Ricardo M. Letelier

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.

Dinitrogen fixation in the world's oceans

The surface water of themarine environment has traditionally beenviewed as a nitrogen (N) limited habitat, andthis has guided the development of conceptualbiogeochemical models focusing largely on thereservoir of nitrate as the critical source ofN to sustain primary productivity. However,selected groups of Bacteria, includingcyanobacteria, and Archaea canutilize dinitrogen (N2) as an alternativeN source. In the marine environment, thesemicroorganisms can have profound effects on netcommunity production processes and can impactthe coupling of C-N-P cycles as well as the netoceanic sequestration of atmospheric carbondioxide. As one component of an integrated ‘Nitrogen Transport and Transformations’ project, we have begun to re-assess ourunderstanding of (1) the biotic sources andrates of N2 fixation in the worldsoceans, (2) the major controls on rates ofoceanic N2 fixation, (3) the significanceof this N2 fixation for the global carboncycle and (4) the role of human activities inthe alteration of oceanic N2 fixation. Preliminary results indicate that rates ofN2 fixation, especially in subtropical andtropical open ocean habitats, have a major rolein the global marine N budget. Iron (Fe)bioavailability appears to be an importantcontrol and is, therefore, critical inextrapolation to global rates of N2fixation. Anthropogenic perturbations mayalter N2 fixation in coastal environmentsthrough habitat destruction and eutrophication,and open ocean N2 fixation may be enhancedby warming and increased stratification of theupper water column. Global anthropogenic andclimatic changes may also affect N2fixation rates, for example by altering dustinputs (i.e. Fe) or by expansion ofsubtropical boundaries. Some recent estimatesof global ocean N2 fixation are in therange of 100–200 Tg N (1–2 × 1014 g N)yr−1, but have large uncertainties. Theseestimates are nearly an order of magnitudegreater than historical, pre-1980 estimates,but approach modern estimates of oceanicdenitrification.

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.

Long-term changes in plankton community structure and productivity in the North Pacific Subtropical Gyre : The domain shift hypothesis

Oceanic productivity, fishery yields and the net marine sequestration of atmospheric greenhouse gases are all controlled by the structure and function of planktonic communities. Detailed paleoceanographic studies have documented abrupt changes in these processes over timescales ranging from centuries to millennia. Most of these major shifts in oceanic productivity and biodiversity are attributable to changes in Earths climate, manifested through large-scale ocean–atmosphere interactions. By comparison, contemporary biodiversity and plankton community dynamics are generally considered to be “static”, in part due to the lack of a suitable time frame of reference, and the absence of oceanic data to document ecosystem change over relatively short timescales (decades to centuries). Here we show that the average concentrations of chlorophyll a (chl a) and the estimated rates of primary production in the surface waters of the North Pacific Subtropical Gyre (NPSG) off Hawaii have more than doubled while the concentrations of dissolved silicate and phosphate have decreased during the past three decades. These changes are accompanied by an increase in the concentration of chl b, suggesting a shift in phytoplankton community structure. We hypothesize that these observed ecosystem trends and other related biogeochemical processes in the upper portion of the NPSG are manifestations of plankton community succession in response to climate variations. The hypothesized photosynthetic population “domain shift” toward an ecosystem dominated by prokaryotes has altered nutrient flux pathways and affected food web structure, new and export production processes, and fishery yields. Further stratification of the surface ocean resulting from global warming could lead to even more enhanced selection pressures and additional changes in biogeochemical dynamics.

Trichodesmium Blooms and New Nitrogen in the North Pacific Gyre

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Microbial oceanography of anoxic oxygen minimum zones

Vast expanses of oxygen-deficient and nitrite-rich water define the major oxygen minimum zones (OMZs) of the global ocean. They support diverse microbial communities that influence the nitrogen economy of the oceans, contributing to major losses of fixed nitrogen as dinitrogen (N2) and nitrous oxide (N2O) gases. Anaerobic microbial processes, including the two pathways of N2 production, denitrification and anaerobic ammonium oxidation, are oxygen-sensitive, with some occurring only under strictly anoxic conditions. The detection limit of the usual method (Winkler titrations) for measuring dissolved oxygen in seawater, however, is much too high to distinguish low oxygen conditions from true anoxia. However, new analytical technologies are revealing vanishingly low oxygen concentrations in nitrite-rich OMZs, indicating that these OMZs are essentially anoxic marine zones (AMZs). Autonomous monitoring platforms also reveal previously unrecognized episodic intrusions of oxygen into the AMZ core, which could periodically support aerobic metabolisms in a typically anoxic environment. Although nitrogen cycling is considered to dominate the microbial ecology and biogeochemistry of AMZs, recent environmental genomics and geochemical studies show the presence of other relevant processes, particularly those associated with the sulfur and carbon cycles. AMZs correspond to an intermediate state between two “end points” represented by fully oxic systems and fully sulfidic systems. Modern and ancient AMZs and sulfidic basins are chemically and functionally related. Global change is affecting the magnitude of biogeochemical fluxes and ocean chemical inventories, leading to shifts in AMZ chemistry and biology that are likely to continue well into the future.

