Effect of algal food (Chlorella vulgaris) concentration and inoculation density on the competition among three planktonic Brachionidae (Rotifera: Monogononta)

 

Efecto de la concentración de algas (Chlorella vulgaris) y la densidad de inoculación sobre la competencia entre tres Brachionidae (Rotifera: Monogononta) planctónicos

 

S.S.S. Sarma¹*, José Luis Franco–Téllez¹ & S. Nandini²

 

¹ Laboratory of Aquatic Zoology, UMF, Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, Av. de Los Barrios No. 1, Los Reyes, Iztacala, A.P. 314, C.P. 54090, Tlalnepantla, State of Mexico, Mexico. * E–mail: sarma@servidor.unam.mx.

² UIICSE, Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, Av. de Los Barrios No. 1, Los Reyes, Iztacala, A.P. 314, C.P. 54090, Tlalnepantla, State of Mexico, Mexico.

 

Recibido: 28 de agosto de 2006
Aceptado: 7 de septiembre 2007

 

ABSTRACT

Competitive outcomes among three rotifer species (Anuraeopsis fissa, Brachionus havanaensis and B. angularis) were quantified with different inoculation densities of two competing species at a time (0, 25, 50, 75 and 100%) and using different algal (Chlorella vulgaris) densities (0.2 x 10⁶, 0.4 x 10⁶ and 0.8 x 10⁶ cells ml^–1). In control cultures, when each species was grown alone, the population growth of rotifers increased with increasing food availability in the medium, but in mixed cultures, decreased with increasing proportion of the competing species. At low food level, compared to B. havanaensis, B. angularis had stronger negative impact on A. fissa. However, with increasing algal density, both species of Brachionus had similar but reduced impact on A. fissa. Population growth of B. havanaensis was more adversely affected by A. fissa than B. angularis at low and intermediate concentrations. At high food level, the impact of either A. fissa or B. angularis on the growth of B. havanaensis was similar. When grown alone, for a given food density, A. fissa was more numerically (4 to 6 times) abundant than the other two species. The rate of population increase (r) of rotifers increased with increasing food levels. Depending on the rotifer species and the test conditions, the r varied from –0.001 to 0.34 d^–1. Results showed that the competitive outcome in the tested rotifers depended on the initial inoculation density of the competing species, the offered food concentration as well as the interaction of these two factors.

Key words: Competition, resource density, zooplankton, inoculation density.

 

RESUMEN

Se evaluaron los resultados de la competencia entre tres especies de rotíferos (Anuraeopsis fissa, Brachionus havanaensis y B. angularis) usando dos especies a la vez con diferentes densidades de inóculo (0, 25, 50, 75 y 100%) y diferentes niveles de alga (Chlorella vulgaris; 0.2 x 10⁶, 0.4 x 10⁶ y 0.8 x 10⁶ células ml^–1). En cultivos monoespecíficos, utilizados como control, el crecimiento poblacional de rotíferos aumentó con el aumento de disponibilidad del alimento en el medio, pero en los cultivos mixtos, disminuyeron con el aumento de la proporción de la especie competidora. En el nivel bajo de alimento, B. havanaensis tuvo un mayor impacto negativo sobre A. fissa en comparación con B. angularis.

Sin embargo, al aumentar la densidad del alga, ambas especies de Brachionus tuvieron un impacto reducido, pero similar. Por otro lado, el crecimiento poblacional de B. havanaensis fue afectado más por A. fissa que por B. angularis en las concentraciones bajas e intermedias de alimento. Con altas concentraciones del alga, el impacto de A. fissa o B. angularis sobre el crecimiento de B. havanaensis fue semejante. Cuando fue cultivado solo, para una densidad dada de alimento, A. fissa alcanzó una mayor abundancia (4 a 6 veces) que las otras dos especies. La tasa de crecimiento poblacional (r) de los rotíferos se incrementó con la disponibilidad del alga. Dependiendo de la especie de rotíferos y de las condiciones del experimento, la r varió entre –0.001 a 0.34 d^–1. Se concluye que el resultado de la competencia entre las especies de estudio depende de la densidad inicial de los competidores, de la concentración de alimento así como de la interacción de los dos factores.

Palabras claves: Competencia, densidad de recursos, densidad inicial, zooplancton.

 

INTRODUCTION

Limnetic rotifers play an important role in the zooplankton productivity (Wetzel, 1981). Compared to crustaceans, rotifers, due to their rapid maturation, faster development rates and higher reproductive output in relatively shorter time, may account for 10 to 40% of the total zooplankton production (Herzig, 1987). Rotifers rapidly occupy all available niches in ponds and lakes and thus help in transferring energy with high efficiency from primary producers (alga and bacteria) to secondary consumers (such as larval insects and fish; Wallace et al., 2006).

