加拿大海盆北部营养盐限制作用研究

利用2008年夏季中国第三次北极科学考察获得的营养盐、叶绿素a、温度和盐度等数据资料,结合现场营养盐添加实验的结果讨论西北冰洋加拿大海盆北部营养盐对浮游植物生长的限制作用。结果表明:由于融冰水稀释作用,加拿大海盆B80站约20m深度存在较强的盐跃层,阻碍了水体上下混合。较低浓度的溶解无机氮(DIN)和硅酸盐(分别为0.31μmol/L和0.94μmol/L)以及严重偏离Redfield比值的N/P、N/Si比值(分别为0.42和0.32)表明加拿大海盆表层水体存在N和Si限制。根据现场营养盐加富实验各培养组叶绿素a浓度变化、营养盐吸收总量差异和浮游植物种群结构,进一步表明氮是北冰洋海盆首要限制营养盐,而Si则抑制了硅质生物的生长。同时,较小的硝酸盐半饱和常数(Ks)证明即使在营养盐充足的情况下北冰洋海盆浮游植物生长速率也处于较低水平。计算得到各培养组营养盐吸收比例(N/P比值)均大于Redfield比值,可能是培养实验过程中以微型、微微型浮游植物为主,硅藻等小型浮游植物为辅造成的。     

石油烃和营养盐的复合污染对海洋浮游植物的影响*Ⅱ. 营养盐对浮游植物吸收石油烃的影响

通过向培养体系中添加不同浓度的营养盐和相同浓度的石油烃, 对中肋骨条藻(Skeletonema costatum)、赤潮异弯藻(Heterosigma akashiwo Hada)和锥状斯氏藻(Scrippsiella trochoidea)进行周期性培养, 研究了营养盐对海洋微藻的石油烃吸收动力学的影响。当营养盐按照Redfield比值添加时, 三种微藻在初始的12h中摄取的石油烃都是最大的。对于三种藻而言, 不同营养盐浓度条件下, 对石油烃的摄取程度从强到弱依次为氮潜在限制>磷潜在限制>硅潜在限制(中肋骨条藻)。在相同营养盐浓度条件下, 在这三种微藻中, 中肋骨条藻对石油烃的摄取程度最大, 在初始的12h里, 石油烃浓度的减少几乎完全是藻将摄取的吸附在表面或吸收入体内的石油烃又重新释放回了水体引起的。     

Simulation of upper-ocean biogeochemistry with a flexible-composition phytoplankton model: C, N and Si cycling in the western Sargasso Sea

We report the first application of a biogeochemical model in which the major elemental composition of the phytoplankton is flexible, and responds to changing light and nutrient conditions. The model includes two phytoplankton groups: diatoms and non-siliceous picoplankton. Both fix C in accordance with photosynthesis-irradiance relationships used in other models and take up NO₃⁻ and NH₄⁺ (and Si(OH)₄ for diatoms) following Michaelis-Menten kinetics. The model allows for light dependence of photosynthesis and NO₃⁻ uptake, and for the observed near-total light independence of NH₄⁺ uptake and Si(OH)₄ uptake. It tracks the resulting C/N ratios of both phytoplankton groups and Si/N ratio of diatoms, and permits uptake of C, N and Si to proceed independently of one another when those ratios are close to those of nutrient-replete phytoplankton. When the C/N or Si/N ratio of either phytoplankton group indicates that its growth is limited by N, Si or light, uptake of non-limiting elements is controlled by the content of the limiting element in accordance with the cell-quota formulation of Droop (J. Mar. Biol. Ass. U.K 54 (1974) 825).We applied this model to the Bermuda Atlantic Time-series Study (BATS) site in the western Sargasso Sea. The model was tuned to produce vertical profiles and time courses of [NO₃⁻], [NH₄⁺] and [Si(OH)₄] that are consistent with the data, by adjusting the kinetic parameters for N and Si uptake and the rate of nitrification. The model then reproduces the observed time courses of chlorophyll-a, particulate organic carbon and nitrogen, biogenic silica, primary productivity, biogenic silica production and POC export with no further tuning. Simulated C/N and Si/N ratios of the phytoplankton indicate that N is the main growth-limiting nutrient throughout the thermally stratified period and that [Si(OH)₄], although always limiting to the rate of Si uptake by diatoms, seldom limits their growth rate. The model requires significant nitrification in the upper 200 m to yield realistic time courses and vertical profiles of [NH₄⁺] and [NO₃⁻], suggesting that NO₃⁻ is not supplied to the upper water column entirely by physical processes. A nitrification-corrected f-ratio (f_NC), calculated for the upper 200 m as: (NO₃⁻ uptake—nitrification)/(NO₃⁻ uptake+NH₄⁺ uptake) has annual values ranging from only 0.05–0.09, implying that 90–95% of the N taken up annually by phytoplankton is supplied by biological regeneration (including nitrification) in the upper 200 m. Reported discrepancies between estimates of organic C export based on seasonal chemical changes and POC export measured at the BATS site can be almost completely resolved if there is significant regeneration of NO₃⁻ via organic-matter decomposition in the upper 200 m.     

