A biophysical NPZ model with iron for the Gulf of Alaska: Reproducing the differences between an oceanic HNLC ecosystem and a classical northern temperate shelf ecosystem

Modeling the coastal Gulf of Alaska (CGOA) is complicated by the highly diverse physical and biological features influencing productivity and energy flow through the region. The GOA consists of the offshore oceanic environment, characterized by iron limitation, high-nutrients and low-chlorophyll. The coastal environment is consistently downwelling, with high iron levels from glacial melt water and runoff, but lower concentrations of macronutrients, and with a spring bloom, nutrient depletion cycle (low-nutrient, high-chlorophyll). Cross-shelf movement of water masses mixes coastal and oceanic ecosystem elements.Simulations and field data indicate that the minimum model complexity necessary to characterize lower trophic-level production and biomass in the offshore and coastal regions includes 10 boxes: iron, nitrate, ammonium, small phytoplankton, large phytoplankton, small microzooplankton, large microzooplankton, small copepods, large oceanic copepods and detritus, with copepod mortality as a model closure term. We present the model structure, equations required (and initial parameters used) to simulate onshore and offshore lower trophic-level production in the Gulf of Alaska, along with the information from field data and simulations used to construct the model. We show the results of simulations with and without iron, and with and without two size classes of phytoplankton. These simulations indicate that our method of inclusion of iron works well to distinguish the coastal and the oceanic ecosystems, and that the inclusion of two size categories of phytoplankton is also necessary to generate the differences between these two ecosystems.     

Quantifying cross-shelf and vertical nutrient flux in the Coastal Gulf of Alaska with a spatially nested,coupled biophysical model

The Coastal Gulf of Alaska (CGOA) is productive, with large populations of fish, seabirds, and marine mammals; yet it is subject to downwelling-favorable coastal winds. Downwelling regions in other parts of the world are typically much less productive than their upwelling counterparts. Alternate sources of nutrients to feed primary production in the topographically complex CGOA are poorly known and difficult to quantify. Here we diagnose the output from a spatially nested, coupled hydrodynamic and lower trophic level model of the CGOA, to quantify both horizontal and vertical nutrient fluxes into the euphotic zone. Our nested model includes both nitrogen and iron limitation of phytoplankton production, and is driven by a fine-scale atmospheric model that resolves the effects of local orography on the coastal winds. Results indicate significant “rivers” of cross-shelf nitrogen flux due to horizontal advection, as well as “fountains” of vertical transport over shallow banks due to tidal mixing. Using these results, we constructed a provisional budget of nutrient transport among subregions of the CGOA. Contrary to expectations, this budget reveals substantial upwelling of nutrients over major portions of the shelf, driven by local wind-stress curl. These effects are large enough to overwhelm the smaller downwelling flux at the coast throughout the growing season. Vertical mixing by winds and tides, and horizontal flux from the deep basin, are other substantial contributors to nutrients above the 15-m horizon. These findings help to explain the productivity of this coastal ecosystem.     

Numerical modelling of spatio-temporal variability of growth of Mytilus edulis (L.) and influence of its cultivation on ecosystem functioning

One of the key needs of the aquaculture industry is the implementation of effective management methods to ensure the sustainability, economic viability and minimization of negative impacts on both human and ecosystem well-being. The authors developed a Fortran 90 implementation of the dynamic energy budget (DEB) model for Mytilus edulis. The model has been further developed to include physiological interactions with the ecosystem and coupled to a biogeochemical nutrient–phytoplankton–zooplankton–detritus (NPZD) model. Phytoplankton and detritus uptakes, oxygen utilisation, CO₂ production, NH₄ excretion, egestion of faeces, and assimilation of food are modelled. A novel approach was derived that accounts for the allocation of C and N in mussel flesh and shell organic fraction. The DEB–NPZD model has been subsequently coupled to a high resolution three dimensional numerical coastal ocean model of the south–west coast of Ireland, where approximately 80% of national rope mussel is produced annually. Simulations have been carried out for the time period July 2010–June 2011, for which the field data on mussel biometrics and ambient seawater properties were collated. The model accurately reproduced the spatio-temporal variability in blue mussel growth. It is also shown that the ecosystem dynamics is affected by the presence of aquaculture farms. The modelling system presented allows for the assessment of the impacts of aquaculture activities on water quality, quantification of the production and ecological carrying capacities and improvement of our understanding of the ecosystem functioning with particular emphasis on interactions between various trophic levels.     

