Community structure and spatio-temporal variation of microbenthos and meiobenthos in sediments off the Yangtze Estuary

Using epifluorescence microscopy and the Ludox-QPS method,we investigated the community composition,abundance,and biomass as well as distribution of microbenthos and meiobenthos in combination with the analysis of benthic environmental variables at four stations off the Yangtze Estuary in the summer( July) and fall( November) of 2010 and2011,respectively. The abundances of bacteria( 108 cells / cm3),cyanobacteria( 106 cells / cm3),and phototrophic( PNF,106 cells / cm3) and heterotrophic nanoflagellates( HNF,105-106cells / cm3) were much higher than those of other components in the upper 5 cm of sediments. The biomass partitioning indicates the primary importance of benthic bacteria( 19-24 [mu]g C/cm3),followed by PNF( 13-31 [mu]g C/cm3),HNF( 5-44 [mu]g C/cm3),cyanobacteria( 1-2 [mu]g C/cm3),meiobenthos( 0. 9-1 [mu]g C /cm3),ciliates( 0. 04-0. 2 [mu]g C /cm3),heterotrophic microflagellates( HMF,0. 02-0. 08 [mu]g C / cm3),and diatoms( 0. 001-0. 008 [mu]g C /cm3). The standing crops of microbenthic and meiobenthic assemblages,except of bacteria and cyanobacteria,were generally higher in 2011 than in 2010 and usually higher in the fall than in the summer. About 84% of diatoms,82% of ciliates,65% of HMF,and 77% of meiobenthos were distributed in the upper 2 cm of the sediments,whereas no distinct vertical distribution was observed for bacteria,cyanobacteria,PNF,and HNF. The vertical distribution had little variation between the years and seasons. The estimated productivity of diatoms ranged from 0. 05 to 0. 4 mg C m- 2d- 1and contributed only 0. 01% -0. 1% of the primary productivity. Thus,most of the primary productivity was contributed by the pico- and nano-sized benthos. The estimated productivity of bacteria was 80-101 mg C m- 2d- 1,about 17% -21% of the primary productivity. The biomass of bacteria was nine to 18 times higher than that of the meiobenthos,whereas the productivity was 30-61 times higher than that of the meiobenthos( 2-3 mg C m- 2d- 1). The biomass of ciliates was much lower than that of the meiobenthos,while the ciliate productivity( 3-23 mg C m- 2d- 1) was one to 14 times higher than that of the meiobenthos. The relative metabolic rates indicate that benthic heterotrophic nanoflagellates constitute the most important group in sediment respiration,accounting for 92. 9% -99. 1% of the estimated combined metabolic rate of the microbenthic and meiobenthic consumers. Among the 166 identified ciliate species,carnivorous ciliates constituted the p