Long-term nutrient variation trends and their potential impact on phytoplankton in the southern Yellow Sea,China

The concentration and composition of nutrients, such as N, P, and Si, respond to biogeochemical processes and in turn, impact the phytoplanktons’ community structure and primary production. In this study, historical data was systematically analyzed to identify long-term variations in nutrient trends, red tide frequency, phytoplankton community abundance, and dominant species succession in the southern Yellow Sea(SYS). Results showed that N/P concentration ratios dramatically increased as a funct...     

An ecosystem model with iron limitation of primary production in the equatorial Pacific at 140°W

We construct a one-dimensional ecosystem model (nitrate, ammonium, phytoplankton, zooplnakton and detritus) with simple physics and biology in order to focus on the structural relations and intrinsic properties of the food web that characterizes the biological regime in the central equatorial Pacific at 140°W. When possible, data collected during the EgPac and other cruises were used to calibrate model parameters for two simulations that differ in the limiting nutrient, i.e. nitrogen or iron. Both simulations show annual results in good agreement with the data, but phytoplankton biomass and primary production show a more pronounced annual variability when iron is used as the limiting nutrient. This more realistically reproduces the variability of biological production and illustrates the greater coupling between vertical physical processes and biological production when the limiting nutrient is iron rather than nitrogen. The iron simulation also illustrates how iron supply controls primary production variability, how grazingbalances primary production and controls phytoplankton biomass, and how both iron supply and grazingcontrol primary production. These results suggest that it is not possible to capture primary production variability in the central equatorial Pacific with biological models using nitrogen as the limiting nutrient. Other indirect results of this modeling study were: (1) partitioning of export production between dissolved and particulate matter is almost equal, suggesting that the importance of DOC export may have been previously overestimated; (2) lateral export of live biomass has to be taken into account in order to balance the nitrogen budget on the equator at 140°W; and (3) preferential uptake of ammonium (i.e. nitrate uptake inhibition by ammonium) associated with high regeneration of nitrogen (low f ratio as a consequence of the food web structure imposed by iron limitation) largely accounts for the surface build-up of upwelled nitrate.     

Biogeochemical cycling of N and Si during the mesoscale iron-enrichment experiment in the western subarctic Pacific (SEEDS-II)

Biogeochemical cycles of N and Si were examined in the surface mixed layer during the mesoscale iron-enrichment (IE) experiment in the high-nutrient low-chlorophyll (HNLC) western subarctic Pacific (SEEDS-II). Although the IEs increased nitrate uptake, silicic acid utilization was not stimulated. The nitrate drawdown in the iron-patch (IN-patch, 140.3 mmol m⁻² in the surface mixed layer, 0–30 m) was only 25% of the initial inventory, which was 1/3–2/5 of the previous IE experiments in the subarctic Pacific. This relatively weak response of nutrient drawdown to IEs was due to the high biomass of mesozooplankton (MZ) dominated by copepod Neocalanus plumchrus. Feeding of MZ (247.2 mmol m⁻² during Day 0–21 from the first IE) in the IN-patch was higher than the nitrate drawdown and prevented further development of the phytoplankton bloom. In the later period of the experiment (Day 14–21), the increase in the feeding activity and resultant decrease in phytoplankton biomass induced the accumulation of dissolved organic nitrogen (DON) and ammonium. Among total growth of MZ (81.6 mmol N m⁻²), 89% (72.8 mmol N m⁻²) was transported to the depth by the ontogenetic downward migration of N. plumchrus. Although silicic acid drawdown was not increased by the IEs, Si export flux increased by 2.7 times. The increase in Si export was also due to the increase in MZ, which egested faecal pellets with higher Si:N ratio and faster sinking speed than diatoms. The export efficiency (78% of new production) and total amount of export flux (143.8 mmol N m⁻², 1392 mmol C m⁻²) were highest records within the IE experiments despite weak responses of nutrient drawdown to the IE. During SEEDS-II, the high biomass of MZ reduced the phytoplankton response and nutrient drawdown to the IEs but via grazing and ontogenetic vertical migration accelerated the export flux as well as accumulations of dissolved forms of N. Results of the present and previous IE experiments indicate that the ecosystem and biogeochemical responses to IEs in the HNLC region are quite sensitive to the ecosystem components, especially for grazers of diatoms such as copepods and heterotrophic dinoflagellates. More attention needs to be paid to the ecosystem components and their biogeochemical functions as well as physical and chemical properties of the ecosystems in order to hindcast or forecast the impacts of changes in atmospheric iron deposition.     

