Picoplankton Associations in an Ultra-Oligotrophic Lake on Vancouver Island, British Columbia

Abstract

Epiphytic associations or "microaggregates" of picoplankton with larger phytoplankton in Sproat Lake, British Columbia, occurred in the epilimnion throughout the 1-yr study, but were most frequently seen at times of severe nutrient limitation in summer and autumn. Both bacterial and algal picoplankton epiphytes were attached to the cell surface of algal hosts by either fine protrusions or by their fibrillar extracellular matrix (glycocalyx). This surface coat increased in thickness as ambient nutrient concentrations became limiting, and harboured a range of heterogeneous organic and inorganic particles and phytoplankters. The phytoplanktonic host cells for these picoplankton associations included filamentous cyanobacteria, dinoflagellates, chlorophytes, coccolithophores, and diatoms. Under controlled nutrient-depleted conditions, the filamentous nitrogen-fixing cyanobacterium Anabaena flos-aquae became a common host of chroococcoid cyanobacterium. Some picoplankters were endophytic within the cytoplasm and vacuoles of protozoa and microflagellates, serving either as prey or as symbionts. These associations are thought to facilitate rapid carbon and nutrient recycling in pelagic ecosystems.