Drivers of coastal bacterioplankton community diversity and structure along a nutrient gradient in the East China Sea

Anthropogenic nutrient discharge poses widespread threats to coastal ecosystems and has increased environmental gradients from coast to sea. Bacterioplankton play crucial roles in coastal biogeochemical cycling, and a variety of factors af fect bacterial community diversity and structure. We used 16 S r RNA gene pyrosequencing to investigate the spatial variation in bacterial community composition(BCC) across five sites on a coast-of fshore gradient in the East China Sea. Overall, bacterial alpha-diversity did not diff er across sites, except that richness and phylogenetic diversity were lower in the of fshore sites, and the highest alpha-diversity was found in the most landward site, with Chl-abeing the main factor. BCCs generally clustered into coastal and of fshore groups. Chl-a explained 12.3% of the variation in BCCs, more than that explained by either the physicochemical(5.7%) or spatial(8.5%) variables. Nutrients(particularly nitrate and phosphate), along with phytoplankton abundance, were more important than other physicochemical factors, co-explaining 20.0% of the variation in BCCs. Additionally, a series of discriminant families(primarily affiliated with Gammaproteobacteria and Alphaproteobacteria), whose relative abundances correlated with Chl-a, DIN, and phosphate concentrations, were identified, implying their potential to indicate phytoplankton blooms and nutrient enrichment in this marine ecosystem. This study provides insight into bacterioplankton response patterns along a coast-of fshore gradient, with phytoplankton abundance increasing in the of fshore sites. Time-series sampling across multiple transects should be performed to determine the seasonal and spatial patterns in bacterial diversity and community structure along this gradient.     

Dissolved oxygen distribution and O2 fluxes across the sea-air interface in East China Sea waters

This study on the sectional and vertical distribution of dissolved oxygen (DO) and the O₂ fluxes across the sea-air interface in East China Sea (ESC) waters shows that the waters were in steady state and that the difference of DO was great in upper and bottom waters in Apr. 1994; but that seawater mixing was strong and the difference of DO was small in upper and bottom waters in Oct. 1994. The above conclusions were specially obvious in continental shelf waters under 100 m. The DO maximum in subsurface layer waters appeared only at several stations and in general the DO in the waters decreased with depth. The horizontal distributions of O₂ fluxes across the sea-air interface appeared in stripes in Leg 9404 when most regions covered were supersaturated with O₂ seawater to air flux was large, and that on section No. 1 was 1.594 L/m²·d. The horizontal distribution of O₂ fluxes across the sea-air interface appeared lumpy in Leg 9410, when most regions covered were unsaturated with O₂·O₂ was dissolved from air to seawater, and the fluxes were 0.819 L/m²·d on section No. 1 in Leg 9310, 0.219 L/m²·d in Leg 9410. The main reasons for DO change in surface layer seawater were the mixture of upper and bottom layer water, and the exchange of O₂ across the sea-air interface. The variation of DO by biological activity was only 20% of total change of DO. Contribution No. 2716 from Institute of Oceanology, Chinese Academy of Sciences.     

Spatial variations of bacterial community composition in sediments of the Jiaozhou Bay,China

