Biogeochemical controls on new production in the tropical Pacific

https://doi.org/10.1016/S0967-0645(02)00051-6Get rights and content

Abstract

Sources of variability in new production (NP) measured during nine cruises in the tropical Pacific Ocean are examined with respect to other biological and chemical properties. NP measured along the equator during the Zonal Flux and Flupac cruises using 15NO3 incubation methods is presented in this paper and compared to similar data from seven previously published cruises to the tropical Pacific. The Zonal Flux cruise found a strong zonal gradient of increasing NP to the east that followed increasing nitrate inventories. NP values ranged from 0.8 and 3.8 mmol N m−2 d−1 from 165°E to 150°W, respectively. During the 7-day Flupac Time Series II at 150°W, NP measurements also showed strong variability, ranging from 1.9 to 3.6 mmol N m−2 d−1, despite relatively uniform nitrate. Both cruises observed a previously measured but seldom discussed trend for f-ratios to increase substantially at the limits of the euphotic zone (0.1% E0).

Multiple linear regression (MLR) analyses of areal, depth-integrated data from 121 stations in the tropical Pacific previously have showed that variability in primary production (or chlorophyll), ammonium, nitrate and temperature together could “explain” 79% of the variability in NP (Aufdenkampe et al., Global Biogeochem. Cycles 15 (2001) 101). In the present study, the MLR method was extended to depth specific data, where the same variables were shown to explain 77% of nitrate uptake variability. MLR was then used to investigate differences between individual cruises in the relationships of NP to these variables. Similar to MLR results with combined data from all cruises, MLR of individual cruises also found primary production (or chlorophyll), ammonium and nitrate to be consistently the best variables to explain variability in areal NP, exhibiting R2 values from 0.45 to 0.92. However, nitrate is consistently a much stronger predictor of NP within cruises than between cruises. Other lines of evidence—including plots of each property vs. NP and vs. standard residuals of the all-cruise MLR, and differences in MLR partial slopes for individual cruises—together demonstrate that the relationship of NP to nitrate exhibits subtle but real differences from one cruise to the next. Zonal Flux and Flupac sampled the two extremes of this observed NP-to-nitrate variability.

Introduction

Processes controlling primary production stimulated by newly available nutrients drive major biological links and feedbacks between oceanic carbon reservoirs and climate (Falkowski et al., 1998). Such new production (NP) (Dugdale and Goering, 1967) supports the biological pump of organic carbon export to the deep ocean (Eppley and Peterson, 1979), and the efficiency with which upwelled nutrients and dissolved carbon dioxide are sequestered by phytoplankton regulates carbon dioxide exchange with the atmosphere (Dugdale et al., 1992; Kurz and Maier-Reimer, 1993). For these reasons, the equatorial Pacific upwelling zone and other high-nitrate, low-chlorophyll (HNLC) regions have held the attention of the oceanographic community over the last decade (Murray et al., 1994; Barber et al., 1996; Feely et al., 1997).

While classically defined as “all primary production associated with newly available nitrogen” in the form of upwelled nitrate (Dugdale and Goering, 1967), the broad definition of NP requires consideration of all fluxes of limiting nutrients into the euphotic zone. These can include vertical and horizontal advective inputs, estuarine fluxes, nitrogen fixation, and atmospheric deposition. The fraction of NP to total primary production (PP) is referred to as the f-ratio. When considering the classical definition of new production, f-ratios are commonly calculated in terms of new and regenerated nitrogen uptake or in terms of carbon uptake.f=newproductionnew+regeneratedproductionNO3uptake(NO3+NH4++NO2+DON)uptake15NO3uptake14CO2uptake×CNplankton.

A corollary to the concept of new production is that, given steady-state nutrient inventories in the euphotic zone, new nutrient uptake must balance nutrient export in the forms of dissolved and particulate organic matter (Eppley and Peterson, 1979). Thus, the net flux of the limiting nutrient into the euphotic zone ultimately controls new production, and new nutrient uptake rates should equal export rates of those nutrients as organic matter when integrated over similar time intervals (Murray et al., 1989).

