Nitrate water column profiles

Figure 34-4 Water column profiles of nitrate concentration (open symbols) and c5 N itrate (filled symbols) in the Eastern Tropical North Pacific (ETNP, coastal Baja California), Southern Ocean and North Atlantic (Sargasso Sea). The ETNP shows a large increase in c5 N i ate in the thermocline owing to local water column denitrification. The Southern Ocean shows little deviation from the global deep mean c5 N it te except at the surface, where partial NO3 assimilation leaves residual nitrate enriched in N. The North Atlantic profile shows low c5 N itrate in the thermocline owing to the nitrification of locally fixed N. Note that the ETNP profile also includes deep measurements from near Hawaii (diamonds) the smooth transition between samples at distant locations emphasizes the homogeneity of the deep Pacific.

Sampling sites are also referred to as station locations. For water column work, depth profiles are constructed from seawater samples collected at representative depths. Temperature and salinity are measured in situ with sensors. Remote-closing sampling bottles deployed from a hydrowire are used to collect water for later chemical analysis, either on the ship or in a land-based laboratory. The standard chemical measurements made on the water samples include nutrients (nitrate, phosphate, and silicate), dissolved O2, and total dissolved inorganic carbon (TDIC) concentrations. 

A notable example of a ooastal system in which intense denitrification occurs seasonaiiy is the west Indian shelf and inshore waters, in this setting, denitrification has been observed to oompletely remove ali nitrate and nitrite, enabiing suifate reduction to proceed. Water column depth profiles documenting the spatiai and temporai deveiopment of these conditions are provided in the supplemental information for Chapter 24.4.5 that is available at http //elsevierdirect.eom/companions/97801230885305. 

Depth profiles of cell densities in the photic zone generally show E. huxleyi to live within the mixed layer. Cortes et al. (2001) studied the seasonal depth distribution of coccohthophorid species off Hawaii. Sampling showed that the main production occurred in the middle photic zone (50-100 m), which lay within the mixed layer for most of the year. While the depth of maximum E. huxleyi density varied during the annual cycle, it generally lay between the shallowest sampling level (10 m) and 100 m. Depth profiles off Bermuda (Haidar and Thierstein, 2001) found that maximum densities of E. huxleyi were nearly always shallower than 100 m, and more commonly within the upper 50 m. The highest cell densities for E. huxleyi recorded were at 1 m depth in March, after the seasonal advection of nitrate into the mixed layer. Seven years of water-column particulate data off Bermuda confirm that alkenone concentrations in the surface mixed layer are 2-4 times higher than in the deep fluorescence maximum at 75-110 m (Conte et al., 2001). 

When the data are plotted versus density, all of the profiles from different locations fall together in a narrow range. The data for dissolved oxygen, sulfide, iron, and manganese are shown in Figure 5, and the data for dissolved nitrate, nitrite, ammonia, and phosphate are shown in Figure 6. Features in the water column occur at different depths at different locations, but they always occur close to the same density surface. The only exceptions appear to occur in the region close to the Bosporus, where the Black Sea inflow interleaves with ambient water. 

For density profiling, 100-cm long density gradient columns were prepared using calcium nitrate-water solutions. The columns were calibrated with standard glass calibration balls. Polymer specimens were sectioned Into pieces whose thicknesses were measured with a micrometer, and the density of each piece determined from the equilibrium floating position In the column (7). 

The nutrient uptake by vegetation contributes to nutrient reduction in the soil profile with time. In low-nutrient systems, plants can sequester nutrients from the subsurface soil layers and deposit them on soil surfaces through detrital accumulation and increasing the connectivity of nutrients with water. Vegetative water uptake and transpiration can increase the solute flux from water column into the soil (Figure 14.30). For example, Martin et al. (2003) showed a greater reduction of surface water nitrate concentration in experimental Typha mesocosms with greater rates of evapotranspiration. 

Vertical profiles were made to 70-m depths with a submersible pump at the anchor station. A thermistor, fluorometer, and the inlet for the rFIA system were placed in line with the effluent from the pump. The temperature and chlorophyll fluorescence were recorded continuously. The concentration of nitrate was determined 75 times per hour. The residence time of water in the pump was about 4 min. A typical vertical profile is shown in Figure 11. The cadmium column was switched out of line at each depth to... 

Based on a detailed study of pore water profiles in sediment cores from the eastern equatorial Atlantic, Froelich et al. (1979) were able to show that oxidants are consumed in the order of decreasing energy production per mole of organic carbon oxidized (O > Mn oxides nitrate > Fe oxides > sulfate). A schematic representation of the profiles is shown in Figure 11.7. From this diagram, it is seen that the reduction and remobilization of Mn takes place in zone 4. This is followed by the upward diffusion and reoxidation of Mn in zone 3. This process enables Mn to be stripped from the sediments as they accumulate and to be redeposited as a discrete layer within the sediment column. An example of this process is given in Figure... 

Fig. 11.7 Schematic representation of trends in pore water profiles for the principal oxidants in marine sediments. Depths and concentrations in arbitrary units (after Froelich et al. 1979). This pattern reflects the sequence of reduction of the principal oxidants in the sediment column (02>Mn oxides=nitrate>Fe oxides>sulfate).

Table 15.5 Worksheet (excerpts) from modeling the decomposition of organic substance by dissolved oxygen and dissolved nitrate in a diffusion controlled pore water profile. The model procedure was performed according to the Press-F9-method with an Excel , worksheet The columns are marked in the uppermost row with alphabetical characters, the rows are numbered in the first column. Details pertaining to the model are further explained in the text.

Fig. 22. Profiles from the water (unfiltered) column in the western North Pacific Ocean. Comparison of Nd, salinity and the nutrients, phosphate and silica from Sta. 271-1 at 24 N with Sta. 39-1 at 47°N. Data from Piepgras and Jacobsen (1992), Nitrate closely follows the distribution of phosphate.

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