Biogeochemistry of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the surface microlayer and subsurface water of the western North Atlantic during spring

Sixteen surface microlayer samples and corresponding subsurface water samples were collected in the western North Atlantic during April–May 2003 to study the distribution and cycling of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) and the factors influencing them. In the surface microlayer, high concentrations of DMS appeared mostly in the samples containing high levels of chlorophyll a, and a significant correlation was found between DMS and chlorophyll a concentrations. In addition, microlayer DMS concentrations were correlated with microlayer DMSPd (dissolved) concentrations. DMSPd was found to be enriched in the microlayer with an average enrichment factor (EF) of 5.19. However, no microlayer enrichment of DMS was found for most samples collected. Interestingly, the DMS production rates in the microlayer were much higher than those in the subsurface water. Enhanced DMS production in the microlayer was likely due to the higher concentrations of DMSPd in the microlayer. A consistent pattern was observed in this study in which the concentrations of DMS, DMSPd, DMSPp (particulate) and chlorophyll a in the microlayer were closely related to their corresponding subsurface water concentrations, suggesting that these constituents in the microlayer were directly dependent on the transport from the bulk liquid below. Enhanced DMS production in the microlayer further reinforces the conclusion that the surface microlayer has greater biological activity relative to the underlying water.     

Dimethylsulfide enrichment in the surface microlayer of the South China Sea

A total of 22 sea surface microlayer samples collected from the Nansha Islands waters of the South China Sea were analyzed for dimethylsulfide (DMS), chlorophyll a and nutrients including nitrate, phosphate and silicate. The DMS concentrations in surface microlayer samples ranged from 82 to 280 ng S/l with a mean of 145 ng S/l. A significant correlation was found between DMS and chlorophyll a data both in the surface microlayer as well as in the subsurface water. However, no correlation was observed between DMS and nutrient concentrations in the surface microlayer. The DMS concentrations were higher in all surface microlayer samples, compared with subsurface samples. The enrichment factor (EF) of DMS in the surface microlayer varied from 1.21 to 3.08 with an average of 1.95. The EF of DMS was significantly correlated with that of chlorophyll a in the microlayer. The enrichment of DMS in the microlayer may be due to two factors, including the in situ production from phytoplankton and the transportation from the underlying seawater. The diel variations in DMS and chlorophyll a concentrations were studied at a fixed station. The highest concentrations of DMS in the surface microlayer and subsurface water were simultaneously observed in the late afternoon (1800 h), while the highest levels of chlorophyll a were simultaneously found at night (0200 h).     

Biogeochemistry of Dimethylsulfoniopropionate (DMSP) in the Surface Microlayer and Subsurface Seawater of Funka Bay,Japan

Twenty-eight sea surface microlayer samples, along with subsurface bulk water samples were collected in Funka Bay, Japan during October 2000–March 2001 and analyzed for dimethylsulfoniopropionate, dissolved (DMSP_d) and particulate (DMSP_p), and chlorophyll a. The aim of the study was to examine the extent of enrichment of DMSP in the microlayer and its relationship to chlorophyll a, as well as the production rate of dimethylsulfide (DMS) from DMSP and the factors that influence this. The enrichment factor (EF) of DMSP_d in the surface microlayer ranged from 0.81 to 4.6 with a mean of 1.85. In contrast, EF of DMSP_p in the microlayer varied widely from 0.85–10.5 with an average of 3.21. Chlorophyll a also appeared to be enriched in the microlayer relative to the subsurface water. This may be seen as an important cause of the observed enrichment of DMSP in the microlayer. The concentrations of DMSP_p in the surface microlayer showed a strong temporal variation, basically following the change in chlorophyll a levels. Moreover, the microlayer concentrations of DMSP_p were, on average, 3-fold higher than the microlayer concentrations of DMSP_d and there was a significant correlation between them. Additionally, there was a great variability in the ratios of DMSP_p to chlorophyll a over the study period, reflecting seasonal variation in the proportion of DMSP producers in the total phytoplankton assemblage. It is interesting that the production rate of DMS was enhanced in the microlayer and this rate was closely correlated with the microlayer DMSP_d concentration. Microlayer enrichment of chlorophyll a and higher DMS production rate in the microlayer provide favorable evidence supporting the view that the sea surface microlayer has a greater biological activity than the underlying water.     

