High-nitrate Low-chlorophyll Regions Research Articles

The impact of ocean deoxygenation on iron release from continental margin sediments

In the oceans’ high-nitrate–low-chlorophyll regions, such as the Peru/Humboldt Current system and the adjacent eastern equatorial Pacific1, primary productivity is limited by the micronutrient iron. Within the Peruvian upwelling area, bioavailable iron is released from the reducing continental margin sediments2. The magnitude of this seafloor source could change with fluctuations in the extension or intensity of the oxygen minimum zones3, 4. Here we show that measurements of molybdenum, uranium and iron concentrations can be used as a proxy for sedimentary iron release, and use this proxy to assess iron release from the sea floor beneath the Peru upwelling system during the past 140,000 years. We observe a coupling between levels of denitrification, as indicated by nitrogen isotopes, trace metal proxies for oxygenation, and sedimentary iron concentrations. Specifically, periods with poor upper ocean oxygenation are characterized by more efficient iron retention in the sediment and a diminished iron supply to the water column. We attribute efficient iron retention under more reducing conditions to widespread sulphidic conditions in the surface sediment and concomitant precipitation of iron sulphides. We argue that iron release from continental margin sediments is most effective in a narrow redox window where neither oxygen nor sulphide is present. We therefore suggest that future deoxygenation in the Peru upwelling area would be unlikely to result in increased iron availability, whereas in weaker oxygen minimum zones partial deoxygenation may enhance the iron supply.

High-resolution opal records from the eastern tropical Pacific provide evidence for silicic acid leakage from HNLC regions during glacial periods

A shift from carbonate- to silica-dominated primary production could significantly affect the oceanic carbon cycle via changes in the particulate carbon rain-rate ratio ( C organic:C inorganic fluxes). An increase in C rain rate ratio has been invoked to explain lower glacial pCO 2; however, firm evidence of an ecological shift towards silica-dominated productivity during the last glacial period is lacking. Here, we present new high-resolution reconstructions of biogenic silica and total production over the past 40,000 yr BP in 3 cores from the eastern tropical North Pacific (ETNP) off Mexico and Nicaragua. These records reveal a clear regional pattern of higher siliceous productivity with higher opal accumulation during the last glacial period compared to interglacial times. Higher Si:C and Si:N ratios of glacial sediments in these records suggest a net increase in siliceous production over total production. We attribute this to the additional supply of silicic acid to the ETNP margins favouring diatoms over other non-siliceous algae. This suggestion for increased supply of Si during glacial periods is consistent with the proposed large-scale redistribution of excess silicic acid from High Nitrate Low Chlorophyll (HNLC) regions like the eastern equatorial Pacific (EEP) and the Southern Ocean by the Silicic Acid Leakage Hypothesis (SALH). In these HNLC regions, the Si-isotope composition of diatom frustules (δ 30Si) has provided evidence for the generation of surplus of silicic acid during diatom growth under conditions of higher Fe availability during glacial periods. We suggest that silicic acid leakage from the HNLC regions to the adjoining oceans may have increased the carbon rain rate ratio and ultimately, contributed to the decrease in glacial atmospheric pCO 2.

Estimating iron and aluminum removal rates in the eastern equatorial Pacific Ocean using a box model approach

