Discussion
The serial discontinuity concept describes how riverine systems are
expected to respond to major disruptions along the course of a river
(Ward and Stanford, 1989; Ward & Stanford, 1995; Stanford & Ward,
2001). The expectations vary depending on the geomorphology of the river
and the responses are described for three different reach types –
constrained, braided, and meandering (Ward & Stanford, 1995). The
portions of the Tallapoosa River sampled in our study fit between the
constrained and braided reach types, characterized by a stable channel,
a narrow riparian corridor, and lotic habitats throughout. As such, the
biodiversity of the Tallapoosa was expected to increase with distance
downstream (Ward & Stanford, 1995). Interestingly, we measured no such
increase in Shannon’s H or species/family richness within the river
section from Harris Dam to the headwaters of Lake Martin. However, we
documented variation in the assemblage structure that is likely
influenced by the presence and operation of Harris Dam.
Sunfishes and minnows were generally the most common fish families in
this part of the Tallapoosa River, and variation in diversity from
upstream to downstream was neither large nor systematic. Catostomids,
centrarchids, and cyprinids were dominant in catches above Harris Dam,
similar to the findings of Travnichek and Maceina (1994) who conducted a
survey (prior to the implementation of the water release management
Green Plan in 2005) of the broader Tallapoosa River from the upper
reaches near Heflin, AL downstream to the Coastal Plain. Overall species
diversity index values for this study area were slightly higher and more
variable in 1994 (1.98 - 3.53) versus our study (2.49 - 2.88), though
this difference may have been driven in part by differences in sampling
techniques (prepositioned electrofishing grids in 1994 versus boat
electrofishing in our study). Trends in fish diversity upstream to
downstream were similar between our findings and those of Travnichek and
Maceina (1994), who found some evidence that river regulation diminished
the number of obligate fluvial specialist species in the Tallapoosa
River. It is important to note that centrarchids were not historically
dominant in the Tallapoosa River (Irwin & Hornsby, 1997). Considering
that catch rates of centrarchids in both our study and in Travnichek and
Maceina (1994) were high downstream of Harris Dam, this supports the
idea that generalist species (such as many centrarchids) may be less
affected by river regulation (Scott, 1951; Swingle, 1953; Kinsolving &
Bain, 1993; Travnichek & Maceina, 1994), and that this trend had not
been altered by implementation of the more recent “Green Plan” flow
modification. While Travnichek and Maceina (1994) observed an increase
in species richness from upstream to downstream across a much broader
study area, their results from within the bounds of this study area did
not yield such a trend, again consistent with our findings.
Irwin and Hornsby (1997) compared rotenone surveys conducted at
Horseshoe Bend in 1951 (pre-Harris Dam) versus 1996 (post-Harris Dam) to
assess the effects of river regulation due to Harris Dam on downstream
fish assemblages. Differences in species composition between these
rotenone studies also suggested that the pre- versus post-dam fish
assemblage at Horseshoe Bend may have shifted from one dominated by
cyprinids and ictalurids to one dominated by centrarchids (Irwin &
Hornsby, 1997). Our results showed a larger relative proportion (by
numbers) of centrarchids versus the 1951 rotenone sample (0.01 in 1951,
0.46 in our study), but a similar proportion to the 1996 sample (0.51 in
1996). In addition, the proportion of cyprinids and catostomids in our
sample was higher than in the 1996 rotenone sample (0.11 in 1996, 0.43
in our study), but similar to the 1951 findings (0.50 in 1951; Irwin &
Hornsby, 1997). Differences in sampling method (electrofishing versus
rotenone), sampling frequency (bimonthly here versus a single sample
historically), and seasonality likely limits direct comparisons.
However, the continued prevalence of centrarchids in the Tallapoosa
River downstream of Harris Dam in our study, Travnichek and Maceina
(1994), and Irwin and Hornsby (1997) suggests that Harris Dam has
affected and continues to affect the downstream fish assemblage.
