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2006 SOUTHEASTERN NATURALIST 5(2):345–354
Rotifer Hatching from the Sediments of a
Fluctuating Mainstem Reservoir
Christopher J. Albritton1 and David S. White1,*
Abstract - Rotifer hatching was examined for moist sediments collected from three
benthic environments in a mainstem reservoir embayment, Kentucky Lake, KY: a
drawdown zone with an established annual drying and wetting cycle, a floodplain
that is only rarely inundated, and the permanently inundated embayment. Hatching
data were compared with previous published data for laboratory-dried sediments
from the same sites and with bi-weekly water-column samples over a one-year
period. Of the total 44 taxa now recorded from Kentucky Lake, 18 species hatched
from moist sediments and six from completely dried sediments. Taxa that emerged
from moist and dried sediments were common to temporary and littoral habitats
indicating that they may be better adapted for surviving diapause in a fluctuating
reservoir environment. Evidence further suggests that the degree of sediment drying
in drawdown and floodplain zones may affect egg hatching and that human-induced
reservoir water level fluctuations may play a role in rotifer community dynamics by
increasing or reducing rotifer community diversity in a single year.
Reservoirs are human-constructed ecosystems (Thornton et al. 1990,
White 1990) with water levels that are dependent on rainfall patterns as well
as dam manipulations (Wetzel 1990). Kentucky Lake is a mainstem (large
river) reservoir on the Tennessee River. Its seasonal water-level fluctuations,
which generally consist of a low water level at winter pool and a high
water level at summer pool, establish extensive littoral areas that are subject
to predictable periods of drying and wetting. Because of relatively mild
winters, exposed sediments rarely experience freezing. Less predictable are
water-level changes in response to flood storage. Flood events may occur at
any time of the year and may inundate surrounding floodplains beyond what
is normally part of the summer pool.
Rotifers are common in freshwater habitats from temporary ponds to
large lakes, including reservoirs (Wallace and Snell 2001). Many species
deposit resting eggs in lake-bottom sediments, creating large diapausing
egg banks. Diapause is an important aspect of rotifer community dynamics
and serves to provide certain species with survival advantages during
unfavorable environmental conditions (Gilbert 1974, Ortega-Mayagoitia
et al. 2000, Ricci 2001). When conditions are favorable, environmental
cues trigger eggs to hatch (Hairston et al. 1995, Pourriot and Snell 1983).
Eggs of some species, including desiccation-tolerant bdelloid rotifers,
1Hancock Biological Station and Center for Reservoir Research, 561 Emma Drive,
Murray, KY 42071. *Corresponding author - firstname.lastname@example.org.
346 Southeastern Naturalist Vol. 5, No. 2
may withstand drying when bottom sediments are exposed and hatch or
excyst when sediments are rewetted, while eggs of other species may
hatch following other specific environmental cues, but may not be able to
withstand prolonged drying (Albritton and White 2004; Gilbert 1974;
Marcus et al. 1994; May 1986, 1987; Snell et al. 1983). Some eggs can
remain viable in sediments for decades and enable populations to survive
for years with little to no reproduction in the water column (Gilbert 1974,
Hairston et al. 1995), perhaps resulting in enhanced species richness and
genetic diversity (Hairston 1996).
Our previous work had determined that reservoir water-level fluctuations
in Kentucky Lake influenced littoral rotifer species hatching from sediments
that have been exposed to drying and wetting cycles (Albritton and White
2004). In that laboratory study, only species common to temporary ponds
and littoral zones were observed hatching following sediment rewetting.
Primarily limnetic taxa that were common in the Kentucky Lake water
column did not appear. A variety of both limnetic and littoral taxa, however,
are known to hatch in culture from lake-bottom sediments that have not been
dried (e.g., May 1986, 1987). Thus, sediment drying due to reservoir seasonal
drawdown cycles may selectively promote establishment of littoral
species populations. While previous experiments had demonstrated that
littoral species may be more adapted to sediment drying and rewetting in
Kentucky Lake, it remains unknown whether both littoral and limnetic
species would hatch from floodplain and littoral (drawdown zone) reservoir
sediment that had not been dried. The primary goal of this study was to
examine the assemblage of species that might hatch from littoral- and
floodplain-zone sediments that had not completely dried. These data were
then compared with hatching data from our previous study on dried sediments
and with species present in the water column throughout the year.
