Bachelor thesis, 15 hp
Examensarbete i biologi, kandidatnivå, 15 hp Vt 2021
The significance of tributary mouths for species richness
and composition in riparian vegetation of regulated
rivers
Walter Gezelius
Abstract
River regulation cause unnatural water flow patterns which disrupt succession, survival and dispersal of riverine plant communities. Riparian zones in regulated rivers are generally more species poor and have a lower vegetation cover compared to free-flowing rivers. Tributary mouths within the impoundment however, are not only affected by processes in the main channel, but also affected by processes such as sediment dynamics and flooding regime from the tributary which may more reflect a natural regime and may therefore not be affected by hydropeaking to the same extent. Hence, tributary sites and the accompanied tributary mouths may represent hotspots for high diversity plant communities in regulated rivers. The riparian habitat is unique in its interaction with adjacent ecological systems and is therefore considered important for the riverine ecosystem’s ecological functioning. The aim of the study was to evaluate the significance of tributary sites and accompanied tributary mouths as a hotspot for diverse plant communities in regulated rivers. Additionally, geomorphological features were analyzed to access the impact of hydropeaking in sheltered and non-sheltered tributary sites. Data was acquired from sampling sites within the Umeå and Luleå rivers, representing both tributary mouths and non-tributary mouth reaches. Inventories included presence of riparian and aquatic vascular plant species, vegetation cover and soil
composition. The results indicate a less extensive impact of hydropeaking in tributary mouths compared to non-tributary mouth reaches. The tributary mouths had a higher species richness, diversity and vegetation cover when compared to that of the non-tributary mouth reaches. This supports the concept of tributary mouths being hotspots for plant diversity. Moreover, the sheltered tributary mouths had a higher species richness than the non-sheltered tributary sites, suggesting tributary shelter as a contributing counter of hydropeaking effects.
Table of contents
1. Introduction...1
1.1 Energy production and hydropower………..1
1.2 Riparian zones and ecological function………1
1.3 Effects of water regulation………..2
1.4 Tributary sites and tributary mouths………...3
2. Method………...4
2.1 Study design……….4
2.2 Sampling……….4
2.3 Data analysis………5
3. Results……….6
3.1 general results...6
3.2Difference in species richness and vegetation cover...8
3.3 Soil type………...13
3.4 Simpson’s diversity index………14
3.5 Multidimensional scaling………15
4. Discussion………16
5. Acknowledgements……….18
6. References……….19
1
1. Introduction and background
1.1 Energy production and hydropower
Hydro electrical power is of vital importance for the switch to a fossil free energy production in Sweden. With a rapid increase of global energy demand due to an increasing human population, much of the energy has globally been produced from non-renewable sources with fossil fuels and nuclear power mainly utilized (REN21, 2020). These types of energy
productions are directly linked to environmental consequences and ecosystem degradation in the form of ecosystem functioning and declining biodiversity (Bejarano, Jansson and Nilsson, 2017). However, during the last decade, much focus has been added towards renewable energy, with hydropower as one of the leading sources (Bejarano, Jansson and Nilsson, 2017;
REN21, 2020). At the end of 2019, the estimated renewable energy share of the global electricity production was 27,3% (REN21, 2020). More than half of which was based on hydropower, which is a substantial portion of the renewable energy sources. Furthermore, hydropower will likely play a large role in the future of energy production in order to reach the goal of conversion to zero-carbon emissions, set by the Paris-agreement (REN21, 2020).
With the worldwide spread of the Covid-19 virus in 2020 and its following detrimental effect on economies and societal functioning, renewable energy sources such as hydropower saw an increase in demand due to low operating costs as well as advantageous access to electricity networks (REN21, 2020). This indicates that renewable energy such as hydropower could be an important energy source in the face of future global instabilities, and may well be an important objective for post-Covid economic turnaround, while simultaneously achieving a broader role in global electricity production.
Geographically the prerequisites for hydropower differ, and therefore only enables certain countries to adequately establish and efficiently produce electricity from this source.
Northern Europe, Asia, eastern Africa, and northern America are places projected with large potential for further development of hydropower (REN21, 2020).
Hydropower is considered a renewable source of energy, have a growing demand and may well be one of the main source for energy demands in the future. Despite this however, the benefits of hydropower pose a great threat towards many riverine ecosystems, where environmental alternations result in habitat loss (Bejarano, Jansson and Nilsson, 2017). To produce energy when it is needed, dams are created to regulate the water flow in rivers. The construction of dams accompanied by hydropower causes a discontinued longitudinal
movement of organisms, submerged landscapes, interruption of biochemical cycles, inundate water temperatures and modified flow patterns (Bejarano, Jansson and Nilsson, 2017). The water regulation associated with hydropower threaten many unique ecosystems, one of which are riparian zones.
1.2 Riparian zones and ecological function
Riparian zones, also known as a transition zone, describes interfaces located between distinct ecological systems separated by spatial, hydrological, or geomorphological differences. The riparian zone is defined as the part of the stream channel located within the highest and lowest water levels in a hydrological network, as well as terrestrial areas upland which may be influenced by the highest elevated water level (Naiman and Decamps, 1997).
