The Role of Diatoms, Iron and Primary Production in the Carbon Cycle of Coastal and Equatorial Pacific Upwelling Systems

F r a n c i s c o P. C h a v e z

G e r n o t E. F r i e d e r i c h

and others

MBARI logo

Monterey Bay Aquarium Research Institute (C) 1996
P.O. Box 628 7700 Sandholdt Road
Moss Landing, CA 95039
(408) 775-1700

introduction buttonMethods and Materials buttonResults button



I N T R O D U C T I O N

The partitioning of carbon dioxide (CO2) between ocean and atmosphere is affected by the pelagic food web of the upper ocean. Photosynthesis, carried out primarily by small microscopic plants (phytoplankton) in the shallow, well-lit layer of the ocean (the euphotic zone), converts dissolved inorganic nutrients (primarily CO2) into particulate organic matter. A major portion of this primary production is recycled within the food web above the thermocline. The remaining fraction escapes from the upper ocean to the thermocline and below, where most of it is recycled and only a minor fraction is deposited in the sediments. It is the escaping fraction that effectively regulates the concentration of CO2 in the upper ocean. Since this layer is in contact with the atmosphere this quantity has important consequences for the global carbon cycle and climate change. In the absence of ocean primary production, surface total carbon dioxide CO2 would be 20% higher, and at equilibrium with such a surface ocean, the atmosphere would have a CO2 concentration close to double present levels (Sarmiento et al., 1990). Biological processes, therefore, have a profound impact on the carbon cycle yet this impact is very poorly understood and the subject of significant debate (Broecker 1991, Longhurst 1991, Banse 1991, Sarmiento 1991).

The fraction of primary production escaping to the thermocline and below is often referred to as new production. New production was originally defined as that proportion of primary production that is supported by allochtonous nutrients (Dugdale and Goering, 1967). In order to maintain steady state (over seasonal and longer time scales) an equivalent amount of nutrient must be exported from the system and thus the use of new production as the export term (Eppley and Peterson, 1979). Most of the new production in the ocean is supported by wind-driven upwelling (Chavez and Toggweiler, 1995). Upwelling is a process by which cool water from below is advected to the surface. Typically these subsurface waters are enriched in inorganic carbon and nitrogen (in the form of nitrate) as well as other essential plant nutrients. Newly upwelled water has a partial pressure of CO2 that is in excess of that of the atmosphere. The exchange of CO2 between ocean and atmosphere is a function of the gradient between the surface layer of the ocean and the atmosphere and newly upwelled water is therefore a natural source of CO2 to the atmosphere. Sunlight warms the newly upwelled water and stimulates photosynthesis. The rate of drawdown of CO2 by plants then determines the amount of degassing from the upwelled water to the atmosphere. The longer the surface water partial pressure is greater than that of the atmosphere the greater the degassing.

Degassing from newly upwelled water changes atmospheric CO2 concentration over relatively short time scales (years) since it is assumed that eventually biological uptake will drawdown CO2 to below atmospheric levels (some cooling also needs to be invoked) and that CO2 lost to the atmosphere by degassing will reinvade the ocean (Sarmiento ____). It is therefore assumed that changes in low to temperate latitude upwelling will have relatively minor impact on atmospheric pCO2 over longer time scales (Sarmiento et al., 1988; Watson, 1995). The high latitudes and particularly the Southern Ocean, are thought to be an exception to this upwelling rule; changes in the nutrient content of the high latitude surface waters are thought to affect atmospheric CO2 on longer (decades, centuries) time scales (Sarmiento and Toggweiler, 1984). The low latitude upwelling assumption may be violated, however, if phytoplankton take up carbon and nitrogen (or phosphorus) or the remineralization rate of organic carbon and nitrogen subsurface is different than that of this same ratio in upwelled water. Chavez and Barber (1985) estimate that a difference between a C:N remineralization ratio of 7.5:1 and 6.6:1 would result in an additional 1/2 gigaton of carbon being removed globally from the surface. This notion is not unreasonable since carbon is typically in excess and other nutrients are limiting.

The rate of drawdown of CO2 is a product of the plant biomass (typically reported as chlorophyll a units of ĩgl-1), and the specific rate of nutrient uptake (Vnutrient in units of day-1). Chlorophyll concentration in the ocean can vary by several orders of magnitude (Chavez and Smith, 1995) while the specific uptake rate of nitrate is reported to vary over an order of magnitude or less (Dugdale et al. _____). The general pattern for biomass distribution in the ocean is for higher biomass close to continents and lower biomass in offshore waters (Chavez, 1989, Berger et al. 19..). This spatial distribution is accompanied by changes in species composition and in specific nitrate uptake rate. Coastal upwelling regions typically are dominated by larger organisms, primarily diatoms and may have higher specific nitrate uptake rates. Open ocean upwelling systems are dominated by small cyanobacteria and may have lower specific nitrate uptake rates. There are therefore large differences in the rate of drawdown of nutrients even among upwelling systems (Chavez and Smith, 1995). In this contribution we present biological and chemical observations from coastal and open ocean upwelling regions in the Pacific Ocean and explore with a very simple model the processes that regulate the concentration of inorganic carbon in surface waters and its exchange with the atmosphere. Our focus is on short term degassing effects and the long term effects of variable C:N ratios.

