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1.
Heavy nutrient loads in coastal waters often lead to excessive growth of microalgal and macroalgal epiphytes on seagrass leaves, with varying effects on the underlying seagrasses. This study evaluates the photosynthetic performance, epiphytic biomass and tissue nutrient content of two tropical seagrasses, Cymodocea serrulata and Thalassia hemprichii, in two intertidal areas along the Dar es Salaam coast in the Indian Ocean, a nutrient-rich region at Ocean Road (near the city centre), and a nutrient-poor region at Mjimwema (south of the city centre). Epiphyte biomass was significantly higher at the nutrient-rich site, and epiphytes were associated with reduced photosynthetic performance in both seagrass species at both sites. Likewise, nitrogen and phosphorus tissue content was higher in both species at the nutrient-rich site than at the nutrient-poor site. Epiphytic species composition on the seagrass leaves varied between seagrass species and between sites. Cymodocea serrulata had a higher number of epiphytic species at Mjimwema than at Ocean Road, whereas Thalassia hemprichii had more epiphytic species at Ocean Road than at Mjimwema. Seagrass photosynthetic performance, epiphytic biomass and nutrient content of the seagrasses were shown to be affected by nutrient concentration in the water column. Thus, for the future monitoring of the seagrass meadow, we recommend the use of combined measures such as seagrass performance, epiphytic biomass, nutrient contents and nutrient concentration levels in the water column.  相似文献   

2.
The photosynthetic activity of Zostera marina, Zostera asiatica and Phyllospadix iwatensis shoots from populations of Hokkaido (Northern Japan) was determined using the pulse amplitude modulated (PAM) fluorometer. Several fluorescence parameters were measured as a function of irradiance and leaf age: electron transport rate (ETR), quantum yield, photochemical quenching (qP) and non‐photochemical quenching (NPQ). The leaf age determined by the leaf position in the shoot bundle strongly influenced the photosynthetic activity of Z. marina, Z. asiatica and P. iwatensis. Young leaves had the maximum electron transport rate (Zm: Leaf 1 = 15.7, Leaf 2 = 16.3; Za: Leaf 1 = 13.0, Leaf 2 = 12.2; and Pi: Leaf 1 = 12.5, Leaf 2 = 11.7) and showed higher photoprotection (NPQ) than old leaves. Among the studied seagrass species, Z. marina had the highest photosynthetic activity (ETRmax = 15.3), in accordance with the highest production in the field in comparison with the other two seagrass species. The PAM fluorometry technique showed to be effective in determining intraspecific (among‐leaves) and interspecific (among‐species) variation in seagrass photosynthetic activity.  相似文献   

3.
利用快速光响应曲线(RLCs)技术研究了不同光背景下的底栖甲藻Cooliatropicalis的光合作用特性,比较了RLCs的不同序列(正序、反序和非序)和不同光照时间(10, 30和60 s)对光合参数的影响。结果表明:正序RLCs由于序列内非光化学淬灭(NPQ)的快速累积,造成曲线的不饱以及光合参数最大相对电子传递速率(rETRmax)和半饱和光强(Ek)的高估。反序和非序RLCs均出现光饱和或光抑制,步长30s和60s的光合参数在不同光背景下的趋势基本一致。在底栖甲藻的光合生理研究中,可考虑采用步长30 s的反序RLCs进行测定。  相似文献   

4.
Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (ρNO–3), regenerated production (ρNH+4), and total carbon production (ρC) as a function of varying light. The results show substantial variability in the photosynthesis–irradiance (P vs E) parameters, with phytoplankton communities at stations that were considered iron (Fe)-limited showing low maximum photosynthetic capacity (PBmax) and low quantum efficiency of photosynthesis (αB) for ρNO3, but high PBmax and αB for ρNH4, with consequently low export efficiency. Results at stations likely relieved of Fe stress (associated with shallow bathymetry and the marginal ice zone) showed the highest rates of PBmax and αB for ρNO3 and ρC. To establish the key factors influencing the variability of the photosynthetic parameters, a principal components analysis was performed on P vs E parameters, using surface temperature, chlorophyll-a concentration, ambient nutrients, and an index for community size structure. Strong covariance between ambient nitrate (NO3) and αB for ρNO3 suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NO3 reservoir. However, the observed relationships between community structure and the P vs E parameters suggest superior performance by smaller-sized cells, in terms of resource acquisition and Fe limitation, as the probable driver of smaller-celled phytoplankton communities that have reduced photosynthetic efficiency and which require higher light intensities to saturate uptake. A noticeable absence in covariances between chlorophyll-a and αB, between PBmax and αB, and between temperature and αB may have important implications for primary-production models, although the absence of some expected relationships may be a consequence of the small dataset and low range of variability. However, significant relationships were observed between ambient NO3 and αB for ρNO3, and between the light-saturation parameter Ek for ρNO3 and the phytoplankton community’s size structure, which imply that Fe and light co-limitation drives access to the surplus NO3 reservoir and that larger-celled communities are more efficient at fixing NO3 in low light conditions. Although the mean PBmax results for ρC were consistent with estimates of global production from satellite chlorophyll measurements, the range of variability was large. These results highlight the need for more-advanced primary-production models that take into account a diverse range of environmental and seasonal drivers of photosynthetic responses.  相似文献   

