130 R. T. Prentki et al. 



to 320 Mg P liter ' day"', corresponding to turnover times of 0.23 to 4.6 

 hours. These are about 200-fold greater than necessary for growth of the 

 planktonic algae. These turnover times were very similar to the 0.7 to 1.7 

 hours found for Char Lake (Rigler 1972) but were greater than the 1 to 8 

 minutes generally found in temperate lakes (Rigler 1964, 1973, Lean 

 1973a). 



While some of the inorganic P is returned to the DRP pool from the 

 particulate phase, most of it is transformed into dissolved organic 

 phosphorus. These fractions in the ponds were first followed by a rapid 

 and simple technique based on that of Kuenzler (1970). Water samples 

 containing ^^P were acidified and extracted with isoamyl alcohol. Next, 

 the orthophosphate reagents and more alcohol were added to the water 

 and the phases separated (Barsdate et al. 1974). The fractions were also 

 studied with a ^^P technique developed by Lean (1973b) which uses 

 Sephadex columns to isolate high and low molecular weight pools. Both of 

 these pools cycle rapidly; the high molecular weight material is called 

 colloidal P and the low weight pool is called XP. Comparison of the 

 extraction technique with Lean's method indicated that the extractable 

 organic P pool was colloidal P and the non-extractable organic P pool was 

 XP (see Prentki 1976 for details). In DRP analyses, pondwater colloidal P 

 was hydrolyzed by the non-extraction but not by the extraction procedure. 



The rate of cycling of the DRP to organic P and back to DRP is 

 extremely rapid (Figure 4-19). The data are given in units of mg P m ^ for 

 concentrations and mg P m^ day"' for rates. This assumes that each m^ 

 is overlain by 100 liters of water; these units are used so that later 

 comparisons may be made with sediments. Some of the exchange between 

 DRP and water particulates may be due to exchange with living cells 

 rather than with detritus as indicated (Figure 4-19). However, in either 

 case, the cycling through the organic P phase is not only rapid but is 

 quantitatively the most important pathway (Figure 4-19); this is the first 

 time in\freshwater that a return through the organic phase to DRP has 

 been found to be greater than the direct return from plankton. 



Watt and Hayes (1963) found that dissolved organic phosphorus 

 production from plankton was approximately 3-fold higher than 

 phosphate production, but in this marine system there was no direct 

 hydrolysis of dissolved organic phosphorus to phosphate. Lean (1973b) 

 found phosphorus compartments and pathways in eutrophic Heart Lake 

 similar to those present in the Barrow ponds. However, the direct return as 

 phosphate from the particulates following uptake was 70-fold greater than 

 dissolved organic phosphorus production. 



The XP and colloidal P both lyse to phosphate. The XP is excreted 

 directly by photoplankton (the dissolved organic phosphorus of Kuenzler 

 (1970)) and bacteria (Barsdate et al. 1974). Lean (1976) hypothesized that 

 colloidal P is composed of filaments broken off algae during filtration. 

 However, in pondwater the concentration of colloidal P is much greater 

 than total algal and bacterial phosphorus (Figure 4-19). Thus an earlier 