渤黄海营养盐结构及其潜在限制作用的时空分布

根据2006-2007年4个季节的现场调查资料,分析探讨了渤海和黄海营养盐结构分布变化特征及其对浮游植物生长的潜在的限制状况.结果表明,渤黄海水体 Si/N/P 比值均偏离 Redfield 比值,季节变化明显;春夏冬季 N/P和 Si/P比值由近岸向远岸海域递减,高值区主要分布在黄河口、鸭绿江口及苏北近岸,秋季上层水体N/P和Si/P比值的分布趋势有所不同,高值区主要分布在南黄海的中部海域.受陆源输入的影响,近岸特别是河口区 N/P和 Si/P比值均较高,温跃层的生消变化和生物活动调控着黄海中部海域营养盐结构的变化.渤黄海浮游植物生长主要受P的潜在限制,部分季节受N、Si的潜在限制;营养盐限制状况存在着明显的时空变化及不同营养盐的同时或交替限制的现象.     

Interactive effects of iron,irradiance and CO2 on Ross Sea phytoplankton

We conducted a factorial shipboard continuous culture experiment to examine the interactive effects of altered iron, irradiance and CO₂ on the summer phytoplankton community of the Ross Sea, Antarctica. After 18 days of continuous incubation, iron enrichment increased phytoplankton biomass, nutrient drawdown, diatom and Phaeocystis abundance, and some photosynthetic parameters. High irradiance significantly increased the number of Phaeocystis antarctica colonies, as well as P. antarctica abundance relative to diatoms. Iron and light had significant interactive effects on diatom and P. antarctica pigment concentrations, P. antarctica colony abundance, and Si:N, Si:C, and N:P ratios. The major influence of high CO₂ was on diatom community structure, by favoring the large centric diatom Chaetoceros lineola over the small pennate species Cylindrotheca closterium. The ratio of centric to pennate diatoms was significantly responsive to changes in all three variables individually, and to all of their possible two- and three-way combinations. These results suggest that shifts in light, iron, and CO₂ and their mutual interactions all play a role in controlling present day Ross Sea plankton community structure, and need to be considered when predicting the possible future responses of biology and biogeochemistry in this region.     

Development of a robust marine ecosystem model to predict the role of iron in biogeochemical cycles: A comparison of results for iron-replete and iron-limited areas,and the SOIREE iron-enrichment experiment

A new mixed layer multi-nutrient ecosystem model, incorporating diatoms, non-diatoms and zooplankton, is described that models the role of iron in marine biogeochemical cycles. The internal cell biochemistry of the phytoplankton is modelled using the mechanistic model of Flynn [2001. A mechanistic model for describing dynamic multi-nutrient, light, temperature interactions in phytoplankton. Journal of Plankton Research 23, 977–997] in which the internal cell concentrations of chlorophyll, nitrogen, silica, and iron are all dynamic variables that respond to external nutrient concentrations and light levels. Iron stress in phytoplankton feeds back into chlorophyll synthesis and changes in photosynthetic unit (PSU) size, thereby reducing their growth rate. Because diatom silicon metabolism is inextricably linked with cell division, diatom population density (cell m⁻³) is modelled as well as C biomass. An optimisation technique was used to fit the model to three time-series datasets at Biotrans (47°N, 20°W) and Kerfix (50°40′S, 68°25′E) and the observations for the Southern Ocean Iron-Release Experiment (SOIREE) iron-enrichment experiment (61°S, 140°E). The model gives realistic simulations of the annual cycles of nutrients, phytoplankton, and primary production at Biotrans and Kerfix and can also accurately simulate an iron fertilisation experiment. Specifically, the model predicts the high values of diatom Si:N and Si:C ratios observed in areas where iron is a limiting factor on algal growth. In addition, the model results at Kerfix confirm previous suggestions that underwater light levels have a more limiting effect on phytoplankton growth than iron supply. The model is also used to calculate C budgets and C and Si export from the mixed layer. The implications of these results for developing biogeochemical models incorporating the role of iron are discussed.     

