IMPLICATIONS OF ZOOPLANKTON STOICHIOMETRY ON DISTRIBUTION OF N AND P AMONG PLANKTONIC SIZE FRACTIONS

Pooled samples from the upper 20 m at five stations in the Oslofjord ( Norway) were size fractionated and analysed for particulate dry weight , carbon, nitrogen and phosphorus. The nano fraction (0.7-20 mu m) dom inated in biomass throughout the sampling period. The C:N ratios of th e fractions did not differ much from each other. The C:P ratios of the nano- and microfraction (20-200 mu m) were considerably higher than t he ratio of the mesofraction (200-2000 mu m) throughout the sampling p eriod. High C:P ratios and low phosphate concentrations above the pycn ocline suggest that the system was P-limited. The stoichiometry of mes ozooplankton was more constant than the stoichiometry of the other fra ctions, and the zooplankton constituted consistently a higher percenta ge of the phosphorus pool than of the carbon pool. This suggests that the mesozooplankton can act as a sink of nutrients due to its invariab le stoichiometry.