Competition Between Native and Exotic Daphnia: In situ Experiments

Abstract

The recently introduced exotic cladoceran Daphnia lumholtzi offers an excellent opportunity to study the interactions between exotic and native species in invaded communities. Lake surveys in Missouri have indicated a seasonal succession between native Daphnia and D. lumholtzi. In the current study, we examined competition between D. lumholtzi and the native Daphnia parvula by conducting seasonal in situ field experiments in 1.6 l enclosures. Competition was assessed by comparing the rates of increase (r) and birth rates (b) of each species when grown alone versus when grown together in these enclosures. At high densities, D. lumholtzi suppressed D. parvula rates of increase during the late summer and fall experiments, but did not appear to suppress D. parvula birth rates. The rates of increase of D. lumholtzi did not appear to be affected by the presence of D. parvula. The results of these experiments indicate that although competition between the two species occurs seasonally at high densities, the effects are asymmetrical. The lack of competitive effects on D. lumholtzi byD. parvula suggests that factors other than competition are involved in explaining the absence ofD. lumholtzi in spring zooplankton assemblages.

INTRODUCTION

Competition has usually been considered to play a secondary role to predation in determining the species composition of zooplankton communities (Dodson, 1974; Bengtsson, 1987; DeMott, 1989). However, recent evidence indicates that competition between zooplankton is important (Bengtsson, 1987; DeMott, 1989; Boersma, 1995; Hu and Tessier, 1995; Schulze et al., 1995) and may help explain the seasonal patterns of zooplankton abundance (DeMott, 1989). The seasonal cycles of temperature and stratification in temperate lakes affect nutrient concentrations and the composition and abundance of phytoplankton (Porter, 1977; Lampert and Sommer, 1997). The rapidly changing phytoplankton community, in turn, changes the quality and quantity of food resources available for zooplankton, potentially leading to changes in the outcome of zooplankton resource competition (DeMott, 1989; Hu and Tessier, 1995). If zooplankton species differ in their environmental optima, shifts in relative competitive abilities can occur whenever environmental conditions change (Threlkeld, 1979; Tessier, 1986; Bengtsson, 1987; Hu and Tessier, 1995). Change in any environmental factor that influences ingestion, assimilation, respiration or growth rates may reverse the relative abilities of competing species to grow and reproduce when resources become scarce (DeMott, 1983).

The idea that seasonal succession within zooplankton assemblages involves temporal reversals in the exploit-ative ability of species has been supported by field experiments. For example, in situ competition experiments with Daphnia rosea and Daphnia pulicaria in large enclosures showed that inter- and intraspecific competition limited their population growth. Daphnia rosea had a competitive advantage in the spring and early summer, but was rapidly replaced by D. pulicaria during the late summer and autumn (DeMott, 1983). The decline of D. rosea during the summer was found to be a direct result of competition with D. pulicaria. Similarly, populations of Daphnia galeata mendotae in a Michigan lake had higher birth rates, higher death rates and higher rates of increase than D. pulicaria when the two were placed in direct competition (Hu and Tessier, 1995). The performance of adult and juvenile D. galeata mendotae increased from June to August. Daphnia galeata mendotae were able to exploit ‘low-quality’ summer resources differently or more effectively than D. pulicaria, whereas D. pulicaria were able to exploit ‘high-quality’ spring resources differently or more effectively than D. galeata mendotae (Hu and Tessier, 1995).

Exotic species are a useful tool for providing new insights into the mechanisms of zooplankton resource competition. These invaders may render previously stable systems unbalanced and unpredictable (Mills et al., 1993; Vitousek et al., 1996), and cause perturbations to native species and the physical environment of invaded systems (Carlton, 1996). Exotic species often lack natural predators and other mechanisms for population control that are present in their native ranges (Johnson and Carlton, 1996), and exotics may subsequently become sufficiently abundant to have profound effects on the native communities they have invaded (Vitousek et al., 1996). Although invading species tend to have a number of life history characteristics that contribute to their ability to invade new environments (Ehrlich, 1986), these invaders must also be successful in their biological interactions with native species. Native species that are superior competitors would probably hinder the establishment of a new invader, whereas superior competitive ability by an invading exotic species would probably facilitate its establishment in the community (Orians, 1986). The recently introduced exotic cladoceran Daphnia lumholtzi (Sorensen and Sterner, 1992; Havel and Hebert, 1993) offers an excellent opportunity to study the interactions between exotic species and their invaded communities.

Daphnia lumholtzi is native to Africa, Asia and Australia (Benzie, 1988). It was first reported in the USA from a lake in eastern Texas (Sorensen and Sterner, 1992) and by 1991 was observed in numerous sites in southwestern Missouri and the southeastern USA (Havel and Hebert, 1993). In reservoirs of the south-central USA, D. lumholtzi is typically undetectable in zooplankton samples until late spring, reaches a population peak during late summer, and is undetectable by late fall (Goulden et al., 1995; Havel et al., 1995; Kolar et al., 1995). In contrast toD. lumholtzi, peak densities of native Daphnia in this region typically occur in the winter and spring (Kolar et al., 1995; Work and Gophen, 1995; J. E. Havel and M. E.Eisenbacher, unpublished data). This seasonal succession suggests that midsummer conditions of south-central lakes (e.g. high temperature and poor food quality) favor the exotic D. lumholtzi over the native Daphnia species, and vice versa in the spring and winter (cooler, better food quality).

