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. Introduction 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 - david.white@murraystate.edu. 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. Study Sites 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). Methods 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 Lake, KY. 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. 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. Results 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. Discussion 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 community assembly. 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. Acknowledgments 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. 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