Discussion
The immediate impact of rotenone treatments in August 2011 and 2012 was
minor for benthic invertebrates in the rotenone treated River Fusta.
Only a few taxa showed a marked decrease in abundance in the treated
river compared to the untreated River Drevjaelva, including the
Ephemeroptera Baetis rhodani and the Plecoptera generaDiura and Isoperla . Because the upstream Lake Fustvatnet
was treated two months after the last river treatment, River Fusta also
received rotenone from the lake. Still, the immediate effect of the lake
rotenone treatment on benthic invertebrate community in the downstream
river was minor. One exception was the Trichoptera Rhyacophila
nubila, which occurred at high abundances in both rivers, but which
disappeared from the River Fusta subsequently to the treatment of the
upstream lake. All the above-mentioned taxa are known to be rotenone
sensitive (Engstrom-Heg et al., 1978, Arnekleiv et al., 2001, Gladsø &
Raddum, 2002) and negative impacts were expected.
The rotenone treatment of Lake Fustvatnet took place in October, just
before the lake ice formed. Breakdown of rotenone was thus slowed down
due to low temperatures and limited light. Consequently, the treated
river was exposed to rotenone from the lake for several months. Even
though the toxic effect of rotenone decreases with decreasing water
temperatures (Meadows, 1973, Kjærstad et al. 2015), the long exposure
apparently had a pronounced negative effect. In June 2013, eight months
after the lake treatment, we recorded the lowest abundance of benthic
invertebrates in the treated river. A similar decrease did not occur in
the untreated river. The benthic invertebrate community in the treated
river at this time were dominated by Chironomidae, Oligochaeta, Nematoda
and elmid beetles. Other taxa occurred at very low abundances and
several taxa were missing, including rotenone tolerant species like the
Ephemeroptera Ephemerella . Moreover, despite that the freshwater
pearl mussel (Margaritifera margaritifera ) is known to be
rotenone tolerant (Dolmen et al., 1995), all known specimens in the
treated river died during the spring of 2013 (Larsen, 2015). The
temporal beta diversity confirms that the changes in taxa composition
were by far highest, and significant, in the treated river from October
2012 to June 2013. This decline in benthic invertebrates was mainly due
to losses both in terms of taxa abundances and presence-absence. This
indicates that rivers may receive rotenone for several months following
treatment of upstream lakes, which can have major impacts. Most taxa in
the treated river, including rotenone tolerant taxa, decreased to very
low abundances and some disappeared, like the freshwater pearl mussel.
Despite the strong negative impacts recorded in the treated river eight
months after the lake rotenone treatment, recolonization was relatively
fast. The highest gains in terms of species occurrence in the treated
river occurred in August 2013. Already in August 2014 the total
abundances exceeded pre-treatment levels. In August and October 2016,
the change in taxa composition, in terms of abundance and
presence-absence of taxa between sampling occasions, were rather similar
for the treated and the untreated river. This indicates that the benthic
fauna of the treated river had recovered to a great extent. Several
taxa, like the Ephemeroptera Heptagenia dalecarlica andEphemerella , the Plecoptera Isoperla and Amphinemura
borealis and Oligochaeta reached much higher abundances in 2013, as
compared to pre-treatment levels. Ephemeroptera and Plecoptera are known
to be among the most abundant groups in the drift of swift-flowing
temperate streams (Brittain & Eikeland, 1988). These groups likely
colonized rapidly from drift in untreated tributary streams, although
some may have survived as eggs. Oligochaeta may have avoided rotenone
exposure by digging deeper into the substratum. It took almost four
years after the last rotenone treatment before most of the taxa had
recolonized. The Plecoptera Taeniopteryx nebulosa were among the
slowest to recolonize and appeared in 2016. Also, two rotenone tolerant
taxa, the Ephemeroptera Ephemerella and elmid beetles did not
reached pre-treatment abundances in the treated river four years after
the treatments.
According to Vinson et al. (2010), the overall invertebrate abundances
generally return to pre-treatment levels quicker than the biodiversity
and taxonomic composition measures. This is in accordance to our
findings. However, the recolonization will greatly depend on the
taxonomic resolution of the data. Biodiversity measured at family and
genera level will recolonize fast, while it may take longer before all
species to recolonize. We have identified some taxa to species level,
such as Ephemeroptera, Plecoptera and Trichoptera, and some taxa to a
higher taxonomic level, such as the specious Chironomidae and
Hydrachnidia. Thus, changes in abundance and recolonization patterns of
many species in this study remain unknown.