The invasion of D. lumholtzi into new habitats provides the potential for competition between this exotic and native Daphnia species. The observation that populations of D. lumholtzi tend to achieve their highest densities during periods when most native Daphnia species are rare is consistent with niche segregation, such as has been described for other Daphnia species (DeMott, 1989). The effects of D. lumholtzi on native zooplankton communities are currently unknown. If D. lumholtzi is common only during periods when native Daphnia are absent, the potential for competition is limited. If, on the other hand, D. lumholtzi becomes common during a period that overlaps with native Daphnia, competition is possible. A preliminary study on the Norris Reservoir, Tennessee, suggested that D. lumholtzi was not displacing other Daphnia, although this invasion may be too recent for researchers to detect ecological impacts (Goulden et al., 1995). Monthly sampling, since 1992, from Stockton Lake, Missouri, also indicates that D. lumholtzi has not displaced native taxa (J. E. Havel and M. E. Eisenbacher, unpublished data). In contrast, samples collected by the Illinois Natural History Survey found that after the 1992 invasion of D. lumholtzi into Lake Springfield, Illinois, native zooplankton populations declined (Kolar et al., 1995).

The current study explored competition between the exotic species D. lumholtzi and Daphnia parvula, a native species common in reservoirs of the south-central USA. Daphnia lumholtzi is generally larger than D. parvula. In lakes of southwest Missouri, minimum and maximum adult core body sizes for the two species are 0.9 and 1.3 mm for D. lumholtzi, and 0.7 and 1.0 mm for D. parvula. Prior studies of other Daphnia species have indicated a high overlap in their diet, providing the potential for food resource competition [reviewed in (DeMott, 1989)].

The current study was designed to test for seasonal reversals in competitive abilities between the two species. Between June 1997 and January 1998, we conducted four in situ experiments in a local reservoir. Since competition depends upon the presence of a limiting resource, knowledge of the food levels in the enclosures was important. We conducted dye tests to determine the water exchange rates of the enclosures and measured algal biomass [as chlorophyll (Chl) a] in the enclosures and in the lake. Since the enclosures allowed exchange of water and food, but excluded large zooplankton and fish, the population dynamics of D. parvula and D. lumholtzi were not affected by predation.

Performance of each species during each experiment was assessed by estimating rates of increase and birth rates. We expected D. lumholtzi to have higher performance than D. parvula during the late summer, when food quality is poor and D. lumholtzi is typically the dominant Daphnia species in the zooplankton. Furthermore, we expected D. lumholtzi to suppress the performance ofD. parvula during this time. We predicted a reversal in performance during the early summer, fall and winter, when food quality is high and D. parvula is ordinarily common. During this time, we also expected D. parvula to suppress the performance of D. lumholtzi.

METHOD

Study site

McDaniel Lake is a eutrophic and monomictic reservoir, located in Greene County, Missouri (37°18′N, 93°18′W). The lake has an area of 1.0 km² and an average depth of 3.6 m (Youngsteadt et al., 1989). Like many reservoirs in the region, summer epilimnetic temperatures are quite warm (~30°C). Thermal stratification in McDaniel Lake is typically established by the end of April and continues through September (Youngsteadt et al., 1989). Depth profiles indicate that the hypolimnion develops anoxia below 5 m (Cacka, 1998). During the summer, the average total phosphorus and Chl a concentrations are 48 and 28.5 μg l^–1, respectively (Youngsteadt and Gumucio, 1994). During 1997, blue-green algae reached peak densities in the lake in June, and comprised ~50% of total phytoplankton cells from June until September (N. W. Youngsteadt and R. J. Gumucio, unpublished data). During the same year, diatom numbers began increasing in the lake in early November and achieved peak densities in December, when they comprised 60% of the total phytoplankton (N. W. Youngsteadt and R. J. Gumucio, unpublished data). McDaniel Lake is one of two reservoirs that provide Springfield, Missouri, with drinking water, and has a history of occasional taste and odor problems (N.Youngsteadt, personal communication). Because of growing demands for water, McDaniel Lake has received supplementary water piped from Stockton Lake, Missouri, since 1992. Since Stockton Lake was previously invaded byD. lumholtzi (Havel et al., 1995) and this exotic first appeared in McDaniel Lake shortly after the pipeline opened(J. Cacka, personal observation), this pipeline was the likely conduit for introducing D. lumholtzi into McDaniel Lake.

Zooplankton succession and dynamics in McDaniel Lake

Zooplankton samples were collected in order to determine annual trends in zooplankton composition, Daphnia abundance and short-term population dynamics of lake Daphnia. These population dynamics data could then be compared with the population dynamics of Daphnia in the experimental enclosures. Zooplankton were collected from the lake by depth (0, 2, 4 and 6 m) at the beginning, middle and end of each experiment using a 12 l Schindler trap with a 100 μm mesh bucket. Zooplankton were anesthetized using Alka-Seltzer® and then preserved using cold sugar-buffered formalin (Prepas, 1978). Sample sites corres-ponded to the location of the (fixed) experimental buoys (see ‘Enclosure design’ below), except during the first experiment. During the first experiment, sample sites in the lake were selected randomly. Zooplankton were identified for each sample date, using established keys to species for the Cladocera (Brooks, 1959; Korínek, 1981; Berner, 1982), and to order for juvenile and adult copepods (Williamson, 1991).

Daphnia population dynamics in the field were examined during each experiment. Population parameters were typically estimated only for the most abundant Daphnia species, since all but one species tended to be rare. However, during August 1997, there were enough Daphnia retrocurva and D. parvula to provide estimates for both species. The Daphnia from all depths were pooled to determine population parameters. The rate of increase (r in day^–1) was determined using the rearrangement of the exponential growth equation:

\[\mathit{r}\ =\ (ln\ \mathit{N}_{\mathit{t}}\ {\mbox{--}}\ ln\ \mathit{N}_{0})\ \mathit{t}^{{\mbox{--}}\ 1}\]

where N₀ is the population density at time zero and N_t is the population density at time t, in days. This equation assumes exponential growth and no immigration (Lampert and Sommer, 1997).

