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1.
Dissolved electroactive concentrations of zinc, cadmium, lead and copper in river Rhine and North Sea samples have been measured at natural and lower pH values by differential pulsed anodic stripping voltammetry using a Kemula-type hanging mercury drop electrode. Average concentrations detected in North Sea samples at salinities ? 32‰ S and their range are (in μgl?1): 3.9 (2.0–7.5) for zinc, 0.23 (0.13–0.31) for cadmium, 0.3 (0.1–0.6) for lead and 0.3 (0.25–0.60) for copper (pH 8.1). The APDC-MIBK extracting/concentrating method, followed by AAS measurement applied to the same samples, resulted in 3.9 (2.0–7.5) for zinc, 0.11 (0.01–0.27) for cadmium, 0.5 (0.2–0.9) for lead and 1.6 (0.7–3.2) for copper. A fraction of the electroactive concentrations at pH 2.7 (6.1 for Zn) is electroactive at pH 8.1. The fractions are 100% for Cd, 20% for Cu, 13% for Pb and 40% for Zn. The remaining fractions are partly composed of organically bound species in solution. The low value for lead may be caused by the presence of particulate lead that is dissolved at low pH.Ionic copper and lead species, added separately to seawater at pH 8.1 are removed from the electroactive form, and taken up in (organic) complexes in the same ratio (at least for copper) as the species already present. Added ionic zinc is not removed within the time scale of the measurements (30 min). North Sea water at the natural pH has a complexing capacity, probably due to the presence of dissolved organic compounds, in a concentration equivalent to 3.10?7M copper. The complexing capacity is zero at pH 2.7. The usual method of standard addition for the determination of electroactive copper and lead concentrations may lead to erroneous results in samples where complexation of this type occurs.  相似文献   

2.
Dissolved cadmium and copper concentrations have been determined in 76 surface water samples in coastal and ocean waters around Scotland by anodic stripping voltammetry (ASV). A trace metal/salinity ‘front’ is observed to the west, north and north-east of Scotland separating high salinity ocean water (>35 × 10−3) with low concentrations of dissolved Cd and Cu from lower salinity (<35 × 10−3) coastal water containing higher concentrations of Cd and Cu. Mean Cd concentrations in ocean and coastal waters are 7 ng dm−3 (0·06 n ) and 11 ng dm−3 (0·10 n ) respectively; for Cu the respective levels are 60 ng dm−3 (0·95 n ) and 170 ng dm−3 (2·68 n ). The observed distribution is attributed principally to freshwater runoff and the advection of contaminated Irish Sea water into the study area.  相似文献   

3.
Cd, Cu, Fe, Ni, Pb and Zn were determined in 123 samples from the Baltic Sea proper. The trace metals were extracted directly on board the vessel, using a dithiocarbamate-Freon procedure. Final analyses of the extracts are performed onshore by atomic absorption spectrometry.Similar trace-metal concentrations are found in different areas of the Baltic proper. Most values fall in the following ranges: Cd, 30–60 ng 1?1; Cu, 0.6–1.0 μg 1?1; Fe, 0.3–0.9 μg 1?1; Ni, 0.6–0.9 μg 1?1; Pb, 0.05–0.2 μg 1?1; and Zn, 1.5–3.5 μg 1?1. The metal-concentrations are generally independent of depth. However, copper exhibits a small but significent decrease in concentration below 80 m.Filtration did not affect trace-metal concentrations, with the exception of iron in waters from lower layers. Similarly, storage under acid conditions was shown to affect only the concentration of iron. An electro-chemical technique was also used to determine Cu in some samples.  相似文献   

4.
The chloroform-extractable ‘lipid’ fraction of dissolved organic matter in seawater was analyzed by gravimetry, liquid chromatography, gas chromatography (GC), and gas chromatography—mass spectrometry (GC—MS). Gravimetric concentrations of dissolved lipids in the Gulf of Mexico were in the range of 60–160 μg 1?1 in near-surface waters and 61–116 μg 1?1 in near bottom waters and accounted for ～4% of the dissolved organic carbon. Over a 12-h sampling period and a 5-d sampling period extensive variability in dissolved lipid quantity and quality were observed. The major percentage of extractable weight was collected in the polar liquid chromatographic fraction (55–95%). Gas chromatographic concentrations of the aliphatic fractions were in the range of 0.014-0.187 μg 1?1. Concentrations derived from gas chromatography were consistently lower than gravimetrically-derived concentrations. A number of compounds were tentatively identified by a combination of GC, GC—MS, and authentic standards. The major components of the analyzable dissolved lipids were n-alkanes (C16C32), pristane, phytane, methyl, ethyl and propyl esters of fatty acids. Minor components included olefins and cycloalkanes, aromatics, short-chained acids, and possibly a lactone and an alcohol. All concentrations and compounds were indicative of a fairly pristine environment. The n-alkane distribution appears to be the result of marine and terrestrial inputs superimposed on a chronic low-level background of oil pollution. It is suggested that the fatty acid esters and other fragment molecules are the résult of the degradation of humic substances. A number of potential indicators of source were isolated.  相似文献   