Spatial variations of dimethylsulfide and dimethylsulfoniopropionate in the surface microlayer and in the subsurface waters of the South China Sea during springtime

Spatial variations in dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) were surveyed in the surface microlayer and in the subsurface waters of the low productivity South China Sea in May 2005. Overall, average subsurface water concentrations of DMS and DMSP of dissolved (DMSPd) and particulate (DMSPp) fractions were 1.74 (1.00-2.50), 3.92 (2.21-6.54) and 6.06 (3.40-8.68) nM, respectively. No enrichment in DMS and DMSPp was observed in the microlayer. In contrast, the microlayer showed a DMSPd enrichment, with an average enrichment factor (EF, defined as the ratio of the microlayer concentration to subsurface water concentration) of 1.40. In the study area, none of the sulfur components were correlated with chlorophyll a. An important finding in this study was that DMS, DMSP and chlorophyll a concentrations in the surface microlayer were respectively correlated with those in the subsurface water, suggesting a close linkage between these two water bodies. The ratios of DMS:Chl-a and DMSPp:Chl-a showed a gradually increasing trend from North to South. This might be due to changes in the proportion of DMSP producers in the phytoplankton community with the increased surface seawater temperature. A clear diurnal variation in the DMS and DMSP concentrations was observed at an anchor station with the highest concentrations appearing during the day and the lowest concentrations during the night. The higher DMS and DMSP concentrations during daytime might be attributed to the light-induced increase in both algal synthesis and exudation of DMSP and biological production of DMS. The mean flux of DMS from the investigated area to the atmosphere was estimated to be 2.06 micromo lm(-2)d(-1). This low DMS emission flux, together with the low DMS surface concentrations was attributed to the low productivity in this sea.     

Enhanced extracellular enzymatic peptide hydrolysis in the sea-surface microlayer

The accumulation of dissolved organic matter (DOM) at the air–sea interface is controlled by dynamic physical processes at the boundary between ocean and atmosphere. Much of the DOM concentrated in the surface microlayer is thought to be protein or glycoprotein. Enzymatic hydrolysis of these and other biopolymers is an important step in the microbial uptake of dissolved and particulate organic matter in many aquatic environments. We employed a sensitive fluorescence technique to investigate differences between extracellular enzymatic peptide hydrolysis in the sea surface microlayer and corresponding subsurface water from Stony Brook Harbor, NY. We separated the microlayer from its underlying water and thus measured hydrolysis potential rather than an in-situ process. Peptide turnover was always faster in the microlayer than in subsurface waters. This was confirmed by allowing a new surface film to form on subsurface water; hydrolysis was still faster in the new surface film. In a year-long study, we found the relative difference between turnover times in the surface film and subsurface waters to vary greatly with season. While rate constants of peptide hydrolysis were generally higher in both microlayer and bulk water samples in spring/summer than in fall/winter, the difference in activity between the two environments was greatest in winter. Enhanced hydrolysis in the sea surface microlayer is likely due to the greater concentrations of DOM in the microlayer. Seasonal changes in distribution of hydrolytic activity between surface film and subsurface water probably reflect seasonal variation in the mechanisms of DOM enrichment, which depend on water temperature, substance and energy fluxes across the water–air boundary, activity of aquatic organisms and other seasonal variables.     

Bursting bubbles and their effect on the sea-to-air transport of Fe, Cu and Zn

The effects of the bubble breaking process on the atmospheric geochemical cycles of the elements Fe, Cu and Zn were investigated, in situ, in the estuarine waters of Narragansett Bay, Rhode Island. Enrichment, as defined by the metal-to-sodium ratios in the aerosols produced compared to their ratio in bulk water, occurred for the three metals investigated. The extent and potential geochemical importance of the process were different for each element. Iron enrichment was quite low (enrichment factor (EF) <100) and constant, and scavenging of iron from the water column and subsequent enrichment on the aerosols produced did not appear to occur. Copper enrichment on the aerosols was 200 and appeared to be influenced by both microlayer and scavenging effects. In addition, copper enrichment appears to be correlated to biological processes. Zinc enrichment was approximately the same as Cu; however, a strong scavening effect appeared to occur, suggesting scavenging of Zn by rising bubbles. Scavenging effects suggest that open-ocean enrichments for Cu may be slightly higher than observed here and a great deal higher for Zn.Geochemical implications of the data, together with other existing data, indicate that the sea is an insignificant source of Fe to the atmosphere. The sea may be a significant source (contributing on the order of 10% or more) of the total annual quantity of Cu and Zn to the atmosphere.     

