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Dive into the research topics where Steve E. Fitzwater is active.

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Featured researches published by Steve E. Fitzwater.


Deep Sea Research Part A. Oceanographic Research Papers | 1989

Vertex: phytoplankton/iron studies in the Gulf of Alaska

John H. Martin; R. Michael Gordon; Steve E. Fitzwater; William W. Broenkow

VERTEX studies were performed in the Gulf of Alaska in order to test the hypothesis that iron deficiency was responsible for the phytoplanktons failure to remove major plant nutrients from these waters. In view of the observed Fe distributions and the results of phytoplankton Fe enrichment experiments, it was concluded that Gulf of Alaska atmospheric Fe input rates are sufficient to support moderately high rates of primary productivity; however, not enough Fe is available to support the high growth rates that would lead to normal major nutrient depletion. Enhanced Fe input does occur along the Alaska continental margin, where normal NO3 surface depletion is observed. Coccolithophorids appear to be best able to cope with low Fe conditions; however, they cannot compete with diatoms when Fe is readily available. Iron may be more important than available N in determining global rates of phytoplankton new production. Offshore Pacific Ocean water, replete with major nutrients, appears to be infertile without supplemental iron from the atmosphere or continental margin.


Deep-sea Research Part Ii-topical Studies in Oceanography | 1993

Iron, primary production and carbon-nitrogen flux studies during the JGOFS North Atlantic bloom experiment

John H. Martin; Steve E. Fitzwater; R. Michael Gordon; Craig N. Hunter; Sara J. Tanner

Primary production was measured every other day towards the end (18–31 May) of the 1989 North Atlantic spring bloom. Rates varied with light and averaged 90.4 mmol C m−2 day−1 at the 47°N, 20°W station. Productivities measured south of Iceland (59°30′N, 20°45′W) were somewhat lower, averaging 83.6 mmol C m−2 day−1. Carbon and nitrogen fluxes were estimated using free-floating, VERTEX type particle trap arrays. To obtain mean rates representative of the North Atlantic spring bloom, flux data from three trap deployments were combined and fitted to normalized power functions: mmol C m−2 day−1 = 14.35 (z/100)−0.946, mmol N m−2 day−1 = 2.34(z/100)−1.02, with depth z in meters. Regeneration rates were: mmol C m−2 day−1 = 0.136(z/100)−1.946, mmol N m−2 day−1 = 0.0239(z/100)−2.02. The carbon export rate from the upper 35 m for the entire NABE study period (24 April to 1 June) was 39 mmol m−2 day−1. This value divided by the averaged productivity for the entire study (86 mmol N m−2 day−1) gave an F-ratio of 0.45. Concentrations of Cu, Fe, Ni, Pb and Zn were determined in water samples provided by JGOFS NABE scientists involved with primary productivity measurements. Although little contamination was observed for Cu, Ni and Pb, relatively large amounts of Zn (10 nmol kg−1) were found in some cases. In subsequent studies it was learned that this quantity of Zn can depress productivity rates by 25%. North Atlantic dissolved Fe concentrations were similar to those occurring in the Pacific (surface = 0.07; deep = 0.5–0.6 nmol kg−1). Although no evidence of Fe deficiency was found in enrichment experiments, the addition of nmol amounts of Fe did increase CO2 uptake and POC formation by factors of 1.3–1.7. In this region, most of the phytoplanktons Fe requirement is probably met via the lateral transport of Fe from distant continental margins.


Eos, Transactions American Geophysical Union | 2007

Developing standards for dissolved iron in seawater

Kenneth S. Johnson; Edward A. Boyle; Kenneth W. Bruland; Kenneth H. Coale; Christopher I. Measures; James W. Moffett; Ana M. Aguilar-Islas; Katherine A. Barbeau; Bridget A. Bergquist; Andrew R. Bowie; Kristen N. Buck; Yihua Cai; Zanna Chase; Jay T. Cullen; Takashi Doi; Virginia A. Elrod; Steve E. Fitzwater; Michael Gordon; Andrew L. King; Patrick Laan; Luis Laglera-Baquer; William M. Landing; Maeve C. Lohan; Jeffrey Mendez; Angela Milne; Hajime Obata; Lia Ossiander; Joshua N. Plant; Géraldine Sarthou; Peter N. Sedwick

In nearly a dozen open- ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters. However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well- known dissolved iron (Fe) concentrations similar to open- ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002]. The Sampling and Analysis of Fe (SAFe) cruise was staged from Honolulu, Hawaii, to San Diego, Calif., between 15 October and 8 November 2004 to collect data and samples that were later used to provide this reference material. Here we provide a brief report on the cruise results, which have produced a tenfold improvement in the variability of iron measurements, and announce the availability of the SAFe dissolved Fe in seawater standards.