These observations should lead to a better understanding of the effects of
reservoir manipulations on plankton composition.
Kentucky Lake, completed in 1944, is the largest and furthest downstream
of nearly 50 reservoirs on the Tennessee River system created by the
Tennessee Valley Authority (TVA). Kentucky Lake is managed for power
generation, flood control, and navigation. The study site was located in
Ledbetter Bay, a mainstem embayment at approximately Tennessee River
km 68 (mile 42) in southwestern Kentucky. The Bay was formed by the
flooding of 4th-order Ledbetter Creek (Fig. 1). Summer pool is maintained at
approximately 109.4 m (359 ft) above MSL from April through August,
while the winter pool is maintained at 107.9 m (354 ft) above MSL from
October through March.
Three sample sites were selected for sediment sampling corresponding
with those in a previous study on dried sediments (Albritton and White
2004). Site 2L was in a permanently inundated portion of the embayment
2006 C.J. Albritton and D.S. White 347
and was covered by about 4 m of water at winter pool. Site 3 was in the
drawdown zone of the embayment that generally is exposed from October–
March. Site 1 was in the floodplain of Ledbetter Creek, at a location flooded
at least twice per year since 1988 (Center for Reservoir Research [CRR],
Murray, KY, unpubl. monitoring data).
The floodplain (Site 1) and the drawdown zone (Site 3) were sampled in
triplicate during winter pool. A 15- x 15-cm box made of Plexiglas 2-cm
deep was pushed into the sediment. All sediment was removed from inside
the box area using a scoop and placed into a 1-L wide-mouth Nalgene
container. The permanently limnetic Site 2L was sampled using a 15- x 15-
cm Ekman grab, from which the top two cm of sediment was carefully
removed and placed in 1-L wide-mouth Nalgene containers.
Samples were transported to the laboratory, and all water from above
the samples decanted. Samples were mixed thoroughly, and 20-ml
subsamples of the mud suspensions were washed into 250-ml Erlenmeyer
flasks. Kentucky Lake water that had been filtered through a 0.7-μm pore
size Whatman GF/F glass fiber filter to remove all zooplankton and eggs
was used to fill the flasks to a total volume of 200 ml, and the flasks were
Figure 1. Sampling site map of the Ledbetter Embayment study area on Kentucky
348 Southeastern Naturalist Vol. 5, No. 2
fitted with foam stoppers. A new airstone was placed in each flask to
saturate the overlying water with oxygen. Bubbling was kept low so as not
to resuspend the sediment. Cultures were kept at 25 ºC and under a 12:12
photoperiod. Lights were 20-watt daylight fluorescent bulbs placed about
30 cm above the cultures.
To collect hatched rotifers, all water overlying the sediment was removed
with a 50-ml pipette and replaced with freshly filtered water (May
1986) every 3–10 days over a 4-month period or until no new species were
observed for at least three weeks. The removed water was filtered using a
0.8-μm pore size GFC filter. The filters were then washed with filtered
Kentucky Lake water into 25-ml glass scintillation vials for initial observations
of taxa present.
Kentucky Lake water-column samples of rotifers were taken from January
to December by filling two 2-L Erlenmeyer flasks with surface water on
a bi-weekly basis. The water was filtered and treated similarly to experimental
Samples were then narcotized using Procaine HCI (May 1985) and
preserved with sugar-buffered Formalin to a final concentration of 4%
(APHA 1998). Individual specimens were slide mounted for final identification
using methods described by Stemberger (1979). A Nikon inverted
microscope was used to examine live specimens and to count and identify
preserved samples. Identifications were based on Edmondson (1959),
Stemberger (1979), and Wallace and Snell (2001). The total numbers of
species and individuals were recorded, and data were graphed and analyzed
using Excel and Systat 9.0.