These zones obtain specific chemical, biotic, energy and nutrient attributes and are unique in their interaction with surroundings and adjacent ecological systems. The cross-system interactions vary, depending on spatial and temporal scales, which are largely controlled by the resource difference between the adjacent ecosystems. Transitions zones regulate energy and nutrient supply as well as material fluxes between ecosystems. As a result, these areas are often prone to high biodiversity and generate critical habitats for rare species dependent on specific environmental conditions (Naiman and Decamps, 1997).
2
Riparian zones are complex ecosystems which function as interfaces between terrestrial and aquatic ecosystems, as well as fluctuation zones for nutrients and energy (Czortek, Duderski and Jagadodzinski, 2020). The species dynamics and composition of riparian zones are mainly determined by fluvial processes associated with river flow patterns, flooding, and hydrological connectivity (Wollny et al.., 2021).
The riparian vegetation, which along rivers consist of large proportions of biomass, has a vital role in numerous functions of the fluvial ecosystem. One of which is the influence on the stream water chemistry. The influence on water chemistry involves functions such as chemical uptake, denitrification, supply of organic material to surrounding soils and water channels, regulating water movement and stabilizing soils through root systems (Dosskey et al.., 2010). The root systems of the riparian vegetation which often trails into the streams, as well as deadwood provided have also been shown to serve as an essential habitat for various macroinvertebrate species (Milner and Gloyne-Phillips, 2005).
Aquatic plants, known as macrophytes, also have a large role in the overall water quality and ecosystem functioning in a river by stabilization sediment, increasing water clarity,
decreasing nutrient overload as well as providing food, habitat and refuge for aquatic organisms such as macroinvertebrates, zooplankton and fish (Jong-Yun et al.., 2014; Zhang et al.., 2017).
1.3 Effects of water regulation
Currently, studies on effects of river regulation have mainly been focused on fish and macroinvertebrates. Here, fish habitat, population size, feeding habits and behavior has changed, leading to species degradation in the form of reproduction and survival (Schmuts et al.., 2014). Macroinvertebrates have seen decreased population sizes and diversity due to hydraulic stress and changes in substrate composition (Leitner, Hauer and Graf, 2017).
However, riparian zones are very, if not more, susceptible to anthropogenic impact, mainly from urbanization and water regulation (Czortek, Duderski and Jagadodzinski, 2020; Wollny et al.., 2021). With the regulation of rivers, large-scale ecological impoverishment has been observed in the form of fragmentation and impediment of natural disturbance dynamics (Omelshuk and Bohadan, 2015; Rivaes et al., 2015, Jansson et al., 2000). Resulting in detrimental effects on the riparian ecosystem, with limited function and scarce
representation of high diversity plant communities (Wollny et al., 2021; Jansson et al., 2000).
These ecological changes can be attributed to geomorphological changes as well as hydrological effects on sediment deposition, connectivity, flow patterns, flooding, and nutrient distribution (Wollny et al., 2021; Rivaes et al., 2015; Jansson et al., 2000b). One of the main causes for the hydrological disturbance effects are hydropeaking, whereby water is discontinuously released from dams in the regulated river, resulting in a non-natural water flow pattern downstream with increased short time fluctuations (Figure 1).
Figure 1. Yearly (A) and weekly (B) water flow in the regulated Umeå River, with occurring hydropeaking, in comparison to the natural flow pattern of Vindel River. (Bejarano, Jansson and Nilsson, 2017).
3
The flow pattern, with factors such as frequency, duration, temporal occurrence and magnitude govern ecological processes in ecosystems connected to the river system.
Modification of the natural flow pattern results in an impairment of native plant species distribution, survival, dispersal, and reproduction (Bejarano, Jansson and Nilsson, 2017).
The dispersal and succession of riverine vegetation relies upon a wide connectivity within the hydrological network creating a cross-habitat corridor. Within regulated rivers, it has been noted that the dispersal of riparian vegetation diminishes, leading to distinct community compositions within different impoundments, compared to unregulated rivers with a more homogenous composition along the rivers (Jansson et al., 2000a).
This is likely due to fragmentation by dams and lower flow velocities in regulated rivers limiting dispersal. (Andersson, Nilsson and Johansson, 2000). Additionally, plant species with different dispersal traits have a largely differentiated colonization capability. Species with good dispersal capacity such as long floating time, are the best colonizers along storage reservoirs in regulated rivers. This is likely due to dispersal barriers of species with short floating time in the onset of site extinction (Jansson et al., 2000b). However, in free-flowing rivers, short floating species would seemingly not be affected to the same extent, since spring floods would transport seeds long distances regardless of floating time (Jansson et al., 2000b).
It is unfortunate that the effects of hydropeaking on riverine and aquatic vegetation have not received more attention since they constitute such a broad role in a river’s ecosystem
functioning.
1.4 Tributary sites and tributary mouths
River tributary sites with associated riparian habitats are commonly characterized by diverse plant communities due to persistent supply of moisture and nutrients as well as variation in discharge and flooding events (Ribeiro, Blanckaert and Schleiss, 2012). The confluence, whereby the tributary meets the mainstream is often referred to as the tributary junction, whereas in this article will be referred to as the tributary mouth.
