Predation And Competition: Interspecies Interactions Of Zooplankton Communities In Liuxihe Reservoir

Predation and competition are the main factors regulating population dynamics in freshwater organisms and also the most important structuring forces in freshwater ecosystems. Liuxihe Reservoir is a large deep oligo-mesotropic sub-tropical reservoir with high diversity of zooplankton species,. The present study investigated structures and dynamics of zooplankton communities in the reservoir from 2006～2009, explored the role of predation and competition in structuring zooplankton communities, vertical distribution patterns and population dyanmics.Liuxihe Reservoir is an oligo-mesotropic water body with low chlorophyll a concentration, the zooplankton suffered from food limitation, and strong competition, consequently, predation can evidently modify competition consequence.Zooplankton biomass composition in Liuxihe Reservoir was predominated by crustaceans, of which copepods accounted for 61.7%～82.6%. Cladocreans contributed a percentage of 15.7%～29.8%. Rotifers had a decreasing trend within the years, and they were mainly composed of small-bodied species. Among crustaceans, Phyllodiaptomus tunguidus and Diaphanosoma orhidani were the most dominant species and filter feeders, followed by Bosmina fatalis, Bosminiopsis deitorsi, and Mesocyclops sp.As the large fitler feeder Daphnia geleata occurred, the seasonal succession pattern of zooplankton in Liuxihe Reservoir was similar to PEG model in temporal water-bodies. The annual average ratio of the maximum biomass to minimum biomass of zooplankton was about 13.4, indicating a distinct seasonal succession pattern which is the main different characteristics different to other tropical water-bodies. On other hand, the zooplankton mainly consisted of typical tropical zooplankton species such as Phyllodiaptomus tunguidus, Diaphanosoma orhidani, Ceriodaphnia and flatworms. Their succession and dynamics were also typical tropical zooplankton species, with peak populations at high temperatures in summer, short development time, and high food concentration requirements.Phyllodiaptomus tunguidus distributed widely in all South China as the most dominant pelagic species and filter feeders in tropical and sub-tropical water-bodies. It had an increasing trend in the investigation years. Its abundance ranged from 0 to 36.5 ind/L, with an annual average value of 6.8 ind/L; the coorepsonding biomass range was from 0 to 926.3μg/L.. It existed a significant positive correlation to water temperature(r=0.330(*), p=0.016, N=42), and a significant negative correlations with chlorophyll a concentration (r=-0.309(*); p=0.020, N=45). P. tunguidus is a K-strategist and strong competitor with large body size, low threshold food concentration, wide food niches, and strong escaping ability. It well adapted to high water temperatures and strong fish predation pressure. The intrinsic increase rate (r) of population was 5 times of Daphnia geleata at low food concentrations. The intrinsic increase rate of Phyllodiaptomus tunguidus increased with higher temperature and food concentration. The ability to avoid predation is a characteristic feature for Phyllodiaptomus tunguidus to adapt to the tropical water with high predation.Diaphanosoma orhidani has a short life generation time, high reproductive rate and high requirements for food concentration, and its abundance showed a significant correlation to water temperatures. The abundance of Diaphanosoma orhidani was from 0 to 15.2 ind/L with an annual mean value of 2.3 ind/L. Diaphanosoma orhidani can speed development rate and enlarge clutch size without shortening life span under high water temperature. Because predation is comparatively stronge in tropical waterbodies, that the most effective adaptive strategies for zooplankton to survive is to shorten life generation time, increase development time and increase reproduction rate to make up for the predation loss, especially when temperature is higher.Many zooplankton species undergo diel vertical migration (DVM) in which they move up and down the water column within a day. The DVM activity patterns for crustaceans were distinct in Liuxuhe Reservoir. The amplitude for DVM of crustaceans in Liuxihe Reservoir were mainly induced by predation pressure, modified by species-specific life strategies, development stages, and food concentration, radiation and seasonal variations.Diaphanosoma orhidani and naupliar has no diel vertical migration in all seasons, regardless of their development stages. The population stayed mainly in the epilimnion, and their distributions were significantly correlated to chlorophyll a concentration Bosmina fatalis and Bosminopsis deitorsi are small body size cladocerans which are most sensitive to invertebrate predation. They showed a reverse diel vertical migration mode in the reservoir, which stayed in the surface water layer during the daytime and descend to deep layers during the night. However, they didn't show any diel vertical migration in winter. Ceriodaphnia, Daphnia geleata and Moina micrura showed strong diel vertical migration in all seasons, independent of their body size.Phyllodiaptomus tunguidus and Mesocyclops are the largest zooplankton in the reservoir. Their diel vertical migration behavior varied in their development stage. In general, the younger stages of Phyllodiaptomus tunguidus and Mesocyclops showed little or no diel vertical migration, and they stayed mostly in the epilimnion where food is abundant. The later stages and adults showed extended vertical migration. Tropocyclops bopingi, however, due to its small size, showed a short-range vertical migration in summer and no vertical migration in winter, with the main population stayed in the surface layers.Testate amoebae are an abundant and diverse polyhyletic group of shelled protozoa which are quite common and distributed widely in tropical and sub-tropical water bodies In the Liuxihe Reservoir, Difflugia are carnivorous or omnivorous which feed on a large range of food like rotifers, Peridinium or even mesozoans, the food selection mainly depended on the aperture diameter of the prey, for example, D. tuberspinifera preyed mainly on a Peridinium species with a diameter of 30μm, while D. biwae preferred the one of 25μm, and it can suppress the population of this Peridinuim. Asplancha, Stentor, Ploesome, and Mesocyclops are the predators of Difflugia, and the positive correlations between Difflugia and Mesocyclops indicated that Difflugia is an important food source for Mesocyclops when they are abundant in the water column. The characteristic of Difflugia is the bentho-planktonic cycle, with peak population in summer. They have distinct temporal and spatial distribution patterns, and they have much higher abundance in the uprivers and transition region with high water turbulence and food source. They produce gas bubble to be suspended in the pelagic and to move and prey. The vertical sample profiles showed that they don't have DVM and they distributed mostly in the epilimnion with peak population at a depth of 8m. Competition has more important effect than predation for Difflugia. There were fierce competition between species and within species, and cannibalism was common to be found in Difflugia. Pseudopodia played an important role for Difflugia. Aperture size decided the pseudopodia activity and the food niche range of Difflugia.In Liuxihe, a freshwater reservoir in South China with artificially reduced fish predation pressure since 2004-2005, the zooplankton showed a single yearly pulse of pelagic flatworms in the early summer periods of 2006-2009. As soon as they appear, Daphnia galeata retreats in dormancy, one month earlier (July) under a flatworm than under a fish predation regime (August), but the related Ceriodaphnia quadrangula does not and has a tendency to replace it. We show, using in situ lake sampling, by experiments in large enclosures and by laboratory observations, that Ceriodaphnia is competitively inferior to Daphnia, presumably in its ability to acquire algal food, but has a much higher tolerance to flatworm toxins. As a result, Ceriodaphnia manages to coexist with the flatworm and the balance of the competition is tipped. Observations in the laboratory suggest that flatworm populations autoregulate by being sensitive to their own toxins and that Ceriodaphnia, even though a prey to the worms, may incur less cost than benefit from their presence.