Chemistry 135 



have found 1 mg P Hter ' in drops on the leaf tips of tundra plants. We 

 have assumed that DRP on the plant surface is washed into the water by 

 rain or dew. The production of phosphorus from this source increases with 

 increasing vascular plant standing stock, reaching a maximum daily 

 production rate of 0.14 mg P m 'of plant stand in early August. This 

 input is important in this low phosphate environment and Carex is a 

 phosphorus pump comparable to the freshwater Nuphar luteum at 0.22 

 mg P m"^ day"' (Twilley et al. 1977). However, both are very poor 

 phosphorus pumps relative to marine Spartina alterniflora at 600 mg P 

 m~^ day ' (Reimold 1972) and Zostera marina at 60 mg P m^ day ' 

 (McRoy et al. 1972) in marine systems. 



Phosphorus leaching from standing dead vascular vegetation is 

 minimal at the mid-summer date of 12 July (Figure 4-19), but is much 

 higher immediately after thaw and again in late summer as leaf senescence 

 increases. Over a 100-day thaw season an average input of 0.72 mg P m ^ 

 day"' has been computed for this pathway. Therefore, when rates are 

 compared over the entire summer, litter decomposition, which is several 

 times greater than runoff and precipitation, is the most important source 

 of phosphorus entering the water column. 



The release of P from the Carex margin of the ponds can be compared 

 to release by the Myriophyllum 5/7Zfa/wm-dominated littoral of Lake 

 Wingra, Wisconsin. Over the ice-free period and on a whole lake basis, 

 weedbeds in Lake Wingra release 760 kg DRP -I- DOP or 2.5 mg P m ^^ 

 day " ' to the central lake basin (Prentki et al. 1979). Thus the contribution 

 of phosphorus by Carex in the ponds is only about 3-fold less than that of 

 macrophytes in a weedy, temperate, eutrophic lake. 



The turnover time for DRP as a result of sediment sorption is 0.8 day, 

 calculated from the disappearance of phosphate in whole-pond 

 fertilizations. The pond water concentration divided by turnover time 

 (0.14 mg DRP m"' - 0.8 day = 0.18 mg P m ' day ') is an estimate of 

 the net uptake and release of DRP by sediment. An independent measure 

 of the flux of DRP between water and sediment comes from a mass 

 balance of other DRP pathways in Figure 4-19. This calculation indicates 

 that there is a net flux of0.17 mg P m ^^ day"' from water to sediment. 



The turnover time of DRP is very short in the ponds. Hayes et al. 

 (1952) reported a range of 2.4 to 40 days for 40 lakes, with the shortest 

 turnover time occurring in the shallowest nonstratified lake. The 0.8 day 

 turnover time in the ponds is due to their shallow depth and rapid 

 convective and wind mixing. 



Unlike the water column, the sediment is dominated by its abiotic 

 components (Figure 4-19). Most of the standing stock of phosphorus in the 

 sediment is sorbed inorganic P, reductant-soluble inorganic P, or dead 

 organic P. In anion exchange resin experiments with shaken sediment, 

 DR ^"P and sorbed P equilibrated within 4 hr. Thus, the rate of exchange in 

 units of Figure 4-19 was greater than 12,600 mg P m ' day '. The in situ 

 rate of exchange in undisturbed sediment would obviously be much slower 



