Euphotic zones

The quantity of primary production that is exported from the upper ocean is said to be equivalent to new production (18, 19) New primary production is that associated with allocthonous nutrients (i.e., those upwelled or mixed into the euphotic zone or input via rivers and rain). In order for steady state to be maintained, an equivalent flux out of the euphotic zone is required. Earlier studies (19) suggested that sediment-trap measurements of particulate organic carbon (POC) flux were equivalent to new primary production however, recently it has become clear that these measurements probably represent only a... 

A schematic representation of biological processes in the marine euphotic zone is shown in... 

Fig. 10-13. The links between the cycling of C, N, and O2 are indicated. Total primary production is composed of two parts. The production driven by new nutrient input to the euphotic zone is called new production (Dugdale and Goering, 1967). New production is mainly in the form of the upward flux of nitrate from below but river and atmospheric input and nitrogen fixation (Karl et al, 1997) are other possible sources. Other forms of nitrogen such as nitrite, ammonia, and urea may also be important under certain situations. The "new" nitrate is used to produce plankton protoplasm and oxygen according to the RKR equation. Some of the plant material produced is respired in the euphotic zone due to the combined efforts...

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... 

Fig. 10-15 Organic carbon fluxes with depth in the water column normalized to mean annual primary production rates at the sites of sediment trap deployment. The undulating line indicates the base of the euphotic zone the horizontal error bars reflect variations in mean annual productivity as well as replicate flux measurements during the same season or over several seasons vertical error bars are depth ranges of several sediment trap deployments and uncertainities in the exact depth location. (Reproduced with permission from E. Suess (1980). Particulate organic carbon flux in the oceans - surface productivity and oxygen utilization, Nature 288 260-263, Macmillan Magazines.)...

The elements Na, K, Cl, SO, Br, B, and F are the most conservative major elements. No significant variations in the ratios of these elements to chlorine have been demonstrated. Strontium has a small (< 0.5%) depletion in the euphotic zone (Brass and Turekian, 1974) possibly due to the plankton Acantharia, which makes its shell from SrS04 (celestite). Calcium has been known since the 19th century to be about 0.5% enriched in the deep sea relative to surface waters. Alkalinity (HCOf") also shows a deep enrichment. These elements are controlled by the formation... 

Indeed Kim et al. (1999) have argued that better agreement is obtained from a one-box model approach, in which the particulate Th residence time is calculated as the difference between the residence times of total Th and dissolved Th (effectively the difference in the l/k values calculated from Eqns. 5 and 6). However, Buesseler and Charette (2000) argued in a response to Kim et al. (1999) that there is abundant evidence to support the notion that residence times of POC and Th are different in the euphotic zone. 

Buesseler et al. (1992b) proposed a method to circumvent these difficulties in comparing residence times. They argued that the deficiency in total " Th with respect to indicates a flux of " Th in association with particles sinking out of the euphotic zone. If the POC (or particulate organic nitrogen, PON)/ Th ratio of these sinking particles is known, a POC (or PON) flux can be calculated as ... 

Pacific Ocean. Upper horizontal lines delineate the base of the mixed layer and lower horizontal lines delineate the base of the euphotic zone. The vertical line at = 1 represents radioactive... 

A major opportunity to test the use of " Th as a proxy for POC flux arose with the Joint Global Ocean Flux Study (JGOFS). JGOFS had as a central goal a better understanding of the ocean carbon cycle, including the flux of POC leaving the euphotic zone. Process studies were carried out in the Atlantic Ocean, Pacific Ocean, Arabian Sea and Southern Ocean. " Th profiles were obtained as a part of each process study. 

The fluxes of POC determined by the " Th method applied to the world s oceans are summarized in Table 1. Where possible we have tabulated the ratio of Th-derived POC export to independent estimates of primary production. As noted above, this ratio, termed the 77i ratio (Buesseler 1998), is important in the euphotic zone carbon balance as it represents the leakage of POC out of the euphotic zone (The ThE ratio is so named to evoke the e ratio, which is defined as the ratio of POC flux measured with sediment traps to primary production). 

Table 1. " Th-derived POC export fluxes from the euphotic zone. 

Coale KH, Braland KW (1987) Oceanic stratified euphotic zone as elucidated by Th-234 U-238 disequilibria. Limnol Oceanogr 32 189-200... 

Fig. 2.8 Time series of mean marine snow concentration [kmolP/m3] (solid), number of aggregates [particles/cm3] (dashed), and the mean slope of the particle size distribution in the euphotic zone (solid), and number of aggregates (dashed) at 175W 55°S.

To assess gravitational settling of DDT, the setding flux Fs, is diagnosed by the amount of DDT bound to detritus [kg(DDT)/kmol(P)] in the lowest euphotic zone layer (75 m - 90 m) and the detritus export production Fe [kmol(P)/m2]... 

Fig. 2.9 Seasonal mean sinking velocity in the euphotic zone [m/d] in the season from June to August (JJA), and in the season from December to February (DJF).

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