DISCUSSION
We found longitudinal trends in geomorphic and ecological parameters to
be very weak in these naturally disconnected study catchments, which
contains disparate channel types that segment the stream. Our study also
finds the geomorphic difference between the catchments to be visible in
the riparian vegetation along the different catchments.
Downstream hydraulic geometry relationships of width- drainage area
power functions explain at most 57% of variability in width for
slow-flowing reaches in Hjuksån, and a minimum of only 4% of
variability in width for slow-flowing reaches in Bjurbäcken. Combining
all reach types, only 8% of variability in width is explained in
Bjurbäcken and 43% of variability is explained in Hjuksån. According to
Faustini et al. (2009), all of these regressions classify as ‘poor’
fits, except for slow-flowing reaches in Hjuksån; and the power
relationships in Bjurbäcken have poorer fits than for any region in the
conterminous United States (Faustini et al., 2009). In the Hjuksån
catchment, located below the FHC, which contains more fine deltaic
sediment in addition to coarse glacial sediment, there are slightly
stronger downstream hydraulic geometry relationships for all reach
types, indicating potential for self-adjustment in response to the
current flow regime as seen in alluvial channels (Singh et al., 2003;
Wohl, 2004). The fact that rapids and the catchment above the FHC show
poorer DHG relationships, fits with Wohl’s (2004) observations from
coarse mountain streams, where low ratios of the stream power to
D84 lead to poor DHG fits. Exponent (β-) values, which
describes the spatial rate at which width increases with increased
drainage area, typically range 0.2-0.4 (Faustini et al., 2009). Combined
reaches and slow-flowing reaches in Bjurbäcken fall within that range
(0.34 and 0.26, respectively) in addition to rapids in Hjuksån (0.39);
however, the remaining reach types have fairly large β-values (ranging
0.56- 0.75) (Figure 3), indicating a fairly rapid downstream increase in
channel width, potentially caused by a y-value >1 (Equation
(2)), due to the more dendritic drainage basin in Hjuksån compared to a
more linear drainage basin in Bjurbäcken (Figure 2) and greater
connections to groundwater (Burgers et al., 2014).
The presence of lakes in the Hjuksån catchment, containing finer
sediment and thus having more alluvial characteristics, create wider
channels directly downstream of the lake than if the reach was further
downstream of a lake (Table 1). Lakes are serving to reset the
longitudinal sediment conveyor belt by trapping fine sediment,
increasing width: depth ratios directly downstream of lakes (Arp et al.,
2007). In the Sawtooth Mountains region of Idaho, US, channel shape
recovered by 50% within 1.0-1.8 km downstream of lakes and required up
to 10-20 km to recover by 90%. Given the high spatial density of lakes
in catchments in northern Sweden, channels may never recover and thus
never reach a non-lake influenced equilibrium form. In addition to the
significant effect of lakes, several surficial geology types, reflecting
erosivity of streambanks affect the drainage area- channel width
relationship, where the coarse till has a narrowing effect and fine
deltaic, and subglacial sediment has a widening effect (Table 1). Peat
has a widening effect on rapids but a narrowing effect on slow-flowing
reaches, likely because the intrinsic cohesivity of clay in peat will
decrease lateral erosion but compared to coarse till found in rapids the
presence of peat will allow greater erosion. Likewise, bedrock has a
narrowing effect in rapids and the slow-flowing reaches in Hjuksån but a
widening effect in slow-flowing reaches in Bjurbäcken; bedrock outcrops
found in slow-flowing reaches are smoothly rounded features composing
the bed or small parts of the banks, whereas bedrock in rapids will lead
to high slopes that naturally form narrower channels or make up
streambanks that will hinder any lateral erosion. In general, the low
R2 values and generally low β-values reflect the low
ability for semi-alluvial channels to adjust their channels to the
contemporary flow regime (Polvi, 2021).
The positive relationships between increased distance between reaches
and reach similarity indicates low connectivity between reaches in terms
of hydrochoric seed or propagule dispersal. However, models like linear
regressions are sensitive to outliers and therefore these models are
significant but with a very weak R2. We therefore
interpret these results a lack of connectivity since reaches closer to
each other are less similar than reaches that are further apart. The
geomorphic trend of weak longitudinal relationships with lakes is also
mirrored in the ecological data by a slight trend in decrease in
longitudinal species density of riparian vegetation in Hjuksån, and no
significant relationships in Bjurbäcken.