Modeling iron limitation of primary production in the coastal Gulf of Alaska

A lower trophic level NPZD ecosystem model with explicit iron limitation on nutrient uptake is coupled to a three-dimensional coastal ocean circulation model to investigate the regional ecosystem dynamics of the northwestern coastal Gulf of Alaska (CGOA). Iron limitation is included in the NPZD model by adding governing equations for two micro-nutrient compartments: dissolved iron and phytoplankton-associated iron. The model has separate budgets for nitrate (the limiting macro-nutrient in the standard NPZD model) and for iron, with iron limitation on nitrate uptake being imposed as a function of the local phytoplankton realized Fe:C ratio. While the ecosystem model represents a simple approximation of the complex lower trophic level ecosystem of the northwestern CGOA, simulated chlorophyll concentrations reproduce the main characteristics of the spring bloom, high shelf primary production, and “high-nutrient, low-chlorophyll” (HNLC) environment offshore. Over the 1998–2004 period, model-data correlations based on spatially averaged, monthly mean chlorophyll concentrations are on average 0.7, with values as high as 0.9 and as low as 0.5 for individual years. The model also provides insight on the importance of micro- and macro-nutrient limitation on the shelf and offshore, with the shelfbreak region acting as a transition zone where both nitrate and iron availability significantly impact phytoplankton growth. Overall, the relative simplicity of the ecosystem model provides a useful platform to perform long-term simulations to investigate the seasonal and interannual CGOA ecosystem variability, as well as to conduct sensitivity studies to evaluate the robustness of simulated fields to ecosystem model parameterization and forcing. The ability of the model to differentiate between nitrate-limited, and iron-limited growth conditions, and to identify their spatial and temporal occurrences, is also a first step towards understanding the role of environmental gradients in shaping the complex CGOA phytoplankton community structure.     

Nutrient regeneration by mussel Mytilus edulis spat assemblages in a macrotidal system

Besides exercising grazing control over phytoplankton populations, suspension-feeding bivalves can also stimulate carrying capacity by regeneration of nutrients. This study provides new data on nutrient uptake and release dynamics, and potential implications for availability and stoichiometry of nutrients, for Mytilus edulis spat collectors in the Netherlands. Uptake and release rates were measured in situ on intact spat collector ropes in a eutrophic macrotidal system in relation to development of ropes in terms of mussel biomass and associated components (fauna, flora, and organic material).There was a good fit between uptake/release rates and mussel weight based on allometric scaling functions, despite the occurrence of a substantial biomass of associated fauna, flora and organic matter on ropes. On a unit biomass basis, nutrient release rates were much higher than reported in other studies, which we attribute to greater activity of small mussels. Accounting for greater weight-specific activity of small mussels, spat collectors released more P than reported for other systems. We show that spat collectors can affect relative availabilities of N, P and Si, and we show that SMCs (Seed Mussel Collectors) likely stimulated phytoplankton production through regeneration of N and of Si, which were at limiting concentrations at different points in time. In the case of Si, stimulation would be restricted to diatoms. We conclude that SMCs are able to stimulate phytoplankton production rates, and thereby carrying capacity, and are able to influence phytoplankton composition.     