Iron limitation plays an important role in maintaining the high-nitrate low-chlorophyll (HNLC) condition in the equatorial upwelling zone. The rate and depth of upwelling control Fe supply to the euphotic zone. This study constrains the transport fluxes and budget of two trace metals, Fe and Al, in the upper ocean. They are co-delivered to the eastern equatorial Pacific surface waters via the Equatorial Undercurrent and upwelling but show distinct biogeochemical cycling processes. We combine the results of the in situ measurements of dissolved Fe and Al (dFe and dAl) with the modeled velocity fields to calculate the physical fluxes. The model calculations are evaluated with the conservation of heat, volume transport, NO 3 and Si(OH) 4 budgets for the equatorial Pacific. The vertical flux due to upwelling provides averaged dFe and dAl supply rates of 1.45 μmol m −2 d −1 and 11.51 μmol m −2 d −1, respectively. The sum of the net physical fluxes in the eastern equatorial Pacific for dFe and dAl are 0.41 μmol m −2 d −1 and 2.77 μmol m −2 d −1, respectively. These estimates are equal to the net biological and chemical removal rates of dFe and dAl. The calculated dFe:C net removal ratio is in the range of 3-9 μmol:mol, which agrees with most other estimates. This suggests that the majority of net dFe removal is due to biological uptake in the upper water column. The results of this box model approach illustrate the usefulness of combining the modeled outputs and in situ measurements, which provide additional constraints on Fe transport and cycling in the equatorial Pacific and possibly other HNLC regions.

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.

Consistent relationship between global climate and surface nitrate utilization in the western subarctic Pacific throughout the last 500 ka

The open subarctic Pacific is, at present, a high nitrate low chlorophyll (HNLC) region, where nitrate is perennially abundant at the surface. Theoretically, the HNLC status of this region is subject to modification by ocean circulation and/or micronutrient supply, with implications for the effectiveness of the biological pump and hence carbon sequestration in the ocean interior. Records of biogenic detritus in sediments from throughout the subarctic Pacific indicate that export production was generally lower during glacial maxima, while nitrogen isotope measurements from the Bering Sea have shown that nitrate consumption there was more complete during the last glacial period than it is today. Here, nitrogen isotopic analyses of bulk sediments (δ15Nbulk) from three deep water sites in the open subarctic Pacific are evaluated in terms of regional nitrate isotopic composition and local relative nitrate utilization. The eastern subarctic Pacific δ15Nbulk record bears great similarity to δ15Nbulk records from the western margin of North America over the last glacial cycle, suggesting that variability in the isotopic composition of subeuphotic zone nitrate, the growth substrate, is reasonably coherent throughout the northeast Pacific and dominates at these sites. However, the two western subarctic Pacific records, which lie at the heart of the HNLC region, display a different pattern, implying that significant changes in local relative nitrate utilization overlie the regional background variability. After a novel correction intended to remove the background signal associated with denitrification in the eastern tropical North Pacific, these nitrate utilization records are correlated with a benthic oxygen isotope stack reflecting global deep ocean temperature and ice volume (r2 = 0.65). The correlation implies a strong link between global climate and subarctic Pacific nitrate utilization, with nearly complete nitrate consumption during glacial periods when export production was low.

Spatial and temporal variability in new production in the equatorial Pacific during 1980–2003: Physical and biogeochemical controls

Abstract The total new production in the equatorial Pacific ocean is still subject to debate. Early estimates range from 0.8 to 1.9 Pg C year−1 for the Wyrtki box (5°N–5°S, 180°W–90°W). More recent studies estimate a rate of less than 0.14 Pg C year−1 for the 1°N–1°S equatorial divergence, which is significantly lower than the previous estimates even when the difference in area is taken into account. To address this issue, we employ a coupled three-dimensional physical-biogeochemical model to simulate ecosystem dynamics and to estimate new production over the period of 1980–2003. The biogeochemical model consists of nine components: two size classes (large and small) each of phytoplankton, zooplankton and detritus, and three nutrients (ammonium, nitrate, and iron). The model is able to reasonably reproduce observed surface chlorophyll, ecosystem structure, primary productivity, and new production. The modeled primary productivity and new production are in the ranges of 20–100 and 5–40 mmol C m−2 d−1, respectively. The modeled primary productivity shows relatively weak spatial and temporal variations whereas the modeled new production shows considerable variability over space and on seasonal-to-interannual time scales. The model simulates two peaks in new production (e.g., in boreal spring and boreal fall) in the high-nitrate low-chlorophyll (HNLC) region. The high rate of new production in spring is associated with the seasonal warming and a shallow mixed layer whereas the high rate of new production during fall is related to the high iron concentrations due to the seasonal upwelling and entrainment in the eastern equatorial Pacific. The new production reveals clear spatial and interannual variability in association with the El Nino/Southern Oscillation (ENSO). The rate of new production is highly related to the mixed-layer concentrations of nitrate and iron, and the 20 °C isotherm depth (Z20) in the HNLC region. However, there is no relationship between the rate of new production and either the concentrations of nitrate or iron, or Z20 in the warm pool. Integrated new production from the model is 0.61±0.14 Pg C year−1 for the Wyrtki box, which is significantly lower than some previous estimates.