Previous studies of new production in the central equatorial Pacific in a variety of conditions have all found low f-ratios, with a mean of 0.16±0.08 (Dugdale et al., 1992; Peña et al., 1992; McCarthy et al., 1996; Navarette, 1998; Raimbault et al., 1999), indicating the importance of nutrient recycling in maintaining primary production. These results fit nicely into the emerging understanding of the equatorial HNLC ecosystem as one in which limiting concentrations of iron and intense microzooplankton grazing jointly control phytoplankton biomass and production (Martin et al., 1994; Price et al., 1994; Fitzwater et al., 1996; Landry et al., 1997; Loukos et al., 1997) and maintain relatively constant rates of chlorophyll-specific primary production (PB) (Barber and Chavez, 1991; Barber et al., 1996). However, variability in measured new production (0.03–6.2 mmol N m−2 d−1) is an order of magnitude greater than that of primary production (5–180 mmol C m−2 d−1) and the range of f-ratios observed in the region, 0.01–0.46, is substantial (Aufdenkampe et al., 2001). The recent appreciation of such strong variability in new production contrasts the earlier paradigm of a biologically stable tropical Pacific ecosystem. Clearly, the biogeochemical and physical controls on new production cannot be identical to those for primary production.

Attempts to explain the variability of new production in the tropical Pacific by comparison with simple parameters has not proven to be straightforward. Previous studies have indeed shown some correlations between nitrate uptake rates vs. nitrate, ammonium, chlorophyll or diatom concentrations (Wheeler and Kokkinakis, 1990; Peña et al., 1992; McCarthy et al., 1996; Landry et al., 1997; Raimbault et al., 1999), yet no single relationship had remained robust from one cruise to another. However, recent multivariate statistical analysis of new production and related data from 121 stations in the tropical Pacific (Aufdenkampe et al., 2001) demonstrates that variability in new production for the entire region is indeed related (R2=0.79) to other properties (primary production (or chlorophyll), ammonium, nitrate and temperature), but only when all are considered simultaneously. These findings advance our ability to extrapolate new production estimates to finer spatial and temporal scales and refine our understanding of what controls new production. However, such statistical models do not directly address the primary controls on new production, which must be the fluxes of bioactive elements into the upper-ocean ecosystem.

A zonal transect along the equator in the Pacific Ocean is in many ways the ideal natural laboratory to study the consequences of varying fluxes of bioactive elements into the euphotic zone. Physical conditions—advective patterns, residence times, stratification, source waters, incident light, and euphotic zone depths—are all generally uniform throughout the upwelling zone, leading to relatively constant primary production, chlorophyll and other biological features (Barber and Chavez, 1991; Chavez et al., 1996; Le Borgne et al (1999), Le Borgne et al., 2002). Underlying these patterns, however, is the classic deepening of temperature and nutrient isolines from east to west (Barber and Kogelshatz, 1990), which results in a strong zonal gradient of upwelling nutrient fluxes to the surface. The general trend of increasing upwelling yet constant productivity offers the perfect opportunity to separate processes that control new vs. primary production. The Zonal Flux cruise in April 1996 sampled such a transect, from 165°E to 150°W (Fig. 1), during mild La Niña conditions in which the nutrient-depleted warm pool was pushed completely west of the study region (Fig. 2a) (Le Borgne et al., 1999). The France-JGOFS Flupac cruise sampled the same transect in October 1994, during moderate El Niño conditions (Eldin et al., 1997).