5.
The MnO2 adsorption method combined with voltammetry is proposed for the direct determination of metal complexation in seawater of various salinities as a more satisfactory alternative to direct voltammetric measurements and bioassay methods. A small quantity of MnO2 is equilibrated with copper ions in filtered seawater. Natural organic ligands in the seawater compete for copper with the MnO2. Total dissolved copper is measured by differential pulse anodic stripping voltammetry after filtration and acidification of the sample. Preconcentration of natural water samples is unnecessary and measurement is performed at the natural equilibrium pH of the aerated sample. The analytical limit of detection of the method depends on contamination from the filtration step, and for copper complexation a ligand concentration of 5 × 10?8 M was obtained. The sensitivity can be increased by use of radioisotopes as tracers. The method is very versatile in that complexation of various metals may be determined by any analytical method that measures total dissolved metal concentrations. Neither organic ligands nor their complexes with copper adsorb on the MnO2 at pH8, but at pH 1.8 MnO2 is an efficient scavenger for electroactive organic material.Samples of surface water from the Irish Sea and the Atlantic Ocean were found to contain ligand concentrations of 1.7 × 10?7 and 1.1 × 10?7 M, with conditional stability constants (log values) of 9.84 ± 0.13 and 9.86 ± 0.23, respectively, at pH 8.0.  相似文献   

6.
The metal load into sediments and the change in the sedimentary environment of Osaka Bay in the Seto Inland Sea have been studied through geochemical analysis of core sediments, using both Pb-210 dating and a selective chemical leaching technique. Analytical results from a 6-m core of sediment show that copper and zinc pollution started in the late 1800's and the present enrichment ratios of copper and zinc, relative to background levels (20 mg kg–1 for Cu and 94 mg kg–1 for Zn), are 2.8 and 4.1, respectively. The present anthropogenic copper and zinc loads into Osaka Bay sediments, are 47 and 368 ton yr–1, while natural copper and zinc loads are 40 and 186 ton yr–1, respectively. Osaka Bay sediment at the present day is considered to be seriously polluted by zinc, now. The vertical profiles of copper and zinc in four successively separated fractions (10% acetic acid soluble fraction: F-HAC, 0.1M hydrochloric acid-soluble fraction: F-HCl, hydrogen peroxide-soluble fraction: F-H2O2 and hydrofluoric acid-soluble fraction: F-HF) from the core sediments indicate that enrichments of copper and zinc in the upper layer of the sediment are dependent on increases in the metal contents of the F-HAC, F-HCl and F-H2O2 fractions. Copper in F-HAC, and zinc in F-HAC and F-HCl, seem to be of anthropogenic origin.Results of sequential studies of the whole Seto Inland Sea can be summarized as follows: At the present time, the sedimentary loads of copper and zinc over the whole Seto Inland Sea area are 630 and 3,500 ton yr–1, respectively, while the natural and anthropogenic loads are 320 and 310 ton yr–1 for copper and 1,800 and 1,700 ton yr–1 for zinc, respectively.  相似文献   

7.
Abstract

Concentrations of cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) were measured in adult barnacles (Elminius modestus Darwin) from Waitemata and Manukau Harbours in the Auckland area, New Zealand. As in studies on sediments reported in the literature, it was possible to identify areas of likely anthropogenic influence, e.g., around the Auckland Harbour Bridge for Pb, Cu, and Zn. Groups of individuals with highest concentrations for these metals showed 19.8–23.8 mg Pb kg?1, 198–266 mg Cu kg?1, and 4460–6530 mg Zn kg?1 (95% confidence limits, dry weight basis). Cd concentrations found for all barnacles from the Auckland area ranged from 0.8 to 3.1 mg kg?1. Two samples from Omaha Beach, 60 km north of Auckland, were used as a reference. Accordingly, groups of individuals with lowest concentrations for Pb, Cu, and Zn could be allocated to this site using the Student‐Newman‐Keuls Multiple Range Test (0.5–1.3 mg Pb kg?1, 8–10 mg Cu kg?1, and 144–214 mg Zn kg?1 ; 95% confidence limits). Only Cd concentrations were highest at Omaha Beach (8.6–12.1 mg Cd kg?1 ). This result may have arisen from “naturally” increased bio‐availabilities of certain metals in mangrove systems which are reported in the literature. Generally, metal concentrations in barnacles from the Auckland Harbour area and from Omaha Beach were within the wider range for E. modestus as well as other barnacle species reported in the international literature.  相似文献   