The tributary mouth is a unique habitat, differing from the upstream and downstream reaches by attaining a larger sediment deposition consisting of erosion-resistant debris deposits, stratified alluvial deposits and high sediment loads from flash floods (Benda L et al., 2004). These sediments often create depositional fans in the tributary mouth, resulting in a low gradient heterogenic habitat with high nutrient supply (Benda L et al., 2004).
The habitat associated with tributary mouths should provide excellent conditions for numerous riparian plant species and may represent vegetation hotspots in riverine ecosystems. However, depending on geomorphological factors in the confluence of the tributary and main river the tributary mouth formation may have different attributes.
Whether the tributary mouth is sheltered from high intensity flow, ice cover and erosion or not, could determine sediment accumulation rate, nutrient supply and establishment of high diversity plant communities (Figure 2). These shelters could involve boulders, fallen trees or material deposits forming shallow banks in proximity of the tributary mouth.
The aim of the study is to determine the significance of tributary mouths as hotspots for plant diversity in regulated rivers, as well as to assess the impact of river regulation on the riparian ecosystem. Additionally, the study aims to provide suggestions for future protective measures to reduce the impact of hydropower on riverine ecosystems.
The hypothesis is that (1); tributary mouths function as a hotspot for complex plant species communities in river systems, (2); sheltered tributary mouths have a higher plant diversity than non-sheltered tributary mouths.
4
Figure 2. Run-off river impoundment with associated sheltered and exposed tributary mouths.
1. Method
2.1 Study design
Five run-off-the river impoundments were selected in the Umeå (Stornorrfors, and Bjurfors övre) and Luleå (Vittjärv, Laxede and Porsi) (figure 3). All impoundments are characterized by hydropeaking (Figure 3). In each impoundment, between three and six tributary mouths with varying size, sediment deposition, depth, and amount of shelter from erosion was selected. A total of 19 tributary mouths was included in the study. Additionally, a matching selection of adjacent non-tributary mouth impoundment shorelines was selected to compare diversity between tributary mouth sites and non-tributary mouth sites (a total of 9 sites).
Further, the tributary mouth sites were divided into sheltered and non-sheltered sites, with 11 non-sheltered sites and 8 sheltered sites. The tributary mouth sites were also divided based on wether the sites were in a delta landscape or not. Here there were 4 non-delta sites and 15 delta sites.
Onwards in the report tributary mouths will be referred to as tributary sites, and adjacent non-tributary mouth impoundment shorelines will be referred to as non-tributary sites.
2.2 Sampling
Sampling at each site consisted of the following:
• Presence and cover of vascular plants in six 0.25-m2 plots distributed at three levels (low, intermediate and high) between the highest and lowest occurring water levels.
Further, percent cover of different substrate size classes was noted.
• Presence of vascular plants in 1, 5 and 25m wide transects of the riparian zone, spanning between the highest and lowest occurring water levels. 1 and 5 m transects were nested in the 25 m transect.
5
• Presence of aquatic macrophytes in a 25 m wide transect, sampled using waders and a rake.
• At each tributary site, sampling of vascular plants of the riparian zone in a 100m reach going up in the tributary, with a starting point at the tributary mouth.
Species nomenclature follows Krok and Almquist (1994) and water dispersal strategy of plants e.g short or long floaters follows Jansson et al., 2000b. Morphological groups also follow Jansson et al., 2000b.
Figure 3. Map of sampling sites in the Umeå and Luleå River.
2.3 Data analysis
The data analysis consisted of differences in species composition, morphological groups, vegetation cover, soil types, species richness and diversity between tributary sites non-
tributary sites, as well as differences in species richness between exposed and sheltered, delta and non-delta sampling sites. (table 1).
The morphological groups used in this study were graminoids, herbs, shrubs and trees.
Specific species were analysed between the tributary sites and the non-tributary sites. The specific species involve riparian specific species and short floating species. Short floating species were defined as species with a seed floating time less than 2 days.
In this report, the definition of a riparian species, or a species tied to the riparian zone, refers to any species which are commonly found in proximity of rivers, streams, lakes or saturated soils (Naturhistoriska riksmuseet 2004).
6
Species richness was defined as the number of species occuring in the used sampling method.
The data analysis was also structured based on the significance of environmental variables for variation in species composition and richness as well as similarities in species
composition.
Method of analysis were Simpson’s diversity index, Multidimensional scaling (MDS), Student T-test and Excel algebra (table 1). Software used was Excel version from 2016 and SPSS version 27.0.0.0. The Alpha value for all the statistical analysis done were set to 0.05.
Simpsons diversity index were used to calculate relative species abundance in the tributary sites and non-tributary sites based on data from the 25m transect (Eq 1).
Eq 1. 𝐷 = ∑ (𝑛𝑖(𝑛𝑖−1)
𝑁(𝑁−1))
𝑅𝑖=1
𝑛𝑖 Stands for the number of individuals of each species. N stands for the total number of individuals for all species.
In this equation, an occurrence of a species in a specific site accounts for one individual. The sum of all the sites therefor represent the population size, not the number of individuals at each site.
Relative abundance for each species were calculated (Dspecies) using equation 1.
The Multidimensional scaling (MDS) were performed in SPSS based on the species composition of the 25m transect in order to observe potential differences in species
composition between tributary sites and non-tributary sites. The multidimensional scaling model gives the species composition of each site a spatial location. Similar species
compositions therefore get a spatial location close to each other. Variation in species composition can be determined based on the spatial locations of the tributary and non- tributary sites.