Deep-sea Research Part Ii-topical Studies in Oceanography | 2000

Trace metal concentrations in the Ross Sea and their relationship with nutrients and phytoplankton growth

Steve E. Fitzwater; Kenneth S. Johnson; R.M. Gordon; Kenneth H. Coale; Walker O. Smith

Abstract Dissolved and particulate trace metal concentrations (dissolved Fe, Zn, Cd, Co, Cu and Ni; particulate Fe, Mn and Al) were measured along two transects in the Ross Sea during austral summer of 1990. Total Fe concentrations in southern Ross Sea and inshore waters were elevated >3.5 times that of northern waters. Dissolved Zn, Cd and Co concentrations were lower by factors of 4.5, 3.5 and 1.6 in southern surface waters relative to northern waters. Dissolved Cu and Ni concentrations were similar in both areas. Elevated Fe concentrations coincided with areas of increased productivity, phytoplankton biomass and nutrient drawdown, indicating that Fe is an important factor controlling the location of phytoplankton blooms in the Ross Sea. Particulate concentrations of Fe, Mn and Al indicate two possible sources of iron to the Ross Sea, resuspension of continental shelf sediments and iron incorporated in annual sea ice and released with meltwaters.


Marine Chemistry | 1995

Analyses of dissolved organic carbon in seawater: the JGOFS EqPac methods comparison

Jonathan H. Sharp; Ronald Benner; Lenore Bennett; Craig A. Carlson; Steve E. Fitzwater; Edward T. Peltzer; Luis M. Tupas

Abstract Results of a dissolved organic carbon (DOC) methods comparison are presented here in which five high temperature combustion (HTC) instruments and a wet chemical oxidation (WCO) method were used on a series of oceanic samples. The samples were collected during US JGOFS Equatorial Pacific Ocean cruises (EqPac) and most of the authors were involved with DOC analyses for the EqPac Program. Samples were collected with a “clean” protocol and were immediately quick frozen in replicate sample bottles. They were distributed by the first author to the other authors for “blind” analyses later on land on the stored samples. Comparable results (±7.5%) were found by three HTC instruments and the WCO method. There were difficulties with the other two HTC methods for which explanations and improvements are offered. The single most critical element for comparable DOC values appears to be assessment and subtraction of the total instrument blank (or reagent and handling blank for WCO methods). A “zero” carbon (very low C) water sample assisted in having all analysts achieve a uniform assessment of individual instrument or methods blanks. “Conditioning” of the catalyst bed in the combustion tube is critical to achieve consistent low instrument blanks. Failure to thoroughly condition the catalyst bed may be a significant error that can give erroneously high DOC values for oceanic samples. Reference standards available to all analysts also allowed comparison of instrument and methods performance. Contamination problems were demonstrated and it was shown that careful preparation and handling can reduce the potential for errors from contaminated samples. Results indicate that Equatorial Pacific oceanic DOC values in near surface waters are on the order of 60–70 μM C and deep water values on the order of 35–40 μM C. Since the “zero” carbon water contained a small, but measurable, amount of DOC, the sample values reported here may be slightly low. Because the lowest instrument blanks were equivalent to about 10 μM C, it is suggested that even if there were no instrument blank at all and all this “blank” were in the “zero” carbon water, the oceanic sample concentrations could not be underestimated by more than 10 μM C.


Deep-sea Research Part Ii-topical Studies in Oceanography | 1996

Iron deficiency and phytoplankton growth in the equatorial Pacific

Steve E. Fitzwater; Kenneth H. Coale; R. Michael Gordon; Kenneth S. Johnson; Michael Ondrusek

Several experiments were conducted in the equatorial Pacific at 140°W during the Joint Global Ocean Flux Study, equatorial Pacific, 1992 Time-series I (TS-I, 23 March–9 April), Time-series II (TS-II, 2–20 October) and FeLINE II cruises (10 March–14 April), to investigate the effects of added Fe on phytoplankton communities. Seven series of deckboard iron-enrichment experiments were performed, with levels of added Fe ranging from 0.13 to 1000 nM. Time-course measurements included nutrients, chlorophyll a and HPLC pigments. Results of these experiments showed that subnanomolar (sub-nM) additions of Fe increased net community specific growth rates, with resultant chlorophyll a increases and nutrient decreases. Community growth rates followed Michaelis-Menten type kinetics resulting in maximum rates of 0.99 doublings per day and a half-saturation constant of 0.12 nM iron. The dominant group responding to iron enrichment was diatoms.