The influence of changes in nitrogen: silicon ratios on diatom growth dynamics

Nitrate loading to coastal waters has increased over recent decades while silicon loading has remained relatively constant or decreased. As the N:Si ratio in coastal waters shifts due to these anthropogenic influences, silicate limitation of diatom biomass may become a feature of the biogeochemistry in coastal waters especially in regions of reduced exchange. Two sets of nutrient enrichment mesocosm experiments were conducted in successive years using a natural planktonic assemblage obtained from the Trondheimsfjord, Norway. The inorganic nutrient concentrations at the start of the experiments were manipulated to give a variety of N:Si concentrations at ratios representative of current and possible future values, should N loading continue. In June 1999 experiments were conducted with a gradient of inorganic N:Si ratios (1:2, 1:1, 2:1, 4:1) to investigate the influence of low and high N:Si ratio conditions and to determine the conditions that would generate Si limitation of diatom growth. In June 2000, based on 1999 data, highly replicated experiments were conducted at N:Si ratios of 1:1 and 4:1 which were expected to result in N and Si limitation of diatom growth, respectively; statistical differences in cellular composition were recorded. N limitation of diatom biomass increase was observed under the three lowest N:Si ratios: particulate carbon (C) accumulation continued to occur following N exhaustion resulting in an increase in the organic C:N ratio. Silicate limitation of diatom biomass increase only occurred at the highest N:Si ratio of 4:1. Silicate exhaustion was followed by continued nitrate uptake for several days, at a slower rate than previously. The resulting increase in organic N was accompanied by an increase in organic C such that the C:N ratio of the organic material at the highest N:Si ratio failed to increase to the extent observed under the N limited conditions. Statistically significant differences in chlorophyll-a yield per unit nitrate, C:chlorophyll-a ratios, C:N ratio and diatom cell yield per unit nitrate or Si were observed in Si compared to N limited conditions. All mesocosms became dominated numerically and in terms of biomass by the diatom Skeletonema costatum. The potential implications of changing N and Si regimes in coastal waters are discussed.     

Oceanic iron fertilization: one of strategies for sequestration atmospheric CO2

Carbon cycle is connected with the most important environmental issue of Global Change.As one of the major carbon reservoirs, oceans play an important part in the carbon cycle. In recent years, iron seems to give us a good news that oceanic iron fertilization could stimulate biological productivity as CO2 sink of human-produced CO2. Oceanic iron fertilization experiments have verified that adding iron into high nutrient low chlorophyll (HNLC) seawaters can increase phytoplankton production and export organic carbon, and hence increase carbon sink of anthropogenic CO2, to reduce global warming. In sixty days, the export organic carbon could reach 10 000 times for adding iron by model prediction and in situ experiment, i.e. the atmospheric CO2 uptake and inorganic carbon drawdown in upper seawaters also have the same magnitude. Therefore, oceanic iron fertilization is one of the strategies for increasing carbon sink of anthropogenic CO2. The paper is focused on the iron fertilization, especially in situ o     

Seasonal and interannual variability in phytoplankton and nutrient dynamics along Line P in the NE subarctic Pacific

We analysed mixed-layer seasonal and interannual variability in phytoplankton biomass and macronutrient (NO₃ and Si(OH)₄) concentrations from three decades of observations, and nitrogen uptake rates from the 1990s along Line P in the NE subarctic Pacific. Chlorophyll a concentrations near 0.35 mg m⁻³ were observed year-round along Line P except at the nearshore station (P4) where chlorophyll a concentrations in spring were on average 2.4 times the winter values. In contrast, the temporal variability in carbon-to-chlorophyll ratios at the two main end members of Line P (P4 and OSP) was high. Large seasonal and interannual variability in NO₃ and Si(OH)₄ concentration were observed along Line P. Highest upper mixed-layer (top 15 m) nutrient concentrations occurred on the continental shelf in late summer and early fall due to seasonal coastal upwelling. Beyond the shelf, maximum nutrient concentrations increased gradually offshore, and were highest in late winter and early spring due to mixing by winter storms. Interannual variations in upper mixed-layer nutrient concentrations beyond the shelf (>128°W) were correlated with E-W winds and the PDO since 1988 but were not correlated with either climate index between 1973 and 1981. Despite differences in nutrient concentration, nutrient utilization (ΔNO₃ and ΔSi(OH)₄) during the growing season were about 7.5 μM at all offshore stations. Variations in ΔNO₃ were correlated with those of ΔSi(OH)₄. The annual cycle of absolute NO₃ uptake (ρNO₃) and NH₄ uptake (ρNH₄) rates by phytoplankton in the upper mixed-layer showed a weak increasing trend from winter to spring/summer for the period 1992-1997. Rates were more variable at the nearshore station (P4). Rates of ρNO₃ were low along the entire line despite abundant NO₃ and low iron (Fe), at the offshore portion of Line P and sufficient Fe at the nearshore station (P4). As a result, new production contributed on average to only 32 ± 15% of the total nitrogen (N) uptake along Line P. NO₃ utilization in the NE subarctic Pacific is probably controlled by a combination of environmental variables, including Fe, light and ambient NH₄ levels. Elevated ambient NH₄ concentrations seem to decrease the rates of new production (and f-ratios) in surface waters of the oceanic subarctic NE Pacific. Contrary to expectation, phytoplankton biomass, nutrient utilization (ΔNO₃ and ΔSi(OH)₄), and nitrogen uptake (ρNO₃ + ρNH₄) varied relatively little along Line P, despite significant differences in the factors controlling phytoplankton composition assemblages and production. Future studies would benefit from including other variables, especially light limitation, to improve our understanding of the seasonal and interannual variability in phytoplankton biomass and nutrients in this region.     