5.
The presences of the common macroalgae Ulva sp. and Jania rubens vary between seasons along the Israeli Mediterranean intertidal zone. To reveal some of the potential acclimation mechanisms of these algae, we examined their photosynthetic traits during the year using rapid light curves (RLC) derived from pulse amplitude modulated (PAM) fluorometry. In addition, the relationships between those photosynthetic traits and the relative abundances of the two algae were investigated. Ulva sp. showed high maximal electron transport rates (ETRmax) (49.7–68.8 μmol electrons m−2 s−1) and onsets of light saturation values (Ek) (75.8–85.6 μmol photons m−2 s−1) in the winter months of December–February, while low values were found in the summer months of June–July (5.2–20.6 μmol electrons m−2 s−1 and 6.0–23.7 μmol photons m−2 s−1, respectively). At noon time (during the highest irradiance of the day), the maximal effective quantum yield (Y0) did not vary significantly during the year. These results indicate that seasonal irradiance influences the number of reaction centres per thallus area. Both algae showed depressions in Y0 at noon (23.8% for Ulva sp. and 20.3% for Jania rubens), indicating an efficient non‐photochemical quenching mechanism. A positive correlation between the relative abundance and the photosynthetic parameter Y0 was found for Jania rubens when Y0 was sampled in the morning or noon, indicating that growth rate and primary production for this alga can be estimated from RLCs. No such correlation was found for Ulva sp., indicating that, in addition to photosynthetic traits, the relative seasonal abundance of this alga is influenced by other factors such as grazing and/or catastrophic events.  相似文献   

6.
A simple exponential equation is used to describe photosynthetic rate as a function of light intensity for a variety of unicellular algae and higher plants where photosynthesis is proportional to (1-e−β1). The parameter β ( ) is derived by a simultaneous curve-fitting method, where I is incident quantum-flux density. The exponential equation is tested against a wide range of data and is found to adequately describe P vs. I curves. The errors associated with photosynthetic parameters are calculated. A simplified statistical model (Poisson) of photon capture provides a biophysical basis for the equation and for its ability to fit a range of light intensities. The exponential equation provides a non-subjective simultaneous curve fitting estimate for photosynthetic efficiency (a) which is less ambiguous than subjective methods: subjective methods assume that a linear region of the P vs. I curve is readily identifiable. Photosynthetic parameters β and a are used widely in aquatic studies to define photosynthesis at low quantum flux. These parameters are particularly important in estuarine environments where high suspended-material concentrations and high diffuse-light extinction coefficients are commonly encountered.  相似文献   

7.
Seagrasses support a great diversity of epiphytic organisms. There are no detailed published accounts of algae and animals growing on seagrasses in South Africa. The seagrass Thalassodendron leptocaule (previously known as Thalassodendron ciliatum) is unique among southern African seagrasses in that it occurs on exposed rocky outcrops along the Mozambican and north-eastern South African coasts; most other seagrasses are restricted to sheltered bays and estuaries. Here we present accounts of species of flora and fauna identified growing epiphytically on this seagrass in northern KwaZulu-Natal. A total of 52 taxa of macroalgae and 204 species of macroinvertebrates were identified as epiphytic on South African T. leptocaule. The most frequently observed macroalgal epiphytes were predominantly Rhodophyta and were common among periodically exposed, intertidal and subtidal habitats. The crustose red coralline algae Pneophyllum amplexifrons and Hydrolithon farinosum were frequently observed, primarily on stems and leaves of the seagrass respectively. The most diverse groups of epiphytic invertebrates were Arthropoda, Annelida and Mollusca, which together comprised 76% of the macroinvertebrate species recorded. This seagrass species is evidently an important substratum and ecosystem that supports a hidden wealth of biodiversity.  相似文献   

