19 Control of primary production in the North Atlantic Ocean

Charles S. Yentsch

Bigelow Laboratory for Ocean Sciences

West Boothbay Harbor, ME

[Editor's note: Yentsch had to leave before his talk could be presented. The following is edited from a message sent after the meeting.]

What I had hoped to talk about concerns my conception of how organic production works in the North Atlantic basin. To be brief, the message is that ocean color analysis from satellite has confirmed the traditional ideas concerning regulation of primary production. What I find interesting is that the baroclinic features created by the circulation in the North Atlantic basin potentially dictate the pattern of primary production. The added input of change in mixed layer depth provides the marked seasonality. It is a truly beautiful system to study carbon dioxide interactions and species diversity and biogeography in general. To study these processes properly, there is a real need for marriage between JGOFS and GLOBEC.

Consider Figure 19.1 showing the 200 meter nitrate concentration in the basin. It fits the pattern of satellite derived chlorophyll very well. It also fits the pattern of general circulation as defined by the Gulf Stream and the Equatorial Currents shown in Figure 19.2. Looking at the section which dissects the basin on its side reveals two mountains of nitrate separated by a valley which is the Sargasso Sea (Figure 19.3). Figure 19.4 shows the familiar nitrate-density relationship for the western part of the basin. Garside has data for the entire basin. In any case the major point is that sigma-t 27.5 is the nitrate maximum, which when viewed from North to South reinforces the dominance of circulation on the deepwater pattern of nitrate (Figure 19.5). Superimposed on the nitrate and density distributions is the productivity which also reflects the bimodal distributions. Note that the equatorial region has as much nitrate at depth but the productivity is lower. This is because the mixed layer is shallower at equatorial latitudes.

The above convinced me that the system looked like the half toroid shown in Figure 19.6. This is consistent with how most physical oceanographers see the system--the top of the beta spiral is sort of an eddy or a badly sprung flywheel (Stommel). I have until recently, been reluctant to assign the relative importance of upwelling in the system--even though the white arrows reflect the general direction of surface winds. Yet these sequences must be extremely important in the establishment of the productivity pattern. The North Atlantic is full of regions of variable upwelling and downwelling which probably convolute the patterns of CO2 shown by Takahashi. Would it not be feasible to identify one of these and examine the time and space change in CO2 fields (JGOFS)? The same type of experiment could be performed examining species diversity (GLOBEC).

Figure 19.1. The pattern of nitrate-nitrogen (um kg-1) at 200 meters in the North Atlantic, superimposed on ocean color composite of the North Atlantic. Yellow indicates a high concentration of chlorophyll, blue is intermediate and purple is low chlorophyll.

Figure 19.2. Circulation in the North Atlantic. The N-S section is along 40 West longitude.

Figure 19.3. Vertical distribution of nitrate nitrogen along 40 W section shown in Figure 19.2.

Figure 19.4. Nitrate-density relationship for the western North Atlantic.

Figure 19.5. Annual latitudinal primary production of particulate nitrogen (PN) and carbon (PP) at Long. 40 West.

Figure 19.6. Schematic representation of wind stress and the resultant convergent circulation. Mass balance is retained through isopycnal transports intercepting the surface layer at 10 and 50 North. Stippling represents areas of high production.