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1.
We have developed an ecosystem model including two nitrogen isotopes (14N and 15N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ15N) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle. Moreover, the model study may indicate a new potential of δ15N as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have applied this model to the Sea of Okhotsk and successfully reproduced the δ15N of nitrate measured in seawater and the seasonal variations in δ15N of sinking particles obtained from sediment trap experiments. Simulated δ15N of phytoplankton are determined by δ 15N of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference between simulated δ15N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of δ 15N values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases much more slowly than the other biogeochemical fluxes. Therefore, simulated δ15N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized with observations of δ15N of ammonium in winter and a sensitive study varying the parameter of nitrification rate.  相似文献   

2.
Because organic matter originating in the euphotic zone of the ocean may have a distinctive nitrogen isotope composition (15N/14N), as compared to organic matter originating in terrestrial soils, it may be used to evaluate the relative nitrogen contribution to marine and estuarine sediment. The nitrogen isotope ratios of 42 sediment samples of total nitrogen and 38 dissolved pore-water ammonium samples from Santa Barbara Basin sediment cores were measured. The range of δ15N values for total nitrogen was +2.89 – +9.4‰ with a mean of +6.8‰ and for pore water ammonium, +8.2 – +12.4‰ with a mean of 10.2‰.The results suggest that the dissolved ammonium in the pore water is produced from bacterial degradation of marine organic matter. The range of δ15N values for total nitrogen in the sediment is interpreted as resulting from an admixture of nitrogen derived from marine (+10‰) and terrestrial (+2‰ marines. The marine component of this mixture, composed principally of calcium carbonate with smaller amounts of opal and organic matter, contains ～ 1.0% nitrogen. The terrestrial component, which comprises over 80% of the sediment, contains ～ 0.1% organically bound nitrogen and accounts for > 25% of the total nitrogen in Santa Barbara Basin sediment.  相似文献   

3.
We present a method for measuring 15N–NH4+ in marine, estuarine and fresh waters. The advantage of this method is that it is broadly applicable to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The procedure is a modification of the ammonia diffusion method and uses large sample volumes (often 4 l) to obtain sufficient N for isotope ratio mass spectrometric analysis. Large volume samples have not previously been used with the diffusion procedure because isotopic fractionation occurs due to incomplete recovery of ammonium. However, the method we present accounts for this fractionation and allows precise correction of measured δ15N values.  相似文献   

4.
I present here a review of my work concerning nitrogen assimilation by marine phytoplankton. This opportunity was provided to me as the recipient of the Okada Prize for 1990 from the Oceanographical Society of Japan. Assimilation of nitrogenous nutrients by phytoplankton has received considerable research effort since it is an essential process in organic matter production in the sea surface. The use of15N technique is necessary for tracing nitrogen assimilation by natural marine phytoplankton, but nitrogen metabolism of heterogenous natural populations significantly complicates flow of isotope. Dilution of15N isotope by heterotrophic regeneration of ammonium causes underestimates of uptake rates. I made an evaluation of isotope dilution effects in available data sets of15N-ammonium uptake experiments in literature. Incorporated15N in particulates might revert back to dissolved organic or inorganic nitrogen. I conducted pulse-chase experiments which can quantify such loss of tracer. From these studies, a short term experiment with sufficient amount of tracer enrichment is found to overwhelm these problems. In such an experiment, however, the elevation of nutrient concentration by tracer addition may likely perturb the uptake process. An initial rapid uptake is expected if the population is nitrogen deficient, but I found that this phenomenon is not common to surface oligotrophic open oceans. Uptake rate from such an experiment, or capacity of nitrogen uptake, was obtained using surface waters from an extended area in the North Pacific, and its regional variability was discussed. In addition to overall15N uptake, time series analysis of intracellular15N partitioning between hot ethanol soluble and insoluble fractions was found to be useful. When15N-ammonium is added to nitrogen deficient cells of phytoplankton,15N is accumulated in the ethanol soluble fraction. Using cultured strains of marine phytoplankton, this accumulation was proved to be caused by the difference of rates of nitrogen uptake and nitrogenous macromolecule synthesis. Uptake rate per cell is relatively constant irrespective of nutritional status, but macromolecule synthesis decreases with nitrogen deficiency. This accumulation of15N in the ethanol soluble fraction was used as an index of nutritional status with respect to nitrogen of the natural populations of phytoplankton from the western North Pacific. The uptake capacity of nitrate was observed to be higher than that of ammonium in the regional upwelling around Izu Islands and during the spring bloom in Alaskan coastal water. The15N partitioning technique revealed that nitrate taken up was rapidly incorporated in the macromolecule fraction. This suggests that ammonium uptake is suppressed to be smaller than intracellular nitrogen assimilation, rather than that nitrate is taken up in excess and accumulates within the cell. Regulation of nitrate uptake by light intensity was also discussed in detail for the Alaskan data. Several other studies currently conducted are also mentioned.  相似文献   

