Meso‐ and microzooplankton grazing in the Amazon River plume and western tropical North Atlantic

Abstract

AbstractLargely due to size differences, mesozooplankton are important exporters of carbon and prey for larger organisms, while microzooplankton are important recyclers of nutrients, dominant grazers of phytoplankton, and a key link in the microbial loop. We investigated the relative importance of meso‐ and microzooplankton grazing in the western tropical North Atlantic Ocean (WTNA) and Amazon River plume. Sampling as part of the ANACONDAS project occurred in spring (May–June) 2010 during the peak outflow of the Amazon River and in fall (September–October) 2011 during the plume seasonal retroflection. Mesozooplankton grazing rates decreased with increasing salinity in both seasons, but during the fall both day and nighttime grazing rates were significantly negatively correlated with salinity. Mesozooplankton grazing was highest in plume‐influenced surface waters (0–25 m), and usually dominated by smaller size classes (0.2–0.5 mm and 0.5–1.0 mm). Microzooplankton grazing accounted for approximately 68% of bulk phytoplankton growth across all stations. Comparison of meso‐ and microzooplankton grazing suggests a transition in food web dynamics from a mesozooplankton dominated “export” structure in the plume transitioning to a microzooplankton dominated “retention” structure at mesohaline and oceanic stations above sea surface salinity of 33. Comparison between the seasons suggests a seasonal planktonic succession of low mesozooplankton grazing during the spring peak discharge followed by higher grazing rates and impact by mesozooplankton during the fall retroflection. These results provide important baseline information required for examining effects of climate change on the planktonic food web of the WTNA and for use in biogeochemical models of the region.