Primary production in the eastern tropical Pacific: A review

Abstract

The eastern tropical Pacific includes 28 million km² of ocean between 23.5°N and S and Central/South America and 140°W, and contains the eastern and equatorial branches of the north and South Pacific subtropical gyres plus two equatorial and two coastal countercurrents. Spatial patterns of primary production are in general determined by supply of macronutrients (nitrate, phosphate) from below the thermocline. Where the thermocline is shallow and intersects the lighted euphotic zone, biological production is enhanced. In the eastern tropical Pacific thermocline depth is controlled by three interrelated processes: a basin-scale east/west thermocline tilt, a basin-scale thermocline shoaling at the gyre margins, and local wind-driven upwelling. These processes regulate supply of nutrient-rich subsurface waters to the euphotic zone, and on their basis we have divided the eastern tropical Pacific into seven main regions. Primary production and its physical and chemical controls are described for each.

Enhanced rates of macronutrient supply maintains levels of primary production in the eastern tropical Pacific above those of the oligotrophic subtropical gyres to the north and south. On the other hand lack of the micronutrient iron limits phytoplankton growth (and nitrogen fixation) over large portions of the open-ocean eastern tropical Pacific, depressing rates of primary production and resulting in the so-called high nitrate-low chlorophyll condition. Very high rates of primary production can occur in those coastal areas where both macronutrients and iron are supplied in abundance to surface waters. In these eutrophic coastal areas large phytoplankton cells dominate; conversely, in the open-ocean small cells are dominant. In a ‘shadow zone’ between the subtropical gyres with limited subsurface ventilation, enough production sinks and decays to produce anoxic and denitrified waters which spread beneath very large parts of the eastern tropical Pacific.

Seasonal cycles are weak over much of the open-ocean eastern tropical Pacific, although several eutrophic coastal areas do exhibit substantial seasonality. The ENSO fluctuation, however, is an exceedingly important source of interannual variability in this region. El Niño in general results in a depressed thermocline and thus reduced rates of macronutrient supply and primary production. The multi-decadal PDO is likely also an important source of variability, with the ‘El Viejo’ phase of the PDO resulting in warmer and lower nutrient and productivity conditions similar to El Niño.

On average the eastern tropical Pacific is moderately productive and, relative to Pacific and global means, its productivity and area are roughly equivalent. For example, it occupies about 18% of the Pacific Ocean by area and accounts for 22–23% of its productivity. Similarly, it occupies about 9% of the global ocean and accounts for 10% of its productivity. While representative, these average values obscure very substantial spatial and temporal variability that characterizes the dynamics of this tropical ocean.

Introduction

Patterns of primary production in the world ocean reflect patterns of thermocline topography. The thermocline is that portion of the water column where temperature decreases maximally with increasing depth (reviewed by Fiedler and Talley, 2006). This thermocline/productivity linkage occurs because the thermocline almost invariably coincides with the nutricline, defined as that portion of the water column where phytoplankton nutrients increase maximally with depth (Barber and Chavez, 1983, Barber and Chavez, 1991, Fiedler and Talley, 2006). Areas of high primary productivity occur where nutrient supply to the euphotic zone is enhanced by mixing and a shallow thermocline (Wyrtki, 1966, Guillén, 1966, Calienes and Guillén, 1981, Guillén and Calienes, 1981a, Barber and Chavez, 1983, Turk et al., 2001, McClain et al., 2002). Because temperature is much easier to measure than nutrients, both from ships and satellites, variations in thermocline depth and even surface temperature are often associated with variations in phytoplankton production.

Two patterns of enhanced nutrient supply occur in the ocean: (1) At high latitudes, typically in winter, deep mixing erodes the thermocline downwards while nutrients are mixed upwards. In these areas phytoplankton often bloom after the winter mixing season when stratification sets in and sunlight in the euphotic zone becomes sufficient; (2) At low latitudes, dynamics of the wind-driven surface flow can cause the thermocline to ‘lift’ or ‘shoal’ towards or into the euphotic zone (reviewed by Barber and Smith, 1981, Barber and Chavez, 1983). The supply of nutrients to phytoplankton is consequently increased, and rates of primary production are elevated. In general, winter mixing and thermocline erosion dominate nutrient supply processes at high latitudes, whereas at low latitudes, such as in the eastern tropical Pacific where seasonality is often weak, thermocline shoaling and upwelling become the main supply processes (Chavez and Toggweiler, 1995). Within the subtropical gyres where neither process operates effectively, the thermocline and nutricline are stable and deep (200–300 m) and phytoplankton in the euphotic zone remain nutrient-impoverished. These biological/chemical/physical relationships have been long-recognized and remain an important paradigm in biological oceanography (Sverdrup et al., 1942, Sverdrup, 1955, Brandhorst, 1958). Fleming (1957) developed a global map of primary production based on upwelling, mixing and depth of the permanent thermocline (Fig. 1A) that is similar to maps created today from satellite imaging of phytoplankton color (Fig. 1B; Behrenfeld and Falkowski, 1997a, Behrenfeld and Falkowski, 1997b, McGowan, 2004), and mathematical models today predict patterns of primary production based in part on the underlying thermocline dynamics.

