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Primary productivity (PP), bacterial productivity (BP) and the uptake rates of nitrate and ammonium were measured using isotopic methods ( 13 C, 3 H, 15 N) during a mesoscale iron (Fe)-enrichment experiment conducted in the western subarctic Pacific Ocean in 2004 (SEEDS II). PP increased following Fe enrichment, reached maximal rates 12 days after the enrichment, and then declined to the initial level on day 17. During the 23-day observation period, we observed the development and decline of the Fe-induced bloom. The surface mixed layer (SML) integrated PP increased by 3-fold, but was smaller than the 5-fold increase observed in the previous Fe-enrichment experiment conducted at almost the same location and season during 2001 (SEEDS). Nitrate uptake rates were enhanced by Fe enrichment but decreased after day 5, and became lower than ammonium uptake rates after day 17. The total nitrogenous nutrient uptake rate declined after the peak of the bloom, and accumulation of ammonium was obvious in the euphotic layer. Nitrate utilization accounted for all the requirements of N for the massive bloom development during SEEDS, whereas during SEEDS II, nitrate accounted for >90% of total N utilization on day 5, declining to 40% by the end of the observation period. The SML-integrated BP increased after day 2 and peaked twice on days 8 and 21. Ammonium accumulation and the delayed heterotrophic activity suggested active regeneration occurred after the peak of the bloom. The SML-integrated PP between days 0 and 23 was 19.0 g C m −2 . The SML-integrated BP during the same period was 2.6 g C m −2 , which was 14% of the SML-integrated PP. Carbon budget calculation for the whole experimental period indicated that 33% of the whole (particulate plus dissolved) PP (21.5 g C m −2 ) was exported below the SML and 18% was transferred to the meso-zooplankton (growth). The bacterial carbon consumption (43% of the whole PP) was supported by DOC or POC release from phytoplankton, zooplankton, protozoa and viruses. More than a half (56%) of the whole PP in the Fe patch was consumed within the SML by respiration of heterotrophic organisms and returned to CO 2 .

The horizontal and vertical distributions of larvae and reproductive timing of euphausiids were investigated in the Ross Sea and its adjacent waters during austral summer 2004–2005. Occurrences of larvae of Euphausia frigida and E. triacantha were confined to the northern oceanic area where the Upper Circumpolar Deep Water prevailed, although their juvenile and adult stages extended the distribution further southward to the area where cooler Lower Circumpolar Deep Water prevailed. Larvae of Thysanoessa spp. were widely distributed within the oceanic to slope areas but did not occur on the shelf as juvenile or adult stages. Eggs and larvae of E. superba occurred with gravid females along the slope, but no juveniles occurred concurrently. Thus the slope does not appear to be a nursery ground for this species. The distribution of E. crystallorophias larvae was mostly confined to the shelf in the juvenile and adult stages. The onset of deepening was from early and later frucilia stages onward for E. triacantha and E. frigida, respectively. However, Thysanoessa spp. were concentrated increasingly within the surface layers from furcilia I onward. The onset of recruitment to calyptopis I appeared to be earlier in the more northern species with the exception of E. crystallorophias, which recruited before E. superba. The intense spawning of E. crystallorophias and E. superba coincided with a period of development of a coastal polynya. Generally, the onset of spawning of euphausiids appeared to be related to the latitudinal distribution and timing of sea-ice melting. Relationships between surface temperatures and development and recruitment of larvae of euphausiids are discussed.

The horizontal and vertical distribution and population structure of euphausiids in the Ross Sea and its adjacent waters were investigated during the summers of 2004/2005 using stratified towed samples. Nine species of euphausiids occurred in the survey area. Among them, Euphausia triacantha was dominant in biomass north of the southern boundary of the Antarctic circumpolar current (SB). Thysanoessa spp. was widely distributed north of the continental slope, while E. superba was distributed from the SB to the slope, where it showed the highest biomass. Juvenile E. superba was distributed offshore near the SB and remained at the surface, but gravid females were dominant in the slope and mainly occurred in the middle layers (400–600 m). Adult and juvenile E. crystallorophias were found at 200–300 m in the colder water of the continental shelf. In general, the peak biomass of euphausiids was found in the mid layers of the Ross Sea area. The life span and the number of spawns for major species are also discussed.