Spatial variations of sediment microbes pose a great challenge for the estimation of anthropogenic influence on biogeochemical processes, yet remain very unclear in coastal ecosystems. Surface sediments in 9 stations from the eutrophic Jiaozhou Bay, China, were sampled, DNA was extracted within the sediments, and the 16 S rDNA was sequenced with the Illumina Hiseq sequencing. Results reveal considerable heterogeneity of sediment bacteria in the Jiaozhou Bay, of which Proteobacteria and Bacteroidetes accounted for over 75%. Bacterial alpha-diversity indices decreased generally from the outside to the inner part of the bay and from the offshore to the nearshore area. Bacterial community structures of S3, S4, S7, and S8 clustered, those of S5, S13, and S14 grouped together, while those of S6 and S10 were distinct from each other and from those of the other stations. Major class Gammaproteobacteria were more abundant at the stations with mesoeutrophic to eutrophic levels(S4, S5, S8, and S10) and less abundant at oligotrophic stations(S6, S13, and S14), while Deltaproteobacteria had an opposite distribution pattern. Overall, bacterial community composition transitioned from being Xanthomonadales-dominant at S4 and S8 to being unidentifed_Gammaproteobacteria-dominant at S5, S6, S13, and S14, while in other stations there were comparable orders. The biogeochemical processes correspondingly changed from being nitrogen cycling-dominant at S4 and S8 to being sulfur cycling-dominant at S5, S6, S13, and S14. The bacterial distribution patterns were especially affected by the factors(dissolved organic phosphorus, DOP) in the overlying seawater due to the habitat status of P-insu fficiency in the bay. Both orders Xanthomonadales and Alteromonadales could serve as bioindicators of anthropogenic pollution to different pollution types. At last, divergent distribution patterns of individual bacterial populations in the bay were revealed, the influential environmental gradients were clarified, and the uncertainty of microbes was reduced, helping to predict environmental functions in coastal areas.     

Effects of organic carbon consumption on denitrifier community composition and diversity along dissolved oxygen vertical profiles in lake sediment surface

At present, the understanding of the dynamics of denitrifiers at different dissolved oxygen (DO) layers under organic carbon consumption within the surface sediments remains inadequate. In this study, high-throughput sequencing and quantitative PCR targeting nirS gene were used to analyze the denitrifier abundance dynamics, community composition, and structure for aerobic (DO 0.5–6.9 mg/L), hypoxic-anoxic (DO 0–0.5 mg/L), and anoxic (DO 0 mg/L) layers in surface sediments under organic carbon consumption. Based on the analysis of nirS gene abundance, the values of denitrifying bacteria decreased with organic carbon consumption at different DO layers. When the bacterial species abundance at the genus level were compared between the high-carbon and low-carbon sediments, there was significant increase in 6 out of 36, 7 out of 36 and 6 out of 36 genera respectively for the aerobic, hypoxic-anoxic and anoxic layers. On the other hand, 14 out of 36, 9 out of 36 and 15 out of 36 genera showed significant decrease in bacterial species abundance respectively for the aerobic, hypoxic-anoxic and anoxic layers. Additionally, 14 out of 36, 20 out of 36, and 15 out of 36 genera had no change in bacterial species abundance respectively for the aerobic, hypoxic-anoxic, and anoxic layers. This indicates that the carbon utilization ability of different denitrifiers on each DO layers was generally different from each other. Diversity of denitrifying bacteria also presented significant differences in different DO layers between the high- and low-carbon content sediment layers. Moreover, under the high-carbon and low-carbon content, the abundance of nirS gene showed a high peak within the hypoxic-anoxic regions, suggesting that this region might be the main distribution area for the denitrifying bacteria within the surface sediments. Furthermore, community of unique denitrifiers occurred in different DO layers and the adaptive changes of the denitrifier community followed the organic carbon consumption.     

Sediment bacterial communities are more complex in coastal shallow straits than in oceanic deep straits

Straits are ideal models to investigate the bacterial community assembly in complex hydrological environments. However, few studies have focused on bacterial communities in them. Here, comparable bacterial communities in costal shallow Bohai Strait(BS) and oceanic deep Fram Strait(FS) were studied. The Shannon and Chao1 indices were both higher in BS than in FS. The relative abundances of the classes Deltaproteobacteria and Bacilli and the family Halieaceae were higher in BS than in FS, in contrast to the families OM1_clade and JTB255_marine_benthic_group, revealing typical characteristics of bacterial communities in coastal and oceanic regions. Cluster analysis based on the Bray-Curtis index showed that samples were clustered by depth layer in FS and BS, indicating that structures of bacterial communities would diff er with increasing water depth in straits. Additionally, the cluster relationships among samples in abundant and rare communities were both similar to those in entire communities. However, the dissimilarities among samples showed a descending order as rare communities, entire communities and abundant communities. Network analysis indicated that the BS network was obviously more complex than the FS network. Filamentous bacteria Desulfobulbaceae exhibited high degree values in BS but not in FS, indicating key roles of Desulfobulbaceae in the BS. Our study provides different and common evidences for understanding microbial ecology in coastal shallow and oceanic deep straits.     