Eighteen rotifer taxa hatched from the moist sediment cultures (Table 1).
Ostracods, copepods, cladocerans (Bosmina longirostris (Müller) and Alona
sp.), nematodes, and larval water mites also appeared. Eight rotifer taxa
emerged from sediments from all three sites: Lecane tenuiseta, L. inermis, L.
obtusa, L. flexilis, Lepadella triptera, Trichocerca porcellus, Keratella
cochlearis var. cochlearis, and K. cochlearis var. tecta. Lecane inermis and K.
cochlearis var. cochlearis, however, were much less abundant than other taxa.
Rotifers that hatched from one or two sites included two additional taxa of
Lecane, one additional taxon of Lepadella and Trichocerca each, as well as
one taxon each of Cephalodella, Brachionus, Notommata, Conochilus,
Lindia, and the bdelloid rotifer Rotaria (Table 1). Thirty rotifer taxa were
observed in the Kentucky Lake water-column samples throughout the year
(Table 2). This group included 10 taxa that hatched from the moist sediment
cultures and 20 that did not. Additionally, seven taxa hatched from the
cultures that were not observed in water-column samples (Table 2). This
brings the total list of Kentucky Lake rotifers to 44 taxa in 19 genera (Table 2).
Lecane flexilis and L. tenuiseta were abundant at all sites. Lecane
closterocerca hatched only from floodplain- and drawdown-zone sediments.
2006 C.J. Albritton and D.S. White 349
Table 2. Total list of rotifer taxa for Kentucky Lake: 1 = hatched from moist sediment this
study, 2 = hatched from dry sediment (Albritton and White 2004), 3 = collected from water
column this study, 4 = additional genera previously reported from water column by Frey (1996).
Ascomorpha ovalis (Bergendal) 3 L. obtusa (Murray) 1
A. saltans Bartsch 3 L. tenuiseta Harring 1, 2, 3
Asplanchna sp. 4 Lepadella acuminata (Ehrenberg) 3
Brachionus angularis (Gosse) 3 L. ovalis (Jakubski) 2
B. budapestinensis (Daday) 3 L. patella (Müller) 1, 2, 3
B. calyciflorus Pallas 3 L. triptera Ehrenberg 1
B. caudatus (Barrois and Daday) 3 Lindia sp. 1
B. patulus Müller 1 Notommata sp. 1
Cephalodella intuta Myers 1 Ploesoma lenticulare (Herrick) 3
Conochilus unicornis Rousselet 1, 3 P. truncatum (Levander) 3
Euchlanis parva Rousselet 2 Polyarthra remata (Skorikov) 3
Filinia sp. 4 P. major (Burckhardt) 3
Hexarthra sp. 4 P. vulgaris Carlin 3
Kellicottia bostoniensis (Rousselet) 3 Rotaria sp. 1, 3
Keratella cochlearis var. cochlearis (Gosse) 1, 3 Synchaeta stylata Wierzejski 3
Keratella cochlearis var. robusta (Lauterborn) 3 S. kitina Rousselet 3
Keratella cochlearis var. tecta (Lauterborn) 1, 3 S. oblonga Ehrenberg 3
Lecane closterocerca (Schmarda) 1, 2, 3 S. pectinata Ehrenberg 3
L. bulla (Gosse) 1, 3 Trichocerca porcellus (Gosse) 1, 3
L. flexilis (Gosse) 1, 3 T. pusilla (Lauterborn) 3
L. inermis (Bryce) 1, 2 T. rattus (Müller) 1
L. inopinata (Harring and Gosse) 2 T. similis (Wierzejski) 3
Table 1. Days to first observations for all species hatching from moist and dried sediment
experiments. Dried sediment data from Albritton and White (2004).
