High-nitrate Low-chlorophyll Waters Research Articles

Iron and Phytoplankton Growth in the Subarctic North Pacific

Iron availability has been shown to have potential controls on phytoplankton growth, nutrient utilization, algal community composition, and the ecosystem structure in the subarctic North Pacific. Recent findings on the lateral iron transport from the surrounding marginal regions to the pelagic waters highlighted the importance of particulate iron in the subarctic North Pacific, but the transformation between dissolved and particulate phases and its interaction with organic ligands are still uncertain. In spite of active researches in the subarctic high-nitrate, low-chlorophyll (HNLC) waters, significant impacts of Asian dust on the phytoplankton productivity have not been detected, suggesting spatial and temporal mismatch between the dust inputs and biological activities. Satisfaction of algal demands for both light and iron is a key for phytoplankton blooming in the HNLC waters. Surprisingly, the community half-saturation constant for growth with respect to iron was found to be similar between the western and eastern gyres; however, differences in the iron supply process and its availability in these two gyres seem to have developed unique phytoplankton populations. It is essential to evaluate iron transport processes that work on a time-scale needed for phytoplankton blooms, and further studies are needed at the central regions of the subarctic North Pacific.

Understanding the transport of Patagonian dust and its influence on marine biological activity in the South Atlantic Ocean

Abstract. The supply of bioavailable iron to the high-nitrate low-chlorophyll (HNLC) waters of the Southern Ocean through atmospheric pathways could stimulate phytoplankton blooms and have major implications for the global carbon cycle. In this study, model results and remotely-sensed data are analyzed to examine the horizontal and vertical transport pathways of Patagonian dust and quantify the effect of iron-laden mineral dust deposition on marine biological productivity in the surface waters of the South Atlantic Ocean (SAO). Model simulations for the atmospheric transport and deposition of mineral dust and bioavailable iron are carried out for two large dust outbreaks originated at the source regions of northern Patagonia during the austral summer of 2009. Model-simulated horizontal and vertical transport pathways of Patagonian dust plumes are in reasonable agreement with remotely-sensed data. Simulations indicate that the synoptic meteorological patterns of high and low pressure systems are largely accountable for dust transport trajectories over the SAO. According to model results and retrievals from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), synoptic flows caused by opposing pressure systems (a high pressure system located to the east or north-east of a low pressure system) elevate the South American dust plumes well above the marine boundary layer. Under such conditions, the bulk concentration of mineral dust can quickly be transported around the low pressure system in a clockwise manner, follow the southeasterly advection pathway, and reach the HNLC waters of the SAO and Antarctica in ~3–4 days after emission from the source regions of northern Patagonia. Two different mechanisms for dust-iron mobilization into a bioavailable form are considered in this study. A global 3-D chemical transport model (GEOS-Chem), implemented with an iron dissolution scheme, is employed to estimate the atmospheric fluxes of soluble iron, while a dust/biota assessment tool (Boyd et al., 2010) is applied to evaluate the amount of bioavailable iron formed through the slow and sustained leaching of dust in the ocean mixed layer. The effect of iron-laden mineral dust supply on surface ocean biomass is investigated by comparing predicted surface chlorophyll-a concentration ([Chl-a]) to remotely-sensed data. As the dust transport episodes examined here represent large summertime outflows of mineral dust from South American continental sources, this study suggests that (1) atmospheric fluxes of mineral dust from Patagonia are not likely to be the major source of bioavailable iron to ocean regions characterized by high primary productivity; (2) even if Patagonian dust plumes may not cause visible algae blooms, they could still influence background [Chl-a] in the South Atlantic sector of the Southern Ocean.