A mesoscale iron-enrichment study (SEEDS II) was carried out in the western subarctic Pacific in the summer of 2004. The iron patch was traced for 26 days, which included observations of the development and the decline of the bloom by mapping with sulfur hexafluoride. The experiment was conducted at almost the same location and the same season as SEEDS (previous iron-enrichment experiment). However, the results were very different between SEEDS and SEEDS II. A high accumulation of phytoplankton biomass (∼18 mg chl m−3) was characteristic of SEEDS. In contrast, in SEEDS II, the surface chlorophyll-a accumulation was lower, 0.8 to 2.48 mg m−3, with no prominent diatom bloom. Photosynthetic competence in terms of Fv/Fm for the total phytoplankton community in the surface waters increased after the iron enrichments and returned to the ambient level by day 20. These results suggest that the photosynthetic physiology of the phytoplankton assemblage was improved by the iron enrichments and returned to an iron-stressed condition during the declining phase of the bloom. Pico-phytoplankton (

This paper reports on the variations in nutrient concentrations and phytoplankton dynamics during the stationary and declining phases of a phytoplankton bloom induced by a mesoscale iron-enrichment conducted in the high-nutrient low-chlorophyll (HNLC) eastern subarctic Pacific. During the 26-d sampling period, the main pycnocline was located between 30 and 45 m with a shallow pycnocline developing at 10 m, 19 d after the first iron-enrichment. The iron-induced bloom dominated by diatoms peaked during days 15 and 18, a period of high chlorophyll a concentrations (ca. 5 mg m−3), and declined thereafter. Nitrogenous nutrients and phosphate were not depleted during the whole experiment. In contrast, silicic acid and iron concentrations became very low during the stationary phase of the diatom bloom (days 15–18) and Fv/Fm declined. These observations suggest that silicic acid and iron limitation probably prevented further development of the diatom bloom. The decline in chlorophyll a concentrations during days 19–24 was mostly due to the decrease in diatom abundance. On the other hand, cell abundances of pico- and nanophytoplankton exhibited little change until day 24. In the layer located between the main and the shallow pycnocline (10–30 m), ammonium and silicic acid concentration increased during days 19–26, suggesting recycling of these nutrients. The amount of silicic acid recycled during that period was estimated at 71.3–99.2 mmol m−2, while the dissolution rate of biogenic silica (BSi) was estimated to be 5.9–9.2% d−1 in the upper 50 m of the water column. These results show that the development of a shallow pycnocline during the experiment accelerated the iron and silicic acid depletion in the upper mixed layer and influenced the recycling of the organic matter assimilated during the iron-induced bloom in the ocean surface.

Primary productivity and uptake rates of nitrate and ammonium were measured during an iron enrichment-experiment in the western subarctic Pacific Ocean using stable 13 C and 15 N isotope methods with simulated, on-board, for 24-h incubations. Nitrate, phosphate and silicate before the iron enrichment in the surface mixed layer (0–10 m) were abundant at 17, 1.5 and 34 μM, respectively. Primary productivity in the upper 20 m ranged from 10 to 30 mg C m −3 d −1 on Day 0 (the day of iron fertilization) and did not change outside the iron patch during 13 days of the observation. Primary productivity in the iron patch increased more than 10-fold after Day 7 and the maximum of 310 mg C m −3 d −1 was observed on Day 9. The elevated primary productivity continued until Day 13 (end of the observation). The integrated primary productivity in the photic zone was about 2 g C m −2 d −1 after Day 7. During 13 days of the observation, 16.5 g C m −2 was fixed in the patch, while C fixation was 4.5 g C m −2 outside. Absolute nitrate uptake at 5 m depth was 0.2 μM d −1 on Day 0, almost same as the ammonium uptake rate. Nitrate uptake sharply increased 20 times after Day 7. The maximum rate of 4.2 μM d −1 was observed on Day 9. Total nitrate uptake at 5 m between Days 4 and 9 was 10.8 μM, which was comparable to the net decrease of nitrate at 8.5 μM at this depth. Uptake rate of ammonium was 0.2 μM d −1 on Day 0 and had not changed by Day 13. The f -ratio (a ratio of nitrate uptake to total nitrogen (nitrate and ammonium) uptake) was 0.4–0.6 before Day 4, but increased to 0.95 on Day 7 and remained high until Day 13. These results suggest that the nitrate assimilation pathway was induced by the iron enrichment to meet the high nitrogen demand from rapid growth of the dominant diatom Chaetoceros dedilis . Relative uptake rate among nutrients was estimated from their apparent concentration change at 5 m in the patch. Nitrate and phosphate decreased at a molar ratio of 15.5, close to Redfield’s stoichiometry. Silicate to nitrate uptake ratio, however, changed sharply on Day 9 from 1.6 to 3.0, indicating physiological stress on growing diatoms. Photosynthetically active radiation (PAR) at 5 m decreased from 400 to −2 s −1 after Day 8 due to the high biomass of phytoplankton within the patch. Dissolved iron concentration also decreased to less than 0.3 nM after Day 9. This concentration was below the half saturation level for iron ( K s ) obtained from on-board iron-addition incubations. These results suggest that low light and low iron concentration caused the physiological stress on diatoms after Day 8.

