Lake eutrophication and the role of phosphours concentration in the eutrophication

In this part we will continue to talk about the eutrophication parameters that affect lakes quality.

One of the major parameter is phosphorus (P), phosphorus is introduced in the lake as a bounded phosphorus to the sediments or as a dissolved phosphorus in the water column.

Holdren investigated the mixing of the water column as a factor that can affect the P release, in the study of Lake Mendota, P concentration increased during mixing times. The increase was attributed to the particulates suspension in water column that lead to an increase in water mixing (Holdren, 1980).

Figure 1 shows the mechanism of phosphorus release and some factors that can affect the releasing of P.

Different seasons influenced the release of P from sediment into the water column. The release of P in oligotrophic lakes (Nürnberg, 1986) was maximum during summer and varied between different summer seasons depending on the extent of anoxic condition of sediments. Iron releasing mechanism did not account for the Soluble reactive phosphorus detected in the water column. Instead, particulate reactive phosphorus – accounted for about 17% of internal phosphorus load (Nürnberg, 1986).

Phosphorus release in eutrophic lakes is usually high during summer. The sediment acts as a source of phosphorus, and the ability of mineral complexes to adsorb phosphorus decreases (Berkheiser, 1980; Perkins, 2011). The phosphorus seasonal patterns are related to the biological activity of lakes. At high temperature. The sediment’s bacteria activity increases and levels of oxygen decrease, such conditions are accompanied by high pHs that favor the phosphate (PO₄^3-) release into water (Redshaw, 1990). During winter the phosphorus retention occurs, and the low biological activity allows for the adsorption of phosphorus into sediments (Sondergaard, 2001). Lake Long in Washington followed that trend during the summer, where at turnover of summer, pH decrease was not followed by the decrease in the phosphorus concentration. This deviation introduced another factor that can participate in increasing P levels, macrophyte decomposition kept P levels high during the turnover of summer (Jacoby, 1982).

Another study of sediment cores from Wisconsin lakes showed the relation between the temperature and the release of phosphorus when the other factors were kept constant. The temperature increase resulted in an increase in P levels in water column. Increasing temperature lead to an increase in the microbial activity in the sediment that resulted in oxygen depletion (Holdren, 1980).

The ice cover during winter has a positive effect on phosphorus concentration. The extent of ice cover increases the possibility of prevailing the reduction condition due to the absence of atmospheric oxygen exchange. The seasonal effect of winter on Lake Peipsi in Russia showed an increase in phosphorus concentration during long severe winters. In meanwhile, the years between 1997 and 2007 experienced mild winters and a shorter ice cover of the lake. The less extent of the ice cover showed a significant decrease in the phosphorus concentration during mild winter (Blank, 2009).

Nicholls 1998 studied the overall concentration of Great Lakes of North America in years where lakes experienced mild winter. His results showed that the overall concentration of phosphorus in lakes increased in years with mild winters. The explanation of decease can be due to the mild winters were shorter, and the lakes were subjected to longer warmer weather that leads to increasing of eutrophication (Nicholls, 1998).

Oligotrophic lakes experience similar low, releasing rates of phosphorus during winter, which enable them to control the phytoplankton of the lakes. Waikaremoan Lake (New Zealand) is an oligotrophic lake that has low phosphorus availability during winter, and the regeneration of phosphors into water takes long periods. The long regeneration enables the lake to control, and limit the phytoplankton growth (Vincent, 1983).

The effect of pH on the phosphorus concentration differs due to the different forms of phosphorus. Phosphorus fractions can be extracted as liable-P form, redox, and metal oxide forms, and calcium form (Fytianos, 2005) (Kaiserli, 2002). Table 1 shows different particulate P forms and pHs at which P release is maximum.

Table 1 Phosphorus forms release in response to different pHs

State	particulate P forms	Releasing pH
Loosely adsorbed (Kronvang, 2012)	NH4Cl-P	High
Calcium bound -P (Hieltjes AH, 1980)	HCl-P	Low
Redox sensitive (Fe-P) (Kaiserli, 2002)	BD-P	High
Metal oxide bound –P (Fe or Al) (Olila, 1995)	NaOH-P	High

The calcium bound-P P(HCl-P) release increases by decreasing pH. At low pH, calcium bound phosphorus becomes unstable. Swan Lake (China) showed very high Ca²⁺ concentrations at acidic pH (below 3). The strong acidic conditions resulted in HCl-P dissociation, and the concentration of P increased in the lake (Gaoa, 2012).

Nur R. et.al studied the effect of pH on the calcium bound P, the study investigated sediments from Lake Carl Blackwell (Oklahoma), where the sediments pHs were altered to see the amount of released fractions at different pHs. The calcium-bound phosphate fraction fixation was minimum at acidic conditions (Nur, 1979).

