Meiobenthic Community Structure As Well As Nematode Diversity And Distribution In The Yellow Sea And East China Sea

Sediment samples were collected from76stations in the Yellow Sea inDecember2009and in the Yangtze Estuary and its adjacent sea waters (East ChinaSea) in November2009and July and December2012for the analyses of thecommunity structure and distribution of meiobenthos in relation to benthicenvironmental parameters. Based on three cruises, the taxonomy and diversity ofnematodes were studied using samples collected from twelve stations in the YangtzeEstuary and its adjacent waters in July. The average individual dry weights ofnematodes in the northern Yellow Sea and the Yangtze Estuary and its adjacentwaters were estimated.Based on the size measurement of a total of1594nematode samples, weobtained the average nematode individual dry weight of0.196μg/ind in the northernYellow Sea in December, and of0.213μg/ind and0.214μg/ind in the YangtzeEstuary and its adjacent waters in November and July, respectively, with themaximum value about1270times of the minimum. The species composition andjuvenile proportion are the main factors regulating the variability of the averageindividual dry weight of nematodes among stations. Accordingly, calculation usingthe simple0.4μg/ind will overestimate the actual biomass of nematodes to about onetime higher than that of direct measurements when meiobenthos as well asnematodes were sorted with a31μm sieve. For better estimation of nematodebiomass, it is suggested to calculate nematode biomass by direct measurement, whilesimple estimation using0.2μg/ind rather than0.4μg/ind is practical for theestimation of nematodes in this sea area.The average abundance of meiobenthos was (1281±1020) ind/10cm2at28stations in the Yellow Sea in December2009, and the biomass was (492±507) μgdwt/10cm2. There was an overall trend in both the abundance and biomassdecreasing from the north to the south and from the inshore to the offshore. The average abundance of meiobenthos was (2878±1279) ind/10cm2at four stations inthe northern Yellow Sea, and was (1014±667) ind/10cm2at24stations in thesouthern Yellow Sea. Among the12meiobenthos groups identified, Nematoda wasthe most abundant group, accounting for93.2%of the total abundance. In terms ofbiomass, nematodes accounted for48.5%. There were about70.8%of the totalmeiobenthos individuals as well as nematodes distributed in the surface0-2cm layer.Spearman correlation analysis showed that meiobenthos abundance was negativelycorrelated with median grain size and silt-clay content of the sediment, water depthand bottom water temperature. BIOENV analysis suggested that the combination ofenvironmental variables that best correlated with meiobenthos community consistsof median grain size, chlorophyll-a concentration, and silt-clay and organic mattercontent of the sediment. Considering all the available data obtained from the YellowSea, we came to the conclusion that: the abundances of meiobenthos in the YellowSea in winter is only about two thirds of those from the same sea area in summer;and there is an overall trend decreasing from the north to the south and from theinshore to the offshore in the same season, the value in the south Yellow Sea is lessthan half of that in the north Yellow Sea.As regards to the samples collected from19stations in November2009and13stations in November2012in the Yangtze Estuary and its adjacent waters, there wasalso an overall trend in both the abundance and biomass decreasing from the north tothe south and from the inshore to the offshore. Due to the influence of the Yangtzediluted water, the abundance of meiobenthos was high in the estuarine community,with the value of (1716±699) ind/10cm2in November2009and (1360±578)ind/10cm2in November2012. In contrast, the abundance was much lower in theoffshore sea areas, with the value of merely (509±234) ind/10cm2in November2009, only about two thirds of the mean abundance detected in the estuarinecommunity. Nematoda was the most abundant group of meiobenthos in both the yearof2009and2012(accounting for94.4%and94.3%, respectively). In terms of biomass, Nematoda was also the most dominant group in2009(accounting for51.6%), while in2012the most dominant group was replaced by Polychaeta(accounting for56.5%). There were on average70.8%and73.4%of the totalmeiobenthos individuals distributed in the surface0-2cm layer. Spearmancorrelation analysis showed that meiobenthos abundance in2009was positivelycorrelated with chlorophyll-a concentration of