Luis M. Tupas

The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific Ocean

Seven years of time-series observations of biogeochemical processes in the subtropical North Pacific Ocean gyre have revealed dramatic changes in the microbial community structure and in the mechanisms of nutrient cycling in response to large-scale ocean–atmosphere interactions. Several independent lines of evidence show that the fixation of atmospheric nitrogen by cyanobacteria can fuel up to half of the new production. These and other observations demand a reassessment of present views of nutrient and carbon cycling in one of the Earth′s largest biomes.

Seasonal and interannual variability in primary production and particle flux at Station ALOHA

A 5-year time-series study of primary production and euphotic-zone particle export in the subtropical North Pacific Ocean near Hawaii (Sta. ALOHA, 22°45′N, 158°W) with measurements collected at approximately monthly intervals has revealed significant variability in both ecosystem processes. Depth-integrated (0–200 m) primary production averaged 463 mg C m−2 day−1 (s = 156, n = 54) or 14.1 mol C m−2 year−1. This mean value is greater than estimates for the North Pacific Ocean gyre made prior to 1984, but conforms to data obtained since the advent of trace metal-clean techniques. Daily rates of primary productivity at Sta. ALOHA exhibited interannual variability including a nearly 3-year sustained increase during the period 1990–1992 that coincided with a prolonged El Nifio-Southern Oscillation (ENSO) event. Export production, defined as the particulate carbon (PC) flux measured at the 150 m reference depth, also varied considerably during the initial 5 years of the ongoing field experiment. The PC flux averaged 29 mg C m−2 day−1 (s = 11, n = 43) or 0.88 mol Cm−2 year−1. A 5-fold variation between the minimum and maximum fluxes, measured in any given year, was observed. During the first 3 years of this program (1989–1991), a pattern was resolved that included two major export events per annum one centered in late winter and the other in late summer. After 1991, export production exhibited a systematic decrease with time during the prolonged ENSO event. When expressed as a percentage of the contemporaneous primary production, PC export ranged from 2 to 16.9%, with a 5-year mean of 6.7% (s = 3.3, n = 40). Contrary to existing empirical models, contemporaneous primary production and PC flux were poorly correlated, and during the ENSO period they exhibited a significant inverse correlation. This unexpected decoupling of particle production and flux has numerous implications for oceanic biogeochemical cycles and for the response of the ocean to environmental perturbations.

Experimental determination of the organic carbon flux from open-ocean surface waters

The flux of biologically produced organic carbon from the euphotic zone of the ocean to the deep waters below—the ‘biological organic carbon pump’—is one of the main controls on the carbon dioxide partial pressure in the atmosphere. Accurate determination of this flux is therefore critically important for understanding the global carbon cycle and its response to climate change. Our goal is to assess how accurately the biological organic carbon pump can be determined at a single location and to constrain estimates of the global value. As there are no standards against which such environmental fluxes can be measured, we assess accuracy by comparing results from three independent experimental approaches for measuring the net annual export of organic carbon from the euphotic zone in the subtropical North Pacific Ocean near Hawaii. Mass balances of dissolved oxygen, inorganic carbon and organic carbon yield estimates of the organic carbon export flux of 2.7 ± 1.7, 1.6 ± 0.9 and 2.0 ± 0.9 mol C m−2 yr−1, respectively. These three estimates are not significantly different, and establish the present analytically attainable accuracy at this location to be about ±50%. If 2.0 mol C m−2 yr−1 is typical of the organic carbon export flux in the subtropical ocean, then this vast region, often considered to be a biological desert, may be responsible for up to half of the global-ocean biological organic carbon pump.

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

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.

Freezing as a method of sample preservation for the analysis of dissolved inorganic nutrients in seawater

It is often desirable or necessary to store collected seawater samples prior to analysis for dissolved inorganic nutrients. It is therefore important to establish preservation and storage techniques that will ensure sample integrity and will not alter the precision or accuracy of analysis. We have performed a series of experiments on the storage of nutrient samples collected at the oligotrophic North Pacific benchmark Station ALOHA, using both standard autoanalyses and low-level techniques. Our results reveal that for oligotrophic oceanic waters, the immediate freezing of an unfiltered water sample in a clean polyethylene bottle is a suitable preservation method. This procedure is simple, it avoids potentially contaminating sample manipulations and chemical additions, and it adequately preserves the concentrations of nitrate + nitrite, soluble reactive phosphate, and soluble reactive silicate within a single water sample.

Dissolved organic carbon in oligotrophic waters : experiments on sample preservation, storage and analysis

Abstract Different methods of preservation and storage of samples for analysis of abundance of dissolved organic carbon (DOC) in oligotrophic waters were evaluated and compared to shipboard measurements. DOC concentrations in samples stored frozen (−20°C) in acid-cleaned polypropylene tubes, high-density polyethylene bottles, and combusted glass ampoules, even for extended periods (up to 5 months after collection), were indistinguishable from those measured on ship at the time of collection. Addition of phosphoric acid (0.025% H 3 PO 4 final concentration in seawater) was necessary to preserve samples at 4°C. Filtration prior to storage was not necessary for the oligotrophic ocean samples analyzed in this study. Removal of dissolved inorganic carbon can be accomplished by bubbling with either high-purity nitrogen or oxygen with no effects on DOC abundance measurements. An estimate of the analytical blank was determined by injecting distilled water which was exposed to high-intensity ultraviolet light, acidified and purged with nitrogen to remove inorganic carbon. The analytical blank measured in this study was 18.7 ± 1.5 μ m C for a 100 μl injection volume. This value was applied as the minimum correction to DOC abundance measurements of seawater. Using the methods described in this paper we observed DOC concentrations of approximately 90–115 μM C for the upper 50 m of the water column at the US-JGOFS Station Aloha (22°45′N, 158°W). DOC concentrations decreased with depth to concentrations of approximately 50 μM C at 500 m and remained relatively constant at greater depths.

Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters

Field studies to examine the in situ assimilation and production of ammonium (NH4+) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing 15N-NH4+ as a tracer showed the bacterial population taking up 0.041–0.128 μg-atoms Nl−1d−1, which was 8–25% of total NH4+ uptake rates. The large bacterial uptake of NH4+ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using 3H-leucine and -adenine were l.0μgCl−1 d−1 before the bloom and 16.2 μg Cl−1 d−1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH4+ was found to supply 35–60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20–50% of the total amino acids taken up were mineralized to NH4+. Bacterial production of NH4+ was occurring simultaneously to its uptake and contributed 27–55% of total regenerated NH4+ in surface waters. Using a variety of 15N-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.

Total dissolved nitrogen in the Northern North Pacific assessed by a high-temperature combustion method

Dissolved organic nitrogen (DON) concentrations in the Northern North Pacific were examined by a high-temperature combustion method using a commercially available instrument (the Yanaco TN-7). Recovery of standard compounds and purified humic acids from freshwater and coastal sediments was greater than 90% except for one humic acid sample. Vertical profiles of DON concentration in the eastern portion of the Northern North Pacific showed no large variations, although the concentration in the upper 100 m (10–12 μg atoms NI−1) was significantly higher than in the middle and deep layers (6–8 μg atoms NI−1). DON analysis of samples from a similar region using a Sumigraph N-200 gave rather low concentrations compared with those obtained using the Yanaco TN-7, i.e. approximately 4–5 μg atoms NI−1 in the upper 0–100 m and 2.0–2.6 μg atoms NI−1 in middle and deep layers.