THE STRENGTH OF THE DISSOLVED OXYGEN MAXIMUM IN THE VERTICAL DISTRIBUTION OF NANSHA ISLANDS WATERS

Observation data from a cruise in the Nansha Islands, in May to June 1990, December, 1993, September to October 1994, and July, 1999, respectively, were used to develop the method presented here to indicate the existing strength of the dissolved oxygen maximum in the vertical distribution of Nansha Islands waters. Its seasonal variation and regional distribution are discussed in this paper. Analysis results showed that the distribution of the strength of dissolved oxygen maximum (DOmax-DOsur) was closely related to the upper layer circulation and the bioactivity of Nansha Islands seawater.     

Bacterial community shifts in different functional zones of a reservoir with ecological purification facilities

Ecological purification in a reservoir is an important strategy to control the level of nutrients in water.The bacterial community of such a reservoir is the main agent for pollutant degradation,which has not been fully documented.Taking the Jinze Reservoir,a freshwater source for Shanghai,China as the case,its spatial distributions of water and sediment bacteria were determined using 16 S rRNA gene-based Illumina MiSeq sequencing,and the environmental parameters were analyzed.The reservoir takes natural river water and functions as an ecological purification system,consisting of three functional zones,i.e.,pretreatment zone,ecological purification zone,and ecological sustaining zone.Results show that the concentrations of both total nitrogen(TN) and total phosphorus(TP) decreased considerably after the ecological treatment,and the concentration of dissolved oxygen(DO) in the ecological purification zone was boosted from that before pretreatment.In addition,patterns of bacterial communities in both water and sediment were similar and consisted of mainly Proteobacteria,Actinobacteria,and Bacteroidetes.However,difference in water bacterial composition was distinct in each functional zone,whereas the bacterial communities in sediment changed only slightly among sediment samples.Network analysis revealed nonrandom co-occurrence patterns of bacterial community composition in water and sediment,and Pseudomonas,unclassified Comamonadaceae,Variovorax,and Dechloromonas were the key taxa in the co-occurrence network.The bacterial taxa from the same module of the network had strong ecological connections,participated in C-cycles,and shared common trophic properties.PICRUSt analysis showed that bacteria were involved potentially in various essential processes;and the abundance of predicted xenobiotic biodegradation genes showed a decreasing trend in water samples from the inlet to the outlet of the reservoir.These results improve our current knowledge of the spatial distribution of bacteria in water and sediment in ecological purification reservoirs.     

Bacterial diversity, composition and temporal-spatial variation in the sediment of Jiaozhou Bay, China

Studies on the diversity and distribution of bacterial populations will improve the overall understanding of the global patterns of marine bacteria and help to comprehend local biochemical processes and environments. We evaluated the composition and the dynamics of bacterial communities in the sediment of Jiaozhou Bay (China) using PCR-denaturing gradient gel electrophoresis (DGGE). Sediment samples were collected from 10 different sites in May, August, and November 2008 and in February 2009. There was significant temporal variation in bacterial community composition at all sites. However, the spatial variation was very small. The DGGE analyses of bacterial communities were used to divide the 10 stations into three types. Canonical correspondence analysis (CCA) revealed that the changes in bacterial communities were driven by sediment properties. Sequence analysis of DGGE band-derived 16S rRNA gene fragments revealed that the dominant bacterial groups in the sediment were of the classes γ-proteobacteria and δ-proteobacteria and phyla Bacteroidetes and Nitrospirae. Our results provide considerable insight into the bacterial community structure in Jiaozhou Bay, China.     