An evaluation of ocean color model estimates of marine primary productivity in coastal and pelagic regions across the globe

Abstract. Nearly half of the earth's photosynthetically fixed carbon derives from the oceans. To determine global and region specific rates, we rely on models that estimate marine net primary productivity (NPP) thus it is essential that these models are evaluated to determine their accuracy. Here we assessed the skill of 21 ocean color models by comparing their estimates of depth-integrated NPP to 1156 in situ 14C measurements encompassing ten marine regions including the Sargasso Sea, pelagic North Atlantic, coastal Northeast Atlantic, Black Sea, Mediterranean Sea, Arabian Sea, subtropical North Pacific, Ross Sea, West Antarctic Peninsula, and the Antarctic Polar Frontal Zone. Average model skill, as determined by root-mean square difference calculations, was lowest in the Black and Mediterranean Seas, highest in the pelagic North Atlantic and the Antarctic Polar Frontal Zone, and intermediate in the other six regions. The maximum fraction of model skill that may be attributable to uncertainties in both the input variables and in situ NPP measurements was nearly 72%. On average, the simplest depth/wavelength integrated models performed no worse than the more complex depth/wavelength resolved models. Ocean color models were not highly challenged in extreme conditions of surface chlorophyll-a and sea surface temperature, nor in high-nitrate low-chlorophyll waters. Water column depth was the primary influence on ocean color model performance such that average skill was significantly higher at depths greater than 250 m, suggesting that ocean color models are more challenged in Case-2 waters (coastal) than in Case-1 (pelagic) waters. Given that in situ chlorophyll-a data was used as input data, algorithm improvement is required to eliminate the poor performance of ocean color NPP models in Case-2 waters that are close to coastlines. Finally, ocean color chlorophyll-a algorithms are challenged by optically complex Case-2 waters, thus using satellite-derived chlorophyll-a to estimate NPP in coastal areas would likely further reduce the skill of ocean color models.

Grazing suppression of dimethylsulphoniopropionate (DMSP) accumulation in iron-fertilised, sub-Antarctic waters

The impact of in situ iron fertilisation on the production of particulate dimethylsulphoniopropionate (DMSPp) and its breakdown product dimethyl sulphide (DMS) was monitored during the SOLAS air–sea gas exchange experiment (SAGE). The experiment was conducted in the high nitrate, low chlorophyll (HNLC) waters of the sub-Antarctic Southern Ocean (46.7°S 172.5°E) to the south-east of New Zealand, during March–April, 2004. In addition to monitoring net changes in the standing stocks of DMSPp and DMS, a series of dilution experiments were used to determine the DMSPp production and consumption rates in relation to increased iron availability. In contrast to previous experiments in the Southern Ocean, DMS concentrations decreased over the course of the 15-d iron-fertilisation experiment, from an integrated volume-specific concentration in the mixed layer on day 0 of 0.78 nM (measured values 0.65–0.91 nM) to 0.46 nM (measured values 0.42–0.47 nM) by day 15, in parallel with the surrounding waters. DMSPp, chlorophyll a and the abundance of photosynthetic picoeukaryotes exhibited indiscernible or only moderate increases in response to the raised iron availability, despite an obvious physiological response by the phytoplankton. High specific growth rates of DMSPp, equivalent to 0.8–1.2 doublings d −1, occurred at the simulated 60% light level of the dilution experiments. Despite the high production rates, DMSPp accumulation was suppressed in part by microzooplankton grazers who consumed between 61% d −1 and 126% d −1 of the DMSPp production. Temporal trends in the rates of production and consumption illustrated a close coupling between the DMSP-producing phytoplankton and their microzooplankton grazers. Similar grazing and production rates were observed for the eukaryotic picophytoplankton that dominated the phytoplankton biomass, partial evidence that picoeukaryotes contributed a substantial proportion of the DMSP synthesis. These rates for DMSPp and picoeukaryotes were considerably higher than for chlorophyll a, indicating higher cycling rates of the DMSP-producing taxa than for the bulk phytoplankton community. When compared to the total phytoplankton community, there was no evidence of selection against the DMSP-containing phytoplankton by the microzooplankton grazers; the opposite appeared to be the case. SAGE demonstrated that increased iron availability in the HNLC waters of the Southern Ocean does not invariably lead to enhanced DMS sea–air flux. The potential suppression of DMSPp accumulation by grazers needs to be taken into account in future attempts to elevate DMS emission through in situ iron fertilisation and in understanding the hypothesised link between levels of Aeolian iron deposition in the Southern Ocean, DMS emission and global albedo.