Table 1. Data analysis and method used.
Analysis Method
Percent occurance (25m transect) Excel algebra
Tributary and non-tributary specific species Excel algebra
Soil type Excel algebra
Simpson's diversity index Excel algebra
Species richness 1m, 5m and 25m transect Student T-test
Species richness aquatic plants Student T-test
Species richness delta and non-delta (25m transect) Student T-test Species richness sheltered and non-sheltered tributary sites (25m transect) Student T-test Tributary sites species dispersal(25m and 100m comparison) Student T-test
0.25-m2 species richness Student T-test
0.25-m2 vegetation cover Student T-test
Morphological groups Student T-test
Short floating species Student T-test
Multidimensional scaling MDS
7
2. Results
3.1 General results
By comparing the total number of species found in tributary sites and non-tributary sites, and dividing them by the number of sampling sites, a percent occurrence of each species was acquired for the tributary sites and the non-tributary sites. 27 species had a 50% or higher occurrence in tributary sites than in non-tributary sites. 14 of these species had habitat tied to the riparian zone, dominantly represented by species belonging to the Poaceae and
Cyperaceae families such as Carex spp, Agrostis gigantea, Juncus filiformis, Poa palustris and Scirpus sylvaticus (table 2).
Table 2. Table showing species with a 50% or higher total percent occurrence in tributary sites in comparison to non-tributary sites. Data based on the 25m transect.
50% or higher occurrence
in tributary sites Riparian species Achillea millefolium L.
Agrostis gigantea Roth yes Cardamine pratensis L. yes Carex junsella (Fr.) Th.Fr yes Carex rostrata Stokes yes Carex vaginata Tansch yes Carex vesicaria L. yes Deschampsia flexuosa (L.)Trin.
Equisetum sylvaticum L.
Geranium sylvaticum L.
Gymnocarpium dryopteris (L:)
Newman
Juncus filiformis L. yes Linnaea borealis L.
Lysimachia thyrsiflora L. yes Melampyrum pratense L.
Melampyrum sylvaticum L.
Orthilia secunda (L.)House Peucedanum palustre (L.)
Moench yes
Picea abies (L.) H.Karst Pinus sylvestris L.
Poa palustris L. yes
Pyrola minor L. yes
Rubus arcticus L. yes
Rubus saxatilis L.
Scirpus sylvaticus L. yes Vaccinium myrtillus L.
Viola epipsila Ledeb yes
In the tributary sites, 56 species were found which had no occurrence in the non-tributary sites. Of these 56 species, 12 had an occurrence in 3 or more tributary sites and 5 of which have a habitat tied to the riparian zone (table 3)(appendix 1). These were Callitriche cophocarpa, Galium uliginosum, Equisetum palustre, Sparganium sp and Phegopteris connectilis.
8
The non-tributary sites had 9 species that had no occurrence in the tributary sites. None of which had a higher occurrence than 1 location and only 1 species with a habitat tied to the riparian zone, Trichophorum alpinum (table 3).
Table 3. Number of species solely found in tributary sites and non-tributary sites. Data based on the 25m transect.
Site Site specific
species Occurrence in 3 or
more sites Species tied to the riparian habitat Tributary
sites 56 12 5
Non- tributary
sites 9 0 1
3.2 Difference in species richness and vegetation cover
There was a significantly higher overall species richness of the 0.25-m2 plots in the tributary sites compared to the non-tributary sites (figure 4D). In the different levels of the riparian zone, there was a significantly higher species richness in the tributary sites compared to the non-tributary sites in all levels of the riparian zone (figure 4A, B and C).
Figure 4. Mean species richness of the 0.25-m2 plots, tributary and non-tributary sites in A) Lower level (mean species richness tributary site= 4.7, and 1.7 for non-tributary sites, p=0.009), B) Middle level (mean species richness tributary site= 8, and 5.9 for non-tributary sites, p=0.03), C) Upper level (mean species richness tributary site= 8.4, and 6.1 for non-tributary sites, p=0.019) and D) all 0.25-m2 plots, tributary and non-tributary sites (mean species richness for tributary sites were 7 and 4.6 for non-tributary sites, p<0.0001). Minimun and maximum values are shown with whiskerbars.
9
There was a significantly higher overall vegetation cover of the 0.25-m2 plots in the tributary sites compared to the non-tributary sites (figure 4D). In the different levels of the riparian zone, there was a significantly higher vegetation cover in the tributary sites compares to the non-tributary sites of the lower level of the riparian zone (figure 5A). In the upper and middle level, there were no significant difference in vegetation cover between tributary and non- tributary sites (figure 5B and C).
Figure 5. Cover of vascular plants from A) lower level of the 0.25-m2 squares (mean coverage tributary sites=
35.9%, and 16.2% for the non-tributary sites, p=0.0017), B) Middle level of the 0.25-m2 squares (mean cover tributary sites=44.2%, and 29.1 for the non-tributary sites, NS), C) Upper level of the 0.25-m2 squares (mean cover tributary sites=49.5%, and 41.7% for the non-tributary sites, NS) and D) all levels of the 0.25-m2 squares (mean cover tributary sites=43.9%, and 26.3% for the non-tributary sites, p=0.039). Minimun and maximum values are shown with whiskerbars.