Particulate iron dynamics during FeCycle in subantarctic waters southeast of New Zealand

The FeCycle experiment provided an SF6 labeled mesoscale patch of high‐nitrate low‐chlorophyll (HNLC) water in austral summer 2003. These labeled waters enabled a comparison of the inventory of particulate iron (PFe) in the 45‐m‐deep surface mixed layer with the concurrent downward export flux of PFe at depths of 80 and 120 m. The partitioning of PFe between four size fractions (0.2–2, 2–5, 5–20, and &gt;20 μm) was assessed, and PFe was mainly found in the &gt;20‐μm size fraction throughout FeCycle. Estimates of the relative contribution of the biogenic and lithogenic components to PFe were based on an Al:Fe molar ratio (0.18) derived following analysis of dust/soil from the nearest source of aerosol Fe: the semi‐arid regions of Australia. The lithogenic component dominated each of the four PFe size fractions, with medians ranging from 68 to 97% of PFe during the 10‐day experiment. The Fe:C ratios for mixed‐layer particles were ∼40 μmol/mol. PFe export was ∼300 nmol m−2 d−1 at 80 m depth representing a daily loss of ∼1% from the mixed‐layer PFe inventory. There were pronounced increases in the Fe:C particulate ratios with depth, with a five‐fold increase from the surface mixed layer to 80 m depth, consistent with scavenging of the remineralized Fe by sinking particles and concurrent solubilization and loss of particulate organic carbon. Significantly, the lithogenic fraction of the sinking PFe intercepted at both 80 m and 120 m was &gt;40%; that is, there was an approximately twofold decrease in the proportion of lithogenic iron exported relative to that in the mixed‐layer lithogenic iron inventory. This indicates that the transformation of lithogenic to biogenic PFe takes place in the mixed layer, prior to particles settling to depth. Moreover, the magnitude of lithogenic Fe supply from dust deposition into the waters southeast of New Zealand is comparable to that of the export of PFe from the mixed layer, suggesting that a large proportion of the deposited dust eventually exits the surface mixed layer as biogenic PFe in this HNLC region.

Controls on ocean productivity and air‐sea carbon flux: An adjoint model sensitivity study

We use the adjoint of a global model of coupled oceanic cycles of carbon, phosphorus and iron to comprehensively and efficiently map the sensitivity of global biological productivity and air‐sea carbon fluxes to local perturbations of the atmospheric iron source sustained for a decade or more. Modeled productivity and carbon flux are found to be most sensitive to enhanced iron sources in high nitrate low chlorophyll regions. The relative response of productivity to an enhanced iron source is greatest in the Equatorial Pacific. Although surface macro‐nutrients are more abundant in the Southern Ocean, nutrient utilization here is critically regulated by light limitation. Our results differ from those of previous studies which imposed depletion of surface nutrients and ignore the availability of light. However, the enhancement of oceanic carbon storage per unit increase in productivity is strongest in the high latitude oceans.

Effect of seafloor depth on phytoplankton blooms in high‐nitrate, low‐chlorophyll (HNLC) regions

We calculated correlations between seafloor depth and phytoplankton blooms in all three main high‐nitrate, low‐chlorophyll (HNLC) regions, but with a particular focus on the subarctic North Pacific area. It has long been known that the central parts of the east and west subarctic North Pacific are HNLC regions. The deep western basin of the Bering Sea is also HNLC, whereas the wide continental shelf of the eastern Bering Sea is not. We carried out a statistical comparison of spatial maps of (1) seafloor depth and (2) chlorophyll a concentration from satellite data. This comparison reveals that shallow waters have, on average, higher peak chlorophyll a concentrations (more intense phytoplankton blooms) than deep waters (p ≪ 0.01). Possible artifacts of the satellite data are considered but it is concluded that the signal is genuine, in part because the same patterns are found in in situ data. There are several possible explanations for the observed correlation; the most probable, we suggest, is that iron diffusing out of the seafloor causes alleviation of iron limitation in shallow waters.