Persistence of iron(II) in surface waters of the western subarctic Pacific

The distribution of dissolved iron(II) [Fe(II)] was studied in surface waters of the western subarctic Pacific during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study‐II (SEEDS II) iron enrichment experiment using highly sensitive flow injection‐based luminol chemiluminescence. Vertical profiles of Fe(II) and total dissolved iron were measured outside of the fertilized patch to investigate the chemical speciation of iron in this high‐nitrate low‐chlorophyll (HNLC) region. Ambient total dissolved iron concentrations ranged from 50 pmol L−1 to 150 pmol L−1 depending on depth and sampling times. Unexpectedly, Fe(II) accounted for up to half of total dissolved iron, with concentrations up to ~50 pmol L−1. Fe(II) concentrations decreased exponentially with depth and were undetectable at depths below 50 m. There was no evidence of increased Fe(II) concentrations associated with the subsurface chlorophyll maximum, indicating that photolysis, rather than biological reduction of Fe(III), was the primary source of Fe(II). Because Fe(II) concentrations in the fertilized patch remained elevated for more than a week after enrichment, Fe(II) oxidation rates at near‐ambient concentrations were measured. Indeed, the temperature‐dependent Fe(II) oxidation rates were significantly slower than predicted by Fe(II) oxidation models and rates measured in ligand‐free seawater. These findings suggest that Fe(II) binding ligands may exist in these HNLC waters, with conditional stability constants on the order of 108‐ 109 with respect to Fe2+. The accumulation of Fe(II) during daylight hours did not alleviate iron limitation of eukaryotic phytoplankton in these waters, contrary to expectations from recent iron uptake models.

Controls on tropical Pacific Ocean productivity revealed through nutrient stress diagnostics

In situ enrichment experiments have shown that the growth of bloom-forming diatoms in the major high-nitrate low-chlorophyll (HNLC) regions of the world's oceans is limited by the availability of iron. Yet even the largest of these manipulative experiments represents only a small fraction of an ocean basin, and the responses observed are strongly influenced by the proliferation of rare species rather than the growth of naturally dominant populations. Here we link unique fluorescence attributes of phytoplankton to specific physiological responses to nutrient stress, and use these relationships to evaluate the factors that constrain phytoplankton growth in the tropical Pacific Ocean on an unprecedented spatial scale. On the basis of fluorescence measurements taken over 12 years, we delineate three major ecophysiological regimes in this region. We find that iron has a key function in regulating phytoplankton growth in both HNLC and oligotrophic waters near the Equator and further south, whereas nitrogen and zooplankton grazing are the primary factors that regulate biomass production in the north. Application of our findings to the interpretation of satellite chlorophyll fields shows that productivity in the tropical Pacific basin may be 1.2-2.5 Pg C yr(-1) lower than previous estimates have suggested, a difference that is comparable to the global change in ocean production that accompanied the largest El Niño to La Niña transition on record.

FeCycle: Attempting an iron biogeochemical budget from a mesoscale SF6 tracer experiment in unperturbed low iron waters

An improved knowledge of iron biogeochemistry is needed to better understand key controls on the functioning of high‐nitrate low‐chlorophyll (HNLC) oceanic regions. Iron budgets for HNLC waters have been constructed using data from disparate sources ranging from laboratory algal cultures to ocean physics. In summer 2003 we conducted FeCycle, a 10‐day mesoscale tracer release in HNLC waters SE of New Zealand, and measured concurrently all sources (with the exception of aerosol deposition) to, sinks of iron from, and rates of iron recycling within, the surface mixed layer. A pelagic iron budget (timescale of days) indicated that oceanic supply terms (lateral advection and vertical diffusion) were relatively small compared to the main sink (downward particulate export). Remote sensing and terrestrial monitoring reveal 13 dust or wildfire events in Australia, prior to and during FeCycle, one of which may have deposited iron at the study location. However, iron deposition rates cannot be derived from such observations, illustrating the difficulties in closing iron budgets without quantification of episodic atmospheric supply. Despite the threefold uncertainties reported for rates of aerosol deposition (Duce et al., 1991), published atmospheric iron supply for the New Zealand region is ∼50‐fold (i.e., 7‐ to 150‐fold) greater than the oceanic iron supply measured in our budget, and thus was comparable (i.e., a third to threefold) to our estimates of downward export of particulate iron. During FeCycle, the fluxes due to short term (hours) biological iron uptake and regeneration were indicative of rapid recycling and were tenfold greater than for new iron (i.e. estimated atmospheric and measured oceanic supply), giving an “fe” ratio (uptake of new iron/uptake of new + regenerated iron) of 0.17 (i.e., a range of 0.06 to 0.51 due to uncertainties on aerosol iron supply), and an “Fe” ratio (biogenic Fe export/uptake of new + regenerated iron) of 0.09 (i.e., 0.03 to 0.24).