8.
Sediment cores from a transect of sealochs, Etive, Sunart, Nevis, Duich, Broom and Torridon in N.W. Scotland were analysed for the trace metals lead, copper, and zinc. In addition, sediment cores from the Clyde Sea Basin (Loch Fyne, the Gareloch, Clyde dumpsite) and from the Irish and Celtic Seas were analysed as possible source areas for contaminated particles. The sediment cores were dated using excess 210Pb and 137Cs, and Al normalisation was used to correct for sediment compositional effects. All cores showed increased concentrations of all trace metals (and M/Al) towards the surface, with a maximum of 280–500% above background being observed in Loch Etive. Only cores from Duich and Torridon (Zn/Al and Pb/Al) and Etive (Zn/Al) did not continue to show an increase in trace metals to the uppermost samples. The burden of atmospherically derived trace metals accumulated over the past 100 years was compared with similar published burdens from a series of freshwater lochs adjacent to the sealochs. Plots of excess 210Pb (used to correct for sediment focusing) against atmospherically derived Pb showed a series of linear relationships of systematically decreasing gradient towards the north-west, away from potential urban sources of lead. It was calculated that 49% (15–77%) of the Pb but only 3% (1–4%) of the Zn and 2% (0–3%) of the Cu reached the sealochs from atmospheric sources. The remaining trace metals deposited in the sealochs came from marine particles. Using trace metal ratios as an indicator of pollutant provenance, it was shown that the trace metals came predominantly from the Irish Sea. There was no evidence of trace metals derived from the Firth of Clyde basin. It was concluded that trace metal contamination from the Clyde and the surrounding industrialised urban area was mainly deposited in the estuary and the Clyde Sea basin with its adjoining sealochs, such as the Gareloch and Loch Fyne.  相似文献   

9.
Radiocaesium isotopes, discharged into the North-east Irish Sea from the Sellafield (formerly Windscale) nuclear fuel reprocessing plant in Cumbria, have been employed as flow monitors to update and extend the record of coastal water movement from the Irish Sea to the Clyde Sea area and, further north, to Loch Etive. The temporal trends in radiocaesium levels have been used to determine the extent of water mixing en route and to define mean advection rates. Flow conditions from the Irish Sea have changed considerably since the mid-1970s, the residence time of northern Irish Sea waters being ～12 months during 1978–1980 inclusive. Average transport times of four and six months are estimated for the Sellafield to Clyde and Sellafield to Etive transects respectively. Sellafield 137Cs levels in seawater were diluted by factors of 27 and 50 respectively during current movement to the Clyde and Etive areas. The decrease in salinity-corrected 137Cs concentrations between the Clyde and Etive suggests that dilution by Atlantic water occurs, the latter mainly entering the Firth of Lorne from the west. The majority (～94%) of the radiocaesium supply to Loch Etive enters the Firth of Lorne via the portion of the coastal current circulating west of Islay, only ～6% arriving via the Sound of Jura.  相似文献   

10.
Benthic fluxes of copper, copper complexing ligands and thiol compounds in the shallow waters of Venice Lagoon (Italy) were determined using benthic chambers and compared to porewater concentrations to confirm their origin. Benthic copper fluxes were small due to small concentration differences between the porewaters and the overlying water, and the equilibrium concentration was the same at both sites, suggesting that the sediments acted to buffer the copper concentration. Thiol fluxes were ～10 × greater at 50–60 pmol cm?2 h?1, at the two sites. Porewater measurements demonstrated that the sediments were an important source of the thiols to the overlying waters. The overlying waters were found to contain at least two ligands, a strong one, L1 (log K′CuL1 = 14.2) and a weaker one, L2 (log K′CuL2 = 12.5). The concentration of L1 remained relatively constant during the incubation and similar to that of copper, whereas that of L2 was in great excess of copper, its concentration balanced by porewater releases and breakdown, probably due to uptake by microorganisms, similar to that of the thiol compounds. Similarity of the thiol and L2 concentrations and similar complex stability with copper suggest that L2 was dominated by the thiols. The free copper concentration ([Cu´]) in the Lagoon waters was lowered by a factor of 105 as a result of the organic complexation.  相似文献   

