Matter In Seawater Research Articles

Polycyclic aromatic hydrocarbon analysis in different matrices of the marine environment

Polycyclic aromatic hydrocarbons (PAHs) were determined in marine samples of various types, i.e. seawater, sediment and mussel homogenate samples. The samples were spiked with standard PAH mixtures in both polar (acetonitrile) and non-polar ( i-octane) solvents, then extracted. Extraction from seawater was performed by liquid/liquid extraction to hexane (LLE) and with solid phase extraction (SPE) discs. The water samples were filtered and unfiltered seawater, and redistilled water for comparison. The discs with PAHs adsorbed from water samples, and also the sediment and mussel homogenate samples, were extracted with acetonitrile by sonication. PAHs in the disc extracts and from the LLE were cleaned-up using TLC and next determined by GC/MS/IT (with ion-trap) and HPLC-DAD/UV. The analytical procedures were verified with deuterated PAH standard mixtures. The large differences in PAH recoveries (from 12 to 86% for sum, and from 3 to 135% for particular PAHs) do not depend solely on the type of matrix and analytical procedure applied (e.g. standard solvent, volume of evaporated sample), but also on the concentration and molecular structure of the analyte. Usually, only a fraction of each PAH content in the matrix is determined, depending on the particulate matter in seawater and the sorption properties of the solid matrix. The recoveries of deuterated PAHs are higher than those of non-deuterated compounds.

Relative molar mass distributions of chromophoric colloidal organic matter in coastal seawater determined by Flow Field-Flow Fractionation with UV absorbance and fluorescence detection

A new method for the characterization of chromophoric colloidal organic matter in seawater has been applied to samples from the Baltic Sea, Kattegatt and Skagerrak seas. Size fractionation of the sample by Flow Field-Flow Fractionation and measurement of the fluorescent and UV absorbing properties of the individual size fractions result in a relative molar mass distribution (RMM) of the optical properties. The RMM distributions have been used to estimate number and weight average relative molar masses, and polydispersity indices. At least two sources of coloured organic matter were identified from the ratio of fluorescence to UV: the Baltic surface water and the Skagerrak deep water. The dominating processes were mixing and dilution, but processes such as photo bleaching of fluorescence are also believed to be important. The RMM distribution derived from UV detection (1150–1300 Dalton) increased with increasing salinity while that derived for fluorescence (1500–1250 Dalton) decreased with increasing salinity. The specific UV absorbance taken as a proxy of the aromaticity of the chromophoric organic material showed decreasing trend with both increasing salinity and increasing UV derived weight average relative molar mass. Increasing polydispersity of the colloidal material was also observed as a function of salinity.

Structure Elucidation and Characterization of Polychlorinated Biphenyl Carboxylic Acids as Major Constituents of Chromophoric Dissolved Organic Matter in Seawater

Chromophoric or colored dissolved organic matter (CDOM) is one of the principal light adsorbing components of seawater, particularly in the ultraviolet, where it attenuates over 90% of downwelling ultraviolet radiation. In highly productive coastal regions and throughout most of the global ocean, in situ biological production is the major source of CDOM. However, little is known about CDOM composition on the molecular level, and there are only a few reports that link CDOM composition to autochthonous biological sources. Here we report the isolation and characterization of CDOM components from one coastal and two open-ocean sites. Each sample contains a complex mixture of light absorbing (300-400 nm) components, including 2,4-dichlorobenzoic acid and a suite of novel, polychlorinated biphenyl carboxylic acids that closely resemble polychlorinated biphenyls (PCBs) of anthropogenic origin. However, the global inventory and isomer distribution of dissolved chlorinated aromatic acids suggest they are derived from in situ biological production rather than anthropogenic contaminants. These novel chlorinated aromatic acids account for a significant amount of CDOM adsorption in the ultraviolet.