The instantaneous birth rate (b) was determined from egg ratio measurements using the equation:

\[\mathit{b}\ =\ (ln\ (\mathit{E}\ +\ 1))\ \mathit{D}^{{\mbox{--}}1}\]

where b is offspring female^–1 day^–1, E is the egg ratio (total eggs/total females) and D is egg development time in days (Paloheimo, 1974). Egg ratios were determined by counting eggs in 100 randomly chosen females in each sample. Egg ratios were not calculated for samples with <100 females. Egg development times at each temperature were based on equations in Hebert (Hebert, 1978). The temperature at which egg development occurred was determined by taking the average temperature from 0–6 m during the course of the sampling period.

Enclosure design

Our field experiments were conducted in 1.6 l clear plastic cylinders (O'Brien and Kettle, 1981), nested in three hinged, locking trays, with each tray holding nine cylinders (Figure 1). Each tray constituted an experimental block and was suspended at a depth of 2 m from a randomly placed anchored buoy. The depth of 2 m allowed the cylinders to be suspended well below the surface, to avoid UV radiation and short-term thermal discontinuity, and still remain in the oxygenated epilimnetic waters. Dissolved oxygen concentrations at 2 m during the experiments never fell below 5.9 mg l^–1 (Cacka, 1998). Both ends of the cylinders were covered with 53 μm Nitex mesh (Research Nets, Bothell, WA), except for the June experiment (100 μm). The 53 μm mesh size prevented colon-ization of the cylinders by most other zooplankton, while still allowing water and phytoplankton exchange between the lake and the cylinders (Cacka, 1998). The 100 μm mesh used in the June experiment was found to allow too high an exchange rate (see Results), as well as a higher rate of colonization by other zooplankton (Cacka, 1998). We also modified the location of the cylinders after the first experiment. During the first experiment, the cylinders were suspended 1 m from a dock, located near the McDaniel Lake dam. The littoral fauna growing on the dock and dam colonized the cylinders (Cacka, 1998), so for the remaining experiments the cylinders were suspended by buoys in three randomly chosen pelagic sites.

The O'Brien and Kettle (O'Brien and Kettle, 1981) field enclosure design was modified (Figure 1) to facilitate sampling of water from the enclosures (Cacka, 1998). Tygon® tubing (0.4 cm diameter) was inserted into each cylinder and extended to the lake surface. Nitex mesh covering the end of the tubing inside the cylinder prevented loss of Daphnia from the cylinders when water was sampled. Water from the cylinders was used to estimate the water exchange rates of the cylinders and to compare Chl a levels.

Water exchange rate and Chl a in enclosures

Kerfoot and DeMott discussed the importance of water exchange in competition experiments (Kerfoot and DeMott, 1980). A rapid water exchange rate decreases the chance of ‘enclosure effects’, but the resulting rapid renewal of resources may prevent intense competition since the animals in the enclosures cannot strongly deplete their food resources (Kerfoot and DeMott, 1980). On the other hand, in a completely closed system with no water exchange, algae rapidly settle out and the enclosed system becomes both biologically and chemically different from the lake water (Kerfoot and DeMott, 1980). Since exchange rates were important in determining the resources available during our competition experiments, we measured water exchange rates with dye tests.

Water exchange rates in the cylinders were estimated at the beginning and end of each experiment by measuring the decrease in dye concentration with time. We expected first that dye concentrations in cylinders would decline with time. This monotonic decline allowed the determin-ation of water exchange rates. Second, we expected that water exchange rates in the cylinders would become slowed over the course of the experiments as the mesh became clogged with debris.

One randomly placed cylinder in each tray was kept free of Daphnia and used for the dye tests and as a control for Chl a measurements. While the cylinders were suspended at 2 m depth, blue food coloring was introduced into these cylinders via the Tygon® tubing. Since the concentration of blue food coloring in sealed containers did not decrease over time (Cacka, 1998), any observed changes in dye concentrations in our open cylinders were attributed to hydraulic exchange with the lake and not to decomposition of the dye. Samples for dye measurements were withdrawn from each cylinder at 0, 0.5, 1.0, 1.5 and 2.0 h after the introduction of the dye (Cacka, 1998). We used simple linear regression of log-transformed concentrations to estimate the slope of the dye dilution function. The antilog of this slope represents the fractional change in volume per unit time. One minus this antilog provides an estimate of the water exchange rate, or the fraction of cylinder water that is being exchanged per unit time.

Chlorophyll a samples were collected from each of the cylinders at the beginning, middle and end of the experiments. For comparison with the open water, Chl a samples were also collected from the lake at a depth of 2 m using a Kemmerer bottle. A 250 ml subsample was filtered through a 1.2 μm GF/C filter (Whatman), stored withdesiccant at –10°C and then analyzed using the method from Golterman et al. [(Golterman et al., 1978), cited in (Havel, 1996)]. Within each experiment (below), Chl a from the cylinders and lake was sampled according to a two-way ANOVA model (lake versus cylinder, date within experiment). However, because of failed homogeneity of variance assumptions (Sokal and Rohlf, 1995), separate one-way comparisons were made. One-way ANOVA and Tukey tests or Kruskal–Wallis and Mann–Whitney U-tests were used to determine whether there were significant differences in Chl a concentrations in the cylinders among dates within an experimental period (season). Two-sample t-tests or Mann–Whitney U-tests were used to determine significant differences between the Chl a concentrations in the cylinders and the lake water on any given date.