11.
This one-year survey conducted in the macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) was aimed at examining the variations of the various dissolved and particulate copper species. Ten field stations along the whole freshwater–seawater mixing zone were sampled each month. Different biogeochemical parameters (SPM, chl-a, pH and DOC) were also measured. The levels in total dissolved and total particulate copper ranged from 1.8 to 9.5 nM and from 5 to 98 μg g−1, respectively; such amounts are indicative of a pollution-free system. Extractable C18 copper (non-polar hydrophobic organic copper species), in winter and spring, accounted for 30–40% of the total dissolved copper. In summer, this contribution rapidly rose to 60% in the salinity range 20–30; over the same period of time, total particulate copper decreased. The change in dissolved copper speciation and the lowering of particulate copper concentrations were attributed to the release of strong organic ligands by phytoplankton. Our field data evidenced a highly variable behaviour for the various copper species over the seasonal cycle, and then led us to identify the following mechanisms: (i) metal desorption from organic river-flown particles (winter and spring), (ii) metal desorption from resuspended sediment in the upstream section (summer), (iii) competition between particles, non-extractable C18 organic ligands and phytoplankton-released extractable C18 organic ligands to complex copper in the downstream section (summer), and (iv) removal of non-extractable C18 organic copper by adsorption (autumn). Dissolved copper species fluxes were assessed: most of metal inputs to the estuary (60–74%) corresponded to non-extractable C18 organic copper. Winter and spring metal output fluxes were mainly constituted of non-extractable C18 organic complexes; on the other hand, extractable C18 organic complexes were predominant in summer and autumn output fluxes.  相似文献   

12.
《Journal of Sea Research》2004,51(3-4):191-197
As a preliminary investigation into the genetic structure of Irish Sea plaice we genotyped samples of juvenile plaice from six inshore areas within the Irish Sea across eight microsatellite loci and compared them with fish from two sites from the Dutch Wadden Sea (North Sea stocks). Genetic variability in all samples was generally low for that typically observed at microsatellite loci. The number of alleles per locus varied between two and nine (average 5.5) and the observed heterozygosity ranged from 0.080 up to a maximum of 0.909 (average 0.382). Few significant heterozygote deficits were observed, even when the data set was pooled. The majority (98%) of genetic variation present was within, rather than between, populations. None of the pairwise comparisons of population differentiation (FST) were significant (P>0.05) and a Bayesian analysis of population structure provided no evidence for a partitioning of the samples. Since juveniles arriving at nursery grounds in the Irish Sea are not distinct, it is likely that adult plaice form a single stock (perhaps with some weak differentiation). However, if plaice eggs and larvae do not disperse as predicted by a particle tracking model, then it is possible that the juveniles represent a mixture of several distinct stocks. Further work is therefore required to determine whether the phenotypic variation observed between female plaice from the eastern and western Irish Sea is the product of reproductive isolation or of the environment.  相似文献   

13.
The oxidation and reduction of nanomolar levels of copper in air-saturated seawater and NaCl solutions has been measured as a function of pH (7.17–8.49), temperature (5–35 °C) and ionic strength (0.1–0.7 M). The oxidation rates were fitted to an equation valid at different pH and ionic strength conditions in sodium chloride and seawater solutions:
The reduction of Cu(II) was studied in both media for different initial concentrations of copper(II). When the initial Cu(II) concentration was 200 nM, the copper(I) productions were 20% and 9% for NaCl and seawater, respectively. The effect of speciation of copper(I) reduced from Cu(II) on the rates was studied. The Cu(I) speciation is dominated by the CuCl2− species. On the other hand, the neutral chloride CuCl species dominates the Cu(I) oxidation in the range of 0.1 M to 0.7 M chloride concentrations.  相似文献   

