Effects Of Nutrient Enrichment And Tilapia (Oreochromis Nilotica) On Zooplankton Community: An Enclosure Experiment

In order to understand trophic cascades effects of omnivorous fish (tilapia) in warm waters, an enclosure experiment was carried out in Liuxihe Reservoir, an oligo-mesotrophic water body, from September 10 to November 14 of 2009, lasting 6 weeks.15 large enclosures in total were used, each enclosure has a water volume of 85 m3. There are 5 treatments, there are three replicates for each treatment. In the control treatment (C), the water was pumped from the reservoir after filtering small fish, other four treatments included:1) adding low nutrients (N,P) to enclosure in the filtered reservoir water (LN),2) adding tilapia with a biomass of 2g/m3 (F); 3) adding nutrients and tilapia (LNF); 4) adding tilapia and high nutrients (three times higher than in LN) (HNF).The N/P (mass ratio) of 10/1 was fixed in all the treatments. Water quality variables and quantitative plankton samplings were measured or taken every week (7 days). The response of plankton to nutrient addition was analyzed through the nutrients enrichment treatments and controls. Top-down effects of tilapia on plankton were analyzed between the fish treatments and only nutrient enrichment treatments.In two no fish treatments (C and LN treatments), the abundance and biomass of phytoplankton increased, but significantly in the LN. In the treatments without fish, the abundance and biomass of zooplankton increased markedly in nutrients enrichment treatments (LN). However, fish treatments only had slight effects on the abundance of zooplankton but decreased their biomass significantly.In all the encousres, zooplankton were dominated by copepods, such as Phyllodiaptomus tunguidus, Mesocyclops thermocyclopoides. The cladocerans were mainly composed of Bosmina longirostris, Moina micrura, Diaphanosoma orghidani and Ceriodaphnia cornuta. In the treatments with fish (F and LNF), the abundance and biomass of cladocerans were reduced, but little influence to Phyllodiaptomus tunguidus was detected.The dominant species of rotifer showed an evident temporal succession in the adding nutrients treatments. In the first 3 weeks, Keratella cochlearis, Trichocerca cylindrica, Trichocerca stylata and Trichocerca chattoni were primary dominant species in abundance, and their abundance sharply decreased from the third week. On the other hand, Brachionus quadridentatus, Brachionus forficula and Brachionus dolabratus increased quickly. Even if they were suppressed by fish predation in the fish treatments, nutrient enrichments improved edible phytoplankton. The rotifer biomass showed a variation pattern similar to the abundance. The copepods were dominated naupli, copepodites, Phyllodiaptomus tunguidus, Mesocyclops thermocyclopoides. The abundance and biomass of Bosmina longirostris, Moina micrura and Ceriodaphnia cornuta was evidently influenced by fish predation. When edible phytoplankton was abundant, the predation pressure seems to be buffered in Bosmina longirostris and Diaphanosoma orghidani.These results indicate that cladocerans could become the dominant population only in water bodies without fish or with low fish biomass in the warm water. By comparing nutrient enrichments, the omnivorous tilapia does not decrease trophic cascade effects when large calanoids replace Cladocera in dominance.