Domoic acid: The synergy of iron, copper, and the toxicity of diatoms

Diatom blooms generated by the alleviation of iron limitation in high nitrate‐low chlorophyll (HNLC) regions of the oceans often are composed of pennate diatoms of the genus Pseudo-nitzschia, many of which periodically produce the potent neurotoxin domoic acid. We show that toxigenic diatoms have an inducible high-affinity iron uptake capability that enables them to grow efficiently on iron complexed by strong organic ligands in seawater. This low-iron adaptive strategy requires copper and domoic acid, a copper chelator whose production increases sharply when both iron and copper are limiting. Addition of either domoic acid or copper to seawater improves the growth of Pseudo-nitzschia spp. on strongly complexed iron during deck incubation experiments with natural phytoplankton. Our findings indicate that domoic acid is a functional component of the unusual high-affinity iron acquisition system of these organisms. This system may help explain why Pseudo-nitzschia spp. are persistent seed populations in oceanic HNLC regions, as well as in some neritic regions. Our findings also indicate that in the absence of an adequate copper supply, iron-limited natural populations of Pseudo-nitzschia will become increasingly toxic.

The uptake and export of silicon and nitrogen in HNLC waters of the NE Pacific Ocean

The high-nitrate, low-chlorophyll (HNLC) waters of the Gulf of Alaska tend towards silicate rather than nitrate depletion as phytoplankton utilize nutrients during summer. This tendency is enhanced when iron supply is elevated through natural inputs such as from coastally generated mesoscale eddies or through artificial enrichment as was carried out in an in situ experiment in July 2002. However, ship-board incubations with iron enrichment demonstrate nitrate rather than silicate depletion for these waters. The difference between in situ and in vitro experiments occurs at least in part because deck incubations do not allow export of particulate Si and N. Due to the more efficient recycling of nitrogen and carbon, export favours the removal of silicon from the upper ocean (the Si pump). Previous measurements at Ocean Station Papa (50°N, 145°W) show that ∼25% of the Si, but only ∼7% of the C and ∼4% of the N utilized during spring growth, is exported to a depth of 200 m. These results in the Gulf of Alaska agree with the present understanding of phytoplankton controls in other HNLC regions and show that any estimates of carbon export from iron enrichment should be based on Si- rather than N-limitation.

The threshold feeding response of microzooplankton within Pacific high-nitrate low-chlorophyll ecosystem models under steady and variable iron input

The equatorial Pacific is an HNLC (High-Nitrate Low-Chlorophyll) region. Modeling and in-situ process studies have confirmed the importance of microzooplankton grazing in this ecosystem. Unfortunately, both the parameters and functions representing microzooplankton grazing within current ecosystem models are poorly constrained. We used a simple 4-component food web model to test the assumption that a lower grazing threshold, which is common in many models, is necessary to achieve the HNLC condition. Without the grazing threshold, the model did not reproduce the HNLC condition. However, by raising the half-saturation constant within the microzooplankton functional response with no threshold, it was possible to reproduce the critical dynamics of the HNLC condition under both steady and moderate seasonal variability in nutrient input. It was also possible to reproduce the HNLC system using a sigmoidal functional response for the microzooplankton, with results somewhere between the other two forms of the model, although this version had the highest sensitivity to changes in its parameters. The three models predicted similar phytoplankton biomass and primary productivity under steady nutrient input, but diverge in these metrics as the amplitude of nutrient input variability increases. These three functional responses also imply certain important differences in the microzooplankton community. Whereas the threshold model had the least sensitivity to parameter choice, the high half-saturation constant, no-threshold model may actually be a better approximation when modeling a community of grazers. Ecosystem models that predict carbon production and export in HNLC regions can be very sensitive to assumptions concerning microzooplankton grazing; future studies need to concentrate on the functional responses of microzooplankton before these models can be used for predicting fluxes in times or regions where forcing is beyond that used to constrain the original model.