In this paper, we first present new production data from the Zonal Flux cruise and Flupac Time Series II (at 150°W), as determined by 15NO3 uptake incubations. These data are used to explore measurement issues that are broadly applicable to all 15N-based nitrate uptake studies—an investigation of day vs. night nitrate uptake rates, a comparison of on-deck vs. in situ incubation methods, and a detailed analysis of procedural and analytical uncertainties associated with 15NO3-based new production estimates. The second objective of the paper is to examine trends in new production with respect to other chemical and biological properties. We build upon previous multivariate statistical analyses (Aufdenkampe et al., 2001) by comparing relationships observed during Zonal Flux and Flupac TS II to those observed during the previous meridional studies of new production at 140°W (McCarthy et al., 1996) and 150°W (Dugdale et al., 1992; Peña et al., 1992; Wilkerson and Dugdale, 1992; Raimbault et al., 1999), and the two time series on the equator at 140°W (Wheeler, 1995). We conclude by making the case that the Zonal Flux and Flupac cruises sampled the extreme end-members of the processes that control new production in the region. In a companion paper (Aufdenkampe and Murray, 2002), the comparisons made here are used as a springboard to explore, with a simple euphotic zone box model of nitrogen and iron fluxes, the role of iron and physical forcing in controlling the relationship of new production to nitrate.

Section snippets

Site description and sample collection for Zonal Flux and Flupac

The Zonal Flux cruise (R./V. Thompson, TTN-060) occupied twelve stations from April 15 to May 14, 1996 (Le Borgne et al., 1999)—ten along the equator from 165°E to 150°W and two stations at 2°N and 2°S at 165°E (Fig. 1). Eight stations were sampled intensively for over 24 h. These stations included deployment of sediment trap arrays and in situ primary and new production incubation arrays in addition to casts for nutrient, chlorophyll, bio-optic, bacteria, zooplankton, TOC, and suspended

Zonal Flux: chemical and biological properties

During the April 1996 La Niña event (Southern Oscillation Index, SOI=0.6) sampled by the Zonal Flux cruise, the cold tongue of equatorial upwelling extended to 158°E (Le Borgne et al., 1999), displacing its boundary with the western warm pool over 20° westward from the climatological mean near 180° (Barber and Chavez, 1991). This strong upwelling condition corresponded in a gradual eastward shoaling of the 25° isotherm and the coinciding 7 μM nitrate isopleth from ∼145 m at 165°E to ∼60 m at 150°W

Patterns in nitrate uptake during Zonal Flux and Flupac

Patterns in new production and nitrate uptake rates found during Zonal Flux and Flupac Time Series II were similar to what might be expected in the idealized equatorial Pacific (Barber and Kogelshatz, 1990). The hypothesized zonal gradient of increasing NP to the east was observed and profiles exhibited patterns typical of previous results. However, the range of values observed for both the transect and the time series was surprisingly large. Furthermore, relationships between NP and other

Conclusions

The ability to remotely monitor ecosystem changes controlling the world's major biogeochemical cycles will be a requirement in future efforts to assess and manage our impact to the global system. Oceanographic new production plays an important role in the global carbon cycle, especially with respect to HNLC environments, where the delayed uptake of upwelled macronutrients results in significant out-gassing of CO2 to the atmosphere (Murray et al., 1994). The high variability of many processes in

Acknowledgements

We thank the crew and scientists aboard the R./V. Thomas G. Thomson and the R./V. Atalante for their good cheer, assistance and thoughtful discussion at sea. Statistical Consulting Services at the University of Washington's Department of Statistics provided invaluable help with MLR analyses. Many thanks to A. Le Bouteiller, R.T. Barber, Z. Johnson, and S. Pegau sharing their data, and to W. Gentleman, J.I. Hedges, E. Laws, J. Newton and two anonymous reviewers for encouragement and comments on

References (64)

  • P.M. Glibert et al.

    Utilization of ammonium and nitrate during austral summer in the Scotia Sea

    Deep-Sea Research

    (1982)
  • D.L. Kirchman et al.

    Uptake of ammonium and nitrate by heterotrophic bacteria and phytoplankton in the sub-Arctic Pacific

    Deep-Sea Research I

    (1998)
  • E.A. Laws et al.

    Carbon cycling in primary production bottle incubationsinferences from grazing experiments and photosynthetic studies using 14C and 18O in the Arabian Sea

    Deep-Sea Research II

    (2000)
  • R. Le Borgne et al.

    Pacific warm pool and divergencetemporal and zonal variations on the equator and their effects on the biological pump

    Deep-Sea Research II

    (2002)
  • A. Le Bouteiller et al.