10
There was a significantly higher total species richness in the tributary sites of the 5m and 25m transects compared to the non-tributary sites (figure 6B and C). There was no significant difference in total species richness between tributary sites and non-tributary sites of the 1m transect (figure 6A).
Figure 6. A) Total species richness from A) the 1m transect (mean species richness tributary sites=19.8, and 14.8 for the non-Tributary sites) NS, B) 5m transect (mean species richness tributary sites=26.5, and 20.2 for the non- Tributary sites, p=0.05), and C) 25m transect (mean species richness Tributary sites=41.7, and 33.3 for the non- Tributary sites, p=0.049). Minimun and maximum values are shown with whiskerbars.
There was no significant difference in species richness of aquatic plants between the tributary sites and non-tributary sites (mean species number for Tributary sites=8.2, and 10.3 for non- Tributary sites, p=0.09).
Comparison between delta and non-delta tributary sites of total species richness of the 25 m transect showed no significant difference (mean species number for delta tributary sites=54, and 60.3 for non-delta tributary sites, p=0.32)
11
There was a significantly higher total species richness in the sheltered tributary sites compared to the non-sheltered tributary sites (figure 7).
Figure 7. Total species richness from the 25m transect of all tributary sites, divided between sheltered and non- sheltered tributary sites (mean species richness sheltered tributary sites=59.4, and 52.4 in the non-sheltered tributary sites, p=0.05). Minimun and maximum values are shown with whiskerbars.
There was no significant difference in the share of species found in the 100 m reach going up the actual tributary that also was found in the tributary mouth (25 m reach) between
sheltered and non-sheltered tributary sites (mean species occurrence in both tributary and tributary mouth of sheltered tributary sites= 48%, and 46% in the non-sheltered tributary sites, p=0.084).
There was a significantly higher amount of short-float species in the tributary sites compared to the non-tributary sites (figure 8).
Figure 8. Short floating species observed in the 25m transect, divided between tributary sites and non-tributary sites (mean species number tributary sites=7, and 5 for the non-tributary sites, p=0.027). Minimun and maximum values are shown with whiskerbars.
12
Of the morphological groups, there was a significantly higher amount of graminoid species in the tributary sites compared to the non-tributary sites (figure 9A). There was no significant difference in species count of herbs, shrubs and trees (figure 9B, C and D).
Figure 9. Morphological classes from the 25m transects in, tributary sites and non-tributary sites for A) Graminoid species observed (mean species number for tributary sites=13.9, and 9.6 for the non-tributary sites, p=0.004), B) Herb species observed (mean species number tributary sites=18.3, and 14.8 for the non-tributary sites, NS), C) Shrub species observed (mean species number tributary sites=5.5, and 6.1 for the non-tributary sites, NS), and D) Tree species observed (mean species number for tributary sites=1.9, and 1.4 for the non- tributary sites, NS). Minimun and maximum values are shown with whiskerbars.
13
3.3. Soil type
The soil types differed between the tributary sites and non-tributary sites where the tributary sites mainly consisted of fine sediment and sand, whereas the non-tributary sites consisted of smaller amounts of fine sediment, and larger amounts of sand, gravel and large stones (figure 12).
Figure 10. Mean percentage soil types in the 0.25-m2 from tributary sites and Non-tributary sites. Tributary sites showing higher percentages in fine sediment peat, blocks and bedrock, whereas the non-tributary sites have higher percentages in sand, gravel and large stones.
0,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0 80,0 90,0 100,0
Blocks Large stones
gravel Small stones
Sand Fine sediment
Peat Bedrock
percent
Substrate Non-tributaries Tributaries
14
3.4 Simpson’s diversity index
A graph was made to visualize the species relative abundance in tributary sites and non- tributary sites (figure 11). The non-tributary sites have less species than the tributary sites, but with higher relative abundances. The tributary sites have more species, but with lower relative abundances (figure 11).
Figure 11. Relative species abundance from all sampling sites, divided between tributary sites and non-tributary sites. Data based on the 25m transect and calculated using Simpson’s diversity index (Eq 1).
0 0,0001 0,0002 0,0003 0,0004 0,0005 0,0006 0,0007 0,0008 0,0009
RELATIVE ABUNDANCE (DSPECIES)
SPECIES
Non-tributaries Tributaries
15
3.5. Multidimensional scaling
The tributary sites have a larger variation in species composition, compared to the non- tributary sites in both Luleå and Umeå river (figure 12B and C), as well as the combined data from both rivers (figure 12A). The non-tributary sites seem to have a higher similarity in species composition.
Figure 12. Multidimensional scaling of 25m transect data from all sampling sites, with spatial distance between points indicating similarities in plant community composition. Sampling sites located in A) Ume and Lule river, B) Lule river, and C) Umeå river.
16
3. Discussion
Previous studies have shown that regulation of flow in hydropeaking reservoirs have a large impact on riverine vegetation patterns (Jansson et al., 2000; Bejarano, Jansson and Nilsson, 2017). However, as seen in this study in the regulated Luleå and Umeå rivers, it seems that the tributary sites are less affected. The difference in plant species composition, species count and cover, all support the hypothesis of tributary sites being hotspots for plant diversity in regulated rivers, where tributary mouths, to some extent, maintain ecosystem functioning and structure, compared to the non-tributary sites which in this study indicate habitat degradation.
