| The occurrence of the giant jellyfish Nemopilema nomurai has been a frequentphenomenon in the Yellow Sea. However, the relationship between the giant jellyfishand protozoa in particular ciliate is largely not known. In the summer of2012, giantjellyfish Nemopilema nomurai bloomed in the Yellow Sea and East China Sea.During June when the occurrence of Nemopilema nomurai began and August whenthere was a mass occurrence of Nemopilema nomurai, water samples were collectedfrom8transects A (37.0°N), C (36.0°N), E (35.0°N), G (34.0°N), I (33.0°N),K (32.0°N), M (31.5°N), PN (30.4-31.1°N), based on which we investigated thechanges of the distribution of planktonic nanoflagellates, ciliates, Noctiluca scintillansand copepod nauplii, as well as the changes of the community structure of ciliates. Forthe first time, significant positive correlations were found between Nemopilemanomurai and ciliates.Nanoflagellates In June and August of2012, the abundance and biomass ofPNFs or HNFs in the Yellow Sea (transects A, C, E, G, I,33.0°N-37.0°N) weregreater than those in the adjacent sea area of Changjiang estuary (transects K, M, PN,30.4°N-32.0°N). In the surface layer, both the abundance and biomass of PNFs (1218±998ind./mL,59.04±58.23μgC/L) in June were bigger than those (640±311ind./mL,29.39±20.69μgC/L) in August. Compared with PNFs, HNFs had lowerabundance and biomass but shared the same time variation, that is293±330ind./mLand10.91±11.63μgC/L in June were larger than173±136ind./mL and10.08±9.57μgC/L in August. The quantity of PNFs and HNFs was larger near shore than offshore in August. The abundance and biomass in the surface layer of nanoflagellates ineach transect (except transects I, PN) in August significantly decreased to25%-80%of those in June.In the10m layer, the quantity of PNFs and HNFs was a little lower than that in thesurface layer. The quantity of PNFs in the10m layer was about2/3of that in thesurface layer, while the quantity of HNFs was only a bit lower than that in the surface layer. The quantity of PNFs and HNFs in the10m layer shared the similarspatio-temporal distribution with that in the surface layer.Ciliates In the surface layer, the abundance, biomass and production of ciliatesshared the similar spatio-temporal distribution, and the quantity was concentrated inthe southern Yellow Sea and the adjacent sea area of Changjiang estuary. Contrary tonanoflagellates, ciliates had much greater abundance, biomass and production in theadjacent sea area of Changjiang estuary than those in the Yellow Sea. In transect I(33.0°N) and the adjacent sea area of Changjiang estuary, the quantity of ciliates wasconcentrated in the center of each transect, in June as well as August. In the YellowSea, in June the quantity of ciliates was concentrated near shore in transect G(34.0°N), however, in August it was concentrated off shore in transect E (35.0°N).In the surface layer, the abundance of ciliates (8466±5972ind./L) in August waslarger than that (6398±5533ind./L) in June, yet the biomass of ciliates in August waslower than that in June (18.99±8.59μgC/L vs.21.03±28.36μgC/L). There weredifferences among time variations of the quantity in each transect. In the surface layerof transects A, C, E and I, the biomass of ciliates in August increased to2-4times ofthat in June. However in the surface layer of transects G, K, M and PN, the biomassof ciliates in August decreased to18%-81%of that in June. Though in half transectsthe biomass of ciliates decreased in August, yet along with the rising temperature theproduction of ciliates increased significantly from24.81±22.34μgC/(L·d) in June to44.77±22.56μgC/(L·d) in August.In the water column of transect I, the biomass (carbon weight) of Nemopilemanomurai had only significant positive correlation with the abundance and productionof ciliates as well as the biomass of tintinnids, indicating a possible prey-predatorrelationship between the jellyfish and ciliates. The BIOENV analysis furthersuggested that the biomass of ciliates and Noctiluca scintillans was the mostexplainable combination for the biomass of Nemopilema nomurai. In addition, duringthe mass occurrence of Nemopilema nomurai in transects A, C, E and I in August, the ciliates in the surface layer still significantly increased. Supposed that Nemopilemanomurai feeds on ciliates, the feeding pressure on the biomass and production ofciliates at the station I3in late June