Prototrophic and apochlorotic components of picoplankton and nanoplankton in the North Atlantic: geographic, vertical, seasonal and diel distributions

Chloroplast-containing and apochlorotic cells of nanoplankton (2 to 20 pm), and chroococcoid cyanobacteria and total bacteria in the picoplankton (0.2 to 2.0 pn) were enumerated by epifluorescence microscopy from 39 estuarine and oceanic stations to assess numerical relations of apochlorotic nanoplankton with other components of the microbial plankton. Apparent relations between different plankton types were found over geographic separation as well as seasonal and diel cycles. Population densities of apochlorotic, heterotrophic nanoplankton (Hnano) and chloroplastcontaining, phototrophic nanoplankton (Pnano) were similar, ranging from 104 cells ml-' in estuarine environments to 10' cells ml-l at oceanic stations. All microbial populations in the euphotic zone showed exponential decreases in concentration with increasing bottom depth. Vertical profiles showed decreasing abundance of all microbial populations with depth. Studies in Narragansett Bay, Rhode Island, indicated that Hnano, Pnano and total picoplankton (Tpico) followed trends similar to each other over winter/spring and summer blooms. Hnano were positively correlated with Tpico populations one week earlier, suggesting a predator/prey relationship. Die1 studies at oceanic stations and in mesocosms in Narrangansett Bay revealed positive correlations between Tpico and Hnano populations which may represent small interval temporal changes. These data suggest a n abundant and dynamic Hnano population in the marine plankton which reflects changes observed in other components of the microbial plankton.