from genera callitriche or Fontinalis antipyretica in patches of other communities were recorded, either.Mechanical dredging and levelling of the river channel eliminated microhabitats favouring the development of such phytocoenoses.Only in the second year after dredging, the presence of callitriche cophocarpa was recorded in the lower course of the river, where it was not encountered before.Diaspores of the species probably came from the upper, non-dredged section of the river.No patches of Elodeetum canadensis were found, although Canadian waterweed appeared in low numbers in other communities in the second half of the summer.Its presence in patches of communities with nuphar lutea preserved during dredging draws particular attention.From this section of the river, it can colonise areas located below by means of its vegetative reproductive organs (Van der Valk 1992;Goodson et al. 2001).
Patches of chenopodietum rubri from class Bidentetea tripartitae developed on the flood terrace, constituting the dominant component of the vegetation already from May of the first year after dredging.They covered the layer of the deposited material, containing terophyte diaspores in the forms of soil deposit, as also observed by Assini (2001).A high contribution in the patches was reached by rush species cutting through the deposited layer, namely phalaris arundinacea and phragmites communis.In the lower course of the river, on the low surface of the terrace, a patch of juncetum bufonii developed.In the second year after dredging, terophytes were again replaced by rush communities, dominated by phalaridetum and phragmitetum.Only one patch of chenopodietum rubri was recorded.It developed on the bank of a canal flowing into the river, freshly uncovered during the maintenance works.Changes in The Vegetation of a Small Lowland River Valley… Communities from association chenopodion are typical of larger rivers (Borysiak 1994).Banks of smaller rivers such as the Krąpiel River are dominated by communities from alliance Bidention (Stępień 2010).A high contribution of species from class Stellarietea mediae and artemisietea, and the presence of alien species suggest their anthropogenic character.
The coverage of willow shrubs considerably decreased.Bushes growing within the river channel and at the banks were removed.Only single small trees and bushes growing further on the terrace remained.Larger fragments of the shrubs were only preserved in the lower course of the river, particularly around the small limnocrene with vegetation with wellhead character -cardamino-chrysosplenietum.
In the first year after dredging, scarce rushes developed at sites with less destroyed surface.Their patches frequently contained high contribution of species from class Bidentetea tripartitae.The cutting and levelling of the channel slopes resulted in a decrease in the diversity of the occurring rush communities.No patches of acoretum calami or Butometum umbellati were encountered, and scarce patches of Glycerietum maximae only developed in the second year.Directly after dredging, radical changes in the cover of riparian tall herb communities from class artemisietea vulgaris were observed.Their small patches only developed on unaffected surfaces.In the second year, abundant reconstruction of the phytocoenoses was observed.A new community also developed dominated by cirsium arvense, not recorded before dredging.

discussion
The investigated stretch of the river valley, despite regulation and its location between fish ponds, has retained many of its valuable natural features.Owing to its relatively high habitat diversity for this type of transformed stretch of river, prior to the dredging 22 plant communities belonging to 10 classes and 14 alliances formed here.The most valuable communities observed in the river channel were of the alliance ranunculion fluitantis, associated with riffles -sites with a fast current and a coarse substrate.Habitats of type 3260 are protected under the European network Natura 2000 (Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora).They represent an impoverished form (Puchalski 2004), without species of the genus Batrachium, but they increase the ecological value of the river.Unfortunately, following the dredging no patches of ranunculo-callitrichetum polymorphae were noted.The works led to the disappearance of sensitive phytocenoses and species, such as callitriche polymorpha and Fontinalis antypyretica, which has also been observed in Danish watercourses (Baattrup-Pedersen et. al. 2002;Pedersen et al. 2006).Patches of a 'ribbon-like leaves' form (community with nuphar lutea fo.submersa) remained, as they were not destroyed during the dredging due to their location near the bridge.It should be emphasized, however, that communities of this type are observed in watercourses subject to human impact (Nowak, Nowak 2010).
Although the floodplain was not very wide, after the spring high water autogenic patches of therophytes of the classes isoëto durieui-juncetea bufonii and Bidentetea tripartitae, which are valuable in the river ecosystem, occupied habitats protected by the European Union (3270 -flooded muddy river banks).The patches of communities of the class Bidentetea observed after the dredging, formed on the surface of the dredging material deposited on the floodplain, had a disturbed structure and species composition, which is indicative of their anthrpogenic character (Brzeg, Ratyńska 1983;Borysiak 2004).
Most of the communities observed in the area studied are characterized by natural syngenesis (Brzeg, Wojterska 2001).Examples include xenospontaneous associations of Elodeetum canadensis and acoretum calami.Some of the communities observed are perdochoric, with their range decreased by human activity.They included the associations nymphaeo albae-nupharetum luteae, cardamino-chrysosplenietum alternifolii and Sagittario-Sparganietum emersi (loc.cit.).However, Pedersen et al. (2006) observed substantial Sparganium emersi cover in watercourses from which vegetation was removed regularly.Following the dredging the Krąpiel river bed quickly became overgrown by arrowhead and European bur-reed.
The dredging caused a decrease in plant diversity, as a direct result of the destruction of plant cover and simplification of the river profile (levelling of the bottom and floodplain and straightening of the banks, as well as deposition of dredging material on the floodplain).A significant factor affecting vegetation is the degree of human impact and the topography of the river bed (Pedersen at all. 2006;Brookes 1986).The transformation and reduction in habitat heterogeneity on the one hand eliminated sensitive phytocenoses and communities associated with a varied littoral zone, while on the other hand it enabled the spread of anthropogenic communities and the penetration of numerous synanthropic species (Stępień et al. 2016).