8.
Spatial variation in the photophysiology of symbiotic dinoflagellates (zooxanthellae) of the scleractinian coral Pocillopora damicornis was examined along an environmental gradient in the Whitsunday Islands (Great Barrier Reef) at two depths (3 m and 6 m). Chlorophyll a fluorescence of photosystem II (PSII) and PAR-absorptivity measurements were conducted using an Imaging-PAM (pulse-amplitude-modulation) fluorometer. Most photophysiological parameters correlated with changes in environmental conditions quantified by differences in water quality along the gradient. For example, maximum quantum yield (Fv/Fm) increased and PAR-absorptivity decreased as water quality improved along the gradient from nearshore reefs (low irradiance, elevated nutrients and sediments) to outer islands (high irradiance, low nutrients and sediments). For apparent photosynthetic rate (PSmax) and minimum saturating irradiance (Ek), the direction of change differed depending on sampling depth, suggesting that different mechanisms of photo-acclimatisation operated between shallow and deep corals. Deep corals conformed to typical patterns of light/shade acclimatisation whereas shallow corals exhibited reduced PSmax and Ek with improving water quality coinciding with greater heat dissipation (NPQ241). Furthermore, deep corals on nearshore reefs exhibited elevated Q241 in comparison to outer islands possibly due to effects of sedimentation and/or pollutants rather than irradiance. These results highlight the importance of mesoscale sampling to obtain useful estimates of the variability of photophysiological parameters, particularly if such measures are to be used as bioindicators of the condition of coral reefs.  相似文献   

9.
In order to investigate the mechanism by which o,p′-DDT disrupts endocrine functioning of Nile tilapia in vivo, the estrogenicity of o,p′-DDT was investigated in conjunction with 17β-estradiol (E2) and tamoxifen. Mature, male tilapia were treated intraperitoneally with o,p′-DDT (60 mg/kg, one dose) or E2 (5 mg/kg, four doses) in the presence or absence of tamoxifen (5 mg/kg, six doses) for 12 days and then plasma vitellogenin (Vtg) (measured as alkaline-labile phosphorous), E2, and testosterone (T) were measured. Vtg levels were increased dramatically by E2 (1744±171 μg/ml) and moderately by o,p′-DDT (82±15 μg/ml) compared with controls (23±3.5 μg/ml). Tamoxifen alone had no effect on Vtg production, but inhibited both E2 and o,p′-DDT stimulated vitellogenesis. T levels were reduced with E2 administration (1688±383 pg/ml) and declined further with the combined treatment of E2 and tamoxifen (281±70 pg/ml), compared with controls (6558±1438 pg/ml). Tamoxifen or o,p′-DDT alone did not affect T levels, but their combined treatment did (2069±647 pg/ml). The results of this study suggest that o,p′-DDT is weakly estrogenic in male tilapia, and that this activity may be mediated through the estrogen receptor.  相似文献   

10.
Acetylene reduction activity at a tropical seagrass bed in Papua New Guinea was studied. Blades of every species of seagrass studied (Enhalus acoroides, Thalassia hemprichii, Halodule uninervis, Syringodium isoetifolium) at the bed showed active acetylene reduction in the rage of 0.025–0.050 mol cm–2d–1. It is suggested that epiphytic blue-green algae would be responsible for active acetylene reduction. Acetylene reducing activity was observed also at rhizosphere of seagrasses, microbial communities on the detritus and periphyton on snail shells. It is estimated that total nitrogen fixed at the seagrass bed will be equivalent to 6.7% of that required for the growth of seagrasses.  相似文献   

11.
Surveys were undertaken on the shallow Bermuda marine platform between 2006 and 2008 to provide a baseline of the distribution, condition and environmental characteristics of benthic communities. Bermuda is located in temperate latitudes but coral reefs, tropical seagrasses and calcareous green algae are common in the shallow waters of the platform. The dominant organisms of these communities are all living at or near their northern latitudinal range limits in the Atlantic Ocean. Among the major benthic autotrophs surveyed, seagrasses were most restricted by light availability. We found that the relatively slow-growing and long-lived seagrass Thalassia testudinum is restricted to habitats with much higher light availability than in the tropical locations where this species is commonly found. In contrast, the faster growing tropical seagrasses in Bermuda, Syringodium filiforme, Halodule sp. and Halophila decipiens, had similar ecological compensation depths (ECD) as in tropical locations. Increasing sea surface temperatures, concomitant with global climate change, may either drive or allow the poleward extensions of the ranges of such tropical species. However, due to latitudinal light limitations at least one abundant and common tropical autotroph, T. testudinum, is able to occupy only shallower depths at the more temperate latitudes of Bermuda. We hypothesize that the poleward shift of seagrass species ranges would be accompanied by restrictions to even shallower depths of T. testudinum and by very different seagrass community structures than in tropical locations.  相似文献   

