N-CYCLING AND P-CYCLING COUPLED BY GRAZERS ACTIVITIES - FOOD QUALITY AND NUTRIENT RELEASE BY ZOOPLANKTON

In order to clarify the role of zooplankton in the cycling of elements and its subsequent effect on algal populations, food elimination and nutrient release rates by zooplankton in a small pond dominated by daphnids were estimated for nitrogen and phosphorus from April to September. Comparison of the elemental contents of food and zooplankton showed that N was the element in shortest supply in the food sources utilized by zooplankton. The net clearance rate of the zooplankton was higher for N than for P and C throughout the study period, indicating that these organisms can eliminate the scarcest element from the food pool with a higher efficiency. Soluble N and P released by the zooplankton consisted mainly of ammonium nitrogen and soluble reactive phosphorus, and on a dry mass basis N release ranged from 0.27 to 1.05 mug.mg-1.h-1 and P release from 0.045 to 0.30 mug.mg-1.h-1. Statistical analysis revealed that the N release rate was most affected by food abundance, whereas the P release rate and N:P release ratio were affected by the N:P ratio of the food. The net production rate of the zooplankton, estimated from the difference between elimination and release rates, showed a quite similar N:P ratio to that of the body tissue, indicating that zooplankton can keep their elemental composition roughly constant even if the N:P ratio of their food supply varies. Thus, the N and P release rates of zooplankton are linked through processes maintaining the elemental composition of body tissues. The present results indicate that the cycling rate of an element in pelagic systems depends highly on the grazer's requirements as well as the relative abundances of other elements. Zooplankton can modify the nutritional environment for algae through accumulation and regeneration processes, as suggested by recent theoretical models.