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.     

Importance of vertical mixing for additional sources of nitrate and iron to surface waters of the Columbia River plume: Implications for biology

The influence of the Columbia River plume on the distributions of nitrate and iron and their sources to coastal and shelf waters were examined. In contrast to other large estuaries, the Columbia River is a unique study area as it supplies very little nitrate (5 μM) and iron (14–30 nM) at salinities of 1–2 to coastal waters. Elevated nitrate and dissolved iron concentrations (as high as 20 μM and 20 nM) were observed, however, in the near field Columbia River plume at salinities of 20. Surface nitrate concentrations were higher than observed in the Columbia River itself and therefore must be added by entrainment of higher nitrate concentrations from subsurface coastal waters. Tidal flow was identified as an important factor in determining the chemical constituents of the Columbia River plume. During the rising flood tide, nitrate and iron were entrained into the plume waters resulting in concentrations of 15 μM and 6 nM, respectively. Conversely, during the ebb tide the concentrations of nitrate and total dissolved iron were reduced to 0.3–3 μM and 1–2 nM, respectively, with a concomitant increase in chlorophyll a concentrations. As these plume waters moved offshore the plume drifted directly westward, over a nitrate depleted water mass (< 0.2 μM). The plume water was also identified to move southwards and offshore during upwelling conditions and nitrate concentrations in this far field plume were also depleted. Iron concentrations in the near-field Columbia River plume are sufficient to meet the biological demand. However, due to the low nitrate in the Columbia River itself, nitrate in the plume is primarily dependent on mixing with nitrate rich, cold, high salinity subsurface waters. Without such an additional source the plume rapidly becomes nitrate limited.     

渤海生态动力过程的模型研究Ⅰ.模型描述

建立了一个NPZD类型的生物化学模型,并将其与原始方程海流模型(POM)、太阳辐射模型和河流输入模型耦合再现渤海的生态动力过程.模拟的初级生产力与实测值吻合较好;此外,该文还首次全方位地检验了渤海f-ratio的特征,通过与莱州湾的实测值比较,模拟结果也显示了相当的准确度.另外,该文在分析f-ratio变化特征的基础上还揭示硝酸盐和铵盐对渤海浮游植物生长的相对贡献率.     

A Comparison of Simulated Particle Fluxes Using NEMURO and Other Ecosystem Models in the Western North Pacific

JGOFS has revealed the importance of marine biological activity to the global carbon cycle. Ecological models are valuable tools for improving our understanding of biogeochemical cycles. Through a series of workshops, the North Pacific Marine Science Organization (PICES) developed NEMURO (North Pacific Ecosystem Model Understanding Regional Oceanography) a model, specifically designed to simulate the lower trophic ecosystem in the North Pacific Ocean. Its ability to simulate vertical fluxes generated by biological activities has not yet been validated. Here compare NEMURO with several other lower trophic level models of the northern North Pacific. The different ecosystem models are each embedded in a common three-dimensional physical model, and the simulated vertical flux of POM and the biomass of phytoplankton are compared. The models compared are: (1) NEMURO, (2) the Kishi and Nakata Model (Kishi et al., 1981), (3) KKYS (Kawamiya et al., 1995, 2000a, 2000b), and (4) the Denman model (Denman and Peña, 2002). With simple NPZD models, it is difficult to describe the production of POM (Particulate Organic Matter) and hence the simulations of vertical flux are poor. However, if the parameters are properly defined, the primary production can be well reproduced, even though none of models we used here includes iron limitation effects. On the whole, NEMURO gave a satisfactory simulation of the vertical flux of POM in the northern North Pacific.     

Remotely sensed variability of temperature and chlorophyll in the southern Benguela: upwelling frequency and phytoplankton response

High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones.     

Strong CO2 outgassing from high nutrient low chlorophyll coastal waters off central Chile (30°S): The role of dissolved iron

Carbonate system parameters (pH and alkalinity) were used to estimate the coastal water CO₂ fluxes off central Chile (30°S) during September 2007. Coastal waters rich in nitrate and silicate were strongly CO₂ supersaturated and normally poor in chlorophyll a. MODIS satellite chlorophyll a data suggest that phytoplankton biomass remained particularly low during September 2007 although coastal waters were highly fertilized with nitrate and silicate. The phytoplankton gross primary productivity in macronutrient-rich waters was very low with the exception of shallow waters (e.g. within or near bays). Several iron-enrichment bottle experiments show that fCO₂ rapidly decreases during iron-enrichment treatments compared to controls. This suggests that iron limitation of phytoplankton growth (mainly diatoms) plays a role in maintaining high-CO₂ outgassing by preventing rapid interception of upwelled CO₂.     

