NITROGEN ISOTOPIC-RATIOS IN FECAL PELLETS PRODUCED BY MARINE ZOOPLANKTON

At each site and in every season studied, Zooplankton in the upper ocean produced fecal pellets that were 1.3%. lower in δ15N than their body tissue but 2.2%. higher than their apparent food source. 14N-containing molecules are evidently preferentially assimilated in Zooplankton intestinal tracts, though other isotopic effects must account for the enrichment in 15N of these organisms relative to their food. These results also show Zooplankton to be important modifiers of nitrogen isotopic ratios for marine particulate matter. δ15N values for sinking particles and fecal pellets are similar, supporting the perspective that macrozooplankton are important factors in producing and processing particles that sink into the ocean's interior and sediments. In contrast, the relationship in δ15N between fecal pellets and suspended particles in the euphotic zone is more variable. It appears that zooplankton select food particles of varying δ15N from the suspended particle pool. These results suggest that both Zooplankton feeding behavior and their digestive chemistry are important in determining the composition of sinking particulate matter in the ocean with respect to the suspended particle source in the euphotic zone. © 1990.