Distribution of microbial populations and their relationship with environmental variables in the North Yellow Sea, China

In order to understand the large-scale spatial distribution characteristics of picoplankton,nanophytoplankton and virio-plankton and their relationship with environmental variables in coastal and offshore waters,flow cytometry(FCM) was used to ana-lyze microbial abundance of samples collected in summer from four depths at 36 stations in the North Yellow Sea(NYS).The data revealed spatial heterogeneity in microbial populations in the offshore and near-shore waters of the NYS during the summer.For the surface layer,picoeukaryotes were abundant in the near-shore waters,Synechococcus was abundant in the offshore areas,and bacte-rial and viral abundances were high in the near-shore waters around the Liaodong peninsula.In the near-shore waters,no significant vertical variation of picophytoplankton(0.2-2μm) abundance was found.However,the nanophytoplankton abundance was higher in the upper layers(from the surface to 10 m depth) than in the bottom layer.For the offshore waters,both pico-and nanophytoplankton(2-20μm) abundance decreased sharply with depth in the North Yellow Sea Cold Water Mass(NYSCWM).But,for the vertical dis-tribution of virus and bacteria abundance,no significant variation was observed in both near-shore and offshore waters.Autotrophic microbes were more sensitive to environmental change than heterotrophic microbes and viruses.Viruses showed a positive correla-tion with bacterial abundance,suggesting that the bacteriophage might be prominent for virioplankton(about 0.45μm) in summer in the NYS and that viral abundance might play an important role in microbial loop functions.     

Macrobenthic assemblages of the Changjiang River estuary (Yangtze River,China) and adjacent continental shelf relative to mild summer hypoxia

To assess the effects of hypoxia, macrobenthic communities along an estuarine gradient of the Changjiang estuary and adjacent continental shelf were analyzed. This revealed spatial variations in the communities and relationships with environmental variables during periods of reduced dissolved oxygen(DO) concentration in summer. Statistical analyses revealed significant differences in macrobenthic community composition among the three zones: estuarine zone(EZ), mildly hypoxic zone(MHZ) in the continental shelf, and normoxic zone(NZ) in the continental shelf(Global R =0.206, P =0.002). Pairwise tests showed that the macrobenthic community composition of the EZ was significantly different from the MHZ(pairwise test R =0.305, P =0.001) and the NZ(pairwise test R =0.259, P =0.001). There was no significant difference in macrobenthic communities between the MHZ and the NZ(pairwise test R =0.062, P =0.114). The taxa included small and typically opportunistic polychaetes, which made the greatest contribution to the dissimilarity between the zones. The effects of mild hypoxia on the macrobenthic communities are a result not only of reduced DO concentration but also of differences in environmental variables such as temperature, salinity, and nutrient concentrations caused by stratification.     

Influence of a Flood Event on Salinity and Nutrients in the Changshan Archipelago Area (Northern Yellow Sea)

River discharge can deliver nutrients to the coastal zone and change the hydrologic properties of the water column. Soon after a flash flood from the Yalu River (Northeast China) in August 2010, we investigated the salinity and nutrient concentrations, as well as other environmental conditions in the Changshan Archipelago area, located approximately 100 km west of the river mouth in the northern Yellow Sea. Diluted water was mainly observed in the upper layers shallower than 15 m, with surface salinity between 18.13 and 30.44 in the eastern study area and between 28.16 and 29.72 in the western area. Surface salinity showed a significant negative correlation with concentrations of dissolved nutrients (P < 0.05), but not with that of Chlorophyll-a (Chl-a), dissolved oxygen (DO), particulate materials or pH. The average concentrations of nitrite, nitrate, and silicic acid decreased from the surface layer to bottom layer and were significantly higher in the east area than in the west area (P < 0.05). In contrast, average ammonium and phosphate concentrations were highest in the bottom layer of both areas, with no significant spatial differences. DO varied between 6.06 and 8.25 mg L-1 in the surface layer, and was significantly higher in the eastern area than in the western area in the surface and middle layers. Chl-a concentration was constantly below 4.09 μg L-1. Our work demonstrated the strong influences of Yalu River on proportions of various nutrient components in the Changshan Archipelago area. Silicic acid and total inorganic nitrogen levels were significantly elevated comparing to phosphate in the eastern area. Such changes can potentially induce phosphate limit to phytoplankton growth.     

