INTRODUCTION
At large spatial scales, a stream’s geomorphic and ecological form and
behavior can be predicted based on longitudinal trends (e.g., Downing et
al., 2012). From a geomorphic perspective, at the catchment scale, the
concept of downstream hydraulic geometry (DHG) shows that several
channel form parameters, including width and depth, increase as a power
function of bankfull discharge (Leopold & Maddock, 1953), which in turn
increases with drainage area. Due to predicted decreases in slope and
thus shear stress, bed material grain size should also decrease
(Gasparini et al., 1999). Within stream ecology, Vannote et al., 1980
built on the idea of downstream hydraulic geometry by extending the idea
of longitudinal patterns or adjustments from the “physical system of
geomorphologists into a biological analog”(Vannote et al., 1980),
developing the river continuum concept (RCC). The RCC predicts
longitudinal patterns in multiple stream ecosystem factors, including
the type of macroinvertebrates present, the ratio of photosynthesis to
respiration and the relative amount of coarse and fine particulate
organic matter, all of which are strongly coupled to the predicted
increase in channel width. Furthermore, biodiversity (species richness)
often shows a predictable downstream increasing trend towards the middle
of the catchment due to dispersal effects and more favorable growing
conditions (e.g., Nilsson et al., 1989, Dunn et al., 2011, Kuglerová et
al., 2015).
Since the establishment of the DHG and RCC concepts, exceptions to
unidirectional
downstream trends have been described, including the geomorphic process
domain concept (Montgomery, 1999), the serial discontinuity concept
(Ward & Stanford, 1995), patch dynamics (Townsend, 1989; Poole, 2002),
the river wave concept (Humphries et al., 2014), and the mighty
headwater hypothesis (Finn et al., 2011). However, these concepts of
downstream trends, implying longitudinal connectivity, still form the
basis for multiple assumptions and concepts within river science (e.g.,
Surian, 2002; Frings, 2008; Dunne & Jerolmack, 2020). For example, the
amount of allochthonous material input into a stream tends to decrease
with increased channel width due to the greater percentage of the
channel covered by canopy compared to larger channels (Vannote et al.,
1980, Doi, 2009). To study these changes, rather than measuring channel
width and canopy cover, studies commonly rely on drainage area as a
proxy (e.g., Jonsson et al., 2018). If these basic longitudinal
gradients do not exist in a certain catchment, then we must re-examine
which geomorphic and ecological processes drive longitudinal stream
patterns. The serial discontinuity concept, predicts local diversity to
increase with increasing distance to dams (Ward & Stanford, 1995), and
Green et al. (n.d.) found the same pattern for invertebrates but along a
lake-stream system with lakes disrupting the continuity. However,
geomorphic and ecological trends are often not examined simultaneously
to explain organization of a stream network, even though they are
intertwined with each other. Hence, the objective of this study was to
examine whether geomorphic and ecological longitudinal trends, as
defined by channel width and riparian vegetation richness and community
composition, exist in a naturally disconnected catchment consisting of
lakes and streams. By understanding the geomorphic and biotic
longitudinal organization of a stream network, we can set expectations
for recovery after stream restoration.