    Size distribution patterns of phytoplankton in the western Pacific, towards a generalization for the tropical open ocean

    Deep-Sea Research I

    (1992)
  • H. Loukos et al.

    An ecosystem model with iron limitation of primary production in the equatorial Pacific at 140°W

    Deep-Sea Research II

    (1997)
  • J.J. McCarthy et al.

    New production along 140°W in the equatorial Pacific during and following the 1992 El Niño event

    Deep-Sea Research II

    (1996)
  • J.J. McCarthy et al.

    Nitrogen dynamics during the Arabian Sea Northeast Monsoon

    Deep-Sea Research II

    (1999)
  • J.W. Murray et al.

    Nutrient assimilation, export production and 234Th scavenging in the eastern equatorial Pacific

    Deep-Sea Research I

    (1989)
  • C. Oudot et al.

    A high sensitivity method for the determination of nanomolar concentrations of nitrate and nitrite in seawater with a Technicon AutoAnalyzer II

    Marine Chemistry

    (1988)
  • M.L Rodier et al.

    Export flux of particles at the equator in the western and central Pacific Ocean

    Deep-Sea Research II

    (1997)
  • A.K. Aufdenkampe et al.

    Estimation of new production in the tropical Pacific

    Global Biogeochemical Cycles

    (2001)
  • R.T. Barber et al.

    Regulations of primary productivity rate in the equatorial Pacific

    Limnology and Oceanography

    (1991)
  • R.T. Barber et al.

    Ground truthing modeled kpar and on deck primary productivity incubations with in situ observations

    Ocean Optics

    (1997)
  • Bonnet, S., 1995. Manuel d’analyses chimiques dans l’eau de mer. Vol. 2, ORSTOM-Nouméa, Notes Techniques Sciences de la...
  • W.P. Cochlan et al.

    Diel periodicity of nitrogen uptake by marine phytoplankton in nitrate-rich environments

    Limnology and Oceanography

    (1991)
  • J.E. Dore et al.

    Nitrification in the euphotic zone as a source of nitrite, nitrate, and nitrous oxide at Station ALOHA

    Limnology and Oceanography

    (1996)
  • Q. Dortch

    The interaction between ammonium and nitrate uptake in phytoplankton

    Marine Ecology Progress Series

    (1990)
  • R.C. Dugdale et al.

    Uptake of new and regenerated forms of nitrogen in primary productivity

    Limnology and Oceanography

    (1967)
  • R.C. Dugdale et al.

    The use of 15N to measure nitrogen uptake in eutrophic oceans; experimental considerations

    Limnology and Oceanography

    (1986)
  • R.C. Dugdale et al.

    Estimating new production in the equatorial Pacific Ocean at 150°W

    Journal of Geophysical Research

    (1992)
  • J.P. Dunne et al.

    Silicon–nitrogen coupling in the equatorial Pacific upwelling zone

    Global Biogeochemical Cycles

    (1999)
  • Cited by (25)

    • Biogeochemical impact of a model western iron source in the Pacific Equatorial Undercurrent

      2009, Deep-Sea Research Part I: Oceanographic Research Papers
    • Primary production in the eastern tropical Pacific: A review

      2006, Progress in Oceanography
      Citation Excerpt :

      Nutrient and phytoplankton interaction within the EU region remains a topic of considerable interest. Given the extremely low nitrate uptake rates observed on the equator, if upwelling stopped it would still take over 6 months to deplete surface nitrate (6–12 μM; Dugdale et al., 1992; McCarthy et al., 1996; Aufdenkampe and Murray, 2002; Aufdenkampe et al., 2002). Upwelled macronutrients, particularly nitrate, are never fully drawn down and chlorophyll and productivity do not reach the levels that seemingly ‘should’ be supported by the available nutrients (Barber and Chavez, 1991).

    • Controls on new production: The role of iron and physical processes

      2002, Deep-Sea Research Part II: Topical Studies in Oceanography
    • Carbon fluxes in the equatorial Pacific: A synthesis of the JGOFS programme

      2002, Deep-Sea Research Part II: Topical Studies in Oceanography
    View all citing articles on Scopus
    View full text