Study On Mesozooplankton Grazing And Reproduction In Pearl River Estuary And North-Central South China Sea

Mesozooplankton is key component in marine ecosystem,and plays vital role in regulating materials and transferring energy.This thesis focus on changes in feeding and reproduction rates of mesozooplankton in estuary and marginal sea,conducted shipboard incubation using food removal and dilution techniches,combined with zooplankton enumeration and identification,analysis of size structure and group composition of phytoplankton,to calculate feeding and reproduction rates of mesozooplankton,and to demonstrate their relationship with food composition and abiotic factors,as well as to understand feeding selectivity,reproduction features and mesozooplankton-mediated trophic cascade.Study area include Pearl River estuarine(PRE)(four cruises,in May,August and November 2015,and January 2016)and the north-central South China Sea(NCSCS)(from July to August 2016).Main results are listed as follows:In PRE surface water,nano-phytoplankton(2-20 ?m)dominated phytoplankton biomass in every season;Mesozooplankton abundance overall showed a pattern of spring>winter>autumn>summer,and copepods dominant mesozooplankton abundance in every season.In NECSC surface water,chl a concentration was lower than PRE,with pico-phytoplankton dominated phytoplankton biomass;Mesozooplankton abundance was lower than PRE,with copepods dominant mesozooplankton abundance.Mesozooplankton showed obvious size-selectivity and group-selectivity from feeding on phytoplankton,and their indirect induced trophic cascade had different effect on each phytoplankton size fraction.For size-selectivity,mesozooplankton had higher clearance rates and ingestion rates on micro-phytoplankton(>20 ?m),and suppressed biomass accumulation of this size fraction in PRE and NCSCS;While they had lower ingestion on nano-sized algae,and indirectly promoted their biomass through trophic cascade;Pico-sized(<2 ?m)algae contributed little to mesozooplankton ingestion,and were either promoted(in PRE)or suppressed(in NESCS)through trophic cascade.For group-selectivity,mesozooplankton had highest ingestion on diatoms,while showed obvious preference on dinoflagellates in PRE;Diatoms and dinoflagellates were preferred in the diet of mesozooplankton as indicated by high clearance rates in NESCS,but their contribution to mesozooplankton ingestion was weak due to low biomass.In all,mesozooplankton-mediated trophic cascade was stronger in spring and summer than in autumn and winter in PRE.In spring and summer,the overall grazing effect of mesozooplankton maintained the biomass of phytoplankton,and mesozooplankton were more herbivorous;in autumn,phytoplankton biomass was partly controlled by mesozooplankton,meantime mesozooplankton were more carnivorous;while in winter,mesozooplankton grazing behavior slowed down,and had little influence on phytoplankton biomass.The main reason causing trophic cascade may attribute to the obvious feeding preference of mesozooplankton on ciliates.However,due to their much lower biomass when compared with phytoplankton,ciliates were not major carbon source for mesozooplankton.The feeding preference on ciliates may due to their promotion effect on copepod reproduction.In PRE,seasonal and spatial distribution patterns of cope-pods reproduction rates were overall spring>summer>autumn>winter,and middle area>upper area>lower area.Copepod reproduction rates were affected by different factors including salinity,temperature,particulate organic carbon,mesozooplankton biomass,food concentration and structure.Elevated proportion or biomass of micro-phytoplankton had a positive feedback on copepod reproduction;ciliates abundance had positive relationship with copepod reproduction rates.Overall,this study will increase our understanding on feeding and reproduction dynamics of mesozooplankton,and their trophic interactions with other planktonic groups.