Heterotrophic and mixotrophic nanoplankton predation on picoplankton in the Sargasso Sea and on Georges Bank

Nanoplankton and picoplankton abundance and community grazing on picoplankt on were determined in summer and autumn at several stations in a productive coastal environment (Georges Bank, NW Atlantic Ocean) and in an oligotroph ic oceanic ecosystem (Sargasso Sea). Ranges of heterotrophic nanoplankton ( HNAN) abundance were 1.2 to 3.6 x 10(3) cells ml(-1) on Georges Bank, and 2 .2 to 6.8 x 10(2) cells ml(-1) in the Sargasso Sea. Ranges of phototrophic nanoplankton (PNAN) abundance in these ecosystems were 1.9 to 6.0 x 10(3) a nd 1.3 to 4.7 x 10(2), respectively. Mixotrophic nanoplankton (MNAN), opera tionally defined here as chloroplast-bearing nanoplankton that ingested flu orescent tracers, comprised an average of 12 to 17% of PNAN in surface wate rs in both environments during August and October. Mixotrophs at specific s tations 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 mu m alga e, FLA). Absolute grazing rates (total picoplankton cells removed d(-1)) on Georges Bank exceeded those in the Sargasso Sea due to the greater abundan ces of predators and prey. However, there was overlap in the specific grazi ng 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 wer e in approximate balance with rates of bacterial production (H-3-thymidine uptake), but production exceeded bacterivory on Georges Bank during the sum mer cruise. These data are among the first documenting the impact of grazin g on picoplankton in these environments, and they are consistent with the p rediction that nanoplanktonic protists are major predators of picoplankton. While the proportion of phototrophs that are phagotrophic was highly varia ble, our study indicates that algal mixotrophy is widespread in the marine environment, occurring in both coastal and oligotrophic sites, and should b e considered quantitatively in microbial food web investigations.