The Microzooplankton And Its Trophic Link With Copepods In The Yangtze River Estuary And Adjacent Waters

The abundance, biomass and growth rate of microzooplankton (ciliate&heterotrophic dinoflagellate) and its contribution to the diet of copepod were studiedin Yangtze Rive estuary and adjacent waters by means of in-situ sampling andincubation in May and August of2013. The results are as follow:1. The surface abundance and biomass of microzooplankton were2318cells/Land5.8μgC/L in nearshore waters in May respectively, which are lower than that inAugust with9610cells/L and13.5μgC/L. Ciliate was the dominant group ofmicrozooplankton and accounted for89%and80%of microzooplankton abundanceand biomass on average respectively. Heterotrophic dinoflagellate was also animportant component of the microzooplankton and was superior in numbers in localarea. Ciliate mainly consisted of the aloricate, and the loricate ciliate abundance onlyaccounted for22.2%(May) and10.7%(August) of the total ciliate abundance. Weidentified41loricate ciliate taxa belonging to17genera, including8genera and21species in spring and16genera and36species in summer. The species fromTintinnopsis spp. were most abundant. From the point of size structure, the loricateciliates were generally larger than the aloricate, and among aloricate ciliates the largerwere prevalent in spring compared to summer when the small was dominant.2. Growth rates of different microzooplankton morphotypes (loricate ciliate,aloricate ciliate, thecate heterotrophic dinoflagellate, naked heterotrophicdinoflagellate) ranged from-0.46to0.89d-1in natural seawater pre-screened on200μm. Among them, the average growth rate of ciliate was0.49d-1which was higherthan that of heterotrophic dinoflagellates,0.38d-1. According to the negative valueoccurred in spring, we inferred that there might be predation within microzooplankton.Therefore, incubation with200μm and50μm filtrating seawater were carried on simultaneously in summer. The results showed that there was no obvious differencebetween two treatments for growth rate of heterotrophic dinoflagellate; However,thegrowth rate of two groups of aloricate ciliate (size <20μm and20~50μm) in50μmfiltering seawater without larger predators had significant increase compared to200μm filtering seawater. The corresponding water column productivity ofmicrozooplakton would rise83%in summer, reaching to218mgC m-2d-1.3. Calanus sinicus, Centropages dorsispinatus, Acartia pacifica and Euchaetaconcinna were the dominant species of copepod in the Yangtze River estuary andadjacent waters in spring and summer. The research on their predation onmicrozooplankton indicated that Euchaeta concinna hardly fed on microzooplankton;the ingestion rate of other three species (Calanus sinicus, Centropages dorsispinatus,Acartia pacifica) on microzooplankton (ciliate+heterotrophic dinoflagellate) reached0.66μgC ind. d-1,0.09μgC ind. d-1and0.59μgC ind. d-1, respectively, accounting for29%,24%and37%of daily carbon intake of copepod on average, respectively.Heterotrophic dinoflagellate, as an important component of microzooplankton, wasresponsible for the30%biomass and28%food intake ascribed to microzooplakton.According to the Chesson’s index of selectivity, although the biomass ofmicrozooplankton was far less than that of phytoplankton, copepods preferred to feedon microzooplankton; These three dominant species accounted for77%of totalabundance of copepod community on average, but they just removed0.8%and1.5%of the microzooplankton standing stock and productivity daily, which suggested thatthe top-down effect copepod had on microzooplankton community was not significantand there was still most of microzooplankton biomass not entering the copepodcommunity through predation.