Competition experiments

Competition experiments were conducted four times during 1997–98: once in June (early summer), once in August (late summer), once in October (fall) and once in December/January (winter). These four experiments encompassed the full range of temperature and food quality typically measured in McDaniel Lake. The dur-ation of each experiment was designed to approximate 1.5 Daphnia generations, based upon the water temperature at the beginning of the experiment and the generation time–temperature function of Hebert (Hebert, 1978). The June experiments (temperature range 24–27°C) lasted10 days, the August experiment (27–30°C) lasted 5 days, the October experiment (20–23°C) lasted 11 days, and the December–January experiment (5–10°C) lasted 57 days.

The experimental cylinders were inoculated in the field with juvenile Daphnia (~48 h old) that had been cultured in the laboratory under conditions that produced individuals with high lipid indices and large clutch sizes (Dodson and Frey, 1991; Havel and Talbott, 1993). These Daphnia were cultured in filtered lake water and were fed once a day with Ankistrodesmus falcatus, supplemented with an artificial food comprised of fermented yeast, Ceraphyll® and trout chow [‘YCT’; (Weber et al., 1989)]. Since the animals were being introduced into the field at temperatures ranging from 10 to 30°C, thermal shock was avoided by first acclimating 2 l cultures for 48 h in incubators set at the lake temperature. Although other in situ competition studies have usedcladocerans captured in situ just prior to the inoculation of field enclosures (Allan, 1973; Smith and Cooper, 1982; DeMott, 1983; Tessier, 1986; Kerfoot et al., 1988; Hu and Tessier, 1995), we used cultured Daphnia for the current study because the different phenologies of D. lumholtzi and native Daphnia would have precluded finding enough of one or the other species in local lakes during most experiments. This method also avoids effects of past food history in the lake on performance of the Daphnia.

Each cylinder was inoculated with Daphnia from one of the following four treatments: each species alone (controls, C) and both species together at low (L) and high (H) densities (Table I). The nine cylinders per tray allowed for two replicates of each species treatment and one empty cylinder, for the dye tests, per tray. At the end of each experiment, the contents of each cylinder were collected and preserved (Prepas, 1978).

Population parameters were estimated as above. Individual biomass estimates were made using published length–dry weight regressions for D. parvula (Rosen, 1981) and D. lumholtzi (Eisenbacher, 1998). Mean core body size was estimated by measuring all individuals in samples with low densities and 100 randomly chosen individuals in samples with high densities.

Statistical methods

Competition was assessed during each experiment by examining differences in each performance variable between single-species controls and low and high initial density combination treatments (Table I). The single-species control treatments represented growth ofD. parvula or D. lumholtzi in the absence of interspecific competition. The low- and high-density dual-species treatments represented growth in the presence of potential inter- and intraspecific competition for food resources. In the absence of competition, rates of increase of Daphnia should be equal in all treatments.

All statistical analyses were conducted separately for each species. Each performance variable was analyzed using a mixed model two-way ANOVA (Sokal and Rohlf, 1995), with species combination (treatment) as the fixed factor and location (block) as the random factor. When the normality and homogeneity of variance assumptions of ANOVA were not met and transformations failed to correct the problem, the non-parametric Friedman test was used (Sokal and Rohlf, 1995). If significant differences in rates of increase (r) were found between treatments and there was no significant block effect, comparisons were also made using one-way ANOVA and Tukey tests (Sokal and Rohlf, 1995). Cylinders with <100 individuals remaining at the end of the experiment were not included in egg ratio and birth rate analyses.

RESULTS

Zooplankton succession and dynamics in the field

During each experimental period, copepods dominated the crustacean zooplankton of McDaniel Lake (Figure 2). Total Cladocera, dominated by Diaphanosoma birgei, had their highest densities in early and late summer, comprising up to 29% of the assemblage. Daphnia had their highest relative densities in the fall and winter, when they dominated the total Cladocera (Figure 2). Daphnia retrocurva was the dominant Daphnia species in the lake during most of the study period, although bothD. retrocurva and D. parvula were common in August. Daphnia ambigua and D. lumholtzi were present, but rare in the plankton, never exceeding 0.01 l^–1 (Cacka, 1998). Mid-day densities of Daphnia in McDaniel Lake showed a pronounced depth distribution. Daphnia avoided the surface, but had similar densities at 2, 4 and 6 m (Cacka, 1998). Thus, the enclosures located at 2 m were at a position where Daphnia tended to occur in the water column.

During our experiments, total Daphnia in the lake achieved their highest abundance in the fall (10.0 l^–1) and the minimum in summer (1.2 l^–1) (Figure 2). Estimates of Daphnia rates of increase in the lake (mostly D. retrocurva) were generally negative during early and late summer, although large variation in estimates of r during most dates led to rates of increase that were not significantly different from zero (Cacka, 1998). Daphnia parvula was chosen for the competition experiments because in four out of five recent years (1991–96) it was the most abundant Daphnia species in McDaniel Lake, with populations typically persisting from October to May. However,D. retrocurva was the most abundant Daphnia species during the course of the current study. Therefore, with the exception of August, in which D. parvula was also abundant enough to estimate population parameters, lake versus cylinder comparisons were made between different species.

Conditions in the cylinders

The water exchange rate has an important consequence for these competition experiments. Because resources are replenished, higher exchange rates would tend to reduce or delay competition. Measurements with dye indicated that concentrations in the cylinders decreased over time (Figure 3). Based upon the exponential decay functions over each 2 h trial, estimated water exchange rates (e_w) ranged from 15 to 99% of the total cylinder volume per hour. Water exchange rates decreased between the beginning and end of the experiments (Figure 3, closed versus open circles). This result is consistent with the observation that sediments and debris accumulated on the mesh over the course of the experiments, which probably impeded flow.

Chlorophyll a concentrations in the lake varied from 4.8 μg l^–1 in June to 31.9 μg l^–1 in October (Table II). Chlorophyll a concentrations in the cylinders were also lowest in June (5.8 μg l^–1), but were maximal in August (14.8 μg l^–1). For each experiment having multiple measurements (June, August and October), Chl a concentrations declined in the cylinders over the course of the experiment (Table II).