长江口理化因子影响初级生产力的探索Ⅰ.营养盐限制的判断方法和法则在长江口水域应用

营养盐限制浮游植物的生长是近年来国际研究的热点。探讨了目前营养盐限制的判断方法，认为仅靠氮和磷比值来得到磷限制或氮限制的结论是不完善的。根据营养盐限制的判断方法和法则，对长江口水域进行综合分析，认为在长江口及其附近海域，磷不是浮游植物生长的限制因子，而且氮、磷的浓度值都高于限制浮游植物生长的阈值，满足浮游植物的生长。通过1985年8月至1986年8月的长江口调查数据分析，发现从长江河口到远海水域的有些断面上，磷酸盐浓度并不一定离岸越远越低，且也没有周期性的季节变化。因此，尚须对河口区磷酸盐来源作进一步研究。     

Variability of nutrients and phytoplankton primary production in a shallow macrotidal coastal ecosystem (Arcachon Bay,France)

Seasonal and spatial variations of phytoplankton primary production were studied using a high frequency sampling strategy in the external (ENW) and internal (INW) part of Arcachon Bay, during 2002 and 2003. In order to better assess the availability of nutrients and their relationship with phytoplankton primary production, nutrient variability was studied in relation to environmental conditions and phytoplankton production. During winter, when primary production rates were the lowest, nutrient concentrations were maximal but did not show excessive levels compared to highly urbanised areas. Seasonal and spatial variations of nutrient concentrations (especially DIN-nitrate + nitrite + ammonium- and Si) were largely influenced by Leyre River loads coupled with high tidal exchange with the Atlantic Ocean creating a nutrient gradient between the INW and ENW. By February, diatom growth leads to an early severe nutrient depletion in the entire bay. Examination of nutrient ratios showed that the potential limiting nutrient during spring was P in 2003, and Si in 2002. During summer 2003, N and Si concentrations reached their lowest values, and nutrient ratios revealed a N-deficient environment, more pronounced in the INW. The high Si:N ratios during this period might be explained by (1) important N-uptake by all autotroph communities and (2) benthic-pelagic coupling with high Si regeneration. This study shows that nutrient levels in Arcachon Bay seem to play an important role in the control of phytoplankton primary production rates during the productive period and explain their spatial, seasonal and inter-annual variability. Our estimates of annual integrated phytoplankton primary production (103 g C m⁻² y⁻¹) place this bay within the low to moderate phytoplankton primary production systems.     

Nutrient regimes and their effect on distribution of phytoplankton in the Bay of Bengal

The seasonal dynamics of nutrient ratios and abundance of phytoplankton cells from the central (CB) and western (WB) Bay of Bengal (BOB) were studied during the fall intermonsoon (FIM; September-October 2002) and spring intermonsoon (SpIM; April-May 2003). The nutrient molar ratios of macronutrients such as nitrate to phosphate (N:P), nitrate to silicate (N:Si) and silicate to phosphate (Si:P) in the top 120m were calculated for both FIM and SpIM. During both the seasons, the N:P ratios along the CB and WB were lower than 16, indicating nitrate deficiency. Whereas, along both transects the N:Si ratio was <1 and Si:P >3 in the top 20 and 40m during FIM and SpIM, respectively, indicating Si enrichment. Relatively greater nutrient concentrations along the WB than the CB appear to contribute to higher phytoplankton abundance. The preponderance of diatoms in the Bay could be attributed to rapid utilization of available nutrients in particular during FIM thus resulting in low N:Si ratios in the water column. Among diatoms, pennales were predominantly controlled by nutrients and their ratios. While, apart from nutrients, physical stratification, light and eddies also seem to influence the distribution and abundance of centrales.     