The colloidal size spectrum of CDOM in the coastal region of the Mississippi Plume using flow field-flow fractionation

Chromophoric dissolved organic matter (CDOM) is a major component of the total dissolved organic matter in seawater, and it can interfere significantly with oceanographic remote sensing in nearshore waters. Coastal marine CDOM comprises an undefined, likely rapidly changing mixture of terrestrially and marine-derived substances. Flow Field-Flow Fractionation (FlFFF) offers a novel approach for separating the colloidal (high molecular weight) fraction of CDOM into broad size continuums that can offer insights to the characteristics and reactivity of CDOM constituents. FlFFF analyses of seawaters in and around the Mississippi Plume were performed during high and low flow conditions and the results compared to analyses of phytoplankton cultures and waters from a coastal estuary having low freshwater inputs. FlFFF fractograms displayed major differences in the size distribution of CDOM between offshore in the Gulf of Mexico (GOMex) and the Mississippi River plume. Colloidal size distributions in nearshore surface waters out of the immediate influence of the Mississippi Plume contained a mixture of features seen in offshore waters, plume waters, and in phytoplankton cultures. Plume waters had a high abundance of larger colloidal phases but continued dilution with coastal waters led to preferential losses of mid-sized colloidal matter. Colloidal size spectra in offshore waters of the GOMex during spring were similar to that observed in estuarine waters after the spring bloom. These preliminary findings suggest that the colloidal size spectrum of CDOM may provide information on its provenance and the relative mixtures of terrestrial and marine components.

Chemical characteristics of amino acid-containing dissolved organic matter in seawater

Total hydrolyzable amino acid and fluorescence properties, using 3-dimensional excitation and emission matrix (3DEEM) spectroscopy of the bulk and four molecular mass fractions (GF/F–0.1 μm, 0.1 μm–10 kDa, 10 kDa–5 kDa and <5 kDa) of dissolved organic matter (DOM), were determined in the surface (10 m) and deep (1000 m) waters of Sagami Bay, Japan. Among the molecular mass fractions, we found a successive decrease in degradation index (DI) derived from amino acid composition with decreasing molecular mass in both surface and deep waters, indicating that relatively fresh and large molecules become smaller in size along a degradation pathway. In the 3DEEM fluorescence patterns of high molecular mass DOM, only a tryptophan-like fluorophore was observed in the surface water. In contrast, both tryptophan- and tyrosine-like fluorophores were observed in the deep water, indicating that amino acid-containing DOM in the high molecular mass fractions was comprised of protein molecules or least degraded proteins (polypeptides) in surface water, but in deep water other forms of amino acids were dominant even though the molecular mass fractions were of the same size. We hypothesize that degradation products (peptides) form high molecular mass fraction in deep water DOM.

Oligotrophic bacterioplankton with a novel single-cell life strategy.

A large fraction of the marine bacterioplankton community is unable to form colonies on agar surfaces, which so far no experimental evidence can explain. Here we describe a previously undescribed growth behavior of three non-colony-forming oligotrophic bacterioplankton, including a SAR11 cluster representative, the world's most abundant organism. We found that these bacteria exhibit a behavior that promotes growth and dispersal instead of colony formation. Although these bacteria do not form colonies on agar, it was possible to monitor growth on the surface of seawater agar slides containing a fluorescent stain, 4',6'-diamidino-2-phenylindole (DAPI). Agar slides were prepared by pouring a solution containing 0.7% agar and 0.5 micro g of DAPI per ml in seawater onto glass slides. Prompt dispersal of newly divided cells explained the inability to form colonies since immobilized cells (cells immersed in agar) formed microcolonies. The behavior observed suggests a life strategy intended to optimize access of individual cells to substrates. Thus, the inability to form colonies or biofilms appears to be part of a K-selected population strategy in which oligotrophic bacteria explore dissolved organic matter in seawater as single cells.

Mobilization of metals from riverine suspended matter in seawater

Suspended matter from rivers in the Russian Far East was leached by seawater to assess the extent of Zn, Cd, Pb, and Cu release from the land-derived solids to estuarine and coastal waters. The concentration of solids in the leaching experiments was 0.25 g/l. The suspended matter used varied in metal content from background levels to heavily contaminated. The concentration of Zn, Cd, Pb, and Cu in the leaching solution after 1, 4, 24, 48, and 96 h was determined by anodic stripping voltammetry. Measurable Cd was transferred from river suspended matter to seawater with both background and elevated Cd concentrations, though the amount of Cd released was different. The increase in Zn and Pb in solution was observed only at the enriched concentration of metals in the suspended solids. The Cu transfer into solution was more pronounced from material with elevated Cu concentration, but the amount of degradable organic matter in the solid phase was more important. The losses of metals from the riverine solids varied from 60% to 80% of total concentration for Cd to negligible for Pb, independent of contamination. The Zn loss depended on the initial concentration in the solids and decreased from 11–16% for the heavily contaminated suspended matter to 3–8% for the moderately enriched ones and to negligible for the pristine solids. Cu loss varied from 1% to 30% of total content, with no clear dependence on concentration in the suspended matter. The significance of additional input of dissolved metals to estuarine and coastal waters from remobilization was assessed by comparison with the initial concentration of dissolved metals in river water. Such experiments could be used to assess the water quality impact of atmospheric fallout of contaminated solids and storm drainage from the urbanized areas, in addition to river suspended matter studies.