Specific oceanographic characteristics and phytoplankton responses influencing the primary production around the Ulleung Basin area in spring

The East Sea(Sea of Japan)is a marginal,semi-closed sea in the northwestern Pacific.The Ulleung Basin area,which is located near the subpolar front of the East Sea,is known to have high primary production and good fisheries in spring season.After episodic wind-driven events during the spring of 2017,horizontal and vertical profiles of physical chemical biological factors were investigated at 29 stations located in the Ulleung Basin area.In addition,growth responses of phytoplankton communities to nutrient additions were evaluated by bioassay experiments to understand the fluctuation of phytoplankton biomass.Because of strong northwestern wind,phytoplankton biomass was scattered and upwelling phenomenon might be suppressed in this season.The phytoplankton abundances in the coastal stations were significantly higher than offshore and island stations.In contrast,the nutrient and chlorophyll a(Chl a)concentrations and the phytoplankton biomass were quite low in all locations.Bacillariophyceae was dominated group(>75.1%for coastal,40.0%for offshore and 43.6%for island stations).In the algal bioassays,the phytoplankton production was stimulated by N availability.The in vivo Chl a values in the+N and+NP treatments were significantly higher than the values in the control and the+P treatments.Based on the field survey,the higher nutrients in coastal waters affected the growth of diatom assemblages,however,little prosperity of phytoplankton was observed in the offshore waters despite the injection of sufficient nutrients in bioassay experiments.The growth of phytoplankton depended on the initial cell density.All of results indicated that a dominant northwestern wind led to a limited nutrients condition at euphotic layers,and the low level of biomass supply from the coasts resulted in low primary production.Both supplying nutrients and introducing phytoplankton through the currents are critical to maintain the high productivity in the Ulleung Basin area of the East Sea.     

The phytoplankton bloom response to wind events and upwelled nutrients during the CoOP WEST study

In the coastal waters off northern California, seasonal wind-driven upwelling supplies abundant nutrients to be processed by phytoplankton productivity. As part of the Coastal Ocean Processes: Wind Events and Shelf Transport (CoOP WEST) study, nutrients, CO₂, size-fractionated chlorophyll, and phytoplankton community structure were measured in the upwelling region off Bodega Bay, CA, during May–June 2000, 2001 and 2002. The ability of this ecosystem to assimilate nitrate (NO₃) and silicic acid/silicate (Si(OH)₄) and accumulate particulate material (i.e. phytoplankton) was realized in all 3 years, following short events of upwelling-favorable winds, followed by periods of relaxed winds. This was observed as phytoplankton blooms, dominated by chlorophyll in cells greater than 5 μm in diameter, that reduced the ambient nutrients to zero. These communities were located over the near-shore shelf (<100 m depth) and were dominated by diatoms. An optimal window of 3–7 days of relaxed winds, following an upwelling pulse, was required for chlorophyll accumulation. The large-celled phytoplankton that result are likely important players in coastal new production and carbon cycling.     

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.     

Model study on Bohai ecosystem 1. Model description and primary productivity

1Introduction TheBohaiSea(BS)(37°10′～40°55′N,117°35′～122°15′E)isthelargestinnerseainChi na,andplaysanimportantroleinsupportingthee conomicdevelopmentoffouradjacentprovincesby meansofitsadvantageouspositionforshippingand fisheryresources.However,     