Biogeochemical evolution of the carbon dioxide system in the waters of long-lived mesoscale eddies in the Northeast Pacific Ocean

Abstract An anticyclonic eddy, named Haida-2000, formed off the southern Queen Charlotte Islands early in the year 2000. The eddy subsequently tracked northwest, transporting fresh, warm, and nutrient-rich water into the Northeast Pacific Ocean. Based on total dissolved inorganic carbon (C T ), total alkalinity (A T ), and nutrients from six seasonal cruises between February 2000 and September 2001, we found that the oceanic carbonate system of the surface waters in the eddy changed significantly. The greatest loss of C T from the surface waters of Haida-2000 occurred during the first year, between February and June 2000. Based on a concomitant loss in nitrate (NO 3 ), the large C T loss we observed was likely due to biological production of organic carbon and probably resulted in a large drawdown of atmospheric CO 2 , as supported by the observed fugacity of carbon dioxide ( f CO 2 ) in the surface waters. During fall, C T , f CO 2 , and NO 3 values increased, probably due to oxidation of organic matter and vertical entrainment of C T and nutrient-rich water from below with fall mixing. As a result, the surface waters of the eddy were highly oversaturated in CO 2 relative to the atmosphere. Another Haida eddy (Haida-2001) had a significantly smaller nitrate drawdown than Haida-2000 during its first spring, but the C T losses were similar in the two eddies. In early summer (mid-June) of the second year, the surface and sub-surface waters in Haida-2000 gained C T , A T , and NO 3 , partly caused by a merging with another 2001 Haida eddy, showing that exchange with surrounding waters took place and was significant at times. The high nitrate/low chlorophyll (HNLC) surface waters at Ocean Station Papa (OSP), showed less f CO 2 undersaturation during summer and less total nitrate and C T loss than Haida-2000, indicating that both eddies were larger sinks for carbon than HNLC waters. The waters surrounding the eddies generally had concentrations of C T , A T , and NO 3 that were similar to or slightly higher than the waters of both Haida eddies throughout the study. Also, the sub-surface waters in these eddies showed no significant change in total carbon content, suggesting that at these depths eddies are recycling, not exporting systems.

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.

Episodic enhancement of phytoplankton stocks in New Zealand subantarctic waters: Contribution of atmospheric and oceanic iron supply

Around 30% of oceanic waters are high nitrate low chlorophyll (HNLC) where low iron levels limit algal growth. HNLC waters have mainly been studied using shipboard and lab experiments. Since 1997, remote‐sensing of phytoplankton via SeaWiFS Ocean Color has permitted monitoring of the constancy of this “HNLC condition,” i.e., spatial homogeneity and low temporal variability of chlorophyll over annual cycles. These trends can be exploited, as episodic iron inputs should be conspicuous by subsequent expression as iron‐elevated algal stocks. Subantarctic (SA) waters near New Zealand are HNLC, and the proximity of the arid Australian landmass, and the iron‐rich Subtropical Front, provide natural laboratories to detect episodic atmospheric and oceanic iron supply, respectively. Two approaches were used: Oceanic supply was inferred from episodic increases in chlorophyll concentrations in SA waters, detected using Ocean Color archives. Additional archives were used to confirm the oceanic provenance of iron supply, and identify supply mechanism(s). Atmospheric supply was assessed using data on source areas and loads for dust storms monitored in central Australia. Dust transport and its fate was assessed using air mass forward trajectories and SeaWiFS Ocean Color and Aerosol Optical Depth maps. During 1997–2001, episodic elevated chlorophyll events occurred in SA waters southeast of New Zealand. There was no evidence of these events being mediated by atmospheric iron supply; however, neither wind‐driven lateral advection or vertical mixing alone could account for these episodes. Dust storms, over this period sent plumes either into high iron SubTropical (ST) waters or into SA waters in early spring, when cells are probably light‐ rather than iron‐limited.