12.
Calcification in the marine environment is the basis for the accretion of carbonate in structures such as coral reefs, algal ridges and carbonate sands. Among the organisms responsible for such calcification are the Corallinaceae (Rhodophyta), recognised as major contributors to the process world-wide. Hydrolithon sp. is a coralline alga that often forms rhodoliths in the Western Indian Ocean. In Zanzibar, it is commonly found in shallow lagoons, where it often grows within seagrass beds and/or surrounded by green algae such as Ulva sp. Since seagrasses in Zanzibar have recently been shown to raise the pH of the surrounding seawater during the day, and since calcification rates are sensitive to pH, which changes the saturation state of calcium carbonate, we measured the effects of pH on photosynthetic and calcification rates of this alga. It was found that pH had significant effects on both calcification and photosynthesis. While increased pH enhanced calcification rates both in the light and in the dark at pH >8.6, photosynthetic rates decreased. On the other hand, an increase in dissolved CO2 concentration to 26 μmol kg−1 (by bubbling with air containing 0.9 mbar CO2) caused a decrease in seawater pH which resulted in 20% less calcification after 5 days of exposure, while enhancing photosynthetic rates by 13%. The ecological implications of these findings is that photosynthetically driven changes in water chemistry by surrounding plants can affect calcification rates of coralline algae, as may future ocean acidification resulting from elevated atmospheric CO2.  相似文献   

13.
Benthic faunal activity and density play an important role in determining the rates of benthic nutrient fluxes, which enrich the water column and contribute to phytoplankton growth. The intensity of nutrient fluxes in the Bay of Brest depends on the density of the invasive gastropod, Crepidula fornicata. In order to study the impact of benthic fluxes on phytoplankton dynamics, realistic daily nutrient inputs simulating various densities of C. fornicata were added to six enclosures during three weeks. The increase in fertilization intensity influenced the phytoplankton biomass. A succession from Chaetoceros spp. to Pseudo-nitzschia spp. and Leptocylindrus danicus was observed in all enclosures, but the dynamics of successions were different. Pseudo-nitzschia spp. was favored in the three more fertilized enclosures, while Chaetoceros spp. persisted longer in less enriched enclosures. Despite an apparent nitrogen limitation, the quantum efficiency of PSII (Fv/Fm) was high (>0.5) and stable in all enclosures. The maximal photosynthetic capacity (PBmax) was also invariable and oscillated around an average value of 2.23 mg C (mg Chl a)−1 h−1. The stability of Fv/Fm and PBmax observed at different nutrient input intensities demonstrates that the daily inputs maintained the physiological balance of the microalgae. The maximal light utilization efficiency (α) and the light saturation parameter (Ek) were also quite stable after day 8, which reveals that photosynthetic parameters were driven by growth constraints due to nutrient availability and not by incident light or species successions. We suggest that our results correspond to an “Ek independent variation” regulation. We propose that such regulation of photosynthetic parameters appears when there are frequent nutrient additions which do not allow replete nutrient conditions to be reached but lead to physiological equilibrium.  相似文献   

14.
A batch sorption technique for the determination of particle–water interactions of hydrophobic organic micropollutants under simulated estuarine conditions is described. Results are presented for the behaviour of 2,2′,5,5′-tetrachlorobiphenyl (2,2′,5,5′-TCB) in river and sea waters, both in the presence and absence of estuarine suspended particles. Adsorption onto particles in sea water was enhanced compared with adsorption in river water owing to salting out of the compound, and possibly of the particulate organic matter, in the presence of high concentrations of dissolved ions. The particle–water distribution coefficient, KD, decreased from about 120×103 to 10×103 ml g−1, and from about 150×103 to 20×103 ml g−1, in river water and sea water, respectively, over a particle concentration range of 10–1000 mg l−1. Incomplete recovery of compound from the reactor walls is partly responsible for a particle concentration effect, while artefacts relating to inadequate sediment and water phase separation were ruled out following further experiments. The particle concentration effect, which is replicated in many field studies of hydrophobic organic micropollutants, including 2,2′,5,5′-TCB, is incorporated into a simple partitioning model and is discussed in the context of the likely estuarine behaviour of such compounds.  相似文献   

