Deep Sea Research Part II: Topical Studies in Oceanography
Biogeochemical controls on new production in the tropical Pacific
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.
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
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