Physical control of biological processes in the central equatorial Pacific Ocean

A five-component (phytoplankton, zooplankton, ammonium, nitrate, detritus) physical–biological model was developed to investigate the effects of physical processes on daily to interannual time scales, on the lower trophic levels of the central equatorial Pacific. Many of the biological processes included in the ecosystem model respond to environmental fluctuations with time scales between 1 and 10 d, which are not typically resolved by basin- to global-scale circulation models. Therefore, the equatorial Pacific ecosystem model is forced using daily information (solar radiation, velocity, temperature) from the Tropical Atmosphere Ocean (TAO) mooring array. The ecosystem model also requires vertical velocity information which necessitated the development of a method for computing daily vertical velocities from the TAO array. Much of the variability in primary production, plankton and nutrient concentrations observed in 1992 during the US Joint Global Ocean Flux Study Equatorial Pacific Process Study time-series cruises (TS1 and TS2), is well reproduced in the model simulations. Simulations demonstrate that lower primary productivities during TS1 as compared to TS2 result from the deeper thermocline that persisted during TS1 as a result of El Niño conditions; however, because of the simultaneous reduction in grazing pressure, simulated chlorophyll levels are similar for these two time periods. Simulations of this single-species ecosystem model successfully reproduce data collected both during and after the El Niño, suggesting that species composition changes are not of first-order importance when examining the effects of the 1991–92 El Niño on the equatorial Pacific ecosystem. A 60–70% increase in chlorophyll concentration and a 400% increase in the chlorophyll contribution by diatoms was associated with the passage of a tropical instability wave (20-d period) across the study site during TS2. This period of high chlorophyll concentration and diatom abundance coincided temporally with strong northward velocities and strong downwelling velocities in the upper euphotic zone. Observations and simulations suggest that this increase in chlorophyll concentration and change in species composition not only results from in situ diatom growth stimulated by increased iron concentrations, but also results from the advection of diatoms toward the convergent front located along the leading (western) edge of the instability wave. Equatorially trapped internal gravity waves can also stimulate in situ phytoplankton growth as high-frequency vertical motions introduce limiting micronutrients, such as iron, into the euphotic zone. Because iron can be taken up by the picoplankton on time scales much shorter than the wave period (6–8 days), these waves may provide a mechanism for effecting a large flux of iron into the euphotic zone. Exclusion of these high-frequency motions results in an iron flux to the euphotic zone that may be underestimated by more than 30%.     

Phytoplankton dynamics in the NE subarctic Pacific

Ocean Station Papa (OSP, 50°N 145°W) in the NE subarctic Pacific is characterised as high nitrate low chlorophyll (HNLC). However, little is known about the spatial extent of these HNLC waters or the phytoplankton dynamics on the basin scale. Algal biomass, production and size-structure data are presented from winter, spring and summer between 1992 and 1997 for five stations ranging from coastal to open-ocean conditions. The inshore stations (P04–P16) are characterised by the classical seasonal cycle of spring and late summer blooms (production >3 g C m⁻² d⁻¹), diatoms are not Fe-stressed, and growth rate is probably controlled by macronutrient supply. The fate of the phytoplankton is likely sedimentation by diatom-dominated spring blooms, with a pelagic recycling system predominating at other times. The offshore stations (P20/OSP) display low seasonality in biomass and production (OSP, mean winter production 0.3 g C m⁻² d⁻¹, mean spring/summer production 0.85 g C m⁻² d⁻¹), and are dominated by small algal cells. Low Fe availability prevents the occurrence of diatom blooms observed inshore. The main fate of phytoplankton is probably recycling through the microbial food web, with relatively low sedimentation compared to inshore. However, the supply of macro- and micro-nutrients to the coastal and open ocean, respectively, may vary between years. Variability in macro-nutrient supply to the coastal ocean may result in decreased winter reserve nitrate, summer nitrate limitation, subsequent floristic shifts towards small cells, and reduced primary production. Offshore, higher diatom abundances are occasionally observed, perhaps indicating episodic Fe supply. The two distinct oceanic regimes have different phytoplankton dynamics resulting in different seasonality, community structure and fate of algal carbon. These differences will strongly influence the biogeochemical signatures of the coastal and open-oceanic NE subarctic Pacific.     

Distribution and vertical dynamics of planktonic communities at Sofala Bank, Mozambique

