BIOLOGICAL PUMP

The sinking of organic material produced by photosynthesis from the upper euphotic layers into the oceans represents a potential long-term sink for atmospheric CO2. This sequestration of carbon into the ocean’s interior is termed the ‘biological pump’ and plays a central role in the global carbon cycle. Its magnitude is regulated by the supply of inorganic nitrogen, primarily nitrate to the euphotic layer. This is because the rate of resupply of nutrients from the deep water to the surface regulates the rate of phytoplankton production globally on the time scales relevant to anthropogenic change. Hence, understanding the spatial and temporal variations of N over basin and global scales in the euphotic zone is an important requirement for ocean biogeochemical and climate studies.

METHOD TO MEASURE ANNUAL NEW PRODUCTION

New Production is the portion of total primary production that is driven by new nutrients (primarily nitrate) that enter the euphotic zone. The method developed by us to measure new production is applicable to high and mid-latitude oceanic regions where injection of nutrients into the euphotic zone takes place mainly during winter mixing. This input of nutrients supports the largest fraction of new production occurring annually in the world’s major ocean basins.
In order to understand the principle underlying our method we need to refer to Sverdrup’s critical depth hypothesis. According to Sverdrup’s (1953) theory, phytoplankton growth season starts in early spring when the mixed layer depth becomes shallower than the deepening critical depth. Critical depth or Sverdrup's critical depth is the depth, above which, the depth-integrated daily gross primary production equals respiration, i.e., the depth above which integrated net daily primary production equals zero. During the period between deepest winter mixing and the onset of the growth season, nutrients are distributed abundantly and homogeneously above the top of the main thermocline. In spring, growth starts with a bloom (new production) in the surface mixed layer which progressively reduces the concentration of nitrate in the mixed layer. Thereafter new production continues in the seasonal thermocline and the consequent consumption of nitrate deepens the nitracline (The water-column depth strata associated with rapid changes in nitrate or nutrients). Because nutrient consumption is light dependent, new production should cease by the end of September when the nitracline and the critical depth intersect.
In this method we rationalize that if the magnitude of winter N consumed over the growth season of phytoplankton (ΔN) can be estimated, new production (P_N) can be calculated using the relationship P_N = RΔN(t), where R is the ratio of carbon to nitrogen for phytoplankton and t is the time (late summer) when the nitracline is the deepest. ΔN is calculated as ΔN = [N(o) -N(t)]*Z_DN(t) where N(o) and N(t) are the concentrations of N (g N m^-3) in winter and at the end of summer respectively, and Z_DN(t) is the depth of the nitracline in meters at time t. This method represents a lower bound estimate for P_N as it does not account for P_N that could result from N input other than winter convective mixing or for P_N that occurs after summer. More details are available in Goés et al. (2000).

ESTIMATION OF NITRICLINE USING SATELLITE SST

In the interest of developing a method that could be used exclusively with satellite data, we established a procedure to measure the depth of the nitracline based solely on SST. We found a highly significant relationship (r²=0.9) between SST and the depth of the nitracline in a data set collected during four Northwest Pacific Carbon Cycle Study (NOPACCS) summer cruises along the 175^oE meridional transect. The relationship between SST and the depth of the nitracline is available in Goés et al. (2000). The relationship which was remarkably similar from year to year enabled modeling of Z_DN with a high degree of accuracy based solely on SST. Assuming its validity for other longitudes it was then applied to monthly composites of SST for the month of September to calculate the depth of the nitracline at the end of the growth season. Then, using the satellite derived ΔN (the difference in SSN between satellite derived SSN in March and in September), the satellite based depth of the nitracline (above which nitrate is assumed to be consumed) Z_DN and a carbon: nitrogen ratio of 106:16, satellite based maps of new production in the North Pacific were developed.