Moist sediment Dry sediment
Site 1 Site 3 Site 2L Site 1 Site 3 Site 2L
Brachionus patulus 4
Cephalodella intuta 51 12
Conochilus unicornis 113
Euchlanis parva 35 48
Keratella c. cochlearis 12 37 1
Keratella c. tecta 12 4 30
Lecane bulla 66
Lecane closterocerca 12 37 48
Lecane flexilis 16 51 37
Lecane inermis 92 37 99 35 42
Lecane inopinata 66
Lecane obtusa 85 12 37
Lecane tenuiseta 16 92 37 48 102
Lepadella patella 92 37 52 31
Lepadella triptera 18 45 12
Lindia sp. 30
Notommata sp. 51
Rotatoria sp. 85 37
Trichocerca porcellus 135 30 4
Trichocerca rattus 66 37
350 Southeastern Naturalist Vol. 5, No. 2
Conversely, L. obtusa hatched primarily from permanently inundated sediment
(Site 2L). Cephalodella intuta did not hatch from floodplain sediment,
but did appear from drawdown-zone (Site 1) and permanently inundated
sediments (Site 3) (Table 1). Three taxa, Lepadella patella, Trichocerca
rattus, and Rotaria sp., hatched from floodplain and permanently inundated
sediments, but not from drawdown-zone sediment. Five primarily limnetic
taxa, Brachionus patulus, Notommata sp., Lecane bulla, Conochilus
unicornis, and Lindia sp., hatched only from permanently inundated sediment
from Site 2L (Table 1).
Hatching patterns varied widely among taxa and across sites. Some taxa
hatched relatively uniformly over time at all three sites (e.g., Lecane
flexilis, L. triptera; Figs. 2, 3). Other taxa (e.g., L. obtusa, K. cochlearis
Figure 2. Hatching patterns for four rotifer species from Kentucky Lake sediments.
Three replicate sediment samples (A, B, C) were incubated from each site.
2006 C.J. Albritton and D.S. White 351
var. tecta [not figured], L. patella, T. porcellus, L. inermis) had distinct
hatching peaks for specific sites or replicates (Figs. 2, 3). The average
times to first hatching times at all sites for three Lecane taxa—L. flexilis, L.
tenuiseta, and L. inermis—were 69, 81, and 87 days, respectively, and were
consistently later than for other taxa. Conversely, Keratella cochlearis var.
tecta hatched relatively soon after rewetting at all sites, with a mean initial
hatching time of 34 days (Fig. 2). Hatching time varied among sediments
from the three sites (Figs. 2, 3). Lepadella triptera hatched with a mean of
30 days at Site 1, 18 days at Site 2L, and 51 days at Site 3. Similarly,
Lecane obtusa appeared early at Site 3 (mean = 33 days) and 2L (mean =
41 days), but not until much later (mean = 85 days) for Site 1 moist
sediments. Trichocerca porcellus appeared early at Site 2L (mean = 15
Figure 3. Hatching patterns for three rotifer species from Kentucky Lake sediments.
Three replicate sediment samples (A, B, C) were incubated from each site.
352 Southeastern Naturalist Vol. 5, No. 2
days), but late at Site 1 (mean = 135 days) and Site 3 (mean = 93 days).
Lecane closterocerca, which hatched at only sites 1 and 3, appeared early
at Site 1 (mean = 12 days) and late at Site 3 (mean = 71 days). Lepadella
patella appeared between 37 and 92 days (Fig. 3, Table 2). The remaining
taxa appeared with no particular pattern with respect to time or site.
The first taxon to hatch appeared at all sites within four days; however,
subsequent taxa hatched more quickly at Site 2L. By day 37, a mean of 7.3
taxa had emerged at Site 2L, compared with only three taxa at Site 1 and 3.3
taxa at Site 3 (Fig. 3). Furthermore, a greater total and mean number of taxa
hatched from Site 2L sediments than from Sites 1 and 3. Seventeen taxa
hatched from Site 2L, 10 taxa from Site 3, and 12 taxa from Site 1 (Table 2).
Overall hatching patterns and total number of taxa hatched were very similar
between Sites 1 and 3.
