MIXOTROPHIC NANOPLANKTON IN OLIGOTROPHIC SURFACE WATERS OF THE SARGASSO SEA MAY EMPLOY PHAGOTROPHY TO OBTAIN MAJOR NUTRIENTS

The vertical distribution and abundance of mixotrophic nanoplankton was examined during two cruises to the Sargasso Sea south of Bermuda. Fluorescently labeled bacteria and cyanobacteria were used as tracers of ingestion in experiments designed to determine abundances of mixotrophic nanoplankton. Phagotrophic nanoplanktonic (2-20 mu m) algae ranged from undetectable to >100 ml(-1) and were more abundant near the surface (up to 140 ml(-1)) than in the deeper euphotic zone. On two occasions, 50% of the phototrophic nanoplankton in surface waters were observed with ingested fluorescent tracers. The contribution of mixotrophic algae to the total phototrophic nanoplankton assemblage in the deep chlorophyll maximum, however, did not exceed 0.5%. It is possible that mixotrophic algae were more abundant in the deep chlorophyll maximum, but were not phagotrophically active. Two 4 day experimental incubations were subsequently carried out to examine the adaptive significance of phagotrophic behavior for algae in surface waters of the Sargasso Sea. Greater mixotrophic nanoplankton abundances were observed in treatments that received no nutrient inputs and were limited by the availability of inorganic nutrients during the experiments. A decrease in the abundance of mixotrophic algae or a decrease in their phagotrophic activity occurred with nutrient enrichment. Based on the experimental results, we suggest that phagotrophy was a mechanism by which these algae supplemented nutrient acquisition during periods of low dissolved nutrient concentrations. Higher abundances of mixotrophic nanoplankton observed in the upper 50 m of the Sargasso Sea may have been due to the generally low nutrient concentrations in these waters.