Phosphorus stratification

Most lakes affected by eutrophication will also have significant amounts of phosphorus in their sediments, which can be recycled into the water column (Section 4). The control of this source can be achieved by treating the sediments with iron salts or calcite to bind the phosphorus more tightly into the sediments. These methods have been used to some effect, but consideration has to be given to the quality of the materials used and whether or not the lake can become de-oxygenated in the summer. In the latter case this can be overcome by artificial de-stratification. 

The most commonly used physical method for long-term eutrophication control in lakes is that of artificial destratification. This method is well tried and understood and uses either jetted water or compressed air bubbles to break down the lake stratification in the summer months. Algal growth is also affected by an increase in circulation. This is due to the artificial shading effect which results from the algae spending less time near the surface and consequently less time in the light. This technique also reduces the redox-dependent phosphorus release from sediments because the sediment surface remains aerobic. 

Fig. 14-5 Typical distribution of P and temperature in a temperate lake in summer. Thermal stratification restricts exchange between surface and deep wafers. Phosphorus is depleted in the surface waters by the sinking of biologically produced particles.

In addition to carbon, DOM also contains large amounts of nitrogen and phosphorus. As shown in Table 23.1, concentrations of DOC are on the order of tens of micromolar, a few micromolar fitr DON, and tenths of micromolar for DOP. The vertical concentration distributions typically exhibit stratification as exemplified by the profiles in Figure 23.4. 

Figure 8. Downward areal flux (sediment-trap-measured) of phosphorus at 29 m during the period of thermal stratification.

Setaro and Melack (1984) hypothesized that the seasonal changes in nutrient limitation in Lake Calado were related to differences in stratification and mixing. At high water, when the lake was deep and stratified, phosphorus-rich particulates... 

In the Central North Pacific Ocean (CNPO) it has also been reported that phosphorus availabiHty limits N2 fixation and primary production (Karl et al, 2001 a,b) (but iron availability may also play a role as suggested by Wu et al (2000). Karl et al (2001a,b) contend that since the mid 1970s there has been an enhanced stratification in the CNPO and a decreased inorganic nutrient avadabihty which selects for diazotrophs and shifts from a N-hmited phytoplankton population to one that is either P or Fe Hmited (see Karl et al. Chapter 16, this volume). 

Figure 8 Depth profile of O2, bio-available phosphorus (SRP), nitrate (NO3), and anunonium (NH4, 1990 only) concentrations in February 1961 and March 1990 during Lake Victoria stratification (reproduced by permission of E. Schweizerbart Science Publishers from Verh. Int. Ver. LimnoL, 1993, 25, 39-48).

After the winter bloom, water column stratification occurs in March-April, which results in the formation of the DCM that is characteristic of the system for the remainder of the year. The stratification starts in the offshore area in the Southeast Levantine Basin and spreads from there to the north and west. In summer the DCM is typically greater than 100 m deep with the waters above it depleted in inorganic N and P but containing significant amounts of dissolved organic nitrogen and phosphorus. 

Total sulfur concentrations of WCA-2A soils were higher in surface layers (0-25 cm depth) and decreased with depth (Bates et al., 1998) (Figure 17.36). These profiles were similar to those measured for phosphorus by Craft and Richardson (1993a, 1993b) and Reddy et al. (1993). Total carbon content showed no vertical stratification. Molar ratios of carbon to sulfur increased with depth, with... 

Chinese scholars have started research into the transportation of nutrients to the ocean via the atmosphere and its influence on the marine ecosystem in recent years. There were clear seasonal variations for most of the ions, and the concentrations of major ions from mban area rainwater were apparently higher than those in remote regions. By in situ incubation experiments in the coastal Yellow Sea, the atmospheric deposition with high nitrogen and low phosphorus in the Yellow Sea area was the major nutrient resource for ph3doplankton in the mixed layer during the water stratification period in summer. 

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