The tributary mouth plant communities had a contrasting species composition when
compared to those of the non-tributary sites. Whilst non-tributary sites seem to have subset of species occurring in the tributary sites (figure 12), there are more species solely occurring in the tributary sites (table 3). Additionally, the tributary sites had a higher abundance of riparian specific species (table 2) and a higher species richness (figure 4 and 6).
Many of the species with a larger percentage occurrence in tributary sites, are also riparian species (table 1). These species had a lower frequency in the non-tributary sites (50% or more), suggesting a loss of habitat or capability of survival and dispersal at these sites.
The 0,25-m2 plots showed a higher species richness in the tributary sites (figure 4). The higher species richness was also present in all levels of the riparian zone (figure 4). Based on the mean species richness, as well as vegetation cover, which also showed a higher overall cover (figure 5) in the tributary sites, it can be seen that the largest difference most likely lie within the lower level of the riparian zone, where the non-tributary sites had less than 50% of the mean vegetation cover and a much lower species richness (tributary sites=4.7 and non- tributary sites= 1.7) compared to the tributary sites (figure 4)(figure 6). Supporting this is the difference in morphological classes, where the tributary sites had a higher graminoid species count (figure 9). The graminoid species, which in this habitat mainly consists of species from the Carex family, tend to inhabit the lower reaches of the riparian zone (Andersson, Nilsson and Johansson, 2001).
The lower level of the riparian zone would be more prone to hydropeaking effects, since low magnitude changes in water flow would result in a transition of the water level in this region, causing flooding which might not affect the middle and upper levels as often. With the natural river flow pattern, the flow changes would occur much more infrequent, with longer time periods of lower water flow (figure 1) (Jansson et al., 2000a). However, because of the water regulation, this timespan has been dramatically altered to hourly changes (figure 1), likely prompting the difference in species richness, composition and vegetation coverage observed (Jansson et al., 2000a).
The likely explanation for the habitat degradation in the non-tributary sites is river flow regulation, with hydropeaking and flush and flow regimes. The non-tributary sites would be more receptive towards flow regulation since majority of river water accumulate in these areas, which in turn would create a more extensive fluctuation of the water levels in the riparian zone (Bejarano, Jansson and Nilsson, 2017). Water level fluctuations would not only lead to irregular and unnatural flooding patterns, which interrupts ecosystem functioning and ecological interactions, but also cause largescale erosion from ice, waves and
geomorphological changes, removing fine sediments and leaving larger soil particles such as sand, gravel and stones behind (Bejarano, Jansson and Nilsson, 2017).
This is supported by the results from the soil types found in the sampling sites of the
tributary sites and non-tributary sites. The tributary sites were found to consist of mostly fine
17
sediments and sand, unlike the non-tributary sites which had more sand, gravel and large stones (figure 10).
As a consequence of irregular and heavy fluvial erosion caused by hydropeaking, the residual soils in the riparian zone of the non-tributary sites would have been exposed to leaching, with nutrients removed along with fine sediments. In addition to hydropeaking, a change in soil texture would inaugurate an impaired habitat condition for many of the riparian-specific species, where new plant communities might proposedly mostly consist of high abundance, disturbance tolerant species (Bejarano, Jansson and Nilsson, 2017). This trend is eminent in the relative abundance between the tributary sites and non-tributary sites of the Simpson’s diversity index (figure 11). The tributary sites seem to have a larger diversity with lower abundance, whereas the non-tributary sites have a lower diversity but with specific species of high abundance. This can be a result of less hydropeaking disturbance leading to a more stable environment for species susceptible to large and irregular disturbance, caused by the river regulation. However, since the diversity index represents all tributary and non-tributary sites, calculations based on species count at each site would likely yield a more precise result with larger differences.
A similar trend could be distinguished from the multidimensional scaling model, where the species composition of the non-tributary sites in the Umeå River seem to be clustered; hence a higher similarity (figure 12c). The species composition of the tributary sites in Umeå River has a larger spread, at least more so than the non-tributary sites, indicating, to some extent, a larger variation in community structure, species composition and environmental variables at these sites (figure 12c).
The results indicate that the effects of hydropeaking is less prominent in the non-tributary sites. In addition to the less extensive impact of hydropeaking, this would lead to an advantageous and beneficial environment for species vulnerable to disruptive water fluctuations.
Given the fact that long-float species are dominant in storage reservoirs of regulated rivers (Jansson et al., 2000b), the difference observed in short-float species between the tributary sites and non-tributary sites suggests tributary mouths as an important habitat for short- floating species in regulated rivers (figure 8).
Whilst the results highlight tributary mouths as hotspots for plant diversity and that the hydrological effects of flow regulation seems to be downscaled at these sites, tributary sites are not unaffected by river regulation. This can be seen from the results of the inter-tributary analysis of sheltered and non-sheltered tributary sites. The t-test from the 25m transect of the tributary sites showed a higher species richness in the sheltered tributary sites,
suggesting less disturbance (figure 7).
