Chemistry 133 



sestonic uptake of released phosphate. The added phosphate had no short- 

 term effect on the Daphnia excretion rate. After 10 minutes, the Daphma 

 were removed from the bottle, the water was filtered and DRP-^^P was 

 extracted from the filtrate. After correcting for sestonic uptake of '^P, 

 60% of the excreted *'P was found to be in the form of PP and 40% in the 

 form of DRP. No excretion of DOP was found; this agrees with data from 

 other species o^ Daphma (Rigler 1961, Peters and Lean 1973). 



In 1971, Daphnia middendorffiana made up 80% of the zooplankton 

 biomass in Pond B in mid-July while fairyshrimp and copepods 

 constituted only 10% each. Like Daphnia, fairyshrimp appear to excrete 

 dissolved phosphorus predominantly as DRP while Lepidurus excretes 

 DOP. However, on a quantitative basis, the movement of phosphorus 

 through zooplankton other than Daphnia is minor and has been calculated 

 from the same ratio of excretion to biomass as for Daphnia. 



The overall turnover time of phosphorus in pond zooplankton is 29 

 days (Figure 4-19), a very low excretion rate. Direct dissolved phosphorus 

 excretion measurements on Lepidurus in 1-hour experiments show the 

 same thing as the rates were only 14-20% of that predicted by the Johannes 

 (1964) regression model for zooplankton. Similar "too low" P excretion 

 rates were reported for animals in coral reefs, another low phosphorus 

 ecosystem, by Pomeroy and Kuenzler (1969). The low amounts of 

 phosphorus in the reefs had their greatest effect on herbivore excretion; 

 herbivorous fishes received just enough phosphorus in their diet to meet 

 growth and reproduction requirements. 



In Figure 4-19, it appears that phosphate excretion from zooplankton 

 is sufficient to meet 100% of algal and bacterial phosphorus requirements 

 for growth (0.13 Mg P liter" ' day"'). Barlow and Bishop (1965) also found 

 that zooplankton in late summer in Cayuga Lake could regenerate 

 sufficient phosphorus to meet growth requirements of phytoplankton 

 during that period. Martin (1968), however, found that zooplankton 

 excretion could exceed phytoplankton demand for phosphorus during 

 periods of simultaneous low productivity and low phosphate, but not 

 during periods of high phytoplankton abundance in Narragansett Bay. 

 The situation varies from lake to lake; studies reviewed by Larow and 

 McNaught (1978) found that zooplankton excretion provided 19-200% of 

 the phosphorus needed for summer algal growth. In our study, the 

 requirement for growth is negligible compared to the total quantity of 

 phosphorus cycling through algae and bacteria. 



Benthic bacteria were studied via decomposer microcosms (details in 

 Barsdate et al. 1974). Phosphorus kinetics were investigated with '^'^P in 

 systems with only bacteria, with bacteria plus the ciliate Tetrahymena 

 pyriformis, and with bacteria plus mixed microfauna populations. The 

 systems were sampled at intervals and the radioactivity fractionated [see 

 Figure 2 of Barsdate et al. (1974)]. Phosphate- '"P first appeared in cells in 

 an organic form that had alcohol and phosphate-reagent extraction 

 characteristics similar to XP. The XP-like organic phosphorus plus the 