挪威海和格陵兰海海水DMS分布特征及影响因素研究

于2012年7—9月现场测定了北极挪威海和格陵兰海区域海水二甲基硫(DMS)及其前体物质二甲巯基丙酸内盐(DMSP,分溶解态DMSPd和颗粒态DMSPp)的含量,研究了其空间分布格局及其影响因素,探讨了表层海水DMS的生物周转和去除途径。结果表明,表层海水DMS、DMSPd和DMSPp的平均浓度分别为5.36nmol/L、15.63nmol/L和96.73nmol/L,受挪威海流和北极深层水影响,表层海水二甲基硫化物浓度呈现出由低纬度向高纬度海域递减的趋势。DMSPd和DMSPp浓度与Chl a浓度均有显著的相关性,说明浮游植物生物量是影响挪威海和格陵兰海二甲基硫化物生产的重要因素。表层海水DMS生物生产和消费速率平均值分别为18.19nmol/(L·d)、15.67nmol/(L·d)。DMS微生物周转时间变化范围为0.03~1.80d,平均值为0.49d,DMS海-气周转时间是微生物消费时间的90倍,说明夏季挪威海和格陵兰海表层海水中DMS微生物消费过程是比海-气扩散更具优势的去除机制。     

胶州湾及青岛近海微表层与次表层中二甲基硫(DMS)与二甲巯基丙酸(DMSP)的浓度分布

以胶州湾及青岛近海为研究区域,利用吹扫-捕集气相色谱法研究了二甲基硫(DMS)和二甲巯基丙酸(DMSP,分为溶解态DMSPd和颗粒态DMSPp)在微表层与次表层中的浓度以及它们在微表层中的富集行为。结果表明,DMS、DMSPd和DMSPp在微表层中的浓度高于次表层,它们在微表层中的富集因子分别为1.17、1.84和1.51。研究发现,DMS及DMSPp浓度与叶绿素a(Chl-a)浓度有很好的相关性,但它们的周日变化与Chl-a并不完全同步。DMS/Chl-a和DMSPp/Chl-a的比值在次表层和微表层分别为4.35、13.47mmol/g和3.99、15.88mmol/g。胶州湾及青岛近海生态环境受人为活动干扰严重,使本海域DMS含量较高,从而贡献出较大的DMS海-气通量。     

Comparison of sampling devices for the determination of polychlorinated biphenyls in the sea surface microlayer

Over 30 sea surface microlayer (SML) samples from two contrasting sites in the North Western Mediterranean -- Barcelona (Spain) and Banyuls-sur-Mer (France) -- were collected using three different sampling devices, namely, glass plate, metal screen (MS) and a surface slick sampler (SS), and compared with the corresponding underlying water (16 samples). The distributions of 41 polychlorinated biphenyl congeners (PCBs) were determined in the different phases: particulate (1.17-10.8 SigmaPCB ng L(-1)), truly dissolved (0.080-16.7 SigmaPCB ng L(-1)) and colloidal matter (1.17-43.0 SigmaPCB ng L(-1)), being the last two estimated from the analysis of the apparently dissolved phase. Concentrations of PCBs in the SML were higher than those in the underlying water (ULW), giving rise to enrichment factors (EF=[C](SML)/[C](ULW)) up to first-order of magnitude. The ANOVA statistical approach was used to assess differences of bulk data (e.g. dissolved organic carbon, DOC; particulate organic carbon, POC; suspended particulate matter, SPM) among sampling devices, whilst p-tailed t paired tests were used in order to compare the enrichments obtained for each sampling date. In this respect, no significantly different enrichment factors were found among sampling devices (p < 0.05), although the surface SS showed lower enrichments, probably due to the dilution of the SML with the ULW during sampling. The MS seemed to be the most suitable device for the determination of PCBs in the SML in terms of sampling efficiency under a variety of meteorological conditions.     