Bacterial community succession in response to dissolved organic matter released from live jellyfish

Jellyfish blooms have increased worldwide,and the outbreaks of jellyfish population not only affect the food web structures via voracious predation but also play an important role in the dynamics of nutrients and oxygen in planktonic food webs.However,it remains unclear whether specific carbon compounds released through j ellyfish metabolic processes have the potential to shape bacterial community composition.Therefore,in this study,we aimed to investigate the compositional succession of the bacterioplankton community in response to the dissolved organic matter(DOM)released by the live Scyphomedusae Cyanea lamarcki'i and Chrysaora hysoscella collected from Helgoland Roads of the North Sea.The bacterial community was significantly stimulated by the DOM released form live jellyfish and different dominant phylotypes were observed for these two Scyphomedusae species.Furthermore,the bacterial community structures in the different DOM sources,jellyfish-incubated media,Kabeltonne seawater,and artificial seawater(DOM-free)were significantly different,as revealed by automated ribosomal intergenic spacer analy sis fingerprints.Catalyzed reporter depo sition fluorescence in situ hybridization(CARD-FISH)revealed a rapid species-specific shift in bacterial community composition.Gammaproteobacteria dominated the community instead of the Bacteroidetes community for C.lamarckii,whereas Gammaproteobacteria and Bacteroidetes dominated the community for C.hysoscella.The significant differences in the bacterial community composition and succession indicate that the components of the DOM released by jellyfish might differ with jellyfish species.     

Inorganic carbon parameters responding to summer hypoxia outside the Changjiang Estuary and the related implications

The eutrophication, hypoxia and coastal acidification are attracting more and more attention. In this study, inorganic carbon parameters, including dissolved inorganic carbon (DIC), total alkalinity (TA) and calculated partial pressure of CO₂ (pCO₂), obtained from a summer cruise in August, 2009, were used to investigate their integrated response to biological processes accompanying the oxygen depletion in the areas off the Changjiang Estuary. According to the observations, the typical hypoxia occurred in the bottom water just outside the Changjiang Estuary with Dissolved Oxygen (DO) lower than 2.00 mg L^?1. The biological uptake in the surface water and the decomposition of organic matter in the bottom water were fully coupled with each other. The high concentration of Chl_a (Chl_a = 10.9 μg L^?1) and DO (9.25 mg L^?1), profoundly decreased DIC concentration (1828 μmol kg^?1) and elevated pH (8.42) was observed in the surface water. The correspondingly increased DIC and depletion of oxygen were observed in the bottom water. The semi-quantitative analysis proved that the locally-produced phytoplankton, determined by primary productivity, was deposited to the bottom and contributed about 76% of total amount of the organic carbon decomposition in the bottom. However, in the bottom hypoxia (DO = 2.05 mg L^?1) area observed in the Southern Zhejiang coastal water, the responding patterns of inorganic carbon parameters deviated from the previous one. The expanding of Changjiang Diluted Water (CDW), the adding of Hangzhou Bay water (with high DIC concentration) and Coastal Current together modify the DIC background value in this area, and the local degeneration and upwelling process may also help to offset the local DIC removed by net biological uptake in surface water. In addition, when the mixing occurring in autumn, which may break the summer stratification, the excess release of high DIC in the bottom water to the subsurface water could have an important influence on coastal acidification and the CO₂ uptake capacity in this area.     

