Euphotic zone photosynthesis

As shown in Fig. 10-13, there is also a flux of O2 produced during net photosynthesis from the ocean to the atmosphere and an export flux of particulate and dissolved organic matter out of the euphotic zone. For a steady-state system, new production should equal the flux of O2 to the atmosphere and the export of organic carbon (Eppley and Peterson, 1979) (when all are expressed in the same units, e.g., moles of carbon). Such an ideal state probably rarely exists because the euphotic zone is a dynamic place. Unfortunately, there have been no studies where all three fluxes were measured at the same time. Part of the difficulty is that each flux needs to be integrated over different time scales. The oxygen flux approach has been applied in the subarctic north Pacific (Emerson et al, 1991) and subtropical Pacific (Emerson et al, 1995, 1997) and Atlantic (Jenkins and Goldman, 1985). The organic carbon export approach has... 

Phosphate is usually considered as a parameter that limits photosynthesis in the Black Sea. According to Sorokin [23], the mean content of phosphate in the upper mixed layer down to the lower boundary of the euphotic zone is close to 0.10-0.20 xM in spring-summer. In autumn its mean content varied between 0.01 and 0.02 xM in cyclonic eddies and between 0.12 and 0.18 iM at their periphery over the slopes. In winter the phosphate content in the upper water layer usually rises due to the vertical mixing, thus attaining 0.15 to 0.40 xM. 

Natural populations of Trichodesmium which are often found in the upper layers of the euphotic zone (see Chapter 16 by Karl et al, this volume), appear to be adapted to high light with a relatively shallow compensation depth (typically 100-200 imol quanta s ) for photosynthesis (Carpenter, 1983a,b LaRoche and Breitbarth, 2005). Several early studies considered the light-photosynthesis relationships o Trichodesmium (e.g., Lewis et al, 1988 Li et al, 1980). Half saturation (4) constants for photosynthesis are reported to be about 300 pmol quanta m s (based on results from four studies, LaRoche and Breitbarth, 2005). See LaRoche and Breitbarth (2005) for a recent comprehensive summary of observed physiological and photosynthetic parameters for Trichodesmium. 

From the standpoint of living organisms, the euphotic zone is probably the most important of all oceanic zones since it is the only place in which any significant amount of photosynthesis takes place. By some estimates, about two-thirds of all the photosynthetic activity that occurs on Earth (on land and in the water) takes place within the euphotic zone. 

As we have shown, the fixation of solar energy by photosynthesis causes a redox disproportionation. Abiotic photochemical processes can similarly induce redox disproportionation. Photochemical reactions produce highly reactive radicals and unstable redox species, which are important in the euphotic zone of natural waters. This will be discussed more in Chapter 12. 

Since photosynthesis requires light, it is confined to the surface layers of waters— the euphotic zone (the region receiving >1% of the irradiance arriving at the water surface). The depth of the euphotic zone varies with the angle of the sun, the amount of light absorbed by suspended matter (including phytoplankton) and the presence of dissolved coloured compounds in the water. 

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