Potential food resource limitation is suggested by comparisons of the Chl a levels in the cylinders with those in the lake. Chlorophyll a concentrations in the cylinders were lower than those in the lake (at 2 m depth) during most of the experiments (Table II). During the August, October and January experiments, sample mean Chl a concentrations in the cylinders were always lower than sample mean Chl a concentrations in the lake, although these differences were significant on only four of nine dates (Table II). Overall, mean Chl a concentrations in the cylinders were reduced relative to the lake by 30% in August, 46% in October and 60% in January. The June experiment was the only one in which Chl a concentrations in the cylinders were never significantly different from the Chl a concentrations in the lake (Table II). The use of a larger mesh size during the June experiment probably allowed for a greater exchange of resources.

Competition experiments

Daphnia parvula had consistently higher final densities than D. lumholtzi (Figure 4). Final densities of each species of Daphnia were highly variable among experiments. In the single-species controls, D. parvula had minimum final densities in August (30 l^–1) and maximum final densities in June (996 l^–1), whereas D. lumholtzi single-species controls had minimum final densities in January (0.5 l^–1) and maximum final densities in June (476 l^–1). Since the individual body mass of D. lumholtzi is greater than that of D. parvula, the greater densities of D. parvula do not always translate into greater biomass. Indeed, in the June and October experiments, D. lumholtzi biomass was greater than D. parvula biomass (Figure 4). Daphnia parvula rates of increase tended to be higher than D. lumholtzi rates of increase, with the exception of the June experiment, in which both species had very similar rates (Figure 4). Daphnia parvula and D. lumholtzi both had minimum rates of increase in December–January (0.01 and –0.03 day^–1, respectively) and maximum rates in June (0.47 and 0.53 day^–1, respectively).

Comparisons of birth rates between species were only possible in October (Table III). In two experiments (August and December–January), D. lumholtzi andD. parvula were too limited in number to make egg ratio estimates and, in June, samples had numerous aborted eggs. In October, D. lumholtzi had significantly higher egg ratios and birth rates than D. parvula (Table III; T = 7.0, P < 0.001). Although D. lumholtzi had high final densities during the June experiment, both males and ephippial females were present in the cylinders. Ephippia were also formed by D. lumholtzi in the cylinders during the August experiment. In contrast, D. parvula ephippia were never observed in the cylinders and D. parvula males were only observed during the August experiment.

During the June experiment, both species achieved high final densities, although no significant treatment effects were found for either species (Table IV; Figure 4). In other words, rates of increase of each species did not depend on the presence of the other species in the cylinders. Since the June 1997 experiment had a different experimental set-up than the remaining three experiments, a follow-up experiment was conducted in June 1998, using the same mesh size (53 μm) and pelagic location as the other experiments. As in 1997, both species achieved high final densities, althoughD. lumholtzi achieved higher densities (619 l^–1) than D. parvula (317 l^–1) (data not shown). Chlorophyll a concentrations in the cylinders during the June 1998 experiment were lower than in the lake and D. parvula final densities were suppressed by the presence of D. lumholtzi (ANOVA, F = 6.27, P = 0.046). Conversely, D. parvula had no significant effect onD. lumholtzi final densities during this experiment (Friedman test, χ² = 2.0, P = 0.157).

During the August experiment, D. parvula populations reproduced successfully in the enclosures, whileD. lumholtzi populations had almost died out by the end of the experiments (final densities 0–4.4 l^–1; Figure 4). However, although D. parvula had higher densities,D. lumholtzi biomass was similar to D. parvula biomass (Figure 4). Analyses of rates of increase suggest thepresence of a treatment effect. Daphnia parvula rates of increase were significantly higher in the single-species control treatment than in either combination treatment, indicating that the presence of D. lumholtzi suppressedD. parvula rates of increase (Table IV; Figure 4). Daphnia parvula rates of increase in the high combination treatment were significantly lower than in the low combination treatment, suggesting additional effects of intraspecific competition. No significant differences in the rate of increase among the D. lumholtzi treatments were found (Table IV), but low final numbers may reduce the power of statistical tests. Although not statistically significant, D. lumholtzi final density, biomass and rate of increase are higher in the control than either combination treatment, and the rate of increase is higher in the low combination treatment than in the high combination treatment (Figure 4). These differences point to both inter- and intraspecific effects.

During the October experiment, both species performed well in the cylinders, with the final density of D. parvula in the controls (264 l^–1) exceeding that of D. lumholtzi (142 l^–1). Daphnia parvula had significantly higher rates of increase than D. lumholtzi (Figure 4; Mann–Whitney, P = 0.012). However, D. lumholtzi had significantly higher birth rates than D. parvula (two-sample t, P < 0.001) (Table III). The difference between results for r and b between species suggests that D. lumholtzi suffered higher death rates in the cylinders than did D. parvula during this period. Competitive effects were evident in the October experiment (Figure 4) as the performance of D. parvula (as N_t) was negatively affected by the presence of D. lumholtzi (Tukey test, P < 0.05). In contrast, D. lumholtzi final densities were not significantly affected by the presence of D. parvula(t-test, P > 0.05), although final density and biomass were lower in the high combination treatment (Figure 4). Rates of increase for both species appear to be lower in the high combination treatment, although no significant differences in r or b were found (Table IV; Figure 4). The unexpected difference between statistics for N_t and r may be an artifact of having used the less powerful Friedman test for inferences on r.