14.
Partitioning of copper, zinc, iron and manganese into oxide, sulfide, organic and silicate fractions has been determined with a selective chemical leaching technique on sediment samples from a core collected in Osaka Bay. The samples have been dated by the210Pb method. Most of the copper and zinc in the polluted surface sediment layer are contained in both oxide and sulfide fractions. This suggests that the transformation of oxides and hydroxides to sulfides under anoxic conditions within the sediment is significant for the fixation of copper and zinc discharged through human activities into the sediment. Manganese is apparently enriched in oxide and hydroxide fractions of the surface layer due to the post-depositional migration of manganese within the sediment. The copper, zinc and manganese contents of the 30 % H2O2 soluble fraction (mostly organic fraction) decrease with depth in the sediment core, and correlate significantly with the organic carbon content. The heavy metal (Cu, Zn, Fe and Mn) contents of the silicate fraction, without exchangeable sites, are almost constant with depth.  相似文献   

15.
《Marine Chemistry》2007,103(1-2):197-208
Biological consumption is a major sink for dimethylsulfide (DMS) in the surface ocean, but the fate of DMS is poorly known. We determined the fate of sulfur from biologically consumed DMS in samples from the upper 60 m of the Sargasso Sea during July 2004. Using tracer levels of 35S-DMS in dark incubations we found that DMS was transformed into three identifiable non-volatile, sulfur-containing product pools: dimethylsulfoxide (DMSO), sulfate, and particle-associated macromolecules. Together, DMSO and sulfate accounted for most (81–93%) of the non-volatile sulfur products. Only a small fraction (∼ 2%) of the consumed DMS-sulfur was recovered in cellular macromolecules, leaving 5–17% of the metabolic products of DMS consumption unidentified. The relative importance of the two major products varied with depth. DMSO was the main sulfur product (∼ 72%) from DMS metabolism in the surface mixed layer, whereas sulfate was the most important product (∼ 74%) below the mixed layer. Changes in temperature and photosynthetically-active radiation (PAR) did not cause shifts in DMS fate in short term incubations (7–12 h), however these or other factors (e.g., exposure to ultraviolet radiation), operating over longer time scales, could potentially influence the observed pattern of DMS fate with depth. Biological DMSO production rates ranged from 0.07 to 0.33 nM day− 1, with the highest rate found at 30 m, just below the surface mixed layer. With DMSO concentrations ranging from 4.0 to 8.6 nM, turnover times for DMSO were long (15–61 days) when only the biological production from DMS was considered. Identification of the main sulfur containing products from DMS metabolism improves understanding of this important process in the marine sulfur cycling. Detection and quantification of DMSO production from biological DMS consumption also provides a more complete picture of DMSO biogeochemistry in the ocean.  相似文献   

16.
Surface intertidal sediments from 35 sites in the Irish Sea have been analysed for their 238Pu and 239,240Pu activities, together with an intensive study of plutonium in sediments of the Esk Estuary (NW England). The range of plutonium activities for the whole survey were 0·14–4118 and 1·3–16 026 Bq kg−1 for 238Pu and 239,240Pu, respectively. The levels of Pu activity, derived from the Sellafield nuclear fuel reprocessing effluents, in sediments are controlled by lithological factors and the influence of transport and post-depositional processes. Grain size distribution is particularly important, the major part of plutonium activity being in the mud fraction of all sediments.The data suggest that over the Irish Sea coastline, dynamic mixing of sediment grains by reworking and resuspension and/or by dispersion in tidal currents are important in determining plutonium distributions. The exponential decrease in sediment plutonium activities away from the Sellafield source is attributed to the progressive mixing with older contaminated and uncontaminated sediments.  相似文献   

17.
A sensitive method for iron determination in seawater has been adapted on a submersible chemical analyser for in situ measurements. The technique is based on flow injection analysis (FIA) coupled with spectrophotometric detection. When direct injection of seawater was used, the detection limit was 1.6 nM, and the precision 7%, for a triplicate injection of a 4 nM standard. At low iron concentrations, on line preconcentration using a column filled with 8-hydroxyquinoline (8HQ) resin was used. The detection limit was 0.15 nM (time of preconcentration = 240 s), and the precision 6%, for a triplicate determination of a 1 nM standard, allowing the determination of Fe in most of the oceanic regimes, except the most depleted surface waters. The effect of temperature, pressure, salinity, copper, manganese, and iron speciation on the response of the analyser was investigated. The slope of the calibration curves followed a linear relation as a function of pressure (Cp = 2.8 × 10− 5P + 3.4 × 10− 2 s nmol− 1, R2 = 0.997, for Θ = 13 °C) and an exponential relation as a function of temperature (CΘ = 0.009e0.103Θ, R2 = 0.832, for P = 3 bar). No statistical difference at 95% confidence level was observed for samples of different salinities (S = 0, 20, 35). Only very high concentration of copper (1000 × [Fe]) produced a detectable interference. The chemical analyser was deployed in the coastal environment of the Bay of Brest to investigate the effect of iron speciation on the response of the analyser. Direct injection was used and seawater samples were acidified on line for 80 s. Dissolved iron (DFe, filtered seawater (0.4 μm), acidified and stored at pH 1.8) corresponded to 29 ± 4% of Fea (unfiltered seawater, acidified in line at pH 1.8 for 80 s). Most of Fea (71 ± 4%) was probably a fraction of total dissolvable iron (TDFe, unfiltered seawater, acidified and stored at pH 1.8).  相似文献   