5.
The uptake of urea, nitrate and ammonium by phytoplankton was measured using 15N isotopes over a one-year period in Great South Bay, a shallow coastal lagoon. The bay is a unique environment for the study of nutrient uptake since ambient concentrations of NO3?NH4+ and urea remain relatively high through the year, and phytoplankton are probably never nutrient limited. Urea nitrogen averaged 52% of the total assimilated, while ammonium represented 33% and nitrate 13%. High rates of ammonium uptake occurred only at low urea concentrations (ca< 1-μg-atom urea l?1). Over the sampling period urea was present in relatively high concentrations, averaging 5·35 μg-atom N l?1, while means for ammonium and nitrate averaged 1·94 and 0·65 μg-atom N l?1, respectively. Total N uptake measured with 15N averaged about 3·3 times the calculated (from elemental ratios and 14C productivity measurements) N needs of the phytoplankton population. Highest nitrogen uptake occurred in the summer and coincided with the primary production maximum.  相似文献   

6.
Nitrification rates, as oxidation of 15N-labelled ammonium and loss of nitrite from N-Serve treated samples, were measured in Kochi backwaters during three seasons. Nitrification rates ranged from undetectable to 166 nmol N L−1 h−1 in the water column and up to 17 nmol N (g wet wt)−1 h−1 in sediments. Nitrification rates were higher in intermediate salinities than in either freshwater or seawater end. Within this salinity range, nitrification rates could be related to ammonium concentrations. As shown by the relation between ammonification and nitrification rates, it is also likely that nitrification is more regulated by renewal rates, rather than by in situ concentrations, of substrate. Among other environmental parameters, temperature and pH may have an influence on nitrification. Potential nitrification rates calculated from loss of nitrite from N-Serve treated, nitrite-enriched samples were about 800 nmol N L−1 h−1 in the water column and 40 nmol N (g wet wt)−1 h−1 in sediments. While these rates are in balance with those of biological ammonium production they may be inadequate to mitigate ammonium pollution in this estuary.  相似文献   

7.
The tropical seagrass Halophila stipulacea is dominant in most regions of the Indo‐Pacific and the Red Sea and was introduced into the Mediterranean Sea after the opening of the Suez canal. The species is considered invasive in the Mediterranean Sea and has been progressively colonizing new areas westward. Growth and photosynthetic responses of H. stipulacea have been described but no information is yet available on the nitrogen nutrition of the species. Here we simultaneously investigated the uptake kinetics of ammonium and nitrate and the internal translocation of incorporated nitrogen in H. stipulacea using 15N‐labelled substrates across a range of Ni levels (5, 25, 50 and 100 μm ). The ammonium uptake rates exceeded the nitrate uptake rates 100‐fold, revealing a limited capacity of H. stipulacea to use nitrate as an alternative nitrogen source. The uptake rates of ammonium by leaves and roots were comparable up to 100 μm 15NH4Cl. At this concentration, the leaf uptake rate was 1.4‐fold higher (6.22 ± 0.70 μmol·g?1 DW h?1) than the root uptake rate (4.54 ± 0.28 μmol·g?1 DW h?1). The uptake of ammonium followed Michaelis–Menten kinetics, whereas nitrate uptake rates were relatively constant at all nutrient concentrations. The maximum ammonium uptake rate (Vmax) and the half‐saturation constant (Km) of leaves (9.79 μmol·g?1 DW h?1 and 57.95 μm , respectively) were slightly higher than that of roots (6.09 μmol·g?1DW h?1 and 30.85 μm , respectively), whereas the affinity coefficients (α = Vmax/Km) for ammonium of leaves (0.17) and roots (0.20) were comparable, a characteristic that is unique among seagrass species. No substantial translocation (<2.5%) of 15N incorporated as ammonium was detected between plant parts, whereas the translocation of 15N incorporated as nitrate was higher (40–100%). We conclude that the Ni acquisition strategy of H. stipulacea, characterized by a similar uptake capacity and efficiency of leaves and roots, favors the geographical expansion potential of the species into areas with variable water‐sediment N levels throughout the Mediterranean.  相似文献   