During the winter experiment, D. parvula populations reproduced successfully, showing a positive rate of increase, while D. lumholtzi populations exhibited negative rates of increase and had nearly died out by the end of the experiment (Figure 4). For both species, final densities were highly variable and thus values of r showed no significant differences among treatments (Table IV). However, the D. parvula rate of increase is higher in the control and low combination treatments than the high combination treatment, suggesting intraspecific competition.

DISCUSSION

The observed seasonal succession of the exotic D. lumholtzi and native Daphnia species in the reservoirs of the south-central USA allows clear predictions about their relative performance in competition experiments. We anticipated that D. parvula would have better performance (higher rates of increase and birth rates) during the competition experiments conducted in the early summer (June), fall (October) and winter (December–January), and thatD. lumholtzi would have better performance during the late summer experiment (August). We also expected thatD. parvula would be competitively superior (i.e. suppressD. lumholtzi) during the June, October and December experiments, and that D. lumholtzi would be competitively superior during the August experiment.

With the exception of the August experiment, the relative performances of D. parvula and D. lumholtzi in the cylinders met our expectations. Both D. parvula andD. lumholtzi performed well in the cylinders during the June and October experiments, and the performance of D. parvula exceeded that of D. lumholtzi. Final densities and rates of increase of both species were significantly higher during these experiments than those during the August and December experiments, consistent with the good growth conditions during June and October. SinceD. lumholtzi are native to the tropics (Benzie, 1988), we anticipated that this species would not perform well in the cold conditions of the December experiment. As expected, D. parvula continued to grow in December, whereas D. lumholtzi was extirpated. The relative performance of the two species in August was unexpected, asD. parvula achieved higher densities than D. lumholtzi. Late summer is the period when D. lumholtzi is ordinarily numerically dominant in Missouri lakes (J. E. Havel and M. E. Eisenbacher, unpublished data).

Daphnia performance is known to be affected by several environmental factors. Food levels that support growth at low temperatures may be inadequate at higher temperatures (Neill, 1981; Orcutt and Porter, 1984). The quality of food resources available can also influence Daphnia survivorship and fecundity (De Stasio et al., 1995). Several species of blue-green algae, for example, are inedible to Daphnia, due to toxins, deficiency in essential nutrients and/or mechanical interference (Moore et al., 1996; DeMott and Müller-Navarra, 1997), and can suppress survivorship, growth and fecundity (Carmichael, 1994; DeMott and Müller-Navarra, 1997). A combination of blue-green algae and high temperatures may have adverse effects on Daphnia populations, as has been found for other zooplankton (Gilbert, 1996). Furthermore, the release of toxic compounds following turnover of the anoxic hypolimnion or infection of Daphnia by parasites may also cause a decrease in Daphnia performance (Schwartz and Cameron, 1993).

During the June, October and December experiments of the current study, food quality in the lake was high, dominated by diatoms and other edible algae (N. W. Youngsteadt and R. J. Gumucio, unpublished data). The presence of high-quality food resources was probably key to the good performance of both D. parvula and D. lumholtzi during the June and October experiments. Blue-green algae were numerically dominant (~60%, primarily Raphidiopsis) during the August experiment (N. W. Youngsteadt and R. J. Gumucio, unpublished data). Since lakes are continually warm and blue-green algae abundant all year in the tropics (Horne and Goldman, 1994), we predicted that D. lumholtzi would have a higher tolerance to the presence of blue-green algae and warm temperatures than Daphnia more common in temperate regions. However, D. lumholtzi declined in the cylinders during the August experiment, whereas D. parvula continued to grow, suggesting that D. parvula was less affected than D. lumholtzi by blue-green algae and the other conditions during August in McDaniel Lake. Nevertheless, D. parvula had significantly lower final densities and rates of increase during this experiment than in the June and October experiments, indicating that D. parvula was also negatively affected by the late summer conditions.

Exploitative competition is dependent on the suppression of a limiting resource by other species. Resourcelimitation is further affected by changes in environmental conditions, such as would affect algal growth rates (Lampert and Sommer, 1997). In the current study, comparisons of Chl a measurements suggest that food resources were at lower concentrations in the cylinders than in the lake and declined during most experiments. Although Chl a concentrations in both the lake and the cylinders were lowest in June, the Daphnia achieved their highest final densities and rates of increase in the cylinders during this experiment. The high water exchange rates during this experiment probably provided a regular influx of new food resources, reducing or delaying competition. In contrast, the other experiments showed slower water exchange rates and significant reductions of Chl a in the cylinders, providing the potential for food limitation. Competitive effects were detected in the August and October experiments. Both of these experiments detected a significant suppression of D. parvula final densities byD. lumholtzi. Furthermore, competitive effects were seen in both the low and high initial density treatments ofD. parvula during the August experiment, suggesting that intraspecific competition also affected D. parvula final densities during this experiment. Intraspecific competition was also evident in the December–January experiment. Nevertheless, the competitive effects detected in the current study were generally small. The lack of strong competitive effects in this series of experiments may be due to the relatively high rate of resource exchange in the cylinders. Although resources were usually more limited in the cylinders than in the lake, they may not have been limiting enough for significant competitive effects to be manifested. These experiments may thus underestimate the potential for competition.

Seasonal reversals in competitive effects were not observed during these experiments. We expectedD. lumholtzi to have an advantage during the period of poor water quality (late summer) and D. parvula to have an advantage during periods of good water quality (early summer, fall and winter). During none of the experiments did D. parvula show a statistically detectable suppression of D. lumholtzi, although trends in the data from the August and October experiments point in that direction (Figure 4). Daphnia lumholtzi suppressed D. parvula rates of increase during the August and October 1997 and June 1998 experiments. These results imply that D. lumholtzi may suppress D. parvula in lakes which it has invaded, but only during warm summer and fall conditions. The combin-ation of poor performance by D. lumholtzi and its suppression of D. parvula during the August experiment is perplexing. This result suggests either a statistical artifact or that the D. lumholtzi in the cylinders were previously at a higher abundance, but suffered high mortality prior to the final collections.

