Heterotrophic, autotrophic, and mixotrophic nanoflagellates: Seasonal abundances and bacterivory in a eutrophic lake

Abstract

Abundances of heterotrophic (HF), autotrophic (AF), and mixotrophic (MF) nanoflagellates in Lake Oglethorpe, Georgia, were in the range 102–104 cells ml−1. Pigmented and nonpigmented flagellate abundances were positively correlated with each other in samples spanning a year, but were not significantly correlated to the same physical parameters (temperature, oxygen, light). The highest density of nonpigmented flagellates (> 1 × 104 ml−1) was found in spring 1986 at the surface, although most abundance peaks ranged from 4 to 7 × 103 cells ml−1 and occurred in the metalimnion during summer stratification. Abundances of pigmented flagellates (AF + MF) were greatest (9 × 103 cells ml−1) in surface waters in early spring and late fall when the lake was not stratified. Up to 38% (avg 10%) of all pigmented flagellates at any depth also ingested particles, while the percentage of HF that were identified as bacterivores ranged from 30 to 100%. In situ grazing rate measurements indicated strong seasonal differences in the relative grazing impact of pigmented and nonpigmented flagellates, with HF dominating grazing on picoplankton except in winter.