The role of iron in the biogeochemistry of the Southern Ocean and equatorial Pacific: a comparison of in situ iron enrichments

Abstract A better understanding of the relationship between iron supply and the biogeochemical functioning of high nitrate low chlorophyll (HNLC) regions may be obtained by comparing and contrasting observations from oceanic provinces. The open polar Southern Ocean and the eastern equatorial Pacific are HNLC regimes, but have different oceanographic properties. Until recently, there have been insufficient datasets on the role of iron (mainly from deckboard iron enrichments) to enable a comprehensive comparison of these regions. However, the recent Southern Ocean Iron RElease Experiment (SOIREE), the first in situ iron enrichment in polar waters, provides a detailed suite of time-series measurements to compare with those from the equatorial Pacific IronEx II study. As expected, a comparison of these polar and tropical studies yielded differences in the timing of iron-mediated responses that are mainly due to the temperature-dependence of biological rates. However, trends from both studies are similar with respect to the magnitude of iron-mediated changes in bulk signals (such as macronutrient uptake), algal physiological responses, and shifts in algal community structure. There are also parallels between these studies in the response of components of the pelagic ecosystem such as heterotrophic bacteria. Such convergence suggests that it is possible to incorporate considerable detail into future generic models investigating the role of the biota in the biogeochemical cycling of iron. There are also significant differences, such as the degree of herbivory, and the fate of the accumulated algal carbon during these two iron-stimulated phytoplankton blooms. Such departures offer a means to understand better important regional differences in the biogeochemical cycling of iron in HNLC waters, and to investigate the possible effects of physical artefacts—caused by mixing with surrounding HNLC waters at the boundaries of these labelled patches—during such mesoscale perturbation experiments.

Iron uptake and physiological response of phytoplankton during a mesoscale Southern Ocean iron enrichment

Iron supply is thought to regulate primary production in high nitrate, low chlorophyll (HNLC) regions of the sea in both the past and the present. A critical aspect of this relationship is acquisition of iron (Fe) by phytoplankton, which occurs through a complex series of extracellular reactions that are influenced by Fe chemistry and speciation. During the first in situ mesoscale Fe‐enrichment experiment in the Southern Ocean (Southern Ocean iron release experiment [SOIREE]), we monitored the uptake of Fe by three size classes of plankton and their ensuing physiological response to the Fe enrichment. Rates of Fe uptake from both inorganic Fe (Fe′) and organic Fe complexes (FeL) were initially fast, indicative of Fe‐limitation. After Fe enrichment phytoplankton down‐regulated Fe uptake and optimized physiological performance, but by day 12 they had greatly increased their capacity to acquire Fe from FeL. The increase in Fe uptake from FeL coincided with a sixfold decrease in Fe′ that followed the production of Fe‐binding organic ligands. Phytoplankton were able to use organically bound Fe at rates sufficient to maintain net growth for more than 42 d. Adaptation to such shifts in Fe chemistry may contribute to bloom longevity in these polar HNLC waters.

Iron-mediated changes in phytoplankton photosynthetic competence during SOIREE

Abstract Active fluorescence (fast repetition rate fluorometry, FRRF) was used to follow the photosynthetic response of the phytoplankton community during the 13-day Southern Ocean Iron RElease Experiment (SOIREE). This in situ iron enrichment was conducted in the polar waters of the Australasian–Pacific sector of the Southern Ocean in February 1999. Iron fertilisation of these high nitrate low chlorophyll (HNLC) waters resulted in an increase in the photosynthetic competence (Fv/Fm) of the resident cells from around 0.20 to greater than 0.60 (i.e. close to the theoretical maximum) by 10/11 days after the first enrichment. Although a significant iron-mediated response in Fv/Fm was detected as early as 24 h after the initial fertilisation, the increase in Fv/Fm to double ambient levels took 6 days. This response was five-fold slower than observed in iron enrichments (in situ and in vitro) in the HNLC waters of the subarctic and equatorial Pacific. Although little is known about the relationship between water temperature and Fv/Fm, it is likely that low water temperatures — and possibly the deep mixed layer — were responsible for this slow response time. During SOIREE, the photosynthetic competence of the resident phytoplankton in iron-enriched waters increased at dissolved iron levels above 0.2 nM, suggesting that iron limitation was alleviated at this concentration. Increases in Fv/Fm of cells within four algal size classes suggested that all taxa displayed a photosynthetic response to iron enrichment. Other physiological proxies of algal iron stress (such as flavodoxin levels in diatoms) exhibited different temporal trends to iron-enrichment than Fv/Fm during the time-course of SOIREE. The relationship between Fv/Fm, algal growth rate and such proxies in Southern Ocean waters is discussed.