A persistent subsurface chlorophyll maximum in the Interpolar Frontal Zone south of Australia: Seasonal progression and implications for phytoplankton‐light‐nutrient interactions

A subsurface chlorophyll maximum (SCM), composed principally of large diatoms, has been consistently observed in summer and autumn between 53° and 58°S along 140°E on Australian World Ocean Circulation Experiment and Joint Global Ocean Flux Study cruises from 1994 to 1998. This region lies in a zone of weak northward flow, between two branches of the Polar Front (hence the Interpolar Frontal Zone (IPFZ)). In the IPFZ, mixed layer nitrate concentrations are high (&gt;24 μM) year‐round, while mixed layer silicic acid is intermediate (about 15 μM) in winter but is depleted to 2 μM or less in late summer. Dissolved Fe concentrations, only available for summer, are low (&lt;0.2 nM). Mixed layer chlorophyll concentrations are generally &lt;0.3 μg L−1 and surface waters thus fit the high‐nitrate low‐chlorophyll (HNLC) definition. In spring and early summer, the SCM is relatively shallow (about 60 m), intense (up to 1.5 μg chl a, L−1), and contributes 30–50% of column production. By March the SCM is deep (100 m or greater), less intense (about 0.5 μg chl a L−1), and contributes at most 20% of column production. The existence of a SCM in a HNLC region is surprising, and we consider a number of possible explanations. The SCM may be partly explained by changes in C:chl a ratios, but phytoplankton species composition in the SCM also differs from that in the mixed layer. Sinking of diatoms, mediated by Fe and/or silicic acid availability, appears to play an important role in the formation and maintenance of the SCM.

Determination of organic complexation of cobalt in seawater by cathodic stripping voltammetry

A method was developed to determine the extent of cobalt complexation with organic ligands in seawater, and applied to samples from the North Eastern Atlantic Ocean. The cobalt speciation was determined using catalytic cathodic stripping voltammetry with ligand competition against the adsorptive ligand (nioxime). Optimised conditions include a nioxime concentration of 200 nM, 0.07 M ammonia/ammonium chloride pH buffer (pH 9.1), and 0.5 M nitrite. The stability of the mixed complex of cobalt, nioxime and ammonia was calibrated against EDTA. The cobalt speciation in the open-ocean surface waters was found to be dominated by complexation to natural organic ligands with conditional stability constants (log KCoL′) ranging between 15.6 and 16.1 and with ligand concentrations between 22 and 60 pM. The cobalt concentrations varied between 25 pM in the open-ocean waters and 103 pM in the English Channel, and were less than the ligand concentrations in many of the surface oceanic waters. The ligand concentration at depths between 30 and 115 m (average 22 pM) was lower than that of cobalt, whereas at other depths (shallower as well as deeper), the ligand concentrations were between 26 and 31 pM, sometimes greater than the cobalt concentrations. The cobalt and ligand concentrations in these waters are finely balanced, strongly binding all cobalt when the ligands are in excess. Free Co2+ concentrations calculated for the open-ocean surface waters are extremely low (<5 fM) which could cause cobalt limitation to certain phytoplankton such as coccolithophores and cyanobacteria if the organic complexes are unavailable. If this organic complexation of cobalt occurs also in other regions characterised by low cobalt concentrations, iron-limited waters in high-nitrate low-chlorophyll regions may be cobalt-limited too.

