Biodiversity and habitat conditions in reaches with high flow velocity

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Aims and hypothesis

The aim of the thesis is to compare the hydrological and morphological effects on fish species richness and composition. I also want to determine which morphological or hydrological variable can explain the variation observed in the species composition and richness of fish populations, aquatic vascular plant populations and riparian vegetation.
My hypothesis is that the diversity and species richness of fish, aquatic vascular plants, and riparian vegetation in rapids across Sweden can be predicted by the level of hydrological and morphological alteration that has occurred in the river. In other words, the less alteration, the more abundance and species richness of fish, aquatic vascular plants, and riparian vegetation. The level of hydrological and geomorphological alteration is assessed by analyzing variables describing discharge and channel bed sediment composition in river reaches in a gradient from rapids in free-flowing reaches to reaches heavily modified for hydropower production.

Methods

Study areas

A total of six rivers were used in this study. They are similar in length and average discharge (table 1). They flow from the mountain range between Norway and Sweden, and four of them empty into the Gulf of Bothnia. The Klar River empties into Vänern, which is drained by the Göta River into Kattegatt, and the Vindel River merges with the Ume River 35 km from the coast of the Gulf of Bothnia. Biogeographically, the rivers run from a subalpine zone with mainly birch forests (Betula pubescens ssp. czerepanovii), to boreal forests dominated by Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) (Jansson et al. 2000).
The only river that is entirely free flowing in this study is the Vindel River. It is 445 km long and has a catchment area of 12,650 km2. The mean annual discharge of the river is 190 m3/s, with large seasonal water-level fluctuations over the year. The river has its annual water-level maximum during the spring or early summer, which is caused by the snowmelt in the mountains, followed by reduced flow during summer and a slight increase in autumn (Jansson et al. 2000).
The other five rivers (Lule-, Skellefte-, Ljungan-, Klar-, and Ume Rivers) are all heavily developed for hydroelectric power production. There are numerous dams in both the main channel and the tributaries.
In the six rivers, reaches with high flow velocity, representing conditions equivalent to rapids in free-flowing rivers, were selected. The aim was to have sites differing in hydrological and geomorphological impact, and that variation in hydrological and geomorphological impact would be orthogonal, i.e., vary independently from each other. Hydrological alteration stemmed from regulation to produce hydropower, which implied reduction in seasonal variation with lower spring flood water levels and higher winter flows compared to in free-flowing rivers, and short-term variation in flow and water-levels as a result of hydropeaking (Bejarano et al. 2018). Geomorphological alteration stemmed from clearing and channelization to reduce channel roughness and increase flow velocity, to increase hydropower production. Reaches may also be affected by channelization to facilitate timber floating (Nilsson et al. 2005). Selecting sites varying independently in geomorphological and hydrological alteration proved not to be possible, and sites were therefore instead selected in four categories:
(1) Rapids in free-flowing reaches of rivers, situated upstream of any dam or hydropower station, and geomorphological alteration only from modifications to facilitate timber floating. These are unaffected by exploitation from hydropower having a pristine flow regime but have been cleared to facilitate timber floating (Nilsson et al. 2005).
(2) Rapids with regulated flow, but with remaining fall height. Flow regimes are generally altered, having lower seasonal variation and varying intensity of hydropeaking, meaning sub daily to weekly shifts in discharge, including frequent periods of zero discharge, whereas geomorphological alteration is limited to clearing to facilitate timber floating in the past. These rapids have regulated flow with reduced seasonal variation, but the fall height remains (Englund and Malmqvist 1996).
(3) Reaches in run-of-river impoundments, where flow velocity is high as a result of narrow sections or some remaining fall height. Geomorphologically, they may be cleared but with some structural complexity of the bottom substrates remaining. Hydrologically, they are affected by hydropeaking, to match variation in hydroelectric production.
(4) Reaches in outlet channels below hydropower stations. These reaches have high flow velocity being artificial channels with low roughness. Hydrologically, seasonal flows are reduced, and hydropeaking intensity can be high since they are situated close to the hydropower station, with little or no dampening offered by intervening reaches with roughness features and volumes of water.
Site selection resulted in four reaches in rapids in free-flowing reaches (hereafter free-flowing), four in rapids with regulated flow (regulated rapids), six in impoundments (impounded reaches) and five in outlet channels. Tentatively, the four types of sites can be ranked from most pristine to most degraded in the same order as presented above, based on magnitude of hydrological alteration, the amount of geomorphological alteration to the channel bed, and finally the fall height along the reach.
