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Polymetallic nodules were discovered on the Pacific Ocean in 1891.They exist at the sediment surface,sometimes in upper layers of sediments,and cover more than 50% of the sea floor at Clarion-Clipperton fracture Zone(CCFZ) in the north Pacific.The huge economic value from nodule mining has brought to many research programs dedicated to resource evaluation and design of collecting tools in the 1970s.China has carried out surveys on polymetallic nodules and sediment in the CCFZ for many years.Since the late 1980s,China has already got 75000 km2 exclusive pioneer areas in COMRA′s contracted area.One of the potential impacts of nodule mining is the destruction of the fauna attached to the partial covering of surrounding epifauna by sediment blanketing.Meiofauna,the important components of benthic ecosystem,are considered to be energetically important in benthic food web.as they play a critical role in the recycling of nutrients.In order to manage and mitigate these impacts,we need to better understand the composition and the distribution of meiofauna and its relationship to the environment. A quantitative study was carried out on "Dayang Yihao" in July 2005.The meiofauna samples were collected from 12 stations in eastern and western sub-area at COMRA's contracted areas,Samples were collected using a multiple corer with four tubes of 9.5cm in diameter,and only undisturbed cores with clear overlying water were used.Thirteen benthic groups were found from the surveyed area,.The average density of meiofaunal were(104.4±20.48) ind/10cm2 and(40.26±25.84) ind/10cm2 in eastern and western sub-area,respectively.The average biomass of nematode were(5.25±0.99)(μg·Dry) /10cm2 and(1.68±0.77)(μg·Dry) /10cm2 in each sub-area.The analysis of spatial distribution has shown that the density of meiofauna and the biomass of nematode in eastern sub-area were substantially higher than that in western sub-area.Nematodes,copepods,polychaeta,and ostracoda were found to be the dominant taxa,and nematodes were the predominant one,constituting 93.13% and 91.36% of total density of meiofauna in eastern and western sub-area,respectively.The analysis of vertical distribution has shown that the density of meiofauna decreased with the depth in the sediment.The top 0—2cm layer of sediment accounted for over 75% of meiofauna.The proportion of meiofauna distributed in the deeper sediment was higher in eastern sub-area than that in western sub-area. In such an oligotrophic environment,low food supply may limit meiofaunal abundance,biomass,,and to a lesser extent species richness..In eastern sub-area,the average concentrations of chlorophyll a and phaeopigment in the top 6cm sediment were 0.004μg/cm2 and 0.169μg/cm2 respectively.In western sub-area,the average concentrations of chlorophyll a and phaeopigment in the top 6cm sediment were 0.002μg/cm2 and 0.082μg/cm2,respectively.There were significant positive relationships between chlorophyll a and phaeopigment in the surface sediment.The correlation between the meiofauna distribution and the environmental factors was investigated.The results indicated that(1) the density of the dominant groups(i.e.nematode,copepods,polychaetes) and the biomass of nematodes were significantly negatively correlated with the latitude and longitude;(2)but were significantly positively correlated with the amount of phaeopigment;(3) the biomass in the sediment had no significant impact on the density of meiofauna and their major groups,but was significantly positively correlated with the amount of phaeopigment;(4) the density of meiofauna,nematodes and polychaetes and the biomass of nematodes were significantly positive correlated with the amount of chlorophyll a;(5) the density of copepods has no sigifnicant correlation with the amount of chlorophyll a.

An investigation of the distribution and size composition of chlorophyll a concentration was carried out in the Southwestern Indian Ocean(47—70°E,25—40°S) in January 2011,and factors that might affect the distribution and size composition were discussed.Fluorometric method was used to obtain total and size-fractioned chlorophyll a concentrations.The survey was carried out during the austral summer,and there were 6 stations altogether,two of which located in the region of the south Indian Ocean Subtropical Gyre(IOSG) and the other four located in the subtropical convergence zone(SCZ).Water samples were collected from 13 layers in 0—200m column of each station and the vertical profiles of chlorophyll a,nutrients(NO-3 、PO3-4 and SiO2-3),temperature and salinity were obtained.The objective of this study was to investigate the spatial distribution and size fraction of chlorophyll a concentration and to relate them to environmental factors.The results showed that the surface chlorophyll a concentration was low and did not exceed 0.07mg/m3,and the depth of the subsurface chlorophyll a maximum was deep,at approximately 100m and sometimes reaching as deep as 150m in the region of IOSG;However,in the SCZ,the surface chlorophyll a concentration was higher(0.164—0.247 mg/m3),and the subsurface chlorophyll a maximum layer occurred between 50—70m.In the region of IOSG,phytoplankton growth is limited by nutrients supply,while the frontal system formed by the Agulhas Return Current(ARC) and the Subtropical Convergence(STC) is a region of intense mesoscale activity presenting enhanced levels of biological production and chlorophyll a concentration.Based on the discussion of the factors affecting the distribution of phytoplankton,it appeared that NO-3 was the main factor that limited the growth of phytoplankton in the southwestern Indian Ocean,and PO3-4 could be the main limiting factor in some areas.Size-fractioned chlorophyll a concentration clearly showed that the study area was a typical oligotrophic open ocean.The picophytoplankton(0.2—2μm) chlorophyll a always dominated,representing 71.1% of the total chlorophyll a concentration on average,while the average contribution of nanophytoplankton(2—20μm) and microphytoplankton(20μm) chlorophyll a to total chlorophyll a concentration were 24.2% and 4.7% respectively.The dominance of picophytoplankton was resulted from the low nutrient concentration environment because picophytoplankton were more competent over the microphytoplankton and nanophytoplankton under oligotrophic condition.There were some differences on the contribution of picophytoplankton to total chlorophyll a concentration between the regions of IOSG and SCZ,which were 77.9% and 66.7% respectively.The differences of the hydrographical characteristics,which in turn influenced the nutrient dynamics in the region of IOSG and SCZ,might lead to the significant difference of the spatial distribution and size composition of chlorophyll a concentration between these two regions.