Iron cycling and nutrient-limitation patterns in surface waters of the World Ocean

Abstract A global marine ecosystem mixed-layer model is used to study iron cycling and nutrient-limitation patterns in surface waters of the world ocean. The ecosystem model has a small phytoplankton size class whose growth can be limited by N, P, Fe, and/or light, a diatom class which can also be Si-limited, and a diazotroph phytoplankton class whose growth rates can be limited by P, Fe, and/or light levels. The model also includes a parameterization of calcification by phytoplankton and is described in detail by Moore et al. (Deep-Sea Res. II, 2002). The model reproduces the observed high nitrate, low chlorophyll (HNLC) conditions in the Southern Ocean, subarctic Northeast Pacific, and equatorial Pacific, and realistic global patterns of primary production, biogenic silica production, nitrogen fixation, particulate organic carbon export, calcium carbonate export, and surface chlorophyll concentrations. Phytoplankton cellular Fe/C ratios and surface layer dissolved iron concentrations are also in general agreement with the limited field data. Primary production, community structure, and the sinking carbon flux are quite sensitive to large variations in the atmospheric iron source, particularly in the HNLC regions, supporting the Iron Hypothesis of Martin (Paleoceanography 5 (1990) 1–13). Nitrogen fixation is also strongly influenced by atmospheric iron deposition. Nitrogen limits phytoplankton growth rates over less than half of the world ocean during summer months. Export of biogenic carbon is dominated by the sinking particulate flux, but detrainment and turbulent mixing account for 30% of global carbon export. Our results, in conjunction with other recent studies, suggest the familiar paradigm that nitrate inputs to the surface layer can be equated with particulate carbon export needs to be expanded to include multiple limiting nutrients and modes of export.

What sets lower limits to phytoplankton stocks in high-nitrate, low-chlorophyll regions of the open ocean?

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 193:19-31 (2000) - doi:10.3354/meps193019 What sets lower limits to phytoplankton stocks in high-nitrate, low-chlorophyll regions of the open ocean? Suzanne L. Strom1,*, Charles B. Miller2, Bruce W. Frost3 1Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Rd, Anacortes, Washington 98221, USA 2College of Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA 3School of Oceanography, Box 357940, University of Washington, Seattle, Washington 98195, USA *E-mail: stroms@cc.wwu.edu ABSTRACT: Phytoplankton biomass in high-nitrate, low-chlorophyll (HNLC) ocean regions exhibits a pronounced stability: variation occurs only within a narrow range of values. The magnitude of this variation has profound ecological and geochemical consequences. While mechanisms believed to set the upper limits to HNLC phytoplankton biomass (iron limitation, microherbivore grazing) have received much recent attention, mechanisms setting the lower limits are largely unknown. The demonstrated importance of planktonic micrograzers, largely protists, in removing phytoplankton biomass in HNLC regions suggests that micrograzer behavioral and physiological capabilities may hold the key. This will be the case at any level of phytoplankton cell division greater than zero, regardless of the extent of growth rate limitation by resource (e.g. iron, light) availability. Indeed, HNLC plankton dynamics models almost universally include several biological responses that set lower phytoplankton biomass limits and confer temporal stability, including substantial feeding thresholds, zero micrograzer metabolic costs, and no micrograzer mortality at low food levels. Laboratory observations of these same biological responses in protist grazers are equivocal. There are no direct observations of substantive feeding thresholds, and many heterotrophic protists exhibit significant rates of respiration and mortality (cell lysis) at very low food levels. We present several candidate explanations for the discrepancy between laboratory observations and model biological 'requirements'. Firstly, laboratory-derived rate measurements may be biased by use of species and prey concentrations that are not representative of HNLC communities. Secondly, model micrograzer features may be a proxy for other stabilizing phenomena such as spatial heterogeneity ('patchiness') or carnivory (top-down control of microherbivores), though a logical analysis indicates that neither is likely to provide robust stabilization of lower phytoplankton biomass limits. Lastly, the highly plastic feeding capabilities of protist grazers, which include switching between phytoplankton and alternative prey such as bacteria, detritus, and other microherbivores, are a probable locus for stabilization of biomass limits. The extent to which such behavioral plasticity functions on the level of individuals or of species assemblages is unknown. We advocate a coupled modeling and experimental approach to further progress in understanding this key feature of HNLC ecosystems. KEY WORDS: Planktonic food webs · Grazers · Feeding behavior · Plankton dynamics models · Microzooplankton · Phytoplankton Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 193. Publication date: February 28, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Mesozooplankton grazing manipulations during in vitro iron enrichment studies in the NE subarctic Pacific

