Photic zone

Photic-zone depletion with Ca, Si, ICOz, NO3, PO4, Cu, Ni Biological uptake and regeneration... 

The two prime mechanisms of carbon transport within the ocean are downward biogenic detrital rain from the photic zone to the deeper oceans and advection by ocean currents of dissolved carbon species. The detrital rain creates inhomogeneities of nutrients illustrated by the characteristic alkalinity profiles (Fig. 11-9). The amount of carbon leaving the photic zone as sinking particles should not be interpreted as the net primary production of the surface oceans since most of the organic carbon is recycled... 

The oceanic biota reservoir (4) is also within the surface layers. Although organisms reside at all depths within the ocean, the overwhelming majority reside within the photic zone where phytoplankton dominate. The oceanic biota reservoir only contains roughly 1 /30 as much P as the land biota reservoir. This is primarily because oceanic biomass is composed of relatively short-lived organisms, while land biomass is dominated by massive long-lived forests. 

The deep ocean (6) is the portion of the water column from 300 m to 3300 m and is the largest ocean reservoir of dissolved P. However, since the deep ocean is devoid of light, this P is not significantly incorporated into ocean biota. Mostly, it is stored in the deep waters until it is eventually transported back into the photic zone via upwelling or eddy diffusive mixing. 

The present average PO4 concentration of deep ocean water is 2.2 /rmol/kg. When a parcel of deep water is transported to the photic zone, this POi is completely incorporated into plants. Note that this assumes that net primary productivity is not limited by the availability of other micronutrients. In shortterm laboratory studies, this assumption is clearly not true in that it has been demonstrated... 

Another layering that occurs within the 1000 metre surface ocean is the distinction between seawater receiving solar irradiation (the photic zone) and the dark water below. The sun provides heat, UVR, and photosynthetically active... 

Life on Earth requires the energy from the Sun as a primary energy source but it must be protected from all of the radiation at shorter wavelengths. Radiation shorter than 323 nm can break the C-C bond and this would lead to mutations or complete photolytic destruction of carbon-based life forms. The protection from the short-wavelength radiation is achieved on Earth in two ways the ozone layer and the photic zone. 

A short-wavelength shield achieves protection of chemistry on the surface of the Earth or the initiation of chemistry in a prebiotic Earth or Titan. Essential it may be but there is an alternative - the photic zone. 

Sunlight can enter a body of water to a depth defined as the photic zone (Figure 7.13). The lower boundary of the photic zone is the region where the light levels have fallen to 1 per cent of their surface value. The photic zone may be as little as 1 m where the water is unclear (due to particulates for example) or may extend up to 200 m. Within this region phytoplankton are capable of photosynthesis whereas below this region no light penetrates and the oceans are dark. Protection from all radiation is possible below 200 m and will allow chemistry and bond formation to occur. 

Photic zone The first 200 m of the oceans through which life penetrates. 

Short-wavelength shield The protection of the surface of a planet from dangerous short-wavelength radiation from the local star. On the Earth, the ozone layer shields the surface, as does the photic zone. 

The Iron Cycle in the Photic Zone of Surface Waters In the photic zone the formation of iron(II) occurs as a photochemical process. The photochemical iron II) formation proceeds through different pathways 1) through the photochemical reductive dissolution of iron(III)(hydr)oxides, and 2) through photolysis of dissolved iron(lll) coordination compounds, Fig. 10.16. 

Schematic representation of the photoredox cycling of iron in the photic zone of surface waters. The important features are the following ...

Dissolved iron(III) is (i) an intermediate of the oxidative hydrolysis of Fe(II), and (ii) results from the thermal non-reductive dissolution of iron(III)(hydr)oxides, a reaction that is catalyzed by iron(II) as discussed in Chapter 9. Hence, iron(II) formation in the photic zone may occur as an autocatalytic process (see Chapter 10.4). This is also true for the oxidation of iron(II). As has been discussed in Chapter 9.4, the oxidation of iron(II) by oxygen is greatly enhanced if the ferrous iron is adsorbed at a mineral (or biological) surface. Since mineral surfaces are formed via the oxidative hydrolysis of Fe(II), this reaction proceeds as an autocatalytic process (Sung and Morgan, 1980). Both the rate of photochemical iron(II) formation and the rate of oxidation of iron(II) are strongly pH-dependent the latter increases with... 

R. Riegman and G.W. Kraay, Phytoplankton community structure derived from HPLC analysis of pigments in the Faroe-Shetland Channel during summer, 1999 the distribution of taxonomic groups in relation to physical/chemical condition in the photic zone. J. Plankton Res. 23 (2001) 191-206. 

In the case of plankton, cell lysis that occurs shortly after death causes ATP to be released into seawater. Like most biomolecules, ATP is rapidly degraded in seawater by microbes. Thus, high surfece concentrations in Figure 22.5 reflect a rapid supply supported by the high rates of plankton production characteristic of the photic zone. Below the surface, concentrations decrease with increasing depth beneath the photic zone and, hence, distance from the biosynthetic source of the ATP... 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

Surface Production and Organic Matter Export from the Photic Zone... 

The depth of the mixed layer is important for two reasons. First, phytoplankton can be carried out of the photic zone and, hence, halt net primary production if the mixed layer is deeper than the photic zone. Second, the bottom of the mixed layer marks the upper limit to which density stratification in the thermocline inhibits upward vertical transport of nutrients. If the photic zone extends into the thermocline, phytoplankton... 

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