东海营养盐结构的时空分布及其对浮游植物的限制

本文根据2013年东海海域(120°—128°E、25°—33°N)春、夏、秋、冬的4个航次调查资料,分析了营养盐结构的时空分布并探讨其对浮游植物生长限制的情况。结果表明:(1)东海DIN(无机氮)/P(磷)、Si(硅)/DIN及Si/P比值受各种水团及浮游植物生长周期的影响较为明显,长江冲淡水与沿岸水的交汇作用控制着全航次DIN/P比值,基本呈近岸高、远海低的分布规律,而Si/DIN比值的分布则相反。春、夏季Si/P高值区主要分布在近岸,而秋、冬季则开始由中部海域向远海扩展。(2)研究海域浮游植物的生长主要受到N和P的限制,126°E以西的近岸及中部海域以P限制为主,而126°E以东的黑潮区受N限制;在季节变化上又以夏季受到营养盐的限制最明显。(3)与2001—2010年同期历史资料相比,2013年夏季航次受P限制站位数量比过往10年有所增加,限制范围由28°—32°N、123°E以西的长江口及浙北沿岸海域扩展到了126°E以西的东海中部及近岸水域;受N限制站位基本集中在126°—127°E以东黑潮区海域,但空间范围比十年前增大。     

Changes in the nutrient ratios and phytoplankton community after declines in nutrient concentrations in a semi-enclosed bay in Hong Kong

Tolo Harbour is a landlocked bay with poor tidal flushings in the northeastern part of Hong Kong. During the 1980s, excessive nutrient loading led to dramatic increase in nutrient concentrations, accompanied by lower N:P ratios, higher algal biomass and shifts in the phytoplankton community. We studied the effects of nutrient loading reduction measures on nutrient concentrations, nutrient ratios and phytoplankton dynamics in Tolo Harbour by comparing data collected before the full implementation of nutrient loading reduction measures (1986-1997) to those after the implementation (1998-2008). Such measures led to declines in nutrient concentrations, changes in N:P and N:Si ratios, lower chlorophyll-a concentrations and fewer algal blooms. Diatoms were the most abundant phytoplankton group in Tolo Harbour both before and after declines in nutrient concentrations. The density of dinoflagellates did not change, but substantial increase in other algal group abundance was recorded.     

Modelling the response of the planktonic food web to iron fertilization and warming in the NE subarctic Pacific

A one-dimensional ecosystem model with two explicit size classes of phytoplankton was developed for the NE subarctic Pacific to investigate variations in the export of organic particles to the ocean interior due to potential changes in the environment. Specifically, the responses of the planktonic ecosystem to permanent removal of iron limitation and to warming (of 2 and 5 °C) were explored. The ecosystem model consists of five components (small and large phytoplankton, microzooplankton, detritus and nitrogen), and includes grazing by mesozooplankton that varies in time according to long-term observations at Ocean Station Papa (OSP). The model addresses the role of iron limitation on phytoplankton growth and includes temperature dependence of physiological rates. The ecosystem model was forced with annual wind and solar heating from OSP. The model best reproduced the low chlorophyll high nitrate conditions of the NE subarctic Pacific when both small and large phytoplankton were limited by iron such that their maximum specific growth rate was reduced by 10 and 70%, respectively. Sensitivity analysis showed that model results depended on the value of the iron limitation parameter of large phytoplankton (L_Fe-L) and the grazing parameters of micro- and mesozooplankton. To explore the effect of iron limitation, simulations were carried out varying the iron limitation parameters while maintaining the nitrogen flux at the base of the model constant and the grazing pressure by mesozooplankton unchanged. In the warming case, simulations were carried out increasing ocean temperatures by 2° and 5 °C applied only to the ecological components, the flux of nitrate at the base of the model was increased to obtain a steady annual cycle, and grazing by mesozooplankton remained constant. When compared with the standard case, model simulations indicated that both permanent removal of iron limitation and warming cause changes in food web structure and the carbon cycle. The response was more dramatic in the iron-replete case where the phytoplankton community structure in spring changed from one dominated by pico- and nanoplankton to one dominated by large phytoplankton, and primary production increased until it consumed all the external nutrient (N) supply to the upper layer. However, reducing iron deficiency actually led to lower annual primary production due to a decrease in the regeneration of nitrogen in the euphotic zone. These changes in food web structure influenced the magnitude, composition and seasonal cycle of sinking particles.