Data on the hydrology, geomorphology, species composition of aquatic and riparian vascular plants, and presence of fish were collected from the 19 reaches in the six rivers (the Lule-, Skellefte-, Vindel-, Ljungan-, Klar-, and Ume Rivers). All the data except the fish and hydrological data were collected during field visits in the summer of 2019. The fish data originated from the Swedish Electrofishing Register, a national database for electrofishing data in Sweden; however, only 10 out of the 19 locations had fish data in the database.
At every site, a 150-m long study site was established and divided into six 25-m long sections. In the middle of each section, a 2-m wide transect running perpendicular to river from the channel edge to the thalweg was established. In the transect, the proportional cover of a number of sediment particle size classes in were judged after visual inspection from the shoreline and by a person investigating the transect using a dry suit. The sediment size classes were silt (<0.06 mm), sand (0.6 – 20.0 mm), gravel (0.2 – 16.0 cm), pebbles and cobbles (16.0 – 25.6 cm), boulders (> 25.6 cm) and bedrock, using the Wentworth scale (Wentworth 1922). In each section, the number of large boulders (larger than 1 m in diameter) were counted, along with the number of pieces of large wood (longer than 1 m and with a diameter exceeding 10 cm). In addition, the degree to which the riparian zone provided shading of the river channel was judged for each section, along with recording the three most common tree species in the riparian zone.
The presence of all aquatic vascular plant species growing along the 150-m reach was recorded. Aquatic vascular plants are defined as species with the ability to grow permanently submerged. Some of these species can be called amphibious, i.e. having different growth forms when growing inundated or out of the water. In a 5o-m long stretch of riparian zone placed in the centre of the 150-m long reach, 50×50 cm large plots were placed in three transects running from the annual high water (marked by the spring-flood peak level in free-flowing rivers) to the summer low water levels, with 20 cm in elevation between each plot. Within each plot, the percentage cover of vegetation and the presence of all vascular plant species rooted in them were recorded.
The data were analyzed with the help of R (R core team 2020), and the additional addons, ”Vegan” and “ggplot”. The hydrological data were analyzed with the program Indicators of Hydrologic Alteration (IHA) version 7.1 (The Nature Conservancy 2009). In my analysis, I utilized 10 of the variables that the IHA produced (1-day minimum flow, small flood peak, small flood duration, small flood timing, number of reversals, rise rate, fall rate, large flood peak, large flood duration and finally large flood timing; table 2).
These 10 variables represent different aspects of the hydrological regime affecting ecosystem functioning in some way (table 3). The “1-day-minimum” variable can have an effect on life cycles for organisms: For example, it can influence stress, give access to special habitats and serve as a migration cue for migratory fish. “Small flood peak”, “Small flood timing”, “Small flood duration”, “large flood peak”, and “large flood duration” all have similar ecosystem functions, being related to flood events. They provide migration cues for migratory fish, trigger new phases in life cycles, enable fish to spawn on floodplains, provide nursery areas for juvenile fish, provide new feeding habitats and more (table 3). The “number of reversals”, “fall rate”, and “rise rate” variables describe rapid shifts in flow or water levels that can cause drought stress on plants, cause entrapment of organisms on islands, and desiccate low-mobility organisms that live near the stream edge. I chose these ten after discussing which variables are important to explain hydrological effects on the fish populations at the different locations.
Correlation tests (Spearman tests) were done between all the geomorphology, aquatic vascular plants, riparian vegetation, fish population and hydrology variables to see if the different variables affect each other, and if the effect was positive or negative. I used the Shapiro test to determine if the data were normally distributed. Afterwards, I used either analysis of variance (ANOVA), for data with a normal distribution, or Kruskal Wallis tests, for non-normally distributed data, to determine if there was a significant difference (p < 0.05) between either fish populations, aquatic vascular plants, or riparian vegetation among the four different groups of sites. To explore differences between the four groups of sites I also created several different NMDS (Non-metric multidimensional scaling) figures to show the variation between the four groups. The figures were created with R.

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Results

The analysis of the geomorphology of the channel of the studied reaches showed that there were differences among the reach types (figure 1, table 4). The free-flowing reaches and regulated rapids were similar in sediment composition, having more boulders and less amount of sand and silt compared to the other two types of reaches. There were also tendencies of larger numbers of large boulders (boulder > 40 cm in diameter) in the free-flowing reaches and regulated-rapids reaches compared to the impounded reaches and outlet channels (table 4). However, only the difference for the cover of boulders was significant (P < 0.0001, one-way ANOVA), whereas the other substrate variables did not differ significantly from each other (P > 0.20, one-way ANOVA; boulders > 40 cm in size not tested).

Table of contents :

1 Introduction
1.1 Aims and hypothesis
2 Methods
2.1 Study areas
3 Results
4 Discussion
4.1 Geomorphology
4.2 Hydrology
4.3 Biological variables
4.4 Conclusion
5 Acknowledgements
6 References

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