Contrast between the climatic states of the warm pool in the Indian Ocean and in the Pacific Ocean

Based on the analysis of Levitus data, the climatic states of the warm pool in the Indian Ocean (WPIO) and in the Pacific Ocean (WPPO) are studied. it is found that WPIO has a relatively smaller area, a shallower bottom and a slightly lower seawater temperature than those of WPPO. The horizontal area at different depths, volumes, central positions, and bottom depths of both WPIO and WPPO show quite apparent signals of seasonal variation. The maximum amplitude of WPIO surface area’s seasonal variation is 58% larger over the annual mean value. WPIO’s maximum volume variation amplitude is 66% larger over the annual mean value. The maximum variation amplitudes of the surface area and volume of WPPO are 20.9% and 20.6% larger over the annual mean value respectively. WPIO and WPPO show different temporal and spatial characteristics mainly due to the different wind fields and restriction of ocean basin geometry. For instance, seasonal northern displacement of WPIO is, to some extent, constrained by the basin of the Indian Ocean, while WPPO moves relatively freely in the longitudinal direction. The influence of WPIO and WPPO over the atmospheric motion must be quite different.     

Research on oxygen-releasing agent for prawn culture

A solid powder oxygen-releasing agent was developed and tested in prawn culture. Dissolved oxygen (DO) is a basic necessity for aquatic life and is affected by water temperature, climate, plankton, etc. DO variation in culture ponds strongly and consequently affects prawn growth and production output. In summer, when air temperature is high, pond water is polluted (by prawn feed leftovers) and cannot be changed fully and as frequently as necessary; especially at night, when there is no sunlight for photosynthesis with oxygen as a by-product, the DO in pond water can decrease rapidly. Under the above circumstances, the prawns experience serious lack of oxygen before dawn and die in large number. To solve this problem, we developed an oxygen-releasing agent with excellent performance. Contribution No. 1857 from the Institute of Oceanology, Academia Sinica.     

Impact of Kuroshio on the dissolved oxygen in the East China Sea region

A marine survey was conducted from 18 May to 13 June 2014 in the East China Sea(ECS)and its adjacent Kuroshio Current to examine the spatial distribution and biogeochemical characteristics of dissolved oxygen(DO) in spring. Waters were sampled at 10-25 m intervals within 100 m depth, and at 25-500 m beyond 100 m. The depth, temperature, salinity, and density(sigma-t) were measured in situ with a conductivity-temperature-depth(CTD) sensor. DO concentrations were determined on board using traditional Winkler titration method. The results show that in the Kuroshio Current, DO content was the highest in the euphotic layer, then decreased sharply with depth to about 1 000 m, and increased with depth gradually thereafter. While in the ECS continental shelf area, DO content had high values in the coastal surface water and low values in the near-bottom water. In addition, a low-DO zone of f the Changjiang(Yangtze) River estuary was found in spring 2014, and it was formed under the combined influence of many factors, including water stratification, high primary productivity in the euphotic layers, high accumulation/sedimentation of organic matter below the euphotic layers, and mixing/transport of oceanic current waters on the shelf. Most notable among these is the Kuroshio intruded water, an oceanic current water which carried rich dissolved oxygen onto the continental shelf and alleviated the oxygen deficit phenomenon in the ECS, could impact the position, range, and intensity, thus the formation/destruction of the ECS Hypoxia Zone.     

Bacterial diversity of gut content in sea cucumber (Apostichopus japonicus) and its habitat surface sediment

This study investigated the bacterial diversity of gut content of sea cucumber(Apostichopus japonicus) and its habitat surface sediment in a bottom enhancement area using PCR-based denaturing gradient gel electrophoresis(DGGE) technique. Bacterial diversity evaluation showed that the value of the Shannon-Wiener index of gut content in different intestinal segments of A. japonicus varied between 2.88 and 3.00, lower than that of the surrounding sediment(3.23). Phylogenetic analysis showed that bacterial phylotypes in gut content and the surrounding sediment of A. japonicus were closely related to Proteobacteria includingγ-, α-, δ-and ε-proteobacteria, Bacteroidetes, Firmicute, and Actinobacteria, of which γ-proteobacteria were predominant. These results suggested that the sea cucumber A. japonicus was capable of feeding selectively, and PCR-DGGE was applicable for characterizing the bacterial community composition in gut content and the surrounding sediment of sea cucumber. Further investigation targeting longer 16S rDNA gene fragments and/or functional genes was recommended for obtaining more information of the diversity and function of bacterial community in the gut content of sea cucumber.     