An analysis of chlorophyll fluorescence algorithms for the moderate resolution imaging spectrometer (MODIS)

Abstract Next-generation ocean color sensors will include channels to measure passive chlorophyll fluorescence as well as traditional channels that use radiance ratios to estimate chlorophyll concentration. Because the chlorophyll fluorescence signal is small, these sensors have significantly higher signal to noise ratios in the channels used to measure fluorescence. Small changes in sensor performance, atmospheric transmissivity, and fluorescence efficiency could potentially result in significant changes in the performance of the fluorescence algorithms. We perform a sensitivity analysis on the present MODIS algorithms and derive the minimum chlorophyll concentrations that can be observed for various combinations of sensor performance, atmospheric conditions, and phytoplankton physiology. We show that the present sensor specifications will allow us to observe fluorescence at chlorophyll concentrations as low as 0.5 mg/m3 at the full resolution of the sensor (nominally 1 km 2 at nadir) under optimum viewing conditions. Although band position changes over a ±4 nm range affect the absolute reading of individual bands by less than 2%, the impact on the total performance of the fluorescence algorithm can be greater than 70%.

Predictable and efficient carbon sequestration in the North Pacific Ocean supported by symbiotic nitrogen fixation

The atmospheric and deep sea reservoirs of carbon dioxide are linked via physical, chemical, and biological processes. The last of these include photosynthesis, particle settling, and organic matter remineralization, and are collectively termed the “biological carbon pump.” Herein, we present results from a 13-y (1992–2004) sediment trap experiment conducted in the permanently oligotrophic North Pacific Subtropical Gyre that document a large, rapid, and predictable summertime (July 15–August 15) pulse in particulate matter export to the deep sea (4,000 m). Peak daily fluxes of particulate matter during the summer export pulse (SEP) average 408, 283, 24.1, 1.1, and 67.5 μmol·m−2·d−1 for total carbon, organic carbon, nitrogen, phosphorus (PP), and biogenic silica, respectively. The SEP is approximately threefold greater than mean wintertime particle fluxes and fuels more efficient carbon sequestration because of low remineralization during downward transit that leads to elevated total carbon/PP and organic carbon/PP particle stoichiometry (371:1 and 250:1, respectively). Our long-term observations suggest that seasonal changes in the microbial assemblage, namely, summertime increases in the biomass and productivity of symbiotic nitrogen-fixing cyanobacteria in association with diatoms, are the main cause of the prominent SEP. The recurrent SEP is enigmatic because it is focused in time despite the absence of any obvious predictable stimulus or habitat condition. We hypothesize that changes in day length (photoperiodism) may be an important environmental cue to initiate aggregation and subsequent export of organic matter to the deep sea.

Seasonal and interannual variations in photosynthetic carbon assimilation at Station

Abstract Autotrophic carbon assimilation measurements using a trace metal-free 14 C technique were performed at near monthly intervals between 1988 and 1992 in the North Pacific subtropical gyre (U.S. JGOFS-WOCE Sta. ALOHA; 22°45′N, 158°00′W). Integrated photosynthetic values ranged from 127 to 1055 mg C m −2 day −1 while the average carbon assimilation number ( P B ), defined as carbon assimilation rate per unit chlorophyll a (chi a ), varied between 1.6 and 12 g C (g chl a ) −1 h -1 in the 0–45 m depth range. Consistently low P B values ( a ) −1 h −1 , averaged in the upper 45 m of the water column) were observed during the first 2 years of this study but increased to > 5 g C (g chl a ) −1 h −1 during 1991–1992. This rise in PB was not associated with an increase in chi a . Furthermore, it occurred during a period of increased water column stability. Reduction in ATP and (NO 3 − + NO 2 − ) concentrations in the upper euphotic zone suggests that nutrient injections due to mixing events were minor or absent after January 1991. Two non-exclusive hypotheses are presented to explain the rise of P B in the absence of an enhancement of inorganic nutrient fluxes from below the euphotic zone: (i) high P B values observed during 1991–1992 are indicative of phytoplankton growth being balanced as a result of a decrease in the variability of nutrient injection due to a reduction in the frequency of mixing events, and (ii) the rise of PB during 1991–1992 is caused by an ecosystem shift from nitrogen to phosphorus limitation. The stability of the water column during 1991–1992 may have increased the availability of reduced nitrogen relative to phosphorus due to the enhancement of nitrogen fixation. Because these hypotheses do not require an increase in algal biomass or elemental fluxes across the base of the euphotic zone to explain an increase in autotrophic carbon assimilation, they imply that nutrient dynamics within the euphotic zone of the North Pacific subtropical gyre need to be understood in order to interpret changes in P B and predict carbon fluxes.

Role of late winter mesoscale events in the biogeochemical variability of the upper water column of the North Pacific Subtropical Gyre

The present research was funded by NSF grants OCE-93-03094 (to Roger Lukas), OCE 93-01368 (to David M. Karl), OCE 96-01850 (to David Karl, Roger Lukas and Pierre Flament) and NASA grant NAGW-4596 (to Mark R. Abbott).