Biologically driven export from the euphotic zone

3 I Biologically driven export from the euphotic zone 

Biologically driven organic matter export from the euphotic zone regulates the pco of the atmosphere and provides the substrate for 

Since there are no standards for flux determinations, as there are for chemical concentration measurements in water or sediments, evaluating their accuracy is difficult. One is forced to rely on comparison of different anal5dical approaches and modeling of nutrient and oxygen distributions to achieve a consensus. Of the different methods used to determine the organic carbon export presented in the following section, the first two (sediment traps and thorium isotope disequilibria) are used to evaluate the POC flux only. They must be combined with estimates of the DOC flux to achieve the total. The second two methods, molecular oxygen and carbon isotope mass balance, determine the sum of DOC and POC export. 

Sediment traps are rain-gauge-like collectors of particles (Fig. 6.9) that have been deployed in the ocean hundreds of times. Although these deplo5mients have revealed a wealth of information about the nature of the sinking particles, it is generally agreed that sediment 

Profiles of dissolved organic carbon (DOC) for four different times of the year from the Bermuda Atlantic Time-series Station (BATS). The arrow indicates the approximate mixed layer depth for these times of the year. The vertical shaded area represents the wintertime values. Redrawn from Carlson et al. (1994). 

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