The eastern tropical Pacific is herein defined as reaching from the coast of Central and South America to 140°W, about halfway across the Pacific, and between the Tropics of Cancer and Capricorn at 23.5°N and S, respectively. As such, this ‘box’ includes 28 million square kilometers, about three times the area of the United States, and accounts for about 23% of Pacific and 10% of global oceanic primary production (Table 1). Because thermocline depth controls nutrient supply and thus biological production, it becomes important to understand the main processes which control eastern tropical Pacific thermocline depth, all of which are related to the wind. In the north [or south] Pacific subtropical gyre, the wind blows clockwise [or counterclockwise] around the basin to drive the surface flow of the gyral circulation (see Fig. 3). First, because these currents export water and accumulated heat from the eastern to the western tropical Pacific, the thermocline tilts down towards the western side of the basin and is much shallower in the east (Barber and Chavez, 1983, Tomczak and Godfrey, 1994). Second, the currents interact with the rotation of the Earth to lift the thermocline at the gyre edges and depress it in the gyre centers, shoaling the thermocline over substantial portions of the eastern tropical Pacific (see Fig. 2C). Third, in some areas, local winds drive additional thermocline shoaling via wind-driven upwelling. The first two of these processes operate on basin-scales while the third is locally-driven, but all three vary spatially within the eastern tropical Pacific. Fig. 2 shows the nutrient response.

As a consequence biological productivity is heterogeneous and cannot be understood without an understanding of the region’s atmospheric forcing, circulation and hydrography. These physical dynamics are briefly introduced in this paper (see Section 3.2, below, for further discussion of thermocline topography) and are reviewed in detail elsewhere in this volume as follows. The NE and SE trade winds, and the Inter-Tropical Convergence Zone (ITCZ) between them, are reviewed by Amador et al. (2006). The eastern boundary currents (Peru and California), their subsurface countercurrents, and the equatorial currents (North and South Equatorial Currents, the Equatorial Undercurrent and the North Equatorial Countercurrent; hereafter NEC, SEC, EUC and NECC) are reviewed by Kessler, 2002, Kessler, 2006. A poorly ventilated, sub-thermocline ‘shadow zone’ develops between the American coast and the NEC and SEC, resulting in the largest zone of low oxygen in the world’s ocean (reviewed by Fiedler and Talley, 2006).

Continental geometry is also important. It positions the confluence of the NE and SE trade winds (the ‘Inter-Tropical Convergence Zone’, or ITCZ) and the NECC not at the geographic equator, but at about 10°N, so that both subtropical gyres are shifted to the north (Philander et al., 1996). As a consequence, the SE trade winds and the SEC penetrate the northern hemisphere to about 5°N. Divergence of Ekman transport on the equator then creates the famed open-ocean equatorial upwelling system (reviewed by Kessler, 2006). Also as a consequence of continental geometry, the eastern Pacific warm pool above the thermocline lies entirely north of the equator, and the below-thermocline oxygen minimum is much more pronounced to the north (Fiedler and Talley, 2006). The above currents, thermocline dynamics and coastal features all affect euphotic zone nutrient supply and hence biological productivity within the eastern tropical Pacific.

This paper provides an overview of phytoplankton production and its controls in the eastern tropical Pacific. The purpose is to review previous work and to provide region-specific estimates of primary productivity, chlorophyll biomass, and nutrient concentrations. The discussion is based on a compilation of ship-collected physical, chemical and biological data which are compared to satellite chlorophyll and modeled primary production estimates (SeaWiFS and the VGP Model of Behrenfeld and Falkowski, 1997b, respectively; analyses detailed in Appendix). We emphasize spatial and temporal patterns of primary production and their relationship to atmospheric forcing (Amador et al., 2006), hydrography (Fiedler and Talley, 2006) and circulation (Kessler, 2006). The focus is on near-surface waters and the sunlit euphotic zone.

Based on the spatial heterogeneity described above, we have divided the eastern tropical Pacific into seven biogeographic regions (Fig. 4). These are (1) an Eastern Margin, (2) an Equatorial Upwelling region (EU), (3–4) two southern and (5–7) three northern hemisphere open ocean regions. The Eastern Margin and EU regions have been additionally subdivided where particular features require (see Fig. 4). Below we define the regions, introduce their hydrography and nutrient dynamics, and describe the spatial and seasonal patterns of phytoplankton biomass and productivity based on ship-collected data. Because the subtropical gyres flow towards the equator near the continents and then westward across the Pacific, the regions are presented in that order, beginning in the northern hemisphere. In a final ‘synthesis’ section we reconnect the regions, compare ship-collected data to satellite and model results, and highlight processes that regulate primary production in the eastern tropical Pacific. During assembly of this paper it became clear that many aspects of biological production in the region are poorly understood and can only be sketched in the broadest outline.