Coastal ecosystem processes are largely influenced by the interaction of different factors operating at various temporal and spatial scales, specifically those responsible for primary production patterns that modulate zooplankton and subsequent trophic levels. Hydrological processes, such as tidal cycles and coastal currents, nutrients availability, phytoplankton groups (studied through algal pigment signatures analysed by HPLC), and zooplankton abundance and distribution were investigated at the Sofala Bank (Mozambique), with special emphasis on their horizontal distribution and vertical dynamics (48 h). Horizontal distribution has shown inshore–offshore gradients in all analysed parameters, as well as inshore waters intrusion probably related to Zambezi River delta runoff. Tidal currents were responsible for major hydrological vertical variations and for horizontal and vertical advection of phytoplankton biomass in the surface and deepest layers, respectively. Nutrient concentrations were typical from oligotrophic regions, and nutrient ratios were strongly influenced by depleted nitrate + nitrite concentrations, indicating low estuarine discharges typical from the dry season. The very low N:P ratio obtained suggests strong nitrogen limitation to phytoplankton communities, supporting the low phytoplankton abundance observed. Both phytoplankton pigments and zooplankton were found mainly near the bottom (40 m depth), despite the latter displayed vertical migrations triggered by light variations. Phytoplankton community was dominated by microflagellates, specifically prymnesiophyceans, and behaved as a whole, except Cyanobacteria that displayed vertical distribution movements different from other phytoplankton groups, being mainly concentrated at mid-water column depths (10–20 m). This investigation enhances physico-chemical phenomena and their importance determining the planktonic communities vertical dynamics at Sofala Bank, a tropical coastal ecosystem of the Western Indian Ocean where planktonic dynamics are still poorly described and understood.     

Persistence of iron limitation in the western subarctic Pacific SEEDS II mesoscale fertilization experiment

The cumulative evidence from more than a dozen mesoscale iron-enrichment studies in high nitrate low chlorophyll (HNLC) waters demonstrates that iron limitation is widespread and very likely affects atmospheric carbon dioxide and thus global climate. However, the responses of microphytoplankton (>20 μm), predominantly diatoms, vary greatly among these mesoscale experiments even though similar amounts of iron were added, making it difficult to quantitatively incorporate iron effects into global climate models. Nowhere is this difference more dramatic than between the massive bloom observed during Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS) I and the order of magnitude smaller ecosystem response in SEEDS II; two mesocale experiments performed in the same HNLC region of the western subarctic Pacific in different years. Deckboard incubation experiments initiated during the early, middle, and late stages of the 32-day SEEDS II experiment show that while the two iron infusions increased phytoplankton growth, diatoms remained significantly limited by iron availability, despite total dissolved Fe concentrations in the patch being well above the diffusion-limited threshold for rapid diatom growth. This iron limitation was apparent <6 days after the initial iron infusion and was not alleviated by the second, smaller iron infusion. In contrast, smaller phytoplankton (<20 μm) showed a more restricted response to further iron amendments, indicating that their iron nutrition was near optimal. Iron complexed to desferrioximine B, a commonly available siderophore produced by at least one marine bacterium, was poorly available to diatoms throughout the patch evolution, indicating that these diatoms lacked the ability to induce high-affinity iron uptake systems. These results suggest that the strong organic complexation of Fe(III) observed in the SEEDS II-fertilized patch was not compatible with rapid diatom growth. In contrast, iron associated with protoporphyrin IX, a weaker iron complexing ligand of a class hypothesized to be representative of recycled iron species, was readily available to diatoms. Our findings demonstrate that a persistence of iron limitation was the primary factor underlying the comparatively small diatom response during SEEDS II. This continued growth limitation would have increased the importance of mesozooplankton grazing as a controlling factor in the SEEDS II ecosystem response.     

Physiological acclimation by phytoplankton explains observed changes in Si and N uptake rates during the SERIES iron-enrichment experiment

During the SERIES iron-enrichment experiment in the eastern subarctic Pacific, after addition of iron and its subsequent depletion, the Si:N drawdown ratio increased at approximately the time that diatoms became iron limited. Laboratory studies have reported that this results from a decrease in the rate of N uptake together with a more moderate decrease in the rate of Si uptake for iron-limited cultures compared to iron-replete cultures. However, for SERIES Boyd et al. (Limnol. Oceanogr. 50 (2005)) reported an unexplained increase in the rate of Si uptake at the onset of iron limitation and suggested that studies of nutrient uptake kinetics should be undertaken in search of an explanation. We compare the classic Michealis–Menten (MM) kinetics to the recently developed optimal uptake (OU) kinetics (the SPONGE: Smith and Yamanaka. Limnol. Oceanogr. 52 (2007)) within a variable-composition model, which employs cell quotas for each relevant nutrient, applied to the multi-element (C, N, Si, Fe) dynamics during SERIES. Using the Monte Carlo Markov Chain, we fit two versions of the model (differing only in the equations for nutrient uptake) to the available data for nutrient concentrations, chlorophyll, biogenic silica and particulate organic carbon and specific growth rates.With either uptake kinetics, the model reproduces observed concentrations well for nutrients and somewhat less well for chlorophyll. The different uptake kinetics yield greater differences in modeled elemental composition of phytoplankton and biomass of phytoplankton and zooplankton, which are not directly constrained by data. MM kinetics cannot reproduce the observed increase in Si uptake rate as a function of the decreasing trend in concentration of silicic acid, and it predicts Si limitation throughout nearly all of the experiment after iron-fertilization. In contrast, OU kinetics reproduces the increase in Si uptake rate and matches the observation-based estimate for the timing of the return to iron limitation. The key assumption of the SPONGE, that uptake rates of all nutrients depend on physiological acclimation by phytoplankton as a function of the ambient concentration of the growth-limiting nutrient, was originally formulated for modeling chemostat experiments. We show that it also agrees with the observations from this field experiment and provides an explanation for the increases in Si uptake rate and Si:N drawdown ratio.     

