Primary Productivity Measurements on NOAA's Long Line Cruises

Introduction

The partitioning of 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 carbon dioxide) 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 regulates the concentration of carbon dioxide in the upper ocean and since this layer is in contact with the atmosphere this process has important consequences for the global carbon cycle and climate change. In the absence of ocean primary production, surface total 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). This simple exercise shows that ocean biological processes have a profound impact on the global carbon cycle yet this impact is very poorly understood and the subject of significant debate (Broecker 1991, Longhurst 1991, Banse 1991, Sarmiento 1991). Unraveling the role of biology in the ocean carbon cycle requires simultaneous observations of biological and chemical properties over many temporal and spatial scales.

As we described the role of the ocean in the global ocean/atmosphere CO2 cycle is intimately related to the biological production system. The primary producers influence ocean/atmosphere exchange of biogenic gases (especially CO2) through the primary production portion of the system. Eppley and Peterson (1979) recognized the need for a relationship between primary production, new production and the export of primary production to the deep ocean. New production is the process whereby new nutrients (e.g., nitrate), advected to the surface, are taken up and, with sunlight, used to synthesize new plant material (Dugdale and Goering, 1967). At steady state this quantity is equivalent to the nitrogen (and carbon) that are removed from the euphotic zone. We proposed to utilize NOAA's commitment to make observations over a large portion of the global ocean and make measurements of primary and new production in an attempt to quantify the effects of the biological system on the carbon cycle.

Several studies have shown that the flux of carbon through the food web and to the deep ocean is a function of total primary production (Betzer et al., 1984; Eppley and Peterson, 1979; Martin et al., 1987; Pace et al., 1987; Suess, 1980), however, it is clear that the functions describing the coupling between the biological and chemical components are not well resolved at present. Recent theoretical and experimental results point toward the structure of the microbial food web as an important variable in regulating the fate of primary production in the ocean (Michaels and Silver, 1988; Berger and Kier, 1984; Frost, 1984; Toggweiller et al., 1987, Sarmiento et al., 1989). Clearly the size distribution of phytoplankton, linked to the supply of new nutrients, plays a role in the flow of particulate matter through the upper ocean pelagic food web. For example, a diatom dominated system, commonly dominated by large particles or aggregates and a short food chain (Ryther, 1969), will have a different biogeochemical character than a picoplankton dominated system, with small individual particles and a complex food web. Much of our research has been geared at quantifying the relationship of major phytoplankton taxa to primary production in Pacific and Equatorial equatorial and eastern Pacific boundary upwelling regions (i.e. Chavez et al., 1990). The NOAA long line program is an opportunity to determine if observations made in Pacific and Atlantic upwelling ecosystems hold for the Indian Ocean.

As our understanding of the relationships between light, photosynthetic pigments, light absorption and primary production has grown, bio-optical instrumentation has come to the forefront of oceanographic research and become a serious candidate to at least partially replace traditional discrete sampling and incubation experiments in measurements of phytoplankton biomass, composition and production. What is presently required, however, are more bio-optical measurements made concurrently with the traditional measurements. These together with other carbon cycle measurements will allow for modeling biogeochemical properties in terms of bio-optical properties. As noted by Platt and others (Platt and Sathyendranath 1988, Mueller and Lange 1989) these models are likely to vary in each of the oceans major biogeochemical provinces. The large-scale coverage provided by NOAA's cruises is ideal for testing these models since several of these biogeochemical provinces, including the equatorial divergence, the low latitude gyres and the high latitude oceans will be sampled

SPECIFIC OBJECTIVES

1. Determine the horizontal and vertical variability in chlorophyll biomass and rates of primary and new production and their relation to chemical ocean carbon properties.

2. Determine chlorophyll and primary productivity attributable to discrete size fractions.

3. Determine the growth rates of the total phytoplankton community and component size fractions, from the estimates of biomass and production.

4. Determine the horizontal and vertical variability in biomass and composition of the various components of the microbial food web including heterotrophic bacteria.

5. Determine the relationship between select bio-optical properties, more traditional discrete measurements and develop models between bio-optical properties and ocean carbon properties.

6. Investigate the relationship between food web structure and new and total primary production using observations from different biogeochemical provinces.

7. Determine grazing rates of the small heterotrophs on picoplankton and bacteria.

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