Seasonal variations of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the sea-surface microlayer and subsurface water of Jiaozhou Bay and its adjacent area

The distributions of DMS and its precursor dimethylsulfoniopropionate, in both dissolved (DMSP_d) and particulate fractions (DMSP_p) were determined in the seasurface microlayer and corresponding subsurface water of the Jiaozhou Bay, China and its adjacent area in May and August 2006. The concentrations of all these components showed a clear seasonal variation, with higher concentrations occurring in summer. This can be mainly attributed to the higher phytoplankton biomass observed in summer. Simultaneously, the enrichment extents of DMSP_d and DMSP_p in the microlayer also exhibited seasonal changes, with higher values in spring and lower ones in summer. Higher water temperature and stronger radiant intensity in summer can enhance their solubility and photochemical reaction in the microlayer water, reducing their enrichment factors (the ratio of concentration in the microlayer to that in the corresponding subsurface water). A statistically significant relationship was found between the microlayer and subsurface water concentrations of DMS, DMSP and chlorophyll a, demonstrating that the biogenic materials in the microlayer come primarily from the underlying water. Moreover, our data show that the concentrations of DMSP_p and DMS were significantly correlated with the levels of chlorophyll a, indicating that phytoplankton biomass might play an important role in controlling the distributions of biogenic sulfurs in the study area. The ratios of DMS/chlorophyll a and DMSP_p/chlorophyll a varied little from spring to summer, suggesting that there was no obvious change in the proportion of DMSP producers in the phytoplankton community. The mean sea-to-air flux of DMS from the study area was estimated to be 5.70 μmol/(m²·d), which highlights the effects of human impacts on DMS emission.     

Measurements of biomass and activity of neustonic microorganisms

Microlayer samples from a coastal marine area near Marseilles (France) collected in 1979 and 1980 have been analysed for POC, PON, chlorophyll a, ATP, cell counts, carbon fixation and in some cases for heterotrophic activity. Comparison of microlayer data with those of underlying water showed in most cases enrichment of organic matter and microorganisms in the surface microlayer. Carbon fixation values were comparable to those observed at 0·50 m except in the slicks, where phytoneustonic activity was higher than that of the underlying phytoplankton. Though total heterotrophic activity was higher in the surface film, the turnover time was not significantly different in both microlayer and underlying water. Our data show that enrichment is highly dependent on sea conditions; whenever slicks are present, the surface film seems to function like an eutrophic system, including active living material, although detritus is regularly present in large amounts.     

Sea-surface microlayer metals enrichments in an urban and rural bay

The sea-surface microlayer is an important interface between the atmosphere and ocean and a collection point for many anthropogenic materials including potentially toxic metals. We developed a glass plate sampler to collect the upper 30 to 55 micrometers of the sea surface. Samples of the microlayer and subsurface bulk water from an urban and rural bay were analysed for concentrations of Pb, Zn, Cu, Cd and Fe. Metal concentrations in both the microlayer and bulk water were generally 2 to 15 times greater in the urban than in the rural bay. Concentrations of metals in the microlayer of both bays averaged 6 to 65 times greater than those in the bulk water. In the urban bay, microlayer concentrations of Pb, Zn and Cu from 10 to > 100 μg 1^?1 were common. Measured microlayer metals concentrations agree well with those predicted from atmospheric deposition rates using a previously derived empirical model developed from laboratory microcosm studies. Further work will be required to determine whether or not these high microlayer metal concentrations contain significant biologically available fractions which could impact fisheries recruitment of larval icthyoneuston.     

Photochemical production of low-molecular-weight carbonyl compounds in seawater and surface microlayer and their air-sea exchange