Geophysical Research Letters | 2001

The annual cycle of iron and the biological response in central California coastal waters

Kenneth S. Johnson; Francisco P. Chavez; Virginia A. Elrod; Steve E. Fitzwater; J. Timothy Pennington; Kurt R. Buck; Peter M. Walz

Iron has been measured for 16 months with ∼21 day resolution at three stations in the upwelling ecosystem of central California, providing the first detailed assessment of the annual iron cycle in the coastal zone. A large pulse of iron occurs during the first spring upwelling event of the year. Iron concentrations then decay up to 100-fold over several months, although upwelling continues. Excess surface nitrate and low iron are the result during the summer and fall at the two stations furthest offshore (20 and 45 km), while nitrate is depleted and iron high nearshore (5 km). Phytoplankton biomass, primary production and community structure appear to be controlled by iron concentrations in offshore waters during this period.


Deep-sea Research Part Ii-topical Studies in Oceanography | 1996

Iron-enrichment bottle experiments in the equatorial Pacific: responses of individual phytoplankton cells

Erik R. Zettler; Robert J. Olson; Brian J. Binder; Sallie W. Chisholm; Steve E. Fitzwater; R. Michael Gordon

Abstract Iron-enrichment bottle experiments were monitored using flow cytometry to investigate the hypothesis that phytoplankton in the equatorial Pacific are iron-limited. Iron-enriched Synechococcus, ultraphytoplankton, nanophytoplankton, pennate diatoms, and coccolithophorids had higher fluorescence and/or forward light scatter per cell than control cells; for Prochlorococcus the trends were the same although the differences were not significant. This suggests that most phytoplankton cells were physiologically affected by the low iron concentrations in this region. However, only pennate diatoms showed significant increases in cell concentrations due to iron enrichment. The sum of chlorophyll fluorescences of individual cells measured by flow cytometry yielded patterns similar to those of extracted bulk chlorophyll, whth increases of up to 10-fold in iron-enriched bottles but as most 3-fold in control bottles; pennate diatoms accounted for most of the increase in chlorophyll in iron-enriched bottles.


Deep-sea Research Part Ii-topical Studies in Oceanography | 1993

Nitrate utilization in surface waters of the Iceland Basin during spring and summer of 1989

Raymond N. Sambrotto; John H. Martin; William W. Broenkow; C. Carlson; Steve E. Fitzwater

Abstract Oceanographic sampling at 59.5°N, 21°W over the spring and summer months of 1989 provided the basis to quantify the amount of new (nitrate) production and to evaluate the effect of selected environmental factors on new production. Surface water nitrate decreased linearly from 14μM in early April to ∼2.5 μM in August, and suggested that new production averaged 5.4 mmol NO3 m−2 day−1, almost double the rate suggested by a similar analysis at Ocean Weather Sta. P. Equivalent carbon export in the northern Iceland Basin would be 63 g C m−2 over this period. During a week in early July, it appeared that regenerated production compensated for a sharp decrease in new production to maintain carbon productivity at a fairly consistent level despite a decrease in F-ratio from 0.46 to 0.20. New production was predominantly associated with particles over 5 μm, although a subsurface (35m) peak in 15NO3 uptake may have been due to bacterial uptake. We suggest that light was the single-most dominant factor regulating nitrate uptake during this time. However the relationship between new production and light was non-linear in that the light efficiency of nitrate uptake varied significantly between stations in addition to variations in available light.


Journal of Geophysical Research | 1991

We still say iron deficiency limits phytoplankton growth in the subarctic Pacific

John H. Martin; Steve E. Fitzwater; R. Michael Gordon

The failure of Banse (1990) to use a reasonable initial particulate organic nitrogen (PON) value resulted in erroneously high, and physiologically impossible, estimates of phytoplankton growth rates. To correct this situation, we used an initial PON value of 1 μmol PON L−1 for both experimentals and controls; rates similar to those expected under prevailing light and temperature conditions were obtained. This reinterpretation of our data again demonstrates the dramatic effects that are observed when small quantities of iron are made available to the phytoplankton inhabiting offshore subarctic Pacific waters.

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John H. Martin

University of Rhode Island

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R. Michael Gordon

Moss Landing Marine Laboratories

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Kenneth S. Johnson

Monterey Bay Aquarium Research Institute

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Kenneth H. Coale

Moss Landing Marine Laboratories

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Sara J. Tanner

Moss Landing Marine Laboratories

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Virginia A. Elrod

Monterey Bay Aquarium Research Institute

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Craig N. Hunter

Moss Landing Marine Laboratories

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Joshua N. Plant

Monterey Bay Aquarium Research Institute

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Zanna Chase

University of Tasmania

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