Based on its larger body size, we would predict competitive superiority by D. lumholtzi over D. parvula. Large zooplankton filter more efficiently and can consume a larger range of particles than smaller zooplankton (Brooks and Dodson, 1965), and have lower threshold food requirements (Gliwicz, 1990). Furthermore, larger Daphnia species are more resistant to starvation than smaller Daphnia species (Goulden et al., 1982; DeMott, 1989). Although D. lumholtzi was not numerically dominant in the cylinders, its larger average body size resulted in biomasses equal to or greater than those of D. parvula (Figure 4). We would thus expect D. lumholtzi to be able to survive and reproduce at lower food concentrations than D. parvula.

During the October experiment, competitive suppression of D. parvula was expressed through depression of rates of increase, but not by a depression of their birth rates, suggesting that the competitive effect was from increased death rates. Similarly, in a field study of Daphnia in a Michigan lake, Hu and Tessier found that seasonal variation in birth rates of lake D. pulicaria and D. galeata mendotae were not correlated to density or rates of increase (Hu and Tessier, 1995). Increased death rates may have arisen by increased juvenile starvation (Tessier and Goulden, 1982) or increased egg mortality linked to high temperatures and nutritional deficiency (Allan, 1973; Boersma and Vijverberg, 1995). In contrast, prior in situ experiments with D. rosea and D. pulicaria by DeMottprovided evidence for correlated density-dependent decreases in both egg production and rates of increase (DeMott, 1983).

Daphnia population dynamics in the cylinders were quite different from those in the lake. The final densities of the Daphnia in the cylinders were up to 1000 times greater than Daphnia densities in the lake (Figure 5). During the course of these experiments, D. lumholtzi was never found in the lake in any significant numbers, although they did well in the cylinders in early summer and fall, and were often abundant in other Missouri lakes following their invasion (J. E. Havel, unpublished data). Daphnia parvula in the cylinders had higher rates of increase than lake Daphnia (primarily D. retrocurva) during all of the experiments. The large difference between Daphnia densities and rates of increase in the cylinders and lake are likely to have been due to differences between these two environments, such as by releasing Daphnia in the cylinders from predation by planktivorous fish and invertebrates (Dodson, 1974; Bengtsson, 1987; DeMott, 1989) or by excluding large colonial blue-green algae, which tend to be inedible and clog Daphnia filters (Moore et al., 1996; DeMott and Müller-Navarra, 1997).

The high Daphnia densities observed in the current study are not seen with larger scale mesocosms, but are instead more similar to results seen in laboratory experiments (de Bernardi, 1979; Goulden et al., 1982; DeMott, 1983; Romanovsky and Feniova, 1985). In a study analo-gous to ours, Smith and Cooper used small in situ enclosures (1 l, 54 μm mesh), and also found that the cladocerans in the enclosures had higher rates of increase than those in the lake (Smith and Cooper, 1982; ). Experimental studies using large enclosures without predators have detected competitive effects at more natural Daphnia dens-ities (DeMott, 1983; Kerfoot et al., 1988; Hu and Tessier, 1995). Future in situ competition experiments in larger enclosures would help determine whether competition between D. lumholtzi and native Daphnia species occurs at natural densities.

Daphnia lumholtzi is native to warmer climates (Havel and Hebert, 1993) and tends to be a summer dominant in Missouri lakes (J. E. Havel and M. E. Eissenbacher, unpublished data). However, the current study showed only weak performance of D. lumholtzi in late summer conditions, and this species proved to be rare in the McDaniel Lake zooplankton. Blooms of blue-green algae and the resulting poor water quality may account for the poor success of D. lumholtzi in this lake.

The results of the current study suggest that resource competition is not influencing the seasonal dynamics of the exotic D. lumholtzi, and an alternative hypothesis may be necessary to explain its seasonal succession. Most freshwater zooplankton species produce resting eggs at some point during the year (Hairston and Caceres, 1996), and some species are known to have different temperature cues to break diapause (Hairston et al., 1990). If warmer temperature cues are used by D. lumholtzi and cooler temperature cues are used by native Daphnia species, then the cue for D. lumholtzi ephippia to hatch may occur later in the year, causing D. lumholtzi to become abundant when native Daphnia species are not. Further research on the cues used by D. lumholtzi and native Daphnia to break diapause would provide further insight into the reasons for the complementary seasonal dynamics demonstrated by D. lumholtzi and native Daphnia species.

The results of the experiments in the current study suggest that competition between D. lumholtzi andD. parvula is not limiting D. lumholtzi in McDaniel Lake. The fact that little seasonal overlap is typically observed between D. lumholtzi and native Daphnia species in Missouri reservoirs suggests that direct competitive interactions rarely occur between D. lumholtzi and native Daphnia species. Such niche partitioning in time is often used as evidence for the ‘ghost of competition past’ [(Connell, 1980), cited in (Lampert and Sommer, 1997)]. Yet interactions with exotic species have a short history, suggesting that other factors may be needed to explain the seasonal dynamics of these Daphnia species.