8.
Nitrate and ammonium uptake rates were measured during a series of cruises in the well-mixed region of the southern North Sea from February to September. Water column-integrated uptake rates ranged between 0.01 and 8.7 mmol N m−2 d−1 and 0.01 and 12.2 mmol N m−2 d−1 for nitrate and ammonium, respectively, with ammonium uptake dominating after the phytoplankton spring bloom in May. A moored buoy continuously measuring nitrate and chlorophyll a and seabed current meters were also deployed in the central southern North Sea in the region of the East Anglian plume—a permanent physical feature which transports nutrients towards continental Europe. This enabled the flux of water and hence of nutrients across the southern North Sea to be determined and an assessment of the contribution of freshwater nutrients to the flux to be made. A simple box model is developed to relate the phytoplankton uptake of nitrate and ammonium to the transport of nitrate, ammonium and particulate organic matter (POM) across the southern North Sea. This showed the importance of the plume region of the North Sea in the processing of nitrogen, with nitrate dominating total nitrogen transport prior to the spring bloom (10 340×103 kg N inflow to the plume in March) and transport of nitrogen as ammonium, nitrate and POM in approximately equivalent amounts during summer (2560, 2960 and 2151×103 kg N inflow to the plume, respectively, in July). The box model also demonstrates more generally the need to assess nitrogen transport as nitrate, ammonium and POM if an improved understanding of the impact of nutrient input in shelf seas is to be achieved.  相似文献   

9.
Relationships between organic carbon, total nitrogen and organic nitrogen concentrations and variations in δ13Corg and δ15Norg are examined in surface sediments from the eastern central Arctic Ocean and the Yermak Plateau. Removing the organic matter from samples with KOBr/KOH and determining residual as well as total N shows that there is a significant amount of bound inorganic N in the samples, which causes TOC/Ntotal ratios to be low (4–10 depending on the organic content). TOC/Norg ratios are significantly higher (8–16). This correction of organic TOC/N ratios for the presence of soil-derived bound ammonium is especially important in samples with high illite concentrations, the clay mineral mainly responsible for ammonium adsorption. The isotopic composition of the organic N fraction was estimated by determining the isotopic composition of the total and inorganic nitrogen fractions and assuming mass-balance. A strong correlation between δ15Norg values of the sediments and the nitrate concentration of surface waters indicates different relative nitrate utilization rates of the phytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low δ15Norg values correspond to high nitrate concentrations, whereas in the central Arctic Ocean high δ15Norg values are found beneath low nitrate waters. Sediment δ13Corg values are close to −23.0‰ in the Yermak Plateau region and approximately −21.4‰ in the central Arctic Ocean. Particulate organic matter collected from meltwater ponds and ice-cores are relatively enriched in 13C (δ13Corg=−15.3 to −20.6‰) most likely due to low CO2(aq) concentrations in these environments. A maximum terrestrial contribution of 30% of the organic matter to sediments in the central Arctic Ocean is derived, based on the carbon isotope data and various assumptions about the isotopic composition of the potential endmembers.  相似文献   