Determination of Soluble Iron Species in Seawater Using Ferrozine

Ferrozine was used for the simultaneous determination of soluble iron(II) and iron(III) species in seawater. It was shown that iron forms complexes with dissolved organic matter in seawater, which hinders the color development of ferrozine chelates. To eliminate this factor, samples were heated and exposed to UV irradiation. The developed procedures were used for determining soluble iron species in samples of surface and silt seawater. The results agree with the data obtained by atomic absorption spectrometry.

A SENSITIVE FLUORIMETRIC METHOD FOR THE DETERMINATION OF NITRITE AND NITRATE IN SEAWATER BY A NOVEL RED-REGION FLUORESCENCE DYE

A novel method for the determination of dissolved inorganic nitrite and nitrate in surface seawater was reported, which is based on the diazotization reaction between nitrite and a novel red-region fluorescent dye, tetra-substituted amino aluminum phthalocyanine (TAAlPc). Nitrate is determined as nitrite after reduction on a cadmium column. Under optimal conditions, the linear range of the calibration curve is 21–840 nM for nitrite. The detection limit is 7 nM for and the relative standard deviation for six replicate measurements of a 350 nM nitrite solution is 3.2%. Since the method employs a red-region fluorescent dye (λex/em = 610 nm/678 nm), the potential fluorescent background resulting from the organic matters in seawater can be eliminated and thus the sensitivity of the method can be enhanced.

Airborne polarized lidar detection of scattering layers in the ocean

A polarized lidar technique based on measurements of waveforms of the two orthogonal-polarized components of the backscattered light pulse is proposed to retrieve vertical profiles of the seawater scattering coefficient. The physical rationale for the polarized technique is that depolarization of backscattered light originating from a linearly polarized laser beam is caused largely by multiple small-angle scattering from particulate matter in seawater. The magnitude of the small-angle scattering is determined by the scattering coefficient. Therefore information on the vertical distribution of the scattering coefficient can be derived potentially from measurements of the time-depth dependence of depolarization in the backscattered laser pulse. The polarized technique was verified by field measurements conducted in the Middle Atlantic Bight of the western North Atlantic Ocean that were supported by in situ measurements of the beam attenuation coefficient. The airborne polarized lidar measured the time-depth dependence of the backscattered laser pulse in two orthogonal-polarized components. Vertical profiles of the scattering coefficient retrieved from the time-depth depolarization of the backscattered laser pulse were compared with measured profiles of the beam attenuation coefficient. The comparison showed that retrieved profiles of the scattering coefficient clearly reproduce the main features of the measured profiles of the beam attenuation coefficient. Underwater scattering layers were detected at depths of 20-25 m in turbid coastal waters. The improvement in dynamic range afforded by the polarized lidar technique offers a strong potential benefit for airborne lidar bathymetric applications.

UV digestion of seawater samples prior to the determination of copper using flow injection with chemiluminescence detection

A purpose built UV digestion system was successfully used for the breakdown of Cu complexing organic ligands in seawater samples, prior to total dissolved Cu determination using flow injection with chemiluminescence detection (FI-CL) and on-line micro-column preconcentration/matrix removal. Residual dissolved organic carbon (DOC) was quantified using a DOC analyser. Humic acid (1.8–7.2 mg l −1 C) in ultra high purity (UHP) water was completely broken down within 4 h in all batch experiments (125 and 400 W lamps; with and without 15 mM H 2O 2 and, as expected, was more rapid with the 400 W lamp, in the presence of H 2O 2, and for lower humic acid concentrations. UV digestion experiments with seawater showed that the residual DOC concentration after batch UV treatment (4 h) was <0.08 mg l −1 C compared with >0.32 mg l −1 C after on-line irradiation (residence time 11.2 min). Therefore, the batch method was more efficient than on-line UV digestion at breaking down added humic acid and naturally present organic compounds in seawater. However, the release of Cu from metal complexing organic matter in seawater and estuarine water was the same using both on-line and batch UV digestion (sample irradiation residence time: 5.6 min and 8 h, respectively). UV digestion is, therefore, a contamination-free approach for seawater pretreatment prior to micro-column preconcentration and FI-CL determination of total dissolved Cu and should also be applicable to the selective determination of the total dissolved fractions of other trace metals in seawater (e.g. Co, Fe, Mn).

