| Ingestion of cyanobacterial biomass by silver carp(Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) is frequently considered as one of the biological methods for the water bloom formation control. However, the introduction of silver carp and bighead carp into blooming waters not always brings about the expected result, namely a decrease in phytoplankton biomass. The photosynthetic activity and growth of cyanobacteria after passage through the digestive tract of silver carp and bighead carp were determined by fluorescence imaging system. A stable N isotope tracer approach was also developed to study faecal origin nitrogen transformation and turnover rate. The study evaluated the ecological effect of nontraditional biomanipulation to reduce nuisance blooms of large algal species from a faecal effect point.The growth and photosynthetic activity of phytoplankton passed through intestine of the two planktivorous fish from Taihu lake were compared with those of phytoplankton taken the directly from the lake. Chlorophyll fluorescence parameters (including maximal optical quantum efficiency of PSII(Fv/Fm), effective optical quantum yield of PSII(Yield), PSII-driven electron transport rate (ETR), photochemical quenching (qP) and nonphotochemical quenching (NPQ)), phytoplankton community structure and biomass were determined by PAM fluorimeter (Dual-PAM-100) during a 13-day in situ dialysis cultivation of fish faeces. The cyanobacteria exhibited a significant reduction in Fv/Fm, Fv/Fo, Yield and ERT after passage through the two fishes (P<0.05), wheras qP and NPQ significantly increased (P<0.05). Then, the metabolic activity of cyanobacteria gradually recovered on the 3rd and 5th day during culture, respectively. The Fv/Fm, Fv/Fo, Yield and qP values were significantly higher in silver carp and bighead carp treatments than in the control at the end of culture (P<0.01), while NPQ was significantly lower on the 13rd day (P<0.01). The phytoplankton cell density and chlorophyll a content in silver carp treatment was growing faster than that in bighead carp, while the exopolysaccharide content in bighead carp treatment was higher than in silver carp treatment during culture. At the end of the experiment, the total biomass of phytoplankton increased 7.78-and 6.55-fold in the two fish treatment compared to the control, respectively. The relative biomass of green algae and diatom increased a little, but their abundance and biomass were too low compared to cyanobacteria (93% of the total phytoplankton biomass).The fate of 15N-enriched cyanobacteria feed fed to silver carp and tilapia was traced through the food web in enclosure. The N isotopic turnover rate in Tilapia tissues was much higher than in silver carp after a 7-day 15N-labeling tracing experiment. The sedimentary debris in the water column became rapidly enriched (within one day) after fish was fed 15N-enriched feed, followed by phytoplankton and zooplankton (within five day). There were no notable changes in the nitrogen isotopic values of snails.15N budge showed that by day 7,20.97% of the added 15N after silver carp were fed 15N-enriched cyanobacteria. Some of the added 15N was lost by rapid remineralization of organic matter by microbial community in the water column. As the experiment continui, The nitrogen in silver carp muscle increased from 11.15% to 20.97%. And a number of the added 15N-nitrogen was lost (21.98%). Tilapia showed significant difference 15N isotopic pathways with silver carp.52.95% of the total was detected in Tilapia muscle, and only 7.23% was lost at the end of the experiment.In conclusion, the silver carp and bighead carp digestion do not cause fatal injury to Microcystis. Photosynthesis activity stimulation and growth of Microcystis cultivation may be as a result of surplus compensation growth. A significant amount of the dietary N was not retained by the natural biota and is likely to be lost in the water column. |
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