10.
Rates for nitrification, phytoplankton uptake of ammonium, and regeneration of ammonium were measured in the Delaware River as functions of irradiance and nutrient concentrations, using 15N labeling methods. Phytoplankton uptake increased and nitrification rates declined with increased light intensity. The irradiance level required for maximum uptake by phytoplankton was similar to that for maximal inhibition of nitrification (about 300μEm−2 s−1). Daily, water-column averaged rates, calculated by integration of the observed rate-intensity relationships, indicate that light plays a key role in regulating the balance between oxidation of NH4+ by bacteria and assimilation by phytoplankton in the Delaware. The results show that uptake of ammonium by phytoplankton in the dark may exceed uptake in the light in optically thick systems.  相似文献   

11.
The loss of added 15N-labelled NH4+ from bottom sediment was studied in situ in a shallow saline Louisiana Gulf Coast lake. 15N-labelled NH4+ was uniformly mixed with sediment at the level of 35 μg N g?1 dry sediment for 24 h before being weighed into polyethylene containers. The 15N-enriched sediment was incubated on the lake bottom for periods up to 337 days. At intervals, triplicate samples were taken and analyzed for inorganic NH4+, organic N, and denitrification rates. Concentrations of NH4+ in the 15NH4+ enriched sediment samples were at levels similar to those measured in vertical intact cores removed from the lake at each sampling date. The initial and final isotopic compositions of NH4+ in the incubated sediments were 44.26 and 0.521 atom % 15N excess, respectively. 15N enrichment in the organic N fraction increased rapidly during the first 15 days of incubation and then increased slowly from 15 to 123 days. Denitrification, estimated by the acetylene blockage technique, ranged from 0.994 to 0.079 ng N g?1 dry sediment per hour. The mean denitrification rate for the 337-day period was 0.28 ng N g?1 h?1. There was no statistical difference in the recovery of 15N between 15 and 337 days of incubation. When the added 15N became incorporated into the organic N pool, little or no further 15N was lost.  相似文献   

12.
Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific Western Subarctic Gyre (WSG) revealed seasonal changes in δ 15N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting sediment traps (DST; 100–200 m) and moored sediment traps (MST; 200 and 500 m). All particles showed higher δ 15N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ 15N(SUS) of 0.4–3.1 ‰ in the euphotic zone (EZ). The δ 15N(SUS) signature was reflected by δ 15N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ 15N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ 15N(DST) variations of 2.4–7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ 15N(DST) vs. PP regression to δ 15N(MST) of 1.9–8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. This new approach to estimate productivity can be a powerful tool for further understanding of the biological pump in the WSG, even though its validity needs to be examined carefully.  相似文献   

13.
Carbon and nitrogen isotopes have been widely used in the investigation of trophic relations, energy pathways, trophic levels and migrations, under the assumption that δ13C is independent of body size and that variation in δ15N occurs exclusively due to ontogenetic changes in diet and not body size increase per se. However, several studies have shown that these assumptions are uncertain. Data from food-webs containing an important number of species lack theoretical support on these assumptions because very few species have been tested for δ13C and δ15N variation in captivity. However, if sampling comprises a wide range of body sizes from various species, the variation of δ13C and δ15N with body size can be investigated. While correlation between body size and δ13C and δ15N can be due to ontogenetic diet shifts, stability in such values throughout the size spectrum can be considered an indication that δ13C and δ15N in muscle tissues of such species is independent of body size within that size range, and thus the basic assumptions can be applied in the interpretation of such food webs. The present study investigated the variation in muscle δ13C and δ15N with body size and body mass of coastal fishes and cephalopods. It was concluded that muscle δ13C and δ15N did not vary with body size or mass for all bony fishes with only one exception, the dragonet Callionymus lyra. Muscle δ13C and δ15N also did not vary with body size or mass in cartilaginous fishes and cephalopods, meaning that body size/mass per se have no effect on δ13C or δ15N, for most species analysed and within the size ranges sampled. The assumption that δ13C is independent of body size and that variation in δ15N is not affected by body size increase per se was upheld for most organisms and can be applied to the coastal food web studied taking into account that C. lyra is an exception.  相似文献   