Concentrations of lipid phosphorus and its abundance in dissolved and particulate organic phosphorus in coastal seawater

Abstract Analytical procedures to determine lipid phosphorus (lipid P) in seawater were developed and tested using a combination of solvent and solid-phase extraction. Lipid P was extracted from particulate and dissolved matter in seawater with a solvent mixture of chloroform and methanol. Silica gel solid-phase extraction (SPE) was evaluated using phospholipid and hydrophilic compounds. SPE exhibited high recoveries of phospholipids from the solvent mixture and was also effective in removing contamination from hydrophilic compounds. The method was applied to coastal seawater samples to investigate the quantitative significance of lipid P in marine organic P. In samples collected from Tokyo Bay, Japan, and Corpus Christi Bay, USA, concentrations of particulate lipid P ranged from 31 to 300 nM, which corresponded to 3–14% of the total particulate P. Concentrations of dissolved lipid P were low ranging from 0.7 to 6 nM, which were less than 1% of the total dissolved organic P concentrations. Thus, lipid P is a significant component of particulate P, but a rather minor component of dissolved organic P.

Determining optical absorption of colored dissolved organic matter in seawater with a liquid capillary waveguide

Optical absorption spectra of 0.2‐μm filtered sea‐water samples originating from diverse oceanic and coastal waters were measured with a long pathlength capillary wave‐guide; results were compared with those of three different lab‐oratory spectrophotometers. The 0.5‐m long 550‐ µm (inside diameter) aqueous waveguide uses only 120 ml of filtered sea‐water, making it convenient for use in flow‐through cells or when sample volumes are restricted. Source light propagates inside the capillary waveguide by total internal reflection because of the lower refractive index of the waveguide walls with respect to the aqueous core. The absorption coefficient of colored dissolved organic matter (CDOM) at 355 nm and S, the slope of the log linearized CDOM absorption spectra, were determined for all samples. The CDOM absorption spec‐tra measured by the capillary waveguide closely matched that measured by spectrophotometers for CDOM concentrations varying over an order of magnitude. The deviations between the absorption spectra obtained with the capillary waveguide and those obtained with the standard spectrophotometers increased with decreasing total absorption and with increase in wavelength, presumably because of the greater baseline offsets observed in the capillary waveguide. The offsets are due to differences in refractive indices between the seawater samples and the freshwater reference. With a suitable reference, the capillary waveguide will be very useful for monitoring surface seawater CDOM absorption semicontinuously.

Abundance, size distribution, and stable carbon and nitrogen isotopic compositions of marine organic matter isolated by tangential-flow ultrafiltration

Tangential-flow ultrafiltration was used to isolate particulate and high-molecular-weight dissolved material from seawater collected at various depths and geographic regions of the Pacific and Atlantic Oceans. Ultrafiltration proved to be a relatively fast and efficient method for the isolation of hundreds of milligrams of material. Optical and electron microscopy of the isolated materials revealed that relatively fragile materials were recovered intact. Depth-weighted results of the size distribution of organic matter in seawater indicated that ~ 75% of marine organic carbon was low-molecular-weight (LMW) dissolved organic carbon ( 100 nm). The distribution of carbon in surface water was shifted to greater relative abundances of larger size fractions, suggesting a diagenetic sequence from macromolecular material to small refractory molecules. The average C:N ratios of particulate organic matter (POM) and HMW dissolved organic matter (DOM) were 7.7 and 16.7, respectively. Differences in C:N ratios between POM and HMW DOM were large and invariant with depth and geographic region, indicating that the aggregation of HMW DOM to form POM must be of minor significance to overall carbon dynamics. The stable carbon isotope composition (δ13C) of POM averaged −22.7%. in surface water and −25.2%. in subsurface water. Several possible explanations for the observed isotopic shift with depth were explored, but we were unable to discern the cause. The δ13C of HMW DOM samples was relatively constant and averaged −21.7%., indicating a predominantly marine origin for this material. The δ15N values of POM were highly variable (5.8–15.4%.), and the availability of nitrate in surface waters appeared to be the major factor influencing δ15N values in the equatorial Pacific. In the upwelling region nitrate concentrations were relatively high and δ15N values of POM were low, whereas to the north and south of the upwelling nitrate concentrations were low and δ15N values were high. The δ15N values of HMW DOM reflected the same trends observed in the POM fraction and provided the first such evidence for biological cycling of dissolved organic nitrogen (DON). Using the observed δ15N values and an estimate of meridional advection velocity, we estimated a turnover time of 0.3 to 0.5% day−1 for HMW DON. These results suggest a major role for DON in the upper ocean nitrogen cycle.