the sediment and negatively correlatedwith water depth and bottom water salinity and temperature. BIOENV analysissuggested that the combination of environmental variables that best correlated withmeiobenthos community in2009consists of water depth and sediment chlorophyll-aconcentration. BIOENV analysis suggested that the combination of environmentalvariables that best correlated with meiobenthos community in2012consists ofsediment chlorophyll-a concentration and organic matter content. It showed that theenvironmal parameters that influcing the abundance and distribution of meiobenthoswere different between the two years.The average abundance of meiobenthos was (1203±191) ind/10cm2at16stations in the Yangtze Estuary and its adjacent waters in July2012, and the biomasswas (496±152) μg dwt/10cm2, both of which were a little lower than that obtainedin November in the same sea area. Spearman correlation analysis showed thatbottom water salinity, silt-clay content and chlorophyll-a concentration of thesediment were the main factors regulating the variation of meiobenthos abundance.Nematoda was the most dominant group in both abundance (94.1%) and biomass(45.6%). There were about78.9%of the total meiobenthos individuals and76.2%ofthe nematodes distributed in the surface0-2cm layer. Generally, our study revealeda tendency of the meiobenthos congregating from the lower to the upper0-2cmsediment layer within the study areaover the past ten years, to some extent indicatingan increasing year by year degraded and anoxic benthic environment. Spearmancorrelation analysis showed that both the biomass of meiobenthos and the abundanceof Copepoda and Turbellaria were negatively correlated with organic nitrogen content of the sediment. BIOENV analysis suggested that the combination ofenvironmental variables that best correlated with meiobenthos community consistsof median grain size, and organic nitrogen content of the sediment. The horizontaldistribution of meiobenthos abundance in July was different from that in November:gradually increasing from the Yangtze Estuary to the eastern sea areas, reaching thehighest value at about40-45m depth contour, and then decreasing with the raisingof water depth. The difference might result from the seasonal change in the directionof Changjiang Diluted Water.The average abundance of nematodes was (1133±178) ind/10cm2at12stations in the Yangtze Estuary and its adjacent waters in July2012. A total of430species belonging to119genera,24families and four orders were identified. Amongthem, one new genus and five new species were discovered and described:Paraelzalia gen. nov., Paraelzalia longiseta sp. nov., Paramonohystera sinica sp.nov., Linhystera breviapophysis sp. nov., Linhystera longiapophysis sp. nov. andTrochamus falciformis sp. nov. The main dominant species were Dorylaimopsisrabalaisi, Quadricoma sp.1, Sabatieria sp.1, Chromadorita sp.1, Prochromadorellasp.1, Daptonema sp.2, Parodontophora sp.3, Daptonema sp.7, Diplopeltoides sp.1and Microlaimus sp.1. Nematode trophic type analysis showed that deposit feeders(1A+1B) were dominant in both species and individual number. Juveniles occupyabout41.5%of the total individuals. At the12stations, the average ratio of thenumber of female to male was1:1.17. CLUSTER analysis showed that thenematodes in this sea area could be classified into two communities: estuarinecommunity and offshore community. As concerns to the average abundance ofnematodes, there was little difference between the two communities, which was(1161±185) ind/10cm2and (1103±163) ind/10cm2, respectively. However, SIMPERanalysis showed much difference in the species composition of nematodes betweenthe two communities. From the aspect of trophic type, the number of non-selectivedeposite feeders in the offshore community was2.4times of that in the estuarine community, and Phaeophytin-a content of the sediment can explain the differencereasonably. The horizontal distribution of nematode diversity was similar to that ofthe abundance of meiobenthos as well as nematodes. Spearman correlation analysisshowed that the diversity of nematodes was positively correlated with bottom watersalinity, that is to say, Changjiang Diluted Water was the fundamental factor thatinflucing the diversity and distribution of nematodes. BIOENV analysis suggestedthat the combination of environmental variables that best correlated with nematodespecies composition and diversity consists of bottom water temperature, water depthand phaeophytin-a concentration of the sediment.