15.
An analysis is presented of snapshot data (eastward and northward velocity components: u and v; tracer such as potential temperature: τ) from an eddy-resolving (Rgrid: 1/12°) ocean model experiment, in order to explore a method for improving eddy-permitting model performance. Horizontal 3 × 3 R-grid averages give the eddy-permitting grid (P-grid: 1/4°) variables: 〈u〉, 〈v〉, and 〈τ〉, where 〈〉 denotes the spatial P-grid scale average. The difference between the horizontal tracer flux across the boundary face of a P-grid and that across the corresponding faces of R-grids is estimated as F2E. It is found that the correlations among the gradients of u, v, and τ give a good approximation F2C to the estimated flux F2E. The approximated flux is a function of these gradients and the grid size. A method is presented for implementing the F2C for density to an eddying ocean model as an additional advection. Practical experiments were conducted with a realistic configuration. It is shown that the zonal mean isotherms in the Kuroshio extension region are more flattened in the run using the proposed method than in another run using the conventional horizontal biharmonic operator, suggesting that the additional flux correction leads to an enhancement of sub-basin scale mixing.  相似文献   

16.
The photosynthetic properties of phytoplankton populations as related to physical–chemical variations on small temporal and spatial scales and to phytoplankton size structure and pigment spectra were investigated in the Northern Adriatic Sea off the Po River delta in late winter 1997. Large diatoms (fucoxanthin) dominated the phytoplankton in the coastal area whereas small phytoflagellates (mainly 19′-hexanoyloxyfucoxanthin, chlorophyll b, 19′-butanoyloxyfucoxanthin) occurred outside the front. The front was defined by the steep gradient in density in the surface layer separating low-salinity coastal waters from the offshore waters.Physical features of the area strongly influenced phytoplankton biomass distributions, composition and size structure. After high volumes of Po River discharge several gyres and meanders occurred in the area off the river delta in February. Decreasing river discharge and the subsequent disappearance of the gyres and the spreading dilution of the river plume was observed in March. The dynamic circulation of February resulted in high photosynthetic capacity of the abundant phytoplankton population (>3.40 mg m−3). In March, the slow circulation and an upper low-salinity water layer, segregated from the deeper layers, resulted in lack of renewal of this water mass. The huge phytoplankton biomass, up to 15.77 mg chl a m−3, became nutrient depleted and showed low photosynthetic capacity. In February, an exceptionally high PmaxB, 20.11 mg C (mg chl a)−1 h−1 was recorded in the Po River plume area and average PmaxB was three-fold in February as compared to the March recordings, 10.50 mg C (mg chl a)−1 h−1 and 3.22 mg C (mg chl a)−1 h−1, respectively.The extreme variability and values of phytoplankton biomass in the innermost plume area was not always reflected in primary production. Modeling of circulation patterns and water mass resilience in the area will help to predict phytoplankton response and biomass distributions. In the frontal area, despite a considerable variability in environmental conditions, our findings have shown that the phytoplankton assemblages will compensate for nutrient depression and hydrographic constraints, by means of size and taxonomic composition and, as a result, the variability in the photosynthetic capacity was much less pronounced than that observed for other parameters.  相似文献   

17.
In March 2011, the Ecophysiology and Genetics Working Groups of the European Science Foundation COST Action ES 0906, entitled Seagrass Productivity: From Genes to Ecosystem Management, organized an exploratory workshop entitled “Linking Ecophysiology and Ecogenomics in Seagrass Systems”. The goal of the workshop was to discuss how to integrate comparative gene expression studies with physiological processes such as photosynthetic performance, carbon and nitrogen utilization and environmental adaptation. The main questions discussed for integrative research related to mechanisms of carbon utilization, light requirements, temperature effects and natural variation in pH and ocean acidification. It was concluded that the seagrass research community is still in the nascent stages of linking ecophysiology with genomic responses, as carbon and nitrogen metabolism of seagrasses have not been sufficiently well studied. Likewise, genomic approaches have only been able to assign meaningful interpretations to a handful of differentially expressed genes. Nevertheless, the way forward has been established.  相似文献   

