Does mesozooplankton respond to episodic P inputs in the Eastern Mediterranean?

An in situ P addition experiment was carried out in May 2002 in the Levantine basin of the P-limited Eastern Mediterranean that lasted for 10 days. Mesozooplankton abundance, biomass, taxonomic composition and gut fullness were studied from net samples (0–150 m) collected daily at midnight at the center of the fertilized patch starting the day before P addition. Bottle-collected small zooplankton, concentrations of suspended fecal pellets of major grazers and copepod eggs were also examined. Composition, abundance and biomass of bottle-collected zooplankton were also estimated. The following changes were associated with the P addition: higher fecal pellet concentrations in the upper 45 m, a peak in total integrated biomass of suspended fecal pellets, copepod egg concentrations and gut fullness of herbi- and omnivores (but not carnivores) between 3 and 5 days after P release, decreasing thereafter to pre-release levels. These are interpreted as a rapid feeding and reproduction response of the mesozooplankton communities of the Eastern Mediterranean to phosphorus addition. The suggestion is launched that P addition in this ultra-oligotrophic pelagic environment supported a microbial- and phytoplankton-based food web that was instantaneously top-down controlled by zooplankton. Almost no increase in the standing stock of food for mesozooplankton (except for an increase in ciliate abundance) was encountered, but the stimulated production was discernable in an increase in gut fullness, concentrations of copepods eggs and fecal pellets of larger zooplankton forms.