Eutrophication -- 8.7 Variability of vertical export in the pelagic zone

All investigations of export of biogenic matter indicate that the export flux decreases more or less exponentially with depth in the upper part of the ocean, with minor decreases below 200-500 m depth (for algorithms predicting the depth variation of vertical carbon flux see Berger et al. [40]). Resuspension and protrusion of advective, particle rich layers or vertical differences in current direction may alter this general feature of vertical flux. The degradation rate of organic matter in the water column and, in particular for fast sinking particles is of pivotal importance for the quantitative regulation of pelagic-benthic coupling. Depending on this rate, the absolute vertical export of organic matter at a certain depth could be small or large, irrespective the size of the new production from which it derives.

The current lack of adequate investigations of the vertical export above the depth of 200-500 m where the majority of long-term sediment traps have been deployed, results in difficulties to understand and model vertical carbon flux. There exists a black box of several hundred metres between the surface layers where measurements and algorithms of primary production exists and where data on the carbon export to the ocean interior are available. In this black box, the twilight zone, we face a lack of basic understanding on how vertical export of biogenic matter in general is regulated, let alone in eutrophic regions. In order to guide future investigations of vertical flux attenuation in eutrophicated regions we present an idealised, conceptual model of vertical carbon export and focus upon the `pelagic mill' and vertical flux regulation in the upper 200 m (Figures 3 and Figure 9). An adequate understanding of carbon cycling demands not only adequate investigations of primary production, but also concomitant research on the functional biodiversity of the pelagic zone, plankton dynamics, vertical flux and its regulation in the twilight zone.

sedFig9

Figure: Schematic presentation of the `pelagic mill' in the upper part of the ocean and its regulation of biogenic vertical flux. The full line assumes a continuous mineralisation of export production, giving raise to a decline in flux that follows a power function. The broken line indicates a step-wise decrease in vertical flux caused by extensive grazing at certain depth horizons. The stippled line indicates that vertical flux can increase intermittently due to repackaging. The recycling by the zooplankton community is schematically indicated to the right.