Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). II. Mesozooplankton abundance, biomass, depth distribution and grazing

Abstract

Mesozooplankton (202 to 2000 μm) biomass, abundance, taxonomic composition, depth distributions and gut pigment contents were measured inside and outside of an iron-enriched patch during the IronEx II study in the eastern equatorial Pacific. Mean carbon biomass remained nearly constant in the ambient community, but increased 2- to 3-fold during early stages of the phytoplankton bloom. The increases were due primarily to small calanoid and cyclopoid copepods and copepod nauplii in the mixed layer and appeared to be the result of 2 processes. First, significantly higher abundances of nauplii in the patch indicated that adult copepods responded reproductively, at least initially, to the increased food. Second, changes in copepod vertical migratory behaviors in response to reduced light penetration and increased food abundance in the patch apparently resulted in an upward displacement of copepods from the lower euphotic zone into the mixed-layer. Mesozooplankton gut pigment content also increased significantly inside the patch, largely in proportion to the increased concentration of phytoplankton chlorophyll a, and estimates of carbon consumed suggest that mesozooplankton standing stock was growing at maximal, or near maximal, temperature-dependent rates (1.0 d-1) at the peak of the patch bloom. Nonetheless, zooplankton abundance and biomass declined, rather than increased, during this period. The premature decline of mesozooplankton in the patch suggests that they might have been cropped by their predators in a tightly coupled trophic network or that their reproductive output may have failed to produce viable young when the food resources were dominated by diatoms.