Table I:

Experimental design for the current study showing the treatments and the number of Daphnia females introduced into each cylinder

Species treatment	Number of Daphnia introduced		Initial density of Daphnia (l–1)	Number of replicates(cylinders)
	Exotic	Native
For all experiments, ‘exotic’ refers to D. lumholtzi and ‘native’ refers to D. parvula. ‘Exotic alone’ and ‘native alone& represent single-species controls. Two replicates of each treatment were placed in each of three trays (blocks), with trays placed at three different pelagic locations in the lake. Positions of treatments within each tray were randomized.
Exotic alone (C)	10	0	6.25	6
Native alone (C)	0	10	6.25	6
Low-density combination (L)	5	5	6.25	6
High-density combination (H)	10	10	12.5	6

Table II:

Chlorophyll a concentrations (μg l^–1) in the cylinders and in McDaniel Lake at 2 m depth for each experiment during 1997–98

Experiment	Date	Cylinder		Lake (2 m)		P	Test
		Mean	SE	Mean	SE
Different superscript letters indicate significant differences in Chl a concentrations among dates within each experiment. P values are reported for two-sample t-tests (T) or Mann–Whitney U-tests (MW) comparing Chl concentrations in the cylinders and in the lake on each date.
1	June 16	–	–	7.23a	0.00	–	–
	June 21–22	6.93a	0.70	4.82b	2.41	0.15	MW
	June 25–26	5.80b	0.32	4.82b	1.21	0.26	MW
2	August 15	14.87a	0.72	24.11a	1.21	0.01	MW
	August 17	12.19b	0.48	14.47b	2.09	0.12	MW
	August 20	7.84c	0.79	10.85b	2.09	0.10	MW
3	October 11	17.51a	1.08	17.51a	0.60	0.50	T
	October 15	12.23b	0.60	31.87b	5.05	0.03	T
	October 22	12.62b	0.80	29.39b	2.08	0.01	T
4	January 30	6.15	0.95	15.44	1.47	0.02	MW

Table III:

Mean and SE for egg ratio and birth rate estimates for D. lumholtzi and D. parvula in the cylinders at the end of the October 1997 experiment

Species	Treatment	Egg ratio		Birth rate
		Mean	SE	Mean	SE
Mean water temperature 21°C, egg development time 15 days. Treatments: C, control; L, low-density combination; H, high-density combination. Estimates based upon six replicate samples per treatment, except for the December–January experiment (four replicates).
D. lumholtzi	C	2.43	0.29	0.41	0.03
D. lumholtzi	L	2.41	0.53	0.39	0.05
D. lumholtzi	H	2.19	0.17	0.39	0.02
D. parvula	C	0.70	0.12	0.17	0.03
D. parvula	L	0.85	0.16	0.20	0.03
D. parvula	H	1.00	0.13	0.23	0.02

Table IV:

F statistics for two-way ANOVA from the competition experiments^a

Experiment	Species	Statistics
		Treatment	Block	Interaction
Degrees of freedom for treatment, block and interaction were 2, 2 and 4, respectively, except for December–January, which were 2, 1 and 2, respectively. Variable codes: r, population growth rate; b, birth rate. **P < 0.01. aException: Friedman statistic.
June	D. parvula
	r	2.00a
	D. lumholtzi
	r	0.81	3.66	0.13
August	D. parvula
	r	17.32**	5.78	4.13
	D. lumholtzi
	r	2.00a
October	D. parvula
	r	2.00a
	b	0.85	0.14	0.73
	D. lumholtzi
	r	2.00a
	b	0.12	3.35	1.00
December–January	D. parvula
	r	1.30	0.08	0.63
	D. lumholtzi
	r	3.00a

Fig. 1.

Locking tray apparatus and cylinders used in the in situ competition experiments. The cylinder design is a modification of the O'Brien and Kettle bioassay chambers (O'Brien and Kettle, 1981). Tubing was added to allow sampling of the cylinders while at depth.

Fig. 2.

Density of Daphnia (number per liter) in McDaniel lake at the beginning, middle and end of the spring, summer and fall 1997 experiments, and the winter 1997–98 experiments. Mean ± 1 SE. Statistics are based upon three samples, each integrated from Schindler trap samples from four depths (0–6 m), except for January (two samples). Pie graphs indicate the composition of crustacean zooplankton in the lake during the course of each experiment.

Fig. 3.

Dye test results. Tests were conducted at the beginning and end of each field experiment, 1997–98. Water exchange rates (e_w) represent the fraction of water replaced per hour. Dye tests were either not conducted or had low replication on several dates due to bad weather or missing cylinders. Statistics are based upon one replicate cylinder for the June test (end only) and the beginning of August test, two replicates for the January test (end only) and three replicates for all other tests. Mesh size was 53 μm on all dates except June (100 μm).

Fig. 4.

Final Daphnia densities (top), biomasses (middle) and population growth rates (bottom) of D. lumholtzi (lum) and D. parvula (par) from in situ competition experiments during 1997–1998. Mean ± 1 SE. Statistics are based on six replicate cylinders for each treatment. The codes C, L and H indicate control, low and high initial competition treatments (details in Table I). Different letters over the bars indicate treatments (within each date and species) that are significantly different from one another. Note the different scales for each month.

Fig. 5.

Comparison of total Daphnia densities in the lake and cylinders during each experiment, 1997–98. Mean ± 1 SE. Cylinder statistics are based on 24 cylinders, except December–January (16 cylinders). Lake statistics are based upon three Schindler trap samples (each integrated from four depths, 0–6 m), except for January (two samples).

1

Present Address: University of Missouri—columbia, School of Natural Resources—fisheries and Wildlife, 302 Anheuser-Busch Natural Resources Building, Columbia, MO 65211-7240, USA

We thank A. Johnson, A. Hickman, L. Soeken, D. Bethune, K. Pattinson and K. Bueter for assistance in the field, and N. Youngsteadt and R. Gumucio for providing unpublished data on McDaniel Lake. J. Stephens drew Figure 1. Comments by V. Smith, W. DeMott and an anonymous reviewer improved the clarity of the paper. Financial support was provided by NSF grant DEB 96-75374 to J. E. H., a Sigma Xi Grant in Aid to J. L. J., and by the Department of Biology, Southwest Missouri State University.