Poorly developed downstream hydraulic geometry relationships with
regards to channel width, combined with low support for connectivity
between riparian vegetation communities in semi-alluvial stream-lake
systems, indicate that these are highly fragmented catchments where
local factors steer geomorphic form and biotic communities. All rivers
are part of a landscape context with varying degrees of connectivity
between segments and reaches. Previous work has shown that dispersal is
strong in well-connected areas compared to isolated headwaters, and
patterns of connectivity among sites in a network can affect population
dynamics (Swan & Brown, 2017). Hence, biodiversity has often been shown
to increase towards the middle of the catchment (Nilsson et al., 1989;
Kuglerová et al., 2015). However, stream networks in northern
Fennoscandia are commonly naturally fragmented and consist of three
types of process domains (rapids, slow-flowing reaches and lakes), which
all differ in morphology and hydraulics that influence their capacity to
facilitate the plant dispersal (Su et al., 2019a, b). In a connected
stream network, one would expect a downstream increase in species
richness density (Andersson et al., 2000; Kuglerová et al., 2015).
However, in our naturally fragmented river system we found the opposite
pattern, with a reduction in species density with increasing distance
downstream. These differences in patterns can partly be explained by the
high spatial resolution and that our study was conducted continuously
along two streams, in comparison to previous studies with lower spatial
resolution and discontinuity in sampling (e.g. Nilsson et al., 1994;
Andersson et al., 2000; Kuglerová et al., 2015). The increased spatial
resolution and sampling continuity allowed us to find an overall
reduction in species density, as a function of the lack of hydrochory
due to the presence of numerous lakes (Su et al., 2019a, b). Lakes trap
~80% of seeds during the spring flood and only allow
downstream transport if the lake outlet is aligned with the wind
direction (Sarneel et al., 2014; Su et al., 2019a). In a connected
stream network, we would also predict higher similarity between reaches
close to each other; however, in our disconnected systems we did not
find such a relationship. Su et al. (2019b) showed that these three
process domains have differing plant species communities, and thus seed
banks may therefore provide a local source for metacommunity control, or
perhaps diversity depends mostly on local site conditions (Green et al.,
n.d.), and not connectivity.
Rather than longitudinal controls of increasing discharge (as a function
of drainage area) and inter-reach connectivity on channel width and
riparian vegetation communities, our results indicate that our study
catchments are driven by local-scale geomorphic and ecological controls.
These local-scale controls, such as surficial geology (presence of
coarse glacial sediment) and local seed banks and source populations,
are therefore more important in recovery of stream processes and
communities than connectivity of flows, sediment, and propagules between
reaches in responding to disturbances, which include habitat
restoration. Therefore, passive ecological recovery, with recolonization
by hydrochory, is not as likely in disconnected stream networks, and
thus manual planting of riparian vegetation may be necessary. Similarly,
geomorphic recovery through channel adjustment based on the current flow
regime is unlikely in disconnected networks with semi-alluvial process
domain segments; thus, physical channel manipulation should play a
larger role in stream restoration. In addition, interactions between
channel width and riparian vegetation communities throughout the
catchment may serve to further shape reach morphology and riparian zones
(Anderson et al., 2004).
With weak downstream hydraulic geometry relationships, traditional
stream ecology concepts, which assume increasing width as drainage area
increases, may not apply. Geomorphic relationships with channel size
have also been used to model nutrient uptake in river networks (Ensign
& Doyle, 2006). Thus, given the abundance of lakes in northern
latitudes (Messager et al., 2016), this study can have widespread
implications on the understanding of fluvial processes and stream
ecosystems in boreal, and (sub)arctic regions. For example, we may not
be able to rely on past cornerstones of river science, such as
downstream hydraulic geometry and the river continuum concept, in
certain catchments in understanding river dynamics, designing stream
restoration projects, and predicting responses of communities after
restoration or other natural disturbances. Furthermore, disconnected
fluvial systems with abundant lakes may also be analogous to
pre-anthropogenic stream networks that contained abundant log jams and
beaver dams, which create various degrees of natural disconnectivity
(Wohl & Beckman, 2014; Green et al., n.d.). These naturally occurring
discontinuities should be distinguished from anthropogenic dams that are
nearly complete barriers to propagule, sediment and water fluxes.
Natural fluvial disconnectivity features buffer fluxes and allow
transport in temporal pulses and through leaky barriers; thus, lakes can
provide a model for how beaver dams and log jams, which are temporally
and spatially heterogeneous, affect functional connectivity of processes
affecting geomorphic form and ecological communities.