18.
Elevated methane (CH4) concentrations (128–2692×103 nM) occur in the hypersaline anoxic brine pools of Bannock and Urania Basins, eastern Mediterranean Sea, compared to low concentrations (17–80×103 nM) in the sediments below the anoxic brines. The CH4 enrichment in the brines might be due to the long residence time of the brine in the Basin. An attempt is made to determine the sources for the enriched dissolved CH4 by considering the influence of hydrothermal activity, the occurrence of sapropel layers (biogenic) and dissolution of gas hydrates. Furthermore, it is suggested that the enriched CH4 in Bannock and Urania Basins is diffused and mixed with the overlying waters by local upward transport mechanisms that selectively move CH4 upward in these Basins.  相似文献   

19.
Molecular weight and trace metal distributions of fulvic and humic acid fractions of marine sediments from the Seto Inland Sea were investigated by using a gel filtration technique in combination with atomic absorption and emission spectrometries. A binary molecular weight distribution was found both in the fulvic acid fraction and in the humic acid fraction. The fulvic acid fractions with molecular weights of less than 1.5×103 and of 5×103?104 accounted for 38–57 % and 25–41 % of the total fulvic acids, respectively, and the humic acid fractions with molecular weights of 102?104 and of over 2×105 accounted for 58–73 % and 16–27 % of the total humic acids, respectively. The components with molecular weights of over 1.5×103 contained 54 %, on average, of the Fe, Zn and Cu in the fulvic acid fraction, and the components with molecular weights of over 104 contained 58 %, on average, of these metals in the humic acid fraction.  相似文献   

20.
The distributions of monomethylamine (MMA), dimethylamine (DMA), trimethylamine (TMA) and ammonium (NH+4) were investigated in the Arabian Sea. The data set presented is the first to describe the distribution of MAs on an oceanic scale. Throughout the region concentrations of NH+4 were up to two orders of magnitude greater than those of the MAs. MMA (0–66 nM) was generally the most abundant MA, whilst TMA was only found at concentrations <4 nM. Low concentrations of MAs in open-ocean meso- and oligotrophic regions contrasted with the elevated levels recorded in the highly productive coastal upwelling waters of the NW Arabian Sea. In total the MAs contributed <1% dissolved organic nitrogen (DON). Depth maxima of MMA and DMA were generally associated with those of Chla, and in offshore regions, also with those of NH+4 (above the thermo-, oxy- and nitrataclines). Maxima of TMA were recorded at the base of the thermo- and oxyclines, resolved from the other analytes. Through correlation studies, a degree of diatom specific MMA production was inferred (R=0.65, p<0.001) and microzooplankton grazing found to influence significantly all aqueous MA concentrations. Enhanced correlation of MMA concentrations with mesozooplankton abundance was attributed to their ability to graze diatoms. These observations are analogous to those made of equivalent oceanographic regimes in the Mediterranean Sea (Gibb et al., 1994) and support the idea that MA concentrations in seawater are primarily regulated by the productive aspects of their biological dynamics. We postulate that the nitrogen taken up in nutrient-rich, diatom-dominated regions of the Arabian Sea will be used both biosynthetically and anabolically. This may be accompanied by introduction of MMA and DMA into the aqueous phase through enzymatic precursor degradation, nitrogen detoxification, senescence or lysis and accelerated through grazing pressures, particularly that of mesozooplankton on diatoms. In contrast, under the more oligotrophic conditions recorded in the remote Arabian Sea, those species of phytoplankton with a lower nitrogen demand are favoured, e.g., prymnesiophytes and dinoflagellates. Correspondingly lower MA concentrations are recorded in these regions.  相似文献   

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