IronEx I demonstrated a rapid and marked response by grazers to Fe-induced increases in phytoplankton stocks, which was thought to be due, in part, to arrested vertical migration by mesozooplankton. These observations prompted an investigation of the relative roles of Fe enrichment and grazing pressure in controlling the magnitude of phytoplankton stocks in the NE subarctic Pacific. The grazing impact of increased mesozooplankton abundance in response to a localised Fe-induced enhancement of algal biomass was simulated by performing in vitro (6 d) grazer perturbation experiments in May 1994 and September 1995 at Ocean Station Papa (OSP), when pelagic mesozooplankton stocks are usually at their annual maximum and submaximal, respectively. Manipulations were designed to increase mesozooplankton stocks in 25L carboys after various lag-times corresponding to grazing pressure greater or equal to that in situ, and to monitor changes in chlorophyll a levels as a proxy for grazing pressure. At the onset of the experiments, in vitro mesozooplankton abundances were comparable to those in situ. Despite the addition of mesozooplankton to selected Fe-enriched carboys in May after 24, 48 and 72 h, corresponding to ca. two-fold increases in their abundances, chlorophyll a increased to ca. 2 μg l −1 in all treatments. In September, chlorophyll a levels increased five-fold to 2 μg l −1 after 4 days – but little thereafter – in the presence of up to ten-fold higher animal abundances (added at t=0) than observed in situ. Thus, Fe-induced increases in diatom growth rates were sufficiently high to escape both initial and additional grazing pressure. If and when Fe is supplied to this region, it is unlikely that mesozooplankton can respond and graze down the resulting elevated algal abundance. Theoretical calculations, based on algal growth and grazing rate data from May in this study, suggested that a greater than five-fold increase in mesozooplankton abundance, after a 48-h lag, is required to exert sufficient grazing pressure to prevent Fe-mediated increases in algal biomass. These findings are discussed in relation to the scale dependency of such events, and the pelagic ecology of other High Nitrate Low Chlorophyll regions.

Manipulating iron availability in nearshore waters

The consideration of iron effects on marine ecosystems has focused mainly on high‐nitrate low‐chlorophyll regions, but iron likely has an equally important regulatory role in coastal waters. Iron requirements of neritic phytoplankton not only are comparatively high but also differ substantially among species, so that iron fluctuations within metal‐replete systems should strongly influence the composition and distribution of phytoplankton assemblages. But unlike the simplicity of testing iron effects in iron‐depleted waters, ascertaining iron effects in apparently replete waters has been forestalled by a lack of experimental tools that can regulate iron availability independent of other bioactive metals in seawater. I present here the results of size‐fractionated shipboard culture experiments using the fungal siderophore desferriferrioxime B (DFB) to regulate iron availability in coastal upwelling waters. Addition of excess DFB essentially eliminated 59Fe uptake by both phytoplankton and heterotrophic bacteria over a 6‐h period and allowed only marginal iron uptake over 5 d of incubation. Carbon uptake by small (0.2–5.0‐ µm) phytoplankton was immediately curtailed upon addition of DFB, signifying a rapid onset of iron stress. In contrast, short‐term (0–6 h) carbon uptake by larger (&amp;5.0‐ µm) phytoplankton was not affected, indicating that larger cells contained significant iron reserves. Nonetheless, carbon assimilation was substantially lower in DFB treatments relative to the controls after 5 d of incubation. Uptake of 54Mn, 65Zn, and 57Co was not immediately affected by DFB but then slowed after 4 h and was significantly lower after 5 d, presumably because iron limitation lowered the cellular requirements for these bioactive metals. These findings demonstrate that DFB can be used to manipulate biologically accessible iron to determine how iron affects algal community structure and carbon cycling in iron‐replete waters.