A reduction of total benthic invertebrate abundance was observed both in
the treated lake and untreated lake immediately after the lake
treatment, but to a higher extent in the treated lake. This indicates
both natural changes and a negative effect of rotenone. Taxa with
clearly decreasing abundances shortly after the treatment in Lake
Fustvatnet, but not in the untreated Lake Drevvatnet, included the
Ephemeroptera Centroptilum luteolum and the PlecopteraCapnia sp. While Capnia is known to be rotenone sensitive,C. luteolum is quite tolerant (Kjærstad & Arnekleiv, 2011). The
reason for the seemingly high sensitivity of C. luteolum to
rotenone in the treated lake is unknown.
The recolonization in terms of abundance of benthic invertebrates in the
treated lake was fast, reaching pre-treatment levels eight months after
the treatment. A few increased in abundance after the treatment in the
treated lake, but not in the untreated lake. This was particularly
evident for the diving beetles (Dytiscidae). Beetles are generally known
to be very rotenone tolerant (Engstrom-Heg et al., 1978). Some will also
benefit from increased prey availability because of the general increase
in invertebrate abundance after the recolonization. Even though most
taxa increased in abundance during the years following the treatments,
the Plecoptera Capnia , the Trichoptera Polycentropodidae and the
Gastropoda Gyraulus acronicus decreased and had not reached their
pre-treatment levels in the treated lake four years after the treatment.
Considering that Gastropoda are rotenone tolerant (Holocombe et al.,
1987, Arnekleiv et al., 1997), the decline of G. acronicus in the
treated lake was surprising. Mangun & Madrigal (1999) found that of
several taxa missing five years after rotenone treatments, most belonged
to rotenone sensitive groups, like Ephemeroptera, Plecoptera and
Trichoptera. Taxa that were tolerant to rotenone were identified as”
non-missing“ or ”briefly missing“ (Mangun & Madrigal, 1999).
Fish may play an important role in structuring benthic invertebrate
communities (e.g. Weyl et al., 2010, Alexiades & Kraft, 2017). Stocking
of Atlantic salmon (S. salar ) took place in the treated river
between 2013 and 2016, whereas Arctic charr (S. alpinus ) and
brown trout (S. trutta ) were stocked in the treated lake in 2014.
A few benthic invertebrate taxa did not reach pre-treatment abundance.
However, if fish was the main driver, we would expect a decrease in
abundance of other common taxa that are eaten by fish in the years after
the treatments. Instead, many taxa increased in abundances and exceeded
pre-treatment levels, indicating a relatively low predation pressure
from fish in the treated river. In the treated lake, contrary to the
untreated lake, we observed a strong decrease in total abundances of
benthic invertebrates in August 2015. This was most likely due to
predation from fish stocked during the previous year. The decrease was
particularly high for Chironomidae and Ephemeroptera. Abundance of large
sized and important fish prey, like the Ephemeroptera Ephemera
vulgata , increased to a high abundance in 2013-2014, but decreased
drastically in 2015 in the treated lake, probably due to predation from
fish. The species was not found before the treatment, but occurred in
all sampling events after the treatment, indicating a lower predation
pressure from fish after the treatment. Despite the rotenone treatments
and the fish removal/fish stocking, the abundance of many taxa varied to
a similar extent in the impact and control sites. As an exception, the
effect was more prominent in the impacted treated river than in the
untreated river after the lake treatment and subsequent months of
rotenone exposure. This illustrates the importance of using control
sites when performing time-series experiments.
It is striking to observe the large fluctuations in the abundance and in
the presence-absence of benthic invertebrates over time of both treated
and untreated localities. Even before the lake treatment, the
invertebrate abundances in both lakes were reduced by approximately 50%
from August 2011 to August 2012.
According to Vinson et al. (2010),
the accumulation curve for benthic invertebrate genera showed little
inclination for flattening out during a ten-year monthly sampling of a
Utah river. We did not see the same pattern in our data, perhaps due to
fewer taxa present. Even though it will vary greatly with habitat type,
latitude etc., a rising taxa accumulating curve would certainly be found
elsewhere and shows how random the community composition may be
registered at any given time.
Our study found minor short-term (i.e. < one month) effects of
the river rotenone treatments, whereas the lake treatment caused major
temporary negative effects on the lotic fauna eight months after the
treatment. No major short- or long-term (i.e. four years) effects on
lake benthic invertebrate communities where detected. Recovery time of
invertebrate abundance after the lake treatment was fast whereas the
recovery in terms of presence-absence of species were comparably much
longer, lasting for several years. The current study underpins the
importance of performing long-terms studies during investigation of the
effects of rotenone on benthic invertebrates.