The competition and predation are the important forces structuring the zooplankton communities in any aquatic ecosystem (DeMott, 1989; Dumont et al., 1990). In newly formed ponds and lakes competition becomes more important for the initial colonization of zooplankton species (DeMeester et al., 2002). Competition studies in zooplankton under experimental conditions have mainly revealed the existence of two common types of interactions: 1) exploitative competition where a larger species usually rapidly depletes limited food conditions and 2) the interference competition, where larger zooplankton species cause mechanical hindrance or damage to the smaller competing ones (Romanovsky & Feniova, 1985; Gilbert, 1988a, 1988b). Under limiting food conditions, the body size of the competing rotifer species is important in the maintenance of a population (Stemberger & Gilbert 1985; Nandini et al., 2007). Since the amount of energy needed to swim diminishes with increasing body size of zooplankton (Gerristen & Kou, 1985), larger species are expected to have higher capacity to filter greater quantities of food from the medium. Larger species have as an additional advantage their higher ability to resist pulsed periods of starvation longer than smaller zooplankton (Threlkeld, 1976).

Competition among zooplankton is influenced by several factors such as food concentration, the nutritional quality of the diet and the relative initial densities of the competing species and temperature (Rothhaupt, 1988, 1990; DeMott, 1989; Sarma et al., 1996, 1999; Fernández–Araiza et al., 2005). The rate of population growth (r) is considered as one of the important determining factors in the competitive superiority of zooplankton species even though it has not received unequivocal support from literature (Lynch, 1978; Sarma et al., 1996). In rotifers the r appears to be body size–dependent, i.e., under optimal culture conditions, larger species generally have higher growth rates than the smaller zooplankton. The minimum quantity of food needed to maintain a population, the zero population growth rate or r = 0, known as the threshold food concentration, is lower for smaller zooplankton than for the larger species (Stemberger & Gilbert, 1985). Hence smaller zooplankton species would maintain a population at a lower food concentration lower than larger species, as the phytoplankton densities diminish in waterbodies. Therefore the competitive edge of both smaller and larger species depends on the availability of food concentration in natural ponds which varies seasonally (Kirk, 1997; Grover, 1997). Since it is difficult to quantify the available phytoplankton in the field, its impact has been largely ignored (Tessier & Goulden, 1982).

Most works on competition among rotifers have been focused on relatively large species like Brachionus patulus (Müller, 1786), B. calyciflorus Pallas, 1766, B. rotundiformis Tschugunoff, 1921, Euchlanis dilatata Ehrenberg, 1832 (Nandini & Sarma, 2002; Sarma et al., 2002). Nevertheless, in natural water bodies from Mexico a great variety of small rotifers, including Brachionus havan a en sis Roussselet, 1911, Brachionus angularis (Gosse, 1851) and Anuraeropsis fissa (Gosse, 1851) are frequently encountered (Nandini et al., 2005). Competition studies among them are either rare or at best fragmentary (Fernández–Araiza et al., 2005).

Therefore, the aim of this study was to quantify the competitive interactions among three small rotifer species (B. angularis, B. havanaensis and A. fissa) under different algal concentrations and initial densities.

 

MATERIAL AND METHODS

For competition experiments, we used three rotifer species (mean ± standard error of the body length, urn, spines excluded): B. havanaensis, (120 ± 5), B. angularis (100 ± 5) and A. fissa (70 ± 5). All of them were isolated from the Lake Xochimilco Channels (Mexico City) and cultured separately on reconstituted moderately hardwater (EPA medium) for at least 2 years before carrying out the experiments. EPA medium was prepared by dissolving 96 mg NaHCO₃,60 mg CaSO₄, 60 mg MgSO₄ and 4 mg KCl in one liter of distilled water (Weber, 1993). Rotifer mass cultures were raised on the single–celled green alga Chlorella vulgaris Beijerinck, 1890 as the exclusive diet. Using Bold's basal medium enriched every 3^rd d with 0.5 g l^–1 of sodium bicarbonate, batch–cultures of C. vulgaris were established in 2 L transparent bottles under continuous fluorescent illumination (Borowitzka & Borowitzka, 1988). Log phase alga was harvested, centrifuged at 4000 rpm for 5 min, rinsed and resuspended in distilled water. Stock algal density was estimated using Neubauer haemocytometer. From the algal stock, the desired algal levels (0.2 x 10⁶, 0.4 x 10⁶ and 0.8 x 10⁶ cells ml^–1) were obtained by serial dilution using fresh EPA medium.