was16%and10%respectively, supporting thatciliates with their high standing crops and high production might serve assupplemental food for Nemopilema nomurai. Moreover, in transects G, K, M and PNwhere Nemopilema nomurai increased by1-2orders of magnitude in August, thebiomass of ciliates sharply decreased, especially large-sized ciliates, indicating thatthe mass occurrence of Nemopilema nomurai influenced ciliate significantly.Ciliate in the surface layer was more diverse in August than in June: there were atotal of40species belonging to18genera identified, consisting of15species ofaloricate ciliates and25species of tintinnids in June; there were a total of54speciesbelonging to25genera identified, consisting of16species of aloricate ciliates and38species of tintinnids in August. Strombidium was the most diverse group of aloricateciliates, and Tintinnopsis was the most diverse group of tintinnids. The dominantspecies included Strombidium sp.1(28μm), Laboea strobila (140μm), Mesodiniumrubrum (24μm) and Lohmanniella sp.(37μm). The aloricate ciliates contributed toabout98%of total ciliate abundance and biomass in June, and in August theabundance and biomass of tintinnids accounted for about10%of total ciliateabundance and biomass.CLUSTER analysis based on species-abundance data of ciliates in the surface layerindicate that, in June, most sampling stations in transects A, C and E had similarcommunity structures and comprised one group. Most sampling stations in thesouthern Yellow Sea and East China Sea comprised the other two groups, which wascoincident with the occurrence of Nemopilema nomurai in June. In August, only afew sampling stations in transects A, C and E comprised one group, while most of therest sampling stations in all transects comprised the other group, which indicated thatthe mass occurrence of Nemopilema nomurai homogenized the community structuresof ciliates. Noctiluca scintillans&Copepod nauplii In May of2012, a red-tide bloom ofNoctiluca scintillans occurred along the China Rizhao coast of the Yellow Sea (35°N).While the red tide declined in June, the standing crops of Noctiluca scintillans werestill high in the southern Yellow Sea and the adjacent sea area of Changjiang estuary,that is in transects I (33.0°N), K (32.0°N) and M (31.5°N). The abundance, biomassand production of Noctiluca scintillans in June was272±487ind./L,533.20±953.40μgC/L and106.64±190.68μgC/(L·d) respectively. In August, the quantity ofNoctiluca scintillans decreased to75±196ind./L,146.95±383.55μgC/L and29.39±76.71μgC/(L·d). In August the abundance of copepod nauplii in the surface layerwas lower than that in June (85±74ind./L vs.100±101ind./L), however, thebiomass (3.81±7.19vs.2.68±7.01μgC/L) and production (1.41±2.70vs.0.68±1.80μgC/(L·d)) were greater than those in June. The quantity of copepod nauplii wasconcentrated in transect I (33.0°N).Food relationships involved in this study are as follows: nanoflagellatesâ†'ciliatesâ†'copepod naupliiâ†'Noctiluca scintillans, and Noctiluca scintillans can directly feed onciliates and nanoflagellates. Spearman correlation analyses indicated that Noctilucascintillans had extremely significant positive correlations with copepod nauplii;Noctiluca scintillans and copepod nauplii both had extremely significant positivecorrelations with ciliates; ciliates had significant positive correlations withnanoflagellates. In the surface layer, all the biotic factors had significant negativecorrelations with the temperature and salinity, however, in the water column oftransect I the biotic factors had extremely significant positive correlations with thetemperature.In general view, Nemopilema nomurai feeds mainly on mesozooplankton likecopepods, however, mesozooplankton cannot provide sufficient food supply for themass occurrence of Nemopilema nomurai. This study indicates that, under thecompetitive predation of Noctiluca scintillans for copepod nauplii, copepod nauplii aswell as adults could not provide sufficient food supply for the mass occurrence of Nemopilema nomurai. For instance, with the peak abundance of Noctiluca scintillansat station I3in June, the feeding pressure of Nemopilema nomurai on the biomass andproduction of copepod nauplii was up to66%and268%, respectively. In suchcircumstances, ciliates with their high standing crops and high production might serveas supplemental food for Nemopilema nomurai. |
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