MECHANISM OF DEGRADATION OF ORGANIC MATTER IN SEAWATER BY EFFECTIVE MICROBES AND ITS UTILIZATION

海水中有機物の分解に優れた細菌 Pseudomonas paucimobilis の分解機構を把握し, その工学的利用性を検討した. まず, P. paucimobilis には, 直接有機物を分解する他に, 他の細菌群の有機物分解活性を著しく増強させる作用があることがわかった. その機構として, はじめに P. paucimobilis が難分解性の有機物を易分解性に変換し, ついで自然生育の混合細菌群が分解するものと推定した. この結果, P. paucimobilis は単独で使用するより, むしろ混合培養系で活用する方が有効であることがわかった. また, P. paucimobilis の沿岸の砂利への付着・定着過程, およびその環境水中での生残性について調べた. さらに, 砂利およびコンクリートを担体として, これに P. paucimobilis を付着させて浄化実験を行い, いずれも有機物分解能が高まることを示した.

Major biochemical composition of dissolved high molecular weight organic matter in seawater

Abstract Ultrafiltered dissolved organic matter (UDOM) was isolated from surface, oxygen minimum, and deep waters of three ocean basins and its elemental as well as molecular-level aldose and amino acid compositions were determined. Surface concentrations account for 23–33% of total dissolved organic carbon, and are a factor of 2–3 greater than those in deeper waters. Oceanic UDOM has an extremely characteristic organic composition, clearly distinct from other marine materials such as fresh plankton, sinking particles or humic substances. Polysaccharides appear to be the major reactive component of UDOM. They have a distinctive aldose distribution rich in galactose and deoxy sugar that is almost ubiquitous regardless of depth or location, suggesting that UDOM carbohydrate is dominated by a very similar suite of polysaccharide throughout the ocean. In contrast, amino acids account for a relatively minor component of both total UDOM and of its organic nitrogen component. Amino acid distributions are similar to those from unfractionated seawater, and are not preferentially remineralized. In O2 minimum and deep ocean water, ultrafiltered material accounts for 18–25% of total dissolved organic carbon. Compositions are nearly invariant in these subsurface isolates, suggesting that ultrafiltered material is stable and unreactive throughout the subsurface ocean. Taken together with large compositional differences between UDOM and sinking particles, this observation suggests that dynamic aggregation is probably not an important formation or removal process for UDOM in the deep ocean. Amino acid and especially carbohydrate concentrations are lower in deep UDOM, but the overall molecular-level compositions remain similar to those from surface waters. This molecular-level homogeneity suggests that the UDOM biopolymers reflected in amino acid and carbohydrate data persist relatively unaltered in the deep ocean.

Accretion of extraterrestrial matter during the last 80 million years and its effect on the marine osmium isotope record

The flux of particulate extraterrestrial (ET) matter to the deep-sea has been calculated using a four-component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the Pacific Ocean. Nineteen published bulk-leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation. The calculated annual particulate flux, averaged over several 100 ka by slow accumulation and bioturbation, ranges from ∼18,000 to ∼67,000 tons, with a mean value of ∼37,000 ± 13,000 tons. The data indicate no significant variability in the flux with time, except at the K-T boundary and are thus compatible with results based on Ir accumulation in deep-sea sediments. The inferred constancy of the ET matter flux is in contrast to recent results based on the ET 3He flux that integrates only a small size fraction of the entire ET matter flux. 3He data indicate variability by a factor of 6 over the past 70 Ma and a fivefold increase in the ET flux between 3 Ma and 1 Ma that is not seen in the bulk ET matter flux based on osmium isotope data.The apparent constancy of the cosmic matter flux over the past 80 Ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record, except at the K-T boundary. A comparison between osmium isotope and Ir data from two pelagic sediment cores from the Pacific (DSDP Site 596 and LL44-GPC3) indicates that the seawater-soluble fraction of ET Os cannot exceed ∼36 kg/a and most likely is significantly smaller. The maximum value of 36 kg/a can account for approximately one half of the seawater-soluble ET matter flux necessary to balance the radiogenic continental runoff of Os without any additional unradiogenic source. Simple mass balance calculations indicate that an additional unradiogenic source of Os to the oceans, most likely alteration of oceanic crust, is required to balance the present-day seawater osmium-isotopic composition. This source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater.

