Chemistry 175 



An experiment in a microcosm demonstrated that bacteria took up 

 DRP and excreted an XP-like organic compound. In a mixed culture of 

 bacteria alone, the rate of uptake was 2.5 x 10 ' Mg P cell ' hr ' but 

 when a ciliate was introduced the rate rose to 16.6 x 1 ' ^g P cell ' hr ~ ' . 

 However, the ciliates could excrete only 0.4 ng P liter"' while bacterial 

 uptake was 9.3 Mg P liter"'. Thus, release of P by the ciliates was not 

 responsible for the increased activity of the grazed bacteria. Instead, the 

 difference is probably due to the physiological differences between the 

 rapidly dividing bacteria in a grazed system and the relatively static 

 bacteria of ungrazed systems. 



The rooted aquatic plants cycle phosphorus from the sediments to the 

 water through leaching from dead plants and by excretion from live plants. 

 Carex leaves harvested when green lost 60% of their phosphorus during the 

 first month of immersion (34 ng P (gdry wt)"' day"'). Live Carex lost 1.1 

 ng P (g dry wt) " ' day " ' or a maximum daily rate of 0. 14 mg P m " of 

 plant stand. This is a large amount relative to the phytoplankton needs as 

 the quantity approximates the amount of DRP present. Leaching from the 

 standing dead plant leaves averaged about 0.72 /ug P m ^ day"' so this 

 pathway is even more important than the excretion pathway. Phosphorus 

 transfer rates from the sediments to the water are rapid enough so that all 

 the DRP can be replaced in about 0.8 day. 



Control of Phosphorus 



To understand the interactions of phosphorus in the ponds, it is not 

 enough to know the quantities that are added to the pond water; we also 

 have to know what happens to this added phosphorus. Why is the DRP 

 concentration so low? To answer these questions requires a detailed study 

 of the sediment because almost all the phosphorus that enters the pond 

 moves rapidly to the sediments. There it is either retained in an organic or 

 occluded inorganic form or is strongly sorbed to the sediments. The sorbed 

 phosphorus is available for interaction with the water phase; most lakes 

 appear to have sorption rather than precipitation as the primary means of 

 phosphate fixation in the sediments. 



Chemical adsorption phenomena can usually be described by an 

 isotherm equation derived from certain assumptions about the energy with 

 which individual sorbed ions or molecules are held. The best fit to our data 

 was given by the Temkin isotherm equation: 



XX^-' = RTb' (In^O 



where X is the sorbed phosphorus per unit of sorbent, X m is the sorption 

 maximum, R is the gas constant, Tis the temperature in °K, b and g are 

 constants, and C is the equilibrium phosphate concentration. This 



