| The microbenthos mainly consist of unicellular prokaryotes, eukaryotic microalgae and protozoa. Meiobenthos usually refer to those small benthic metazoans, such as nematodes, polychaetes, and copepods. Micro- and meiobenthos are regarded as one complex, which is referred to as the benthic microbial food web, and play an important role in the energy flow from primary producers to higher trophic levels. However, because of methodological difficulties in the quantitative extraction of these fragile organisms from sediments and qualitative analysis, as well as the complex functions and forms of microbenthos, studies of microbenthos are scant and are mainly concentrated in intertidal habitat. Few studies concern the sediments in the shelf sea. This study focus on the Yellow Sea, which has suffered the green macroalgal Ulva prolifera and giant jellyfish Nemopilema nomurai blooms in recent years. Previous studies indicate the macroalgal bloom could have positive or negative effects on the ecosystem. The giant jellyfish sink to the seafloor and decompose after death. However, the patterns of micro- and meiobenthos during and after these events have never been evaluated. In combination with analyses of benthic environmental factors, using epifluorescence microscopy and the Ludox-QPS method, we investigated the distribution, community composition and diversity of micro- and meiobenthos in the sediments from the Yellow Sea in July and November of 2010 and 2011. The responses of micro- and meiobenthos to the blooms were also discussed.In the 0?5 cm sediments, the abundance of bacteria(108 cells/cm3) was about 2?3 orders of magnitude higher than that of cyanobacteria(105?106 cells/cm3), phototrophic(PNFs, 106 cells/cm3) and heterotrophic nanoflagellates(HNFs, 105–106 cells/cm3), and was much higher than those of diatoms, heterotrophic microflagellates(HMFs) and ciliates. Bacteria was also dominant in biomass. The biomass of PNFs and HNFs were in the same order or 1 order of magnitude lower than bacterial biomass. The meiobenthic abundance was far lower than those of pico-(bacteria and cyanobacteria) and nano- sized(PNFs and HNFs) benthos, and also lower than abundance of diatoms, but higher than those of HMFs and ciliates. The meibenthic biomss was also much lower than those of bacteria, PNFs and HNFs, close to that of cyanobacteria, and higher than other microbenthos.The samples collected in 2010 were placed for a long time before analyzed, which might lead to undervalue of the organisms. So the results in 2010 was regarded as background data. There were green macroalgal and giant jellyfish blooms in the summer 2011, so we mainly focused on the data in 2011.Different from the usual pattern, the median grain size decreased and silt?clay content increased in July 2011 in the inshore area, where macroalgae accumulated and subsided, thus the sediments in the inshore area became close to that in the central area in the southern Yellow Sea. While in July 2010, in the inshore area where macroalgal bloom also occurred, the sediment particles were still large and the silt?clay contents were still low, which were similar to the usual pattern. These data indicate the effects of macroalgal bloom on the benthic environment are different in the two years.In July 2011, different from the distribution in June 2007 when no macroalgal bloom happened, the standing crops of cyanobacteria and PNFs were low in the inshore area of the southern Shandong Peninsula where macroalgae accumulated and subsided. Though diatoms had high quantity in the inshore area of Qingdao. The average abundance in the southern Yellow Sea in July 2011 were obviously lower than that in June 2007. These data indicate the macroalgal blooms inhibit the growth of phototrophic organisms in the sediments. On the other side, in July 2011, the standing crops of bacteria, HNFs and ciliates were high in the coastal area near the southern Shandong Peninsula, which were different from their distribution patterns in June 2007. The accumulation and decomposition of macroalgae in the inshore area might stimulate the microbial degradation and detritic trophic webs within the sediments, and induced the growth of bacteria. HNFs and ciliates are the predators of bacteria, and ciliates are also the predators of HNFs, so the quantities of HNFs and ciliates increased by cascade effects.In July 2011, the distribution patterns of meiobenthos were similar to that in June 2007, and standing crops were high in the inshore area and low in the offshore area. The average meiobenthic abundance in the southern Yellow Sea in July 2011 was higher than that in June 2007. The Chl-a concentrations were high in the inshore area in the southern Shandong Peninsula, the enough food might guarantee the