Atmospheric iron supply and enhanced vertical carbon flux in the NE subarctic Pacific: Is there a connection?

Recent studies have confirmed the relationship between iron supply and phytoplankton growth rates in all three high‐nitrate low‐chlorophyll (HNLC) oceanic provinces. However, there is little evidence, so far, of the role of iron in altering the efficiency of the biological pump via increased downward export of particulate organic carbon (POC). The NE subarctic Pacific is unique among HNLC regions in that long time series pelagic observations and deep‐moored sediment trap records exist which may provide the best opportunity thus far to test aspects of the iron hypothesis. Episodic elevated levels of chlorophyll a (&gt; 2.0 μg L−1) were observed 6 times between 1964 and 1976 at the former site of Ocean Station Papa (OSP). In addition, between 1984 and 1990 on at least three occasions, concurrent pulses of POC and biogenic silica were recorded in deep‐moored traps at OSP. Possible explanations for these events, such as lateral advection of more productive waters, iron‐mediated blooms, or grazing by salp swarms are discussed and tested using an existing downward POC flux model. Owing to the episodic nature of such events, no available data are sufficiently comprehensive to unequivocally rule out any of these explanations. Nevertheless, from the data available, the occurrence of pelagic or deep water pulses, approximately once every 3 years, are most consistent with iron‐mediated diatom blooms, and of the sinking of POC and biogenic silica (from such a bloom) to depth, respectively. A comparison of the timing of these iron‐mediated pulses with that of the transport probabilities of atmospheric dust supply from Asia and Alaska provides an opportunity to assess the likelihood of a coupling between the atmosphere and the ocean.

Near-surface phytoplankton pigment from the Coastal Zone Color Scanner in the Subantarctic region southeast of New Zealand

Primarily based on satellite images, the phytoplankton concentration southeast (down- stream) of New Zealand in the High Nitrate - Low Chlorophyll (HNLC) Subantarctic water between the Subtropical Convergence (STC) and the Polar Front (PF) is believed to be higher than in the remainder of the Pacific Sector. Iron enrichment is assumed to be the reason, To study the question, near-surface phytoplankton pigment estimates from the Coastal Zone Color Scanner for up to 7 yr were reprocessed with particular attention to interference by clouds. Monthly mean images were created for the U,S. JGOFS Box along 170 deg W and means for individual dates calculated for 7 large areas between 170 deg E and 160 deg W, 45 deg and 58 deg S, well offshore of New Zealand and principally between and away from the STC and PF. The areal means are about as low as in other HNLC regions (most values between 0.1 and 0.4 or 0.5 mg/ sq m, with very few winter images; median of seasonal means, 0.26 mg/sq m) except at times near the STC, The higher means tend to occur in late summer and autumn, However, contrary to expectations, neither the PF nor the environs of the Subantarctic Front are distinguished by a zone of increased pigment. Also, of 24 spring-summer images of oceanic islands in mostly pigment-poor water, 17 yielded no recognizable elevated pigment; islands were 5 times surrounded by approximately doubled concentrations (ca 100 km in diameter), and 2 cases may have been associated with an extensive bloom. Inspection of offshore images showed concentrations of 1 greater than or equal to(up to 5) mg/sq m in rare patches of 65 to 200 km size on approximately one-tenth of the dates; such patches were not seen in Sub-antarctic waters of the eastern Pacific Sector. A case is made for Australian airborne iron supply being the cause that, presumably, would enhance large-celled phytoplankton. Since, however, the putative iron supply from the seabed around the oceanic islands or the near-by Campbell Plateau normally does not lead to phytoplankton increase, patches of neritic mesozooplankton advected from the shelves might be another mechanism that generates blooms of small-cetled phytoplankton, but there are no data. These alternatives can easily be field-tested from concentration and size composition of the phytoplankton.