Sheltered tributary sites would likely not be affected by hydropeaking to the same degree.
With lower flow velocities, water depth and width in comparison to the main river, along with boulders, fallen trees or material deposits in proximity of the tributary mouth as shelter, less erosion and flow disturbance would take place. With a sheltering border, the tributary mouth of the sheltered tributary sites would retain much of its sediment deposits and nutrients, whilst not being exposed to the same intensity flow as the non-sheltered tributary sites, leading to a natural countering mechanism towards river flow regulation. With less disturbance from hydropeaking, plant communities would be able to further populate the tributary mouth, leading to even higher resistance against erosion due to root systems holding sediment. This is supported by the fact that species richness was higher in sheltered compared to non-sheltered tributary sites (Figure 7).
The percent of species shared between the actual tributary and the tributary mouth did not differ between sheltered and non-sheltered tributary sites, indicating that even though there
18
seems to be a decreased hydropeaking disturbance in the sheltered tributary sites with a higher species richness, this is not driven by species influx and retention from the actual tributary.
The flow pattern of the sheltered tributary sites could possibly be comparable to that of a non-regulated river, since hydropeaking disturbance seems to be decreased. However, as long as there is regulation, there will always be an unnatural disturbance to some extent, which affects the ecosystem dynamics.
The results show no difference between tributary aquatic plants and non-tributary aquatic plants. An explanation for this could be that aquatic plants survival and dispersal are not directly dependent upon the status of the riparian zone. As for the delta and non-delta sites, it seems that there are no connections to the status of the riparian zone. However, the small sampling size of non-delta sites could be a margin of error for this analysis and the
insignificant result.
Conclusion
The results in this study indicate that the negative impact of river regulation on vegetation seem to be less extensive in tributary sites than in non-tributary sites, even more so in sheltered tributary sites.
With a growing demand for renewable energy, and with hydropower as one of the largest renewable energy sources, further hydropower plants are likely to be established in many parts of the world (Zarfl et al. 2015). In Sweden, where the capacity for hydropower
production is almost fully developed and where development has caused negative ecological effects, there is a requirement to re-licensing existing hydropower plants to meet modern environmental standards (SFS 1998:808). Since river regulation is a necessity in order to generate efficient electricity from hydropower plants, it’s important to acknowledge the adverse effects of river regulation on riverine ecosystems.
Suggested mitigation, as well as restauration measures involve recognition of tributary mouths, specifically sheltered tributary mouths as an important driver of plant diversity in riverine ecosystems, and further, anthropogenic sheltering of tributary mouths, such as boulders, as a counter measure for river regulation in order to preserve these ecologically important sites.
5. Acknowledgments
I would first and foremost like to thank Birgitta Malm-Renöfält for her helpful insights on study design, statistical measures and interpretation of results. Her knowledge of riverine hydrology, effects of water regulation and plant ecology have been much appreciated.
I would also like to thank Birgitta Malm-Renöfält and Roland Jansson for letting me be a part of this project. It has been incredibly inspiring to have had the opportunity to work with such an important topic. I have gained much knowledge of riverine ecology and river regulation throughout this project, which has sparked an interest with hopes of further broadening my knowledge.
19
6. References
Andersson E, Nilsson C and Johansson M. 2000. Effects of river fragmentation on plant dispersal and riparian flora. Regulated rivers: research and management 16(1): 83-89. Doi:
10.1002.
Andersson E, Nilsson C and Johansson M. 2001. Plant dispersal in boreal rivers and its relation to the diversity of riparian flora. Journal of Biogeography 27 (5): 1095-1106. Doi:
10.1046.
Bejarano M, Jansson R and Nilsson C. 2017. The effects of hydropeaking on riverine plants: a review. Biological review 93: 658-673. Doi: 10.1111.
Benda L, Poff N, Miller D, Dunne T, Reeves G, Pess G and Pollock M. The Network Dynamics Hypothesis: How Channel Networks Structure Riverine Habitats. 2004. Bioscience 54: 413- 427. Doi:10.1641.
Czortek P, Duderski M and Jagadodzinski. 2020. River regulation drives shifts in urban riparian vegetation over three decades. Urban forestry and urban greening 47. Doi: 10.1016.
Dosskey M, Vidon P, Gurwick N, Allan C, Duval T and Lowrance R. 2010. The Role of Riparian Vegetation in Protecting and Improving Chemical Water Quality in Streams.
Journal of the American Water Resources Association 46: 261-277. Doi: 10.1111.
Jansson R, Nilsson, C and Renöfält, B. 2000a. Fragmentation of riparian floras in rivers with multiple dams. Ecology 81: 899—901. Doi: 10.1890
Jansson R, Nilsson C, Dynesius M and Andersson E. 2000b. Effect of river regulation on river-margin vegetation: A comparison of eight boreal rivers. Ecological applications 10:
203-224. Doi: 10.1890.
Jong-Yun C, Kwang-Seuk J, Seong-Ki K, Geung-Hwan L, Kwang-Hyeon C and Gea-Jaa J.
2014. Role of macrophytes as microhabitats for zooplankton community in lentic freshwater ecosystems of South Korea. Ecological Informatics 24: 177-185. Doi: 10.1016.