The Observations

Observations of carbon properties, nutrients, chlorophyll and species composition were made during a Joint Global Ocean Flux Studies (JGOFS) process study to the equatorial Pacific (Feely et al., 1995; Wanninkhof et al., 1989; Chavez et al., 1996), a study of air-sea fluxes in the coastal upwelling system off Monterey Bay, California (Friederich et al.,1995), and open ocean iron fertilization experiments (Coale et al., 1996; Sakamoto et al., 1996)(Figure 1).

Figure 1. Add text here

The concentration of total CO2 (TCO2), the partial pressure of CO2 (pCO2), nitrate (NO3), chlorophyll and phytoplankton biomass was determined for surface waters. Methods and data have been described in Feely et al., 1995, Wanninkhof et al., 1995, Sakamoto et al., 1996, Coale et al., 1996; Chavez et al., 1996 and Walz and Friederich, 1996. Methods are described briefly below.

TCO2 includes inorganic carbonate, bicarbonate and CO2 ions. During analysis the equilibrium conditions are modified by acidification and the water analyzed for the "total" CO2 concentration.

Chlorophyll a and phaeopigments were determined by the fluorometric technique using a Turner Designs Model 10-005 R fluorometer that was calibrated with commercial chlorophyll a (Sigma). Samples for determination of plant pigments were filtered onto 25-mm Whatman GF/F glass fiber filters and extracted in 90% acetone in a freezer for between 24 and 30 hours (Venrick and Hayward, 1984). Phytoplankton were enumerated and sized using epifluorescence microscopy (Chavez et al., 1991). Phytoplankton biomass was estimated from the enumerations and carbon to volume conversions (Chavez et al., 1991).

The Model

A simple model was constructed to examine the timeline of changes of inorganic carbon during nitrate depletion in a parcel of upwelled water. Horizontal and vertical mixing were not incorporated in these calculations. Only carbon uptake associated with nitrate uptake was considered. Carbon and nitrate uptake were parameterized as net community productivity with a growth rate directly proportional to chlorophyll and modified by a Michaelis Menten relationship to nitrate. In this case net community production is equivalent to new production (see Williams, 1993 for a review of definitions relating to plankton production terms). The growth rate, the half saturation constant for nitrate uptake and the carbon to nitrogen uptake ratio were held constant for each set of calculations. Calculations for the coastal upwelling system in accordance with observation allowed the accumulation of chlorophyll, while chlorophyll levels were held constant for the equatorial system. Calcium carbonate precipitation was scaled as a constant percentage of biological carbon uptake. Calculations of the carbonate system used the constants of Roy et al, Marine Chemistry, 44:249-267, 1993. Chemical and physical starting parameters were taken from shipboard measurements. For the coastal scenario data from the central California coast during April 1995 were used. Equatorial upwelling conditions were representative of waters near ..????????....... The only physical processes that these waters were allowed to undergo was heating and air-sea gas exchange. Heating was approximated by a proportional approach to an upper temperature limit that was set to the highest temperature usually observed in that system. Gas exchange was calculated using a steady wind speed and the relationship of 'Wanninkhof, R. 1992. Relationship Between Wind Speed and Gas Exchange Over the Ocean' JGR, Vol. 97, p7373-7382. Calculation time steps were 0.1 days but no diurnal effects were included.

ENVIRONMENTAL SETTING

The equatorial Pacific has been the recent focus of increased research (Murray et al., 1995) driven by the fact that it is the largest natural source of CO2 to the atmosphere (Tans et al. 1990) and that it may contribute a large fraction of global new production (Chavez and Barber, 1987). In addition to its role in global carbon and nitrogen cycles the equatorial Pacific exhibits enigmatic biological characteristics. Concentrations of macronutrients are well above those that are considered limiting, yet phytoplankton in the equatorial Pacific rarely if ever bloom and biomass levels are low and uniform(Figure 2).

Figure 2. Add text here

A large positive gradient of CO2 between ocean and atmosphere is almost permanently maintained and leads to the large loss to the atmosphere. In sharp contrast the coastal upwelling system off the west coast of North America exhibits large fluctuations in both macronutrients and phytoplankton concentrations (Figure 3). Phytoplankton concentrations reach very high levels and drawdown of nitrate to near-zero levels is often observed. In the equatorial Pacific the upwelling process is more or less continuous throughout the year (Chavez and Smith, 1995) while off California the upwelling process is seasonal (Chavez, 1996). Both are subject to El Niņo-related interannual variations. Interestingly the El Niņo effects, at least during the weak 1991-1993 event, were more notable in the physical and chemical fields; effects on the primary producers were not as strong (Chavez, 1996; Chavez et al., 1996)(Figures 2 and 3).

Figure 3. Add text here