MEPS 192:103-118 (2000)  -  doi:10.3354/meps192103

ABSTRACT: Nanoplankton and picoplankton abundance and community grazing on picoplankton were determined in summer and autumn at several stations in a productive coastal environment (Georges Bank, NW Atlantic Ocean) and in an oligotrophic oceanic ecosystem (Sargasso Sea). Ranges of heterotrophic nanoplankton (HNAN) abundance were 1.2 to 3.6 x 10³ cells ml^-1 on Georges Bank, and 2.2 to 6.8 x 10² cells ml^-1 in the Sargasso Sea. Ranges of phototrophic nanoplankton (PNAN) abundance in these ecosystems were 1.9 to 6.0 x 10³ and 1.3 to 4.7 x 10², respectively. Mixotrophic nanoplankton (MNAN), operationally defined here as chloroplast-bearing nanoplankton that ingested fluorescent tracers, comprised an average of 12 to 17% of PNAN in surface waters in both environments during August and October. Mixotrophs at specific stations constituted as much as 38% of total PNAN abundance on Georges Bank and 30% in the Sargasso Sea. Mixotrophs represented up to 39% of the total phagotrophic nanoplankton abundance (MNAN/[MNAN + HNAN]). Community grazing impact was estimated from the disappearance of fluorescent prey surrogates (fluorescently labeled bacteria, FLB; cyanobacteria, FLC; and <\3 µm algae, FLA). Absolute grazing rates (total picoplankton cells removed d^-1) on Georges Bank exceeded those in the Sargasso Sea due to the greater abundances of predators and prey. However, there was overlap in the specific grazing losses at the 2 sites (ranges = 0.08 to 0.38 d^-1 in the coastal ocean and 0.05 to 0.24 d^-1 in the oligotrophic ocean). Rates of bacterivory were in approximate balance with rates of bacterial production (³H-thymidine uptake), but production exceeded bacterivory on Georges Bank during the summer cruise. These data are among the first documenting the impact of grazing on picoplankton in these environments, and they are consistent with the prediction that nanoplanktonic protists are major predators of picoplankton. While the proportion of phototrophs that are phagotrophic was highly variable, our study indicates that algal mixotrophy is widespread in the marine environment, occurring in both coastal and oligotrophic sites, and should be considered quantitatively in microbial food web investigations.

KEY WORDS: Nanoplankton/picoplankton interactions · Mixotrophy · Bacterivory · Herbivory · Microbial food web · Flagellates · Cyanobacteria