Rotifer competition experiments were conducted in 50 ml transparent glass jars containing 20 ml EPA medium with one of the selected algal densities. The general test conditions were: pH 7.0–7.5, temperature 28 ± 1 °C and continuous but diffuse illumination. The initial density of rotifers (alone or in mixed cultures) was 1 ind. ml^–1. For competitive interactions, we used two rotifer species at a time and five combinations of their initial densities (e. g., A. fissa (A.f.) vs. B. angularis (B.a.): 100 % A.f. + 0 % B.a., 75 % A.f. + 25 % B.a., 50 % A.f. + 50 % B.a., 25% A.f. + 75 % B.a., Wo A.f. + 100% B.a.). For each rotifer species we used 45 test jars (3 food levels x 5 combinations x 3 replicates). For initiation of growth experiments, the chosen rotifer species were individually introduced into each test jar using Pasteur pipette under ste–reomicroscope at 20x (Nikon SMZ645).

Following initiation of the experiments, we daily counted (total count or two aliquot samples of 1 to 2 ml each) the number of live rotifers in each jar. Following the estimation of the density, rotifer populations in each jar were transferred to fresh medium containing appropriate food concentration. The experiments were terminated on day 20, when the test populations in most replicates began to show declined growth.

From the data collected, we derived the rate of population increase (r) using the equation: r = (ln N_t – ln N₀)/t, where, N₀ = initial population density, N_t = density of population after time t (days) (Krebs, 1985). The r was obtained from a mean of 3–5 values during the exponential phase of the population growth from each replicate. For a given rotifer species, the differences in the growth rates in the presence and in the absence of a competitor and under different algal food concentrations were quantified using two–way analysis of variance (ANOVA; Sokal & Rohlf, 2000). For multiple comparisons of the growth rates, we further subjected the data to post hoc analysis (Tukey test) using the statistical software Statistica ver. 6 (StatSoft, Tulsa, OK, USA).

 

RESULTS

When grown alone, A. fissa increased with increasing food density. Regardless algal food concentration and inoculation density of the competing species, in the presence of B. angularis or B. havanaensis, population growth of A. fissa was reduced. At low food level, B. angularis had stronger negative impact on A. fissa than B. havanaensis and with increase in algal density, both Brachionus species had similar but reduced impact on A. fissa (Fig. 1). Population growth curves of B. angularis in relation to different concentrations of algal diet and in the presence and absence of A. fissa or B. havanaensis are presented in Figure 2. When cultured alone, an increase in the concentration of Chlorella resulted in increased population density of B. angularis. The presence of the competing species, however, reduced its growth. In general, higher algal level and higher initial density of competing species had more adverse effect on B. angularis. Population growth of B. havanaensis was more adversely affected by A. fissa at low and intermediate concentrations than B. angularis. At high food level, the impact of A. fissa or B. angularis on the growth of B. havanaensis was similar (Fig. 3).

At a given food density, the peak densities of the tested rotifers when grown alone were different. The peak population densities of A. fissa were 156 ± 3, 425 ± 14 and 698 ± 55 ind. ml^–1, when cultured on Chlorella of 0.2 x 10⁶, 0.4 x 10⁶ and 0.8 x 10⁶ cells ml^–1 respectively. The corresponding values for B. angularis and B. havanaensis were much lower (38 to 170 ind. ml^–1 and 41 to 91 ind. ml^–1 respectively. Thus, A. fissa was numerically more abundant than the other two species. Peak population densities of all tested rotifer species increased with an increase in food density, as well as the rate of population increase (r). For a given rotifer species the rate of population increase was significantly influenced by the food concentration and inoculation density of the competing species (two–way ANOVAs, p <0.05; Table 1). However, for A. fissa in the presence of B. havanaensis, the inoculation density was not significant (p >0.05). Except for two cases (B. angularis vs. B. havanaensis and B. havanaensis vs. A. fissa), the interaction of food level x inoculation density was also significant for the tested rotifer species (p <0.05). Depending on the rotifer species and the test conditions, the r varied from –0.001 to 0.34 d^–1. Post hoc tests revealed further the differences in the growth rates of each rotifer species grown alone and together with a competitor (Table 2).