14.
Stable nitrogen isotopes have customarily been used to delineate trophic position with only infrequent regard to source variability. A compilation of literature data supports a previous tentative hypothesis that marine fish are enriched in15N relative to those inhabiting freshwaters. Estuarine and anadromous fish were also confirmed to have intermediate δ15N values depending on their respective time spent feeding in either fresh- or salt water. The use of fish δ15N as a measure of continental-marine coupling in complex coastal environments will therefore provide additional support for analyses based on the more traditionally utilized isotopes δ34S or δ13C.  相似文献   

15.
利用15 N示踪法实测南海水体反硝化速率的研究发现,培养水样在长时间密闭放置过程中也会受到外界空气的污染,且其29N2/28N2比值恒定为0.007 35。根据空气背景中29N2/28N2比值恒定的特征,提出基于质量平衡关系校正空气N2污染的方法,通过将样品实测29N2浓度扣除由外界空气贡献的29N2浓度,可获得由生物反硝化作用所产生的29N2准确浓度,进而可计算出准确的反硝化速率。经空气29N2背景校正后,29N2浓度的偏差明显小于未经校正的结果,且29N2浓度与培养时间之间的线性相关性显著加强,凸显出空气29N2背景校正是获取准确反硝化速率的关键。鉴于15 N示踪法已被广泛应用于海洋水体与沉积物反硝化速率的测定中,所提出的空气29N2背景校正方法具有重要的意义。  相似文献   

16.
Upwelling and nitrogen (N) fixation provide new N for primary production off southern central Vietnam. Here we evaluate the roles of both N sources for zooplankton nutrition by comparing δ15N and δ13C values in nitrate, particulate organic matter (POM), and six net-plankton size fractions from monsoon and intermonsoon seasons. The δ13C values in POM and the net-plankton size fractions differed by 2–4‰ at any time. We assume that plankton from the POM filters was dominated by nano-and picoplankton as opposed to micro- and mesoplankton in the net-samples. The implications of this are discussed in terms of size differential pathways of C and N in the planktonic food web. We used δ15N to estimate the differences in N nutrition between the actual upwelling region and the oligotrophic area further offshore. The δ15N values of the net-plankton size fractions were depleted in δ15N by ca. 2‰ outside compared to inside the upwelling area during the monsoon season. We attribute these patterns to the additional utilization of N derived from N fixation. The concomitant findings of high N fixation rates reported earlier and low subthermocline nitrate (nitratesub) values of 2.9–3.6‰ support this conclusion. Net-plankton δ15N values increased with size, pointing to the dominance of higher trophic levels in the larger size fractions. According to a two source mixing model N fixation may have provided up to 13% of the N demand in higher trophic levels.  相似文献   

17.
An 8-year record of N fluxes and δ15N of sinking particles from the deep southern Bay of Bengal, northern Indian Ocean, is presented. Fluxes and δ15N vary between ∼0.1 and 3 mg m−2 day−1 and ∼2‰ and 8‰, respectively. The seasonal variation is determined mainly by oceanographic processes coupled to the Indian monsoon system. The annual pattern of δ15N is characterized by minima during spring intermonsoon (∼March–May), when nutrient inputs to the euphotic zone should be low because of stratification, and lighter nitrate/particulate matter is expected to be advected from the central Bay. Highest δ15N are associated with peak fluxes during southwest monsoon (∼June–September), when the southern Bay comes under the influence of the SW monsoon current, which appears to advect particulate matter with distinctly higher δ15N. The impact of this process, however, varies interannually under the influence of factors such as ENSO and the Indian Dipole Mode. Weakened advection leads to relatively low N fluxes and reduced δ15N. The data highlight the necessity of multi-annual studies to comprehend the natural variability of a system.  相似文献   