Most taxa hatching from the rewetted sediments, including the numerically
dominant Lecane tenuiseta and L. flexilis, are common to littoral
habitats (Kuczynska-Kippen 2000, Pennak 1940, Stemberger 1979). Taxa
hatching from the previous experiments on dried sediment were solely
littoral zone taxa (Albritton and White 2004). Only three limnetic genera
(Trichocerca, Keratella, and Conochilus), representing five taxa, emerged
from sediment (Table 2). Of those five taxa, however, only Keratella
cochlearis var. tecta and Trichocerca porcellus were present in sediments
from all sites. One taxon of Brachionus, which is considered a littoral genus
but is often common among the limnetic plankton (Stemberger 1979), also
hatched from dried sediments from the permanently inundated Site 2L.
The dominance of littoral taxa hatching from both dried (Albritton and
White 2004) and moist sediment experiments, despite a large number of
limnetic taxa in the total population, may be the result of a number of
factors. The greater number of taxa hatching from moist sediments suggests
that sediment drying may inhibit hatching for some species. Further, the
abundance of littoral taxa in both treatments suggests that the eggs of littoral
taxa may be better adapted to surviving drying, but some littoral taxa may
not require drying as a hatching cue. Conversely, 14 taxa, many of which
were limnetic, hatched only from moist sediments. Three littoral taxa—
Euchlanis parva, Lepadella ovalis, and Lecane inopinata—hatched from
dried sediments, but not from moist sediments. Many of the taxa hatching
under both dry and moist treatments and under dry treatment alone are
common to wetlands that experience wide hydrologic fluctuation (Ortega-
Mayagoitia et al. 2000). Thus, sediment drying followed by rewetting may
be a hatching cue for taxa commonly found in hydrologically variable
habitats, but not for limnetic taxa. Furthermore, the presence of bdelloids,
taxa with known abilities to withstand desiccation (Ricci 2001, Wallace and
Snell 2001), along with additional taxa of drying-tolerant genera in the
2006 C.J. Albritton and D.S. White 353
non-dried sediments suggested that the degree of sediment drying might
affect which taxa will emerge. Extremely dry conditions at winter pool or an
extended maintenance of winter-pool water level may alter the composition
of rotifer taxa recruited from sediments in the next spring compared with the
composition following more normal winter seasons. Thus reservoir-level
fluctuations may add further complexity to the mechanisms determining the
The first rotifers appeared within 12 days at all sites in the moistsediment
incubations, but not until at least 30 days in the dry sediment
experiment (Albritton and White 2004). Therefore, drying may delay hatching
or decrease the responsiveness of eggs that are able to survive dry
conditions in the upper two cm of sediment. The responsiveness of older
eggs from greater sediment depths, however, appeared to be less affected by
being dried (Albritton and White 2004). The increased number and abundance
of taxa hatching early from permanently inundated moist sediments
(Table 1) suggested also that the eggs in these sediments might respond
quickly to environmental cues.
Studies have shown that temperature, light, oxygen concentration, salinity,
and maternal diet may be cues to breaking diapause (Gilbert 1974, May
1986, Pourriot and Snell 1983); thus, our focus solely on wetting-drying
cycles, may not accurately produce all of the diapausing species in Kentucky
Lake sediment. Multiple cues are likely responsible for hatching in many
species, and the annual community structure will reflect the cues provided in
a given year. Reservoir water-level fluctuations appear to have a definite
influence on the composition of rotifer hatching from the sediments.. Rotifer
egg banks, while providing a unique and vital means of population maintenance,
are highly sensitive to changing environmental conditions. Despite
environmental variability, the existence of large and diverse egg banks
maintains rotifer diversity over periods of years.
We appreciate the assistance of Gary Rice in collecting sediments and other
logistical support. Peder Yurista (US EPA, Duluth, MN) and Jeff Jack (University
of Louisville) gave a number of suggestions in sampling design and in the handling,
mounting, and identification of rotifers. Carl Woods assisted with graphics.
Funding was provided by National Science Foundation grant DBI 9978797. This
paper is Contribution Number 109 from the Center for Reservoir Research,
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