 

DISCUSSION

The theoretical foundations of zooplankton competition have been mainly quantified on two modalities; mechanical interference and exploitative competition (Romanovsky & Feniova, 1985; Gilbert, 1988a; Ciros–Pérez et al., 2001), and both are focused on large–bodied species or those with great differences in the body sizes. Gilbert (1985) has observed that Daphnia caused a significant mortality to eggs and the neonates of rotifers in the laboratory competition experiments. In later studies, Gilbert (1988 a, 1988b) showed that mechanical competition is of great importance in regulating the zooplankton community structure of natural waters. Of the three species selected in this study, two species (A. fissa and B. angularis) do not have posterior or poste rolate ral spines. Although B. havanaensis has long posterior spines, they are much reduced in the absence of predators (Pavón–Meza et al., 2007). Because of this reason, we consider the role of mechanical interference by such spines as probably insignificant. In exploitative competition, larger rotifer species apparently consume food with a greater rate than the smaller species and thus eliminate the smaller ones. Some works have confirmed this tendency under field and under laboratory conditions (Ooms–Wilms et al., 1999). Here we did not observe this consistently. For example, B. havanaensis which is relatively larger, was strongly reduced (including negative growth rates) in the presence of A. fissa at low and intermediate algal densities. Then, an important conclusion derived from this work is that in addition to body size, factors such as food level and the inoculation density of the competing species play a decisive role. Other studies such as Matveev (1985) and Sarma et al. (1996) also showed that the competitive advantage of a zooplankton species over the other depends on its initial population densities. Our results also demonstrated that the initial densities of the competing species and the food concentrations interacted to decide which species would prevail under competition pressure. Competitively inferior species may also show higher incidence of sexual reproduction (Ciros–Pérez et al., 2002). We have not quantified the male production in our study.

When two or more different species compete for the same resources under stable environmental conditions, one of them is eliminated of the competition, in agreement with the principle of competitive exclusion (Gause, 1934). In the lake Xochimilco a natural shallow waterbody in Mexico City (Mexico), Nandini et al. (2005) observed the co–existence of as many as 5 species of Brachionus at any sampling station. Further laboratory tests indicated that variable food density and temperature permitted this coexistence (Fernández–Araiza et al., 2005). In a related study on the competition between two species of cladocerans of similar size, Sarma et al., (2002) have also observed that under some combination of food level and initial density, the competing species were able to coexist together for some time. In the present study in most treatments coexistence of two rotifer species (with different densities) was noticed. The size difference among the rotifer species used here was probably not enough so as to exclude one of the two species from the competition. Similar situation has been observed between A. fissa and B. calyciflorus (Sarma et al., 1996), where in low food concentrations the latter was suppressed but at high concentration the former was adversely affected, but in either case complete elimination was not observed.

In competition studies peak population abundance and the rate of population increase are sensitive variables (Sarma et al., 1996). Independently of the food level, A. fissa was able to reduce the peak population abundance and growth rates of B. hava–naensis more severely than B. angularis. It should be noted that peak abundances and the r of A. fissa were also affected by the presence of a competitor. At a given algal density, the smaller zooplankton species have higher peak abundances due to lower quantity of food required for maintenance and reproduction i.e., threshold food concentration (Nandini et al., 2007). In the present study this tendency was also visible. Within the selected species, at any given food level, A. fissa being the smallest, had higher peak abundances than the other two species. The peak densities of the three species in general agree with the range reported in literature (A. fissa: ~2000 ind. ml^–1 by Dumont et al., 1995); B. havanaensis: 50–500 ind. ml^–1 (Pavón–Meza et al., 2004). Stemberger & Gilbert (1985) found that larger rotifer species have higher growth rates than smaller ones under comparable conditions. However, this does not seem to be a general trend (see Sarma et al., 2001). In the present work, we also did not find a positive relation between body size and the r. This may be due to the food concentrations selected here. We have not tested a wide range of food concentrations to derive growth rates. It is however known that r increases with increasing food levels (Nandini et al., 2007), as also observed here. Regardless of the rotifer species and the food concentration, the r–values observed here are within the range reported for Anuraeopsis and Brachionus(0.1–2.0 d^–1; Sarma et al., 2001).

This study showed that the competitive outcome in the tested rotifers depended on the initial inoculation density of the competing species, the offered food concentration as well as the interaction of these two factors. While all the three species were adversely affected by the presence of competitor, A. fissa was strongly suppressed by B. angularis at low food level. Competition between these brachionid rotifers showed that B. angularis was affected by the presence of B. havanaensis, especially at higher food level. The body size–related traits of rotifers were evident in the peak abundances where large–bodied taxa had lower peak population density, A. fissa being the smallest species had highest abundance while under similar conditions, both Brachionusspecies had lower densities. The rate of population growth increased with increasing food concentrations.

 

ACKNOWLEDGMENTS

We thank three anonymous reviewers for critically evaluating our presentation. SSSS and SN thank PAPIIT IN201907 and PAPIME PE201406 for financial support.

 

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