Krok T. O. B. N and Almquist S. 1994. Svensk flora. Fanerogamer och ormbunksväxter (27th ed). Liber utbildning.
Leitner P, Hauer C and Graf W. 2017. Habitat use and tolerance levels of macroinvertebrates concerninghydraulic stress in Hydropeaking Rivers–A case study at the Ziller Riverin
Austria. Science of total Environment 575: 112-118. Doi: 10.1016.
Milner A and Gloyne-Phillips I. 2005. The role of riparian vegetation and woody debris in the development of macroinvertebrate assemblages in streams. River research and Applications 21: 403-420. Doi: 10.1002.
Naiman R and Decamps H. 1997. THE ECOLOGY OF INTERFACES: Riparian Zones. Annual Review of Ecology and Systematics 28: 621–658. Doi: 10.1146.
Naturhistoriska riksmuseet. 2004. Den virtuella floran: Cardamine pratensis L. - Ängsbräsma (nrm.se) (retrieved 2021-04-23).
Nilsson C, Ekblad A, Dynesius M, Backe S, Gardfjell M, Carlberg B, hellqvist S and Jansson R. 1994. A comparison of species richness and traits of riparian plants between a main river channel and its tributary sites. Journal of ecology 82: 281-295.
20
Omelshuk O and Bohadan P. 2015. Effects of River Regulation on Plant Dispersal and
Vegetation. Transylvanian review of systematical and Ecological research 16. Doi: 10.1515.
REN21. 2020. Global status report. Renewable Energy Policy Network for the 21st century, paris, France.
Ribeiro M, Blanckaert K and Schleiss A. 2012. Hydromorphological implications of local tributary widening for river rehabilitation. Water resources research 48. Doi: 10.1029.
Rivaes R, Rodriguez-Gonzalez P, Abuquerque A, Pinheiro A, Egger G and Ferreira M. 2015.
Reducing river regulation effects on riparian vegetation using flushing flow regimes.
Ecological engineering Volume 81: 428-438. Doi: 10.1016.
Schmuts S, Bakken T, Freidrich T, Greimel F, Harby A, Jungwirth M, Melcher A, Unfer G and Zeiringer B. 2014. Response of fish communities to hydrological and morphological
alterations in Hydropeaking Rivers of Australia. River research and applications 31: 919- 930. Doi: 10.1002.
SFS 1998:808. Swedish Environmental Code.
Wollny J, Bergmann W, Otte A and Harvolk-Schöning S. 2021. River regulation intensity matters: Riverbank vegetation is characterized by more typical riverbank plant species with increasing distance from weirs. Ecological engineering Volume 159. Doi: 10.1016.
Zarfl C, Lumsdon A.E, Berlekamp J, Tydecks L And Tockner K. 2015. A global boom in hydropower dam construction. Aquatic Sciences Volume 77: 161–170. Doi: 10.1007
Zhang Y, Jeppesen E, Liu X, Qin B, Shi K, Zhou Y, Thomaz S and Deng J. 2017. Global loss of aquatic vegetation in lakes. Earth-Science Reviews 173: 259-265. Doi: 10.1016.
21
Appendix
Apendix 1. Species which solely occurred at the tributary sites and the non-tributary sites, as well as the number of sites they occurred in.
Species solely occuring in
tributary sites Number of
sites Species solely occuring in
non-tributary sites Number of
sites
Juniperus communis L 3 Lonicera caerulea 1
Empetrum hermaphroditum 2 Ribes rubrum 1
Lycopodium annotinum L. 1 Salix cinerea 1
Vaccinium oxycoccos L. 1 Botrychium multifidum 1
Vaccinium microcarpum 1 Galium album Mill 1
Alchemilla ssp 1 Paris quadrifolia 1
Bistor vi 4 Stellaria longifolia 1
Callitriche cophocarpa 3 Calamagrostis lapponica 1
Callitriche palustris 1 Scirpus hudsonianus 1
Circium palustre 1
Corallorhiza trifida Chatel 1
Euphrasia stricta 1
Fragaria vesca 1
Galium uliginosum 3
Gymnadenia conopsea 1
Leucantemum vulgare 2
Menyanthes trifoliata 1
Moneses uniflora 1
Oxalis acetocella 2
Potentilla erecta 2
Pyrola rotundifolia 1
Ranunculus acris 5
Rorippa palustris 2
Saussurea alpina 1
Subularia aquatica 2
Utricularia intermedia 1
Utricularia vulgaris 1
Veronica serphyllifolia 2
Viola canina 1
Agrostis canina 1
Agrostis stolonifera 1
Alopequrus aequalis 1
Carex globularis 1
Carex loliacea 1
Carex pallescens 1
Carex panicea 1
Eleocharis acicularis 2
Eleocharis palustris 1
Eleocharis mamillata 2
Elytrigia repens 1
22
Equisetum palustre 3
Eriophorum angustifolium 1
Hierochloe hirta 1
Juncus bulbosus 1
Luzula multiflora 5
Luzula pilosa 9
Melica nutans 3
Phalaris arundinacea 1
Phleum pratense 1
Poa nemoralis 1
Poa pratensis 4
Poa trivialis 1
Sparganium spp 6
Phegop co 5
Callitriche coph/pal 2
Calla Palustre 1