Coastal and oceanic surface microlayer samples were collected using a stainless steel screen, along with subsurface bulk seawater, and were analyzed for low-molecular-weight (LMW) carbonyl compounds, including formaldehyde, acetaldehyde, propanal, glyoxal, methylglyoxal, glyoxylic acid and pyruvic acid. The enrichment factor in surface microlayer compared to corresponding subsurface seawater ranged from 1.2 to 21. A time-series measurement at a coastal site showed strong diurnal variations in concentrations of the LMW carbonyl compounds in the surface microlayer and in the enrichment factor, with maxima in the early afternoon and minima in the early morning. Exposure of samples to sunlight resulted in the higher yields of these compounds in the surface microlayer than in the bulk seawater, by a factor of 1.1–25, suggesting that the higher photoproduction rate of LMW carbonyl compounds in the surface microlayer accounts for the majority of the observed enrichment in these samples. Potential sinks include biological uptake and mixing. Air-sea exchange may be a source for soluble compounds and a sink for less soluble compounds. The enrichment of the LMW carbonyl in surface microlayer may alter their net air-sea exchange direction e.g., from the ocean as a potential sink to a source for atmospheric acetaldehyde and acetone. The residence times of the LMW carbonyl compounds in the microlayer were estimated to be on the order of tens of seconds to minutes using a modified two-layer model. However, to maintain the observed microlayer enrichment factor, the residence time should be on the order of ˜ 1 hour. This prolonged residence time may be due to organic enrichment in the surface microlayer (‘organic film’) which inhibited molecular transfer of carbonyl compounds into and out of the microlayer. The deviated behavior from model prediction may also be due to changes in the apparent partition coefficients of these species as a result of thier physical and chemical interactions with organic matrix in the surface microlayer.     

Characterization of the proteinaceous matter in marine aerosols

Marine aerosols play a dominant role in the transfer of oceanic material to the atmosphere. Most marine aerosol originates when air bubbles burst at the sea surface ejecting material from the sea surface microlayer and bubble surface layers into the air. Concentrations of chemical compounds in these surface layers often differ from their concentrations in bulk water. We examined the enrichment of aerosols with proteinaceous matter and attempted to characterize the physical nature and sources of this matter. We measured concentrations of dissolved free (DFAA), dissolved combined (DCAA), and particulate (PAA) amino acids, transparent stainable particles (TSP), and bacteria and virus-like particles as carriers of protein, in natural and simulated aerosols. We also evaluated D/L ratios certain amino acids in all amino acid fractions.DFAA and DCAA enriched the aerosols we sampled by 1.2–20 times compared to bulk seawater; PAA enrichment was usually higher (up to 50-fold). Aerosols contained particles typical of seawater, e.g., microorganisms, organic debris, inorganic particles with adsorbed organic matter, but also a large number of semitransparent gel-like particles, which all contained amino acids. Some of these particles were probably scavenged from bulk water, but new particles produced as bubbles burst at the surface comprised at least 10% of total proteinaceous matter in the aerosol. D/L ratios of certain amino acid suggested that the particles were most likely made from dissolved polymers secreted by phytoplankton that were concentrated on bubble surfaces and in the microlayer. Examination with Alcian Blue (a dye that targets carbohydrates) and Coomassie Blue (a dye that targets proteins) showed that most TSP in the aerosols contained both proteins and polysaccharides. Microorganisms enriched the aerosols by up to two orders of magnitude, but contributed less than 4% to the total protein pool.     

Evidence for the decoupling of dissolved, particulate and surface microlayer hydrocarbons in Northwestern Atlantic continental shelf waters

Detailed sampling of the water column and subsequent high-resolution gas chromatographic analyses of surface film, particulate and dissolved hydrocarbons from the Georges Bank region off the New England coast have revealed large spatial and temporal heterogeneity in the source of hydrocarbons. Dissolved hydrocarbons, largely of a petroleum-related origin, surface film hydrocarbon also of a petroleum-related origin but different from the dissolved fraction, and bulk-water particulate matter of a mixed biogenic and petrogenic origin are all decoupled with respect to source and quantity in the region.Surface film (microlayer) hydrocarbons are found in greater concentrations than the dissolved fraction, which in turn is greater than the particulate hydrocarbon quantities in the water column. Although the microlayer appears enriched in hydrocarbons relative to the bulk water, the nature of the hydrocarbons is quite different, indicating that the use of a microlayer enrichment factor may not be appropriate in these waters.     

Transport of no. 2 fuel oil between water column,surface microlayer and atmosphere in controlled ecosystems

No. 2 fuel oil hydrocarbons put into the bulk water columns of controlled estuarine ecosystems were found to accumulate in the surface microlayer at the air-water interface. The alkane hydrocarbons were disproportionately enriched in the microlayer compared with the aromatic hydrocarbons. A comparison of hydrocarbon boiling point distributions between bulk water, microlayer and air samples indicated that the oil hydrocarbons underwent extensive weathering by evaporation upon reaching the air-water interface. No evidence was found of increased biodegradation in the microlayer compared with that in the underlying water. A fraction of the high molecular weight alkanes, the least water soluble and least volatile constituents of the oil, appeared to be coated out from the microlayer onto the inner walls of the ecosystems.     