The presence of significant differences in fish assemblage composition
across sites in ordinal space and the formation of upstream versus
downstream site blocks on the Tallapoosa River suggests that discharge
regulation or habitat modification by Harris Dam was responsible for
much of the variation in fish assemblage structure. Although riverine
fish assemblages naturally vary longitudinally, regulation of the
Tallapoosa River by Harris Dam changes the quality of habitat by
altering temperatures and flow regimes, and by fragmenting the river,
changing natural patterns of fish movement, persistence, and
colonization (Vannote et al., 1980; Irwin & Freeman, 2002; Kiraly et
al., 2014, Irwin et al., 2019). Our study describes patterns that are
influenced by Harris Dam and the resulting flow regulation. The first
multidimensional scaling axis (MDS-1) correlated strongly with the CPE
of ictalurids, percids, and centrarchids, and the tailrace had the
highest scores of any site for MDS-1. The species within those families
that largely influenced this trend tended to be smaller bodied
ictalurids and darters – species that tend to be more rheophilic. This
suggests that the tailrace is primarily occupied by species that are
able to persist in the variable flow conditions, potentially at the
expense of other species including minnows and suckers, that might be
less well-suited for conditions in the tailrace. The number of indicator
species at a site can be considered a measure of how unique that site is
relative to the other sites considered. The presence of more indicator
species at the upstream site supports the conclusion that the Lee’s
Bridge fish assemblage was distinct from the sites downstream of the
dam.
The MDS analysis indicated both spatial and temporal variation in fish
assemblage structure throughout the mainstem Tallapoosa River. Several
previous studies did not include seasonal variation when quantifying the
Tallapoosa fish assemblage (Travnichek & Maceina, 1994; Irwin et al.,
2019). Our results indicate that conclusions drawn from Travnichek and
Maceina (1994) and Irwin et al. (2019) need to be limited to the time
scales they encompassed. Our inability to sample at Lee’s Bridge in
winter unfortunately hampered even broader generalization given that a
model of complete annual variation in fish assemblage might identify
additional patterns.
It is difficult to determine exactly how much Harris Dam has affected
the fish assemblage in the Tallapoosa River given the paucity of data
predating its construction. Longitudinal variation in fish assemblage
structure is inevitable in river systems as energy and nutrient inputs
change (Vannote et al., 1980). Overinterpreting the single rotenone
sample taken before Harris Dam began operation is also questionable
given the habitat heterogeneity of the Tallapoosa River and the limited
spatial and temporal coverage of rotenone sampling (Swingle, 1953).
Outside of this study area, literature indicates that flow regulation,
and especially peaking flow, negatively impacts fish recruitment and
spawning (Weyers et al., 2003; Rolls et al., 2013). The persistent
regulation of the Tallapoosa River by Harris Dam likely resulted in a
changes in the fish assemblage driven by the inability of certain
species to adapt their spawning and feeding habits to the rapid
temperature and flow fluctuation (Rolls et al., 2013).
Releases of water from dams can strongly affect habitat conditions for
fish and other aquatic organisms (Freeman et al., 2005; Young et al.,
2011). Impacts that affect fish at the individual scale can also be
manifested at the population and assemblage scales. Our sampling spanned
a longitudinal gradient that included a site above Harris Dam and three
sites at increasing distances downstream of the dam, allowing us to
examine whether patterns in fish communities are consistent with
expected effects of the dam, namely a recovery gradient in the diversity
or assemblage composition (Travnichek & Maceina, 1994; Ward &
Stanford, 1995; Ward & Stanford, 2001), as well as whether the
implementation of the modified flow regime has had any effects on the
downstream fish community. Previous studies (see below) have quantified
assemblage structure and responses of particular fish populations across
this same reach, allowing comparisons that span a range of temporal
scales.
Across the sampled sites downstream of Harris Dam, we expected to
observe an increase in biodiversity as hypothesized under the serial
discontinuity concept (Ward & Stanford, 1995). Interestingly, while we
did see a recovery gradient in multivariate space downstream of the dam
(where site assemblages became increasingly similar to the upstream,
unregulated site), there was no such gradient in species richness or
Shannon’s H. These species diversity results are similar to those from
other previous studies of the Tallapoosa River fishes. Clearly the use
of simple diversity indices did not capture the changes in fish
assemblage structure that we observed downstream of the dam. Quantifying
more than just simple diversity indices will be required to capture the
recovery gradient in response to disturbance.