Effects of hypoxia on survival,behavior,and metabolism of Zhikong scallop Chlamys farreri Jones et Preston 1904

Hypoxia,a frequent occurring threat in coastal regions,often results in mass mortalities of marine organisms and brings a serious ecological problem.The commercially important Zhikong scallop Chlamys farreri is being under such a threat as the risks of eutrophication and hypoxia have risen in their culture areas.However,little information has been known concerning their tolerance to hypoxia and their strategy for survival.In the present study,a 20-day experiment was conducted to determine the ef fects of hypoxia on the survival,behavior,and metabolism of Zhikong scallop.With the LC_(50) for dissolved oxygen(DO) being estimated as 1.8 mg/L,the survival of Zhikong scallop can be greatly challenged even under the moderate hypoxic condition of around 2.0 mg/L DO.The survival rate ranged from 69% to 59% when DO dropped from 3.0 to 2.0 mg/L,and it was further reduced to 47% at 1.5 mg/L DO.In hypoxic conditions,the scallops became significantly active,which may be explained as escape attempts to avoid hypoxic water.To save energy,Zhikong scallop would depress their respiration.However,when DO dropped from 3.0 to 2.0 mg/L,the oxygen consumption rate hardly changed.The upregulation of lactate dehydrogenase activity and the unrepressed phosphofructokinase activity,which often result in the unbalanced cellular homeostasis and energy budget,may account for the observed increase in the mortality rate of Zhikong scallops.In general,Zhikong scallop is sensitive to hypoxia events,though possible escape attempts,depressed respiration,and oxaloacetate-pathway may increase their survival chance.     

Seasonal variation in abundance and species composition of a planktonic diatom assemblage including viable cells on the bottom in matsushima bay,northeastern japan

Seasonal variation in abundance and species composition of a planktonic diatom assemblage distributed in the water column and also settled on the bottom was investigated for the shallow coastal water in Matsushima Bay on the Pacific coast of northeastern Japan during the period from October 1999 to September 2000. A spring bloom of diatoms began in April when nutrient concentrations started to increase, indicating the importance of nutrients. Viable cells of Skeletonema costatum and Thalassiosira spp., which were the dominant species in the water column throughout the year, were also always abundant in the bottom sediment. Both populations in the water column and on the bottom fluctuated essentially in parallel. For the planktonic diatoms in shallow coastal waters to maintain their vegetative populations in the water column, it would be advantageous for them to have a seeding population of viable cells on the bottom that are easily resuspended into the upper photic layer.     

A THEORETICAL SOLUTION FOR THE THERMOHALINE CIRCULATION IN THE SOUTHERN YELLOW SEA

Application of the thermocline equations in the thermocline areas and the boundary layer and the asymptotic matching techniques in each boundary in order to satisfy the surface and bottom conditions yielded a theoretical 2- D solution of the vertical thermohaline circulation of the Southern Yellow Sea in summer when the quasi-statically varying seasonal thermocline (density layer) is the background density structure , the deviations from which cause the secondary vertical circulation . The results show that the thermocline can be considered as an internal boundary or a barrier to the vertical heat advection so that in the central areas of the Southern Yellow Sea or the center of the Yellow Sea Cold Water Mass(YCWM)> the downwelling in the upper layer and upwelling in the lower or bottom layer form a double cell vertical circulation . The solution is similar to Hu's conceptual model ( 1986) in the central areas of the YCWM and is consistent with observed temperature . salinity and dissolved oxygen distri