Dimethyl sulfide in the atmospheric surface layer of the Equatorial Pacific Ocean

This paper reports a case study of atmospheric stability effect on dimethyl sulfide(DMS) concentration in the air. Investigation includes model simulation and field measurements over the Pacific Ocean. DMS concentration in surface sea water and in the air were measured during a research cruise from Hawaii to Tahiti. The diurnal variation of air temperature over the sea surface differed from the diurnal cycle of sea surface temperature because of the high heat capacity of sea water. The diurnal cycle of average DMS concentration in the air was studied in relation to the atmospheric stability parameter and surface heat flux. All these parameters had minima at noon and maxima in the early morning. The correlation coefficient of the air DMS concentration with wind speed (at 15 m high) was 0. 64. The observed concentrations of DMS in the equatorial marine surface layer and their diurnal variability agree well with model simulations. The simulated results indicate that the amplitude of the cycle and the mean     

Effects of solar radiation on dimethylsulfide cycling in the western Atlantic Ocean

The influence of solar radiation on springtime rates of photochemical and biological consumption of dimethylsulfide (DMS) in surface waters from the western Atlantic Ocean was examined by exposing 0.2 μm filtered and unfiltered surface seawater to natural sunlight at five depths in the upper 30 m. Parallel deck incubations of 0.2 μm filtered seawater under various long-pass optical filters were also carried out to aid in assessing the wavelength dependence of DMS photolysis. DMS photolysis rate constants for mid-day exposure (∼10:30–17:30 local time) to surface irradiance ranged from 0.026 to 0.086 h⁻¹ and were highest in coastal and shelf waters. Photolysis rate constants decreased with increasing irradiation depth, in accordance with the attenuation of ultraviolet radiation (UVR, 280–400 nm). Total DMS consumption rates (photochemical+biological) in unfiltered surface samples also decreased with increasing incubation depth and were larger than photolysis rates at nearly all depths and all stations. The decrease in photolysis rate constants with exposure depth was mirrored by biological DMS consumption rate constants that were severely inhibited at surface irradiances, and approached or exceeded dark rate constants at deeper exposure depths. Photolysis rates were 2–19 times greater than estimated biological consumption rates in the surface light exposed samples, while biological consumption rates were significantly larger than photolysis rates at incubation depths below the 1% light level for UV–B radiation (280–320 nm). Total DMS loss rates increased up to nine-fold with UVR exposure, but changes in DMS concentrations were not strongly correlated to light dose, presumably due to parallel, light-mediated DMS production processes. The primary loss process for DMS depended mainly on the depth interval considered and the attenuation of UVR; in general, photochemical removal dominated shallow layers characterized by high UV–B intensities, whereas biological removal dominated in deeper layers where UV–B was absent, but UV–A (320–400 nm) and visible (400–700 nm) light fluxes were still relatively high. These results demonstrate that UVR exposure significantly influences the spatial and temporal pattern of DMS production and loss processes, and ultimately the DMS flux to the atmosphere.     

The concentration and distribution of dimethyl sulfide in the marine atmospheric boundary layer near the equator

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大亚湾微表层和次表层海水营养盐的研究

根据1998年秋季（10、11月）、1999年春、秋季（4、5、10月）5个航次对大亚湾海区微、次表层的调查结果，分析了微表层海水对氮、磷、硅营养盐的富集概况，讨论了营养盐与环境因子的关系。结果表明：大亚湾海区微表层海水对氮、磷、硅营养盐均有富集作用，因海况及季节不同，富集因数（EF）与其他海区的有所差别；无论夏季或秋季，大亚湾海区微表层海水中无机氮都以NH4－N为主要存在形态；其余水层则以NO3－N为主要存在形态。结果还表明，微表层、次表层海水中NH4－N与BOD5、COD测值都呈高度显著正相关，与PO4－P和SiO3-Si含量均无相关，说明大亚湾海区水中含氮有机物较含磷有机物丰富。