Primary Producers 213 



FIGURE 5-23. Productivity of epipelic algae 

 in Pond D after enrichments with P in July 

 and August 1971. (After Stanley 1974.) 



Phosphorus limitation in both the sediments and water of the pond 

 results from strong binding of phosphorus to iron in the sediments (see 

 Chapter 4). The importance of the phosphate exchange reaction between 

 the sediments and water as the rate-limiting step in phytoplankton 

 productivity is seen in the correlation of primary production with 

 phosphorus sorption indices for various Barrow ponds (Figure 4-31) and 

 also in Pond B 1971 experiments where sub-ponds with plastic bottoms 

 were used to eliminate sediment-water exchange processes. An extreme 

 example of the effect of sediment binding capacity occurred in Pond G, a 

 recently formed thermokarst pond with no accumulated pond sediments. 

 Primary productivity in Pond G in late August 1970 was 77 ^lg C liter' 

 hr'' and the total phosphorus concentration was 593 ^g P liter '. This 

 contrasts with 26.2 ^g total P liter ' and 0.31 Mg C liter ' hr ' fixed in 

 the nearby Pond E at about the same date. In the Pond B sub-ponds with 

 plastic bottoms, phosphate concentrations were more than double and 

 phytoplankton biomass 10 times larger than other control sub-ponds and 

 Pond B proper (Table 5-6, Figure 5-2). This presumably occurred because 

 in the sub-ponds with no sediments the phosphate,recycled into the water 

 via bacterial mineralization, zooplankton excretion, and soluble organic 

 phosphate hydrolysis, was utilized for phytoplankton growth rather than 

 sorbed to the sediments as in the controls. 



Phosphorus is the principal cause of the spring phytoplankton bloom 

 and decline. While light and temperature show no major increase or 

 decline over the spring bloom period, there are drastic changes in the PO4 

 concentrations in the water (Chapter 4, Figure 4-16). The /o 5 values 

 (Figure 5-17) and ratios of production to biomass (corrected for 