growth of meiobenthos. Furthermore, the Ludox-QPS method used in this study could estimate higher abundance than the centrifugation method, which might be another reason for the higher abundance in July 2011 than June 2007.Generally, the abundance and biomass of benthic ciliates were lower in the southern than in northern Yellow Sea. In July 2011, the average biomass was still lower but the average abundance was higher in the southern than in northern Yellow Sea, indicating small sized ciliates became dominant in the southern Yellow Sea in July 2011 when macroalgal bloom happened. Compared with those in June 2007, the proportions of bacterivores, for example, Metacystis spp., Euplotes spp. and scuticociliates, in abundance and biomass increased in July 2011, which might be the response of ciliates to the decomposition of macroalgae.In spite of no significant difference according to the T-test, the average total nitrogen content in the southern Yellow Sea in November 2011 both increased 22% when compared to those in July 2011 and November 2010. The increased total nitrogen content might be the result of the decomposition of jellyfish. The average biomass of PNFs in the southern Yellow Sea increased 17% in November 2011 compared to July 2011. In 2010 when no giant jellyfish blooms in summer, the average biomass in the southern Yellow Sea decreased 16% in November compared to July. According to the data in the two years, the decomposition of jellyfish probably released nutrients to the sediments and thus increase the growth of PNFs in November 2011.In the area off the Shandong Peninsula and Yangtze Estuary where large quantity of jellyfish occurred in summer, the standing crops of bacteria, PNFs, HNFs and ciliates were high in November 2011. The patterns were different from those in November 2010 when no jellyfish occurred in summer. Similar to the decomposition of macroalgae, the decay of jellyfish also could induce the benthic detritic trophic webs. However,the bacterial biomass decreased in November compared to July. The low bacterial biomass was likely due to the increased predation pressure of HNFs, whose biomass was largely increased.Compared to those in July 2011, the quantities of PNFs, HNFs and ciliates were all increased, but that of meiobenthos decreased, This was likely related to the decrease in Chl-a concentrations, an important indicator of primary productivity, which serves as one of the main food supplies of meiobenthos.Consistent with June 2007, the biomass and species numbers of carnivorous ciliates were the most in July and November 2011. Carnivores were the primary feeding type, followed by bacterivores, algivores and omnivores. Prostomateans and karyorelicteans consistently constituted the first and second most important ciliate groups in biomass. Compared to those in July 2011, the proportion of carnivores in biomass increased, but the proportion of bacterivores decreased. The rise in carnivores is largely due to the substantial increase of haptorians, which are typically rapacious ciliates. Thus, carnivorous ciliates constituted the primary trophic type at all stations in November. The increasing dominance of carnivorous ciliates is likely a response to the increase of predominant heterotrophic nanoflagellates in the sediments of the Yellow Sea. The latter situation might be ascribed to the subsequent effects of the green macroalgal and giant jellyfish blooms.Compared to those in June 2007, the relative contributions of heterotrophic flagellates in biomass and metabolic rate increased, but those of meiobenthos decreased. Then the relative contributions of heterotrophic flagellates in biomass and metabolic rate further increased in November 2011, and those of meiobenthos further decreased. The YSCWM and the northern Yellow Sea showed a similar trend in the relative contribution, while the sea area outside the YSCWM and the southern Yellow Sea were similar. The data indicate that the green macroalgal bloom together with the giant jellyfish bloom induced regime shifts in the benthic community structure from larger benthos towards smaller sizes and in benthic metabolism from autotrophic to heterotrophic in the southern Yellow Sea as well as the sea area outside the YSCWM. Such environments may favour the occurrence of the obligatorily rapacious giant jellyfish Nemopilema nomurai in the surroundings of the southern Yellow Sea and Yangtze Estuary, where the giant jellyfish medusae were initially sighted. The accumulation of micro- and meiobenthic biomass in this region may provide a good food supply for the obligatorily carnivorous polyps of Nemopilema nomurai and likely initiate the mass occurrence of giant jellyfish the following year. |
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