18.
The particulate beam attenuation coefficient (cp) is proportional to the concentration of suspended particles in a size domain overlapping that of the phytoplankton assemblage. cp is largely insensitive to changes in intracellular chlorophyll concentration, which varies with growth irradiance (a process termed ‘photoacclimation’). Earlier studies have shown that the ratio of cp:chlorophyll (i.e., cp*) exhibits depth-dependent changes that are consistent with photoacclimation. Similar relationships may likewise be expected in the horizontal and temporal dimensions, reflecting changes in mixing depth, incident irradiance, and light attenuation. A link between cp* and more robust photoadaptive variables has never been explicitly tested in the field. Here we use five historical field data sets to directly compare spatial and temporal variability in cp* with two independent indices of photoacclimation: the light-saturated, chlorophyll-normalized photosynthetic rate, Pbopt, and the light-saturation index, Ek. For the variety of oceanographic conditions considered, a first-order correlation emerged between cp* and Pbopt or Ek. These simple empirical results suggest that a relationship exists between a bio-optical variable that can potentially be retrieved remotely (cp*) and physiological variables crucial for estimating primary productivity in the sea.  相似文献   

19.
Polar bears bioaccumulate lipophilic pollutants, including polychlorinated biphenyls (PCBs), into their bodies from their exclusive diet of marine organisms. Hydroxylated PCB metabolites (OH-PCBs) have been found in plasma, presumably due to CYP-dependent biotransformation of PCBs in liver. Little is known about the phase 2 metabolism of hydroxylated xenobiotics in polar bears. The objective of this study was to examine UDP-glucuronosyltransferase (UGT) activity with OH-PCBs and a hydroxylated polycyclic aromatic hydrocarbon, 3-hydroxy-benzo(a)pyrene (3-OH-BaP), in polar bear liver. Samples of frozen polar bear liver were used to prepare microsomes. UGT activity with 3-OH-BaP in Brij-treated microsomes, measured by a fluorescence assay, was readily measurable with protein concentrations in assay tubes of up to 10 μg/ml, but dropped off very sharply at higher protein concentrations. The apparent Km for 3-OH-BaP was 1.71 ± 0.04 μM, and Vmax 1.26 ± 0.16 nmol/min/mg protein (mean ± SD, n=3). UGT activities with a model tetrachloro-OH-PCB (4′-OH-CB72) and a model hexachloro-OH-PCB (4′-OH-CB159) were assayed with [14-C]-UDPGA and separation of the [14-C]-glucuronide by ion-pair extraction and thin-layer chromatography. [14-C]-glucuronide conjugates were readily formed by polar bear liver microsomes in the absence of added substrate, apparently from contaminants present in liver. This phenomenon was not observed using hepatic microsomes from laboratory-held catfish. Glucuronidation efficiency was much higher with 4′-OH-CB72 (Km 7.3 μM; Vmax 1.55 nmol/min/mg) than 4′-OH-CB159 (Km 16.1 μM; Vmax 0.46 nmol/min/mg). The identities of the aglycones present in polar bear liver are not known, but could include OH-PCBs or hydroxylated metabolites of other persistent organic pollutants. This study demonstrates that UGT with high activity for 3-OH-BaP and other substrates is present in polar bear liver.  相似文献   

20.
The spring diatom bloom characterizes the plankton and nutrient dynamics in the Oyashio region, the westernmost part of the subarctic Pacific. Previous studies have shown that NO3 was not depleted during the spring bloom, and an increase in the consumption ratio of Si(OH)4 to NO3 (ΔSi(OH)4:ΔNO3) was observed as the spring bloom progressed. The increase in ΔSi(OH)4:ΔNO3 has been suggested to be caused by growth stresses of diatoms, e.g. light limitation by self-shading. In the present study, incubation experiments of sea-surface water from the Oyashio region under saturated irradiance showed that NO3 was depleted first and ΔSi(OH)4:ΔNO3 was more or less constant until the NO3 depletion occurred. The increase in ΔSi(OH)4:ΔNO3 was observed after the NO3 depletion had occurred in contrast with the field observation. This result of the increase in ΔSi(OH)4:ΔNO3 under saturated irradiance after NO3 depletion suggests that the in situ increase in ΔSi(OH)4:ΔNO3 before the NO3 depletion might be caused by light limitation for diatoms. Responses to a reduction in irradiance were examined using diatom species isolated from the Oyashio region. Variable responses to a reduced irradiance were observed for cell specific C, N, Si and chlorophyll a (Chl) contents. However, the examined diatom species showed similar tendencies for increases in Si:C and Si:N and decreases in C:Chl ratios with the reduction in irradiance. We conclude that light limitation changes the uptake ratio of nutrients and the elemental composition of diatoms and that light limitation is one of the factors influencing the physiology of diatoms and nutrient dynamics in the Oyashio region during the spring bloom.  相似文献   

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