Modeling the bloom evolution and carbon flows during SOIREE: Implications for future in situ iron-enrichments in the Southern Ocean

The impact of a mesoscale in situ iron-enrichment experiment (SOIREE) on the planktonic ecosystem and biological pump in the Australasian-Pacic sector of the Southern Ocean was investigated through model simulations over a period of 60-d following an initial iron infusion. For this purpose we used a revised version of the biogeochemical SWAMCO model (Lancelot et al., 2000), which describes the cycling of C, N, P, Si, Fe through aggregated chemical and biological components of the planktonic ecosystem in the high nitrate low chlorophyll (HNLC) waters of the Southern Ocean. Model runs were conducted for both the iron-fertilized waters and the surrounding HNLC waters, using in situ meteorological forcing. Validation was performed by comparing model predictions with observations recorded during the 13-d site occupation of SOIREE. Considerable agreement was found for the magnitude and temporal trends in most chemical and biological variables (the microbial food web excepted). Comparison of simulations run for 13- and 60-d showed that the e!ects of iron fertilization on the biota were incomplete over the 13-d monitoring of the SOIREE bloom. The model results indicate that after the vessel departed the SOIREE site there were further iron-mediated increases in properties such as phytoplankton biomass, production, export production, and uptake of atmospheric CO , which peaked 20}30 days after the initial iron infusion. Based on model simulations, the increase in net carbon production at the scale of the fertilized patch (assuming an area of 150 km) was estimated to 9725 t C by day 60. Much of this production accumulated in the upper ocean, so that the predicted downward export of particulate organic carbon (POC) only represented 22% of the accumulated C in the upper ocean. Further model runs that implemented improved parameterization of diatom sedimentation (i.e. including iron-mediated diatom sinking rate, diatom chain-forming and aggregation) suggested that the downward POC #ux predicted by the standard run might have been underestimated by a factor

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.

Iron and manganese in surface waters of the Australian subantarctic region

Iron and manganese concentrations were measured in seawater from the upper water column (≤350 m water depth) in the subantarctic region to the south and west of Tasmania, Australia, during January 1995. Dissolved and total-dissolvable Fe and dissolved Mn were determined in samples from four stations along 140°E ranging from polar to subtropical waters, and from one station over the continental slope off north-west Tasmania. Dissolved Fe concentration were generally low (<1 nM) for all stations, ranging from minimum concentrations between 0.1 and 0.2 nM at the southernmost stations to maximum values near 0.8 nM in subtropical waters and over the continental slope. Total-dissolvable Fe concentrations ranged from 0.17 to 1.3 nM, and were more than twice the dissolved Fe concentrations in some samples, implying a significant proportion of particulate Fe in the water column. Dissolved Mn concentrations were generally low (<0.5 nM) along the 140°E transect, with lowest values of around 0.1 nM measured at the southernmost stations, and much higher concentrations (> 2 nM) observed in shallower waters over the continental slope. Vertical concentration profiles suggest biological removal of dissolved Fe from the photic zone at the deep-ocean stations, except in the subantarctic front. The higher mixed-layer concentrations of Fe and Mn observed over the continental slope and in the deep ocean at 40°S are thought to reflect inputs from the continental shelves or atmospheric deposition. Water-column Fe and Mn profiles from 45°S, 50°S and 53°S show no evidence for significant inputs of these metals via atmospheric deposition or upwelling of metal-rich deep waters, although there was evidence of shelf-derived particulate Fe at 45°S. The low concentrations of dissolved Fe measured at 45°S, 50°S and 53°S indicate that iron deficiency could limit summertime primary production in these high-nitrate lowchlorophyll waters.