On-board bottle incubation experiments were carried out by adding different amounts of iron as part of the SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) experiment to elucidate the phytoplankton community response to various iron concentrations. A temperature gradient from 5 to 18 °C was also applied to the incubation experiment because temperature also affects growth rate and metabolic functions such as enzyme reactions. Chlorophyll-a increased with increasing iron concentration, while the increase in chlorophyll-a in the control bottles (no iron addition) was nominal. Size-fractionated chlorophyll-a indicated that micro-phytoplankton (>10 μm) became dominant in the iron-enriched bottles below 13 °C, which was consistent with the results from in situ observations in the simultaneous mesoscale iron-enrichment experiment and similar experiments for the other high nitrate and low chlorophyll (HNLC) waters. Macronutrients (silicate, nitrate and phosphate) were consumed in the bottles below 13 °C. The consumption ratio of silicate to nitrate was around 1:1 for iron-enriched bottles, but this ratio was slightly higher for control bottles, indicating a physiological stress due to low iron concentration on the diatoms. In contrast, nanophytoplankton ( The Chlorophyll-a specific increase rate for micro-plankton (>10 μm) doubled from 5 to 8 °C and was the highest between 8 and 13 °C at the same iron concentration. The surface mixed layer temperature was 8 °C at the beginning of iron fertilization, but it had been 5 °C just 2 weeks before the experiment. This dramatic increase in growth rate with temperature is hypothesized to be the reason for SEEDS 2001 biomass changes being the highest reported response to a mesoscale Fe-enrichment experiment in HNLC waters.

We have performed an in situ test of the iron limitation hypothesis in the subarctic North Pacific Ocean. A single enrichment of dissolved iron caused a large increase in phytoplankton standing stock and decreases in macronutrients and dissolved carbon dioxide. The dominant phytoplankton species shifted after the iron addition from pennate diatoms to a centric diatom, Chaetoceros debilis , that showed a very high growth rate, 2.6 doublings per day. We conclude that the bioavailability of iron regulates the magnitude of the phytoplankton biomass and the key phytoplankton species that determine the biogeochemical sensitivity to iron supply of high-nitrate, low-chlorophyll waters.

Phaeodactylum tricornutum Bohlin (Bacillariophyceae) was maintained in exponential growth under Fe-replete and stressed conditions over a range of temperatures from 5 to 30° C. The maximum growth rate (GR) was observed at 20° C (optimal temperature) for Fe-replete and -stressed cells. There was a gradual decrease in the GR decreasing temperatures below the optimum temperature; however, the growth rate dropped sharply as temperature increased above the optimum temperature. Fe-stressed cells grew at half the growth rate of Fe-replete cells at 20° C, whereas this difference became larger at lower temperatures. The change in metabolic activities showed a similar pattern to the change in growth rate temperature aside from their optimum temperature. Nitrate reductase activity (NRA) and respiratory electron transport system activity (ETS) per cell were maximal between 15 and 20° C, whereas cell-specific photosynthetic rate (Pcell) was maximal at 20° C for Fe-replete cells. These metabolic activities were influenced by Fe deficiency, which is consistent with the theoretical prediction that these activities should have an Fe dependency. The degree of influence of Fe deficiency, however, was different for the four metabolic activities studied: NRA > Pcell > ETS = GR. NRA in Fe-stressed cells was only 10% of that in Fe-replete cells at the same temperature. These results suggest that cells would have different Fe requirements for each metabolic pathway or that the priority of Fe supply to each metabolic reaction is related to Fe nutrition. In contrast, the order of influence of decreasing the temperature from the optimum temperature was ETS > Pcell > NRA > GR. For NRA, the observed temperature dependency could not be accounted for by the temperature dependency of the enzyme reaction rate itself that was almost constant with temperature, suggesting that production of the enzyme would be temperature dependent. For ETS, both the enzyme reactivity and the amount of enzyme accounted for the dependency. This is the first report to demonstrate the combined effects of Fe and temperature on three important metabolic activities (NRA, Pcell, and ETS) and to determine which activity is affected the most by a shortage of Fe. Cellular composition was also influenced by Fe deficiency, showing lower chl a content in the Fe-stressed cells. Chl a per cell volume decreased by 30% as temperature decreased from 20 to 10° C under Fe-replete conditions, but chl a decreased by 50% from Fe-replete to Fe-stressed conditions.