Bacterial chemotaxis and its potential role in marine dimethylsulfide production and biogeochemical sulfur cycling

Dimethylsulfide (DMS) gas constitutes 90% of biogenic sulfur emissions from oceans and is an important agent in climate regulation. DMS in seawater arises primarily via lyase degradation of dimethylsulfoniopropionate (DMSP), an osmolyte produced in high concentrations by some marine phytoplankton. After being induced to synthesize DMSP lyase, cells of a marine bacterium (Alcaligenes strain M3A) significantly reduce their rate of change in direction and tumbling frequency, and they are attracted to DMSP at levels similar to those occurring near senescing phytoplankton cells (10‒7–10‒6 M). In contrast, genetically identical bacteria without lyase induction are not attracted to DMSP. Combined with lyase activity, bacterial chemotaxis to DMSP could increase the rate of DMS production and therefore play a critical role in biogeochemical sulfur cycling between dissolved organic matter in seawater and the atmosphere.

Automatic photocatalytic method for the determination of dissolved organic carbon (DOC) in natural waters

An automatic method for the determination of dissolved organic carbon in natural waters is described. The method relies on the catalytic oxidation of dissolved organic matter to CO 2 by UV radiation and using a titanium dioxide semiconductor as a catalyst. Several compounds including urea and fulvic acid were mineralised completely. Compared with direct photolysis, the semiconductor catalytic photolysis gave consistently higher mineralisation yields for natural organic matter in seawater. The fraction resistant to direct UV photolysis (i.e. UV-refractory) was found to be in the range of 2%–5% of total DOC and it is suggested that this consists of organic macromolecules that fragment to non-UV absorbing species and escape further oxidation in direct UV photolysis. Organic macromolecules concentrated by molecular filtration contained 10%–50% such material, the higher proportion was found during the productive season in the sea, suggesting their primary biological origin (rather than degraded organic matter). The throughput of the method can be up to 30 samples per h and it is capable of online real-time analysis of DOC. The method gives linear calibration of up to 1000 μM C and blanks (instrument, reagent and distilled water blanks) can be readily and easily evaluated. Attempted validation of the method against CHN elemental analysis, highlighted several inherent basic problems. Initial validation showed that within the analytical errors, the two methods were in close agreement.

Distribution of heterotrophic nanoflagellates and their importance as the bacterial consumer in a eutrophic coastal seawater

Seasonal and vertical changes in abundances of bacteria and heterotrophic nanoflagellates (HNF), and HNF grazing on bacteria were investigated in a small eutrophic inlet of Uranouchi-Wan throughout the years. Bacterial densities in the surface water ranged from 1.2 to 11 (average 4.3)×106 cells ml−1 with a couple of maxima following the algal blooming. Densities of HNF ranged from 0.54 to 73 (average 16.4)×103 cells ml−1 in the surface, and showed almost similar fluctuation pattern to that of bacteria with a time lag of about 1 to 2 weeks. Grazing rates of HNF on bacteria obtained by FLB method were 4.78 to 16.9 (average 10.3±SD 4.8) cells HNF−1h−1 in the surface layer in summer, and consequent total bacterial consumption rates by HNF fluctuated from 4 to 99×104 cells ml−1h−1. In deeper layers, however, as HNF densities and grazing rates on bacteria were low, the grazing pressure of HNF on bacteria was small. Turnover times of bacteria by HNF grazing in the surface layer were calculated as relatively constant values of 40 to 60 h, however, it decreased to as low as 6 to 7 h when the HNF activity was highest. These results indicate that bacteria grew so actively by consuming organic matter in seawater as to compensate high HNF grazing pressure, and that bacteria and HNF in the microbial loop play important roles on the turnover of substrates in coastal ecosystems.