Iron,macronutrients and diatom blooms in the Peru upwelling regime: brown and blue waters of Peru

Surface water transects and vertical profiles for dissolved iron, macronutrients, chlorophyll a (Chl a), and hydrographic data were obtained in the Peru upwelling regime during August and September 2000. The supply of the micronutrient iron, relative to that of the macronutrients nitrate, phosphate and silicic acid, is shown to play a critical role in allowing extensive diatom blooms to develop in the Peru upwelling system. The extremely high-chlorophyll “brown waters of Peru” (with Chl a concentrations between 20 and 45 μg/l) result from massive diatom blooms with maximal photochemical efficiencies (F_v/F_m >0.6) occurring in the iron-rich upwelling region observed over the broad continental shelf off northern and central Peru. The source of the upwelled water in this region is the nutrient-rich subsurface countercurrent in contact with the organic-rich shelf sediments. This subsurface shelf water is suboxic and has extremely high concentrations of dissolved Fe (>50 nM) in the near-bottom waters. In marked contrast, relatively low-chlorophyll “blue waters” (Chl a <2 μg/l) with low concentrations of dissolved Fe (<0.1 nM) and high unutilized macronutrient concentrations are observed in the coastal upwelled waters along the southern coast of Peru and in the offshore regions of the Peru Current. Southern Peru is a region without a wide shelf to serve as a source of iron and, as a result, dissolved Fe concentrations in the near-bottom suboxic waters of this region are an order-of-magnitude lower than observed off northern and central Peru. In addition, the offshore Peru Current is a broad, Fe-limited, high-nitrate, lower than expected chlorophyll region extending hundreds of kilometers offshore into the northeast region of the South Pacific subtropical gyre and northwestward into the South Equatorial Pacific.     

Influences of initial plankton biomass and mixed-layer depths on the outcome of iron-fertilization experiments

Several in situ iron-enrichment experiments have been conducted, where the response of the phytoplankton community differed. We use a marine ecosystem model to investigate the effect of iron on phytoplankton in response to different initial plankton conditions and mixed-layer depths (MLDs). Sensitivity analysis of the model results to the MLDs reveals that the modeled response to the same iron enhancement treatment differed dramatically according to the different MLDs. The magnitude of the iron-induced biogeochemical responses in the surface water, such as maximum chlorophyll, is inversely correlated with MLD, as observed. The significant decrease in maximum surface chlorophyll with MLD results from the difference in diatom concentration in the mixed layer, which is determined by vertical mixing. The modeled column-integrated chlorophyll, on the other hand, is the highest with intermediate MLD cases, suggesting difference in iron-induced biogeochemical responses between volume and area considerations. The iron-induced diatom bloom is severely restricted below the compensation depth due to both light limitation and grazing pressure, irrespective of the MLD. Sensitivity of the model to initial mesozooplankton (as grazers on diatoms) biomass shows that column-integrated biomass, net community production and export production are strongly controlled by the initial mesozooplankton biomass. Higher initial mesozooplankton biomass yields high grazing pressure on diatoms, which results in less accumulation of diatom biomass and may account for notably lower surface chlorophyll during SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) II than during SEEDS. The initial diatom biomass is also important to the outcome of iron enrichment but is not as crucial as the MLD and the initial mesozooplankton biomass. This modeling study suggests that not only MLD but also the initial biomass of diatoms and its principle grazers are crucial factors in the response of the phytoplankton community to iron enrichments, and should be considered in designing future iron-enrichment experiments.