The loosely P(NH₄Cl-P) is another form of P bound forms, P(NH₄Cl-P) associated with hard-water lakes, CaCO₃ and the pore water-P  that comes from leached P after decaying bacterial cells during summer stratification (Rydin, 2000; Pettersson, 2001). Inorganic orthophosphate in NH₄Cl-P is the readily form used by algae, this means increasing this soluble inorganic form would promote algae blooms (Ribeiro, 2008). Increasing the depth results in decreasing the soluble NH₄Cl-P, due to the fact that acidity increases by increasing that depth. Moreover, some loosely adsorbed P would rebounds with CaCO₃ at the bottom of the lake. (Gonsiorczyk, 1997)

 The other form of P is the reductant soluble phosphorus (BD-P). This form is the form of inorganic phosphorus bound to iron hydroxide or manganese. During anaerobic condition (low oxygen) the BD-P dissolved form becomes available for algal (Kaiserli, 2002).

Usually, the amount of metallic oxide bound to organic phosphorus in eutrophic lakes is higher than in oligotrophic lakes. The high percentage of released P comes from the role of metallic oxide in internal loading (Ribeiro, 2008). Metal oxide bound –P (Fe or Al) represents the largest portion responsible for P release in lakes, thus causing lake eutrophication. The metal bound phosphate (NaOH-P) is Al,

 or Fe oxides, for Al oxide the adsorbed NaOH-P occurs at the surface of the metal oxide, where adsorption on Fe oxide is interior. In NaOH-P form, P is bound to metal oxide, and the exchange between NaOH and P allows for the release or retention of P in metal complexes. These processes are pHs and oxic states dependent. During aerobic condition, bounding sites of Al and Fe complexes are reduced, and the desorbed phosphate is released into water column leading to the internal loading (Jacoby, 1982; Jensen, 1992).

pH controls the ion exchange between OH^- and PO₄^3- , at high pH the ligand exchange occurs, where OH^- replaces PO₄^3- in the metal complex, leading to high release of NaOH-P in water (Christophoridis, 2006). Most likely, OH^- and PO₄^3-  ion exchange occurs at the surface of the sediment due to the redox condition during summer where anaerobic state prevailed. Lake Swan (China) experienced a high release of total phosphorus as a result of releasing phosphorus (NaOH-P+BD-P) forms at alkaline conditions (Gaoa, 2012).

Besides the effect of pH on the oxide bound –P, the oxic state of the surface sediment affects the ferric oxyhydroxide oxidation states thus affects the phosphorus levels.

Patrick et.al. study of P sorption/desorption on the surface of sediment ferric oxy-hydroxide showed the relation between aerobic/anaerobic conditions and sediment reduction state. The limit oxygen availability at water-sediment interface (anaerobic state) enhanced the reduction of gel-like ferrous compounds, and lead to the formation of P ferric soluble form. In the study, it was argued that the effect of the anaerobic environment has a greater influence on the releasing P than pH. The reason was that, soluble ferric oxyhydroxide form dominates at aerobic condition and it would bound to P firmly than the ferrous oxyhydroxide (Patrick, 1974).

Boström et. al. investigated the combined conditions role on phosphorus metabolism of lakes (Boström, 1988). The phosphorus is fixed to the sediment by bounding to Fe(III), but once the iron is reduced, the phosphorus is released into the water column. The high concentration of sediment internal loading often leads to high phosphorus release conditions. The study of pH effect on the sediment release showed that increasing pH in the water column increases the released phosphorus in water. This effect of pH was different in lakes that contains CaCO₃. At high pH, the high concentration of released phosphorus bounds to CaCO₃ forming hydroxyapatite, or it can be adsorbed to the precipitate in lakes that contains a high concentration of CaCO₃ (Boström, 1988). Figure (2) show the relationship between phosphorus concentration and different aeration states. 

Figure 2. Phosphorus exchange between sediment and water column in different conditions (Boström, 1988)

The Natural nitrogen as seen above concentration in lakes is limited, the excess concentration of nitrogen comes from agriculture, and NO₃^- nitrification pollution from the atmosphere. In agriculture, the excess use of fertilizer used to increase the nitrogen input in solids can end up in streams and lakes. Still, the eutrophication does not show direct relation due to the nitrogen input in the lake system. For only TP (total phosphorus) between 30-100 mg/m³ to consider a lake to experience eutrophic conditions, around 650 – 1200 mg/m³ of TN (total nitrogen) concentration is needed (Smith, 1999). Lake Colorado Front Range experienced large input of NO₃^- due to heavy agriculture activities, the high input did not reflect any increase in lake’s nutrient. Only the increase in the nutrient concentrations was seen either by the addition of P or as combined N+P addition. While Lake Wyoming Snowy Range showed an increase in the photosynthetic rate due to the increase of the N or N+P input to the lake, the enrichment of N alone altered the biotic life that can act indirectly to eutrophication (Fenn, 2003).

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