18.
By transforming fixed nitrogen (N) into nitrogen gas, the biochemical processes that support denitrification provide a function critical to maintaining the integrity of ecosystems subjected to increased loading of N from anthropogenic sources. The Louisiana coastal region receives high nitrate (NO3?) concentrations (> 100 µM) from the Mississippi–Ohio–Missouri River Basin and is also an area undergoing high rates of wetland loss. Ongoing and anticipated changes in the Louisiana coastal region promise to alter biogeochemical cycles including the net rate of denitrification by ecosystems. Projecting what these changes could mean for coastal water quality and natural resources requires an understanding of the magnitude and patterns of variation in denitrification rates and their connection to estuarine water quality at large temporal and spatial scales under current conditions. We compile and review denitrification rates reported in 32 studies conducted in a variety of habitats across coastal Louisiana during the period 1981– 2008. The acetylene inhibition and 15N flux were the preferred techniques (95%); most of the studies used sediment slurries rather than intact sediment cores. There are no estimates of denitrification rates using the N2/Ar ratio and isotope pairing techniques, which address some of the problems and limitations of the acetylene inhibition and 15N flux techniques. These studies have shown that sediments from estuaries, lakes, marshes, forested wetlands, and the coastal shelf region are capable of high potential denitrification rates when exposed to high NO3? concentrations (> 100 µM). Maximum potential denitrification rates in experimental and natural settings can reach values > 2500 µmol m2 h? 1. The lack of contemporary studies to understand the interactions among critical nitrogen transformations (e.g., organic matter mineralization, immobilization, aquatic plant assimilation, nitrification, nitrogen fixation, dissimilatory nitrate reduction to ammonium (DNRA) and anaerobic ammonium oxidation (annamox) limits our understanding of nitrogen cycling in coastal Louisiana, particularly the role of respiratory and chemolithoautotrophic denitrification in areas undergoing wetland restoration.  相似文献   

19.
The stable nitrogen isotope ratio (δ 15N) in macroalgae is effectively used as a time-integrated bioindicator to record nitrogen sources for primary producers during their growing periods in aquatic ecosystems. However, the utility of this tool is limited because the occurrence of these organisms is often restricted in space and time. To investigate the potential of chemical composition in sedimentary organic matter (SOM) as a proxy for time-integrated environmental conditions, nitrogen (N) and carbon (C) contents and their stable isotope ratios (δ 15N and δ 13C) were determined, and systematically cross-checked against corresponding values in macroalgae at the Shiraho fringing reef in Okinawa, Japan. Preliminary trials showed that δ 15N in SOM processed by the “wash-out method” for δ 13C analysis yielded similar δ 15N values to the bulk sediment, despite the loss of some SOM during the process. The amounts of organic matter and the ratio of the HCl-insoluble portion were variable within the reef, probably reflecting local vegetation and subsequent decomposition. The distribution of δ 15N and δ 13C in SOM showed similar trends to those of macroalgae, with mostly constant differences of 1.4‰ and −6.7‰, respectively. These differences throughout the reef appeared to be explained in terms of mixed contributions from macrophyte and epibenthic microalgae growing in different seasons and years, with their debris undergoing diagenetic alteration. Therefore, macroalgae and SOM δ-values can be used in a complementary manner, over various time scales, as indicators of the integrated effect of dissolved inorganic nitrogen (DIN) sources on coral reef ecosystems.  相似文献   

20.
The stable nitrogen isotope ratio (δ15N) is an established indicator of trophic hierarchy in marine food-web studies. Most of these studies presume that spatial variation in the primary food source is negligible, although a water-depth-related increase in δ15N of particulate organic matter (POM) has been found in many systems. We used the high-Antarctic Weddell Sea shelf and slope ecosystem to test whether such a depth-related change in δ15N is reflected at higher trophic levels, i.e., benthic consumers of POM. In suspension feeders (SF) we found a significant increase in δ15N with water depth of up to 9.8‰, whereas in deposit feeders (DF) a depth effect was barely detectable. Particle-size preferences of the two feeding guilds combined with particle-size-dependent sinking velocities and biogeochemical reworking of POM are discussed as the major causes of these differences. It is essential to marine food-web studies to take into account the general depth effect on POM δ15N as well as potential feeding-guild-specific differences in the response of POM consumer tissue δ15N to avoid serious bias and misinterpretation of stable-isotope-based trophic information.  相似文献   

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