Abundance and productivity of heterotrophic nanoplankton in Georgia coastal waters

Abstract

The population abundances and rates of biomass production of heterotrophic nanoplankton (HNAN) in Georgia coastal waters were evaluated by epifluorescence microscopy. HNAN populations (mostly non-pigmented microflagellates <10 μm in diameter) ranged from 0.3 × 10 3 cells ml −1 in shelf waters 15 km offshore to 6.3 × 10 3 cells ml −1 in waters 0.25 km from the coast. There was a strong correlation ( r = 0.83) between HNAN and free bacterioplankton population abundances, but no apparent relation ( r = 0.38) between HNAN and phototrophic nanopLankton (PNAN) abundances. HNAN biomass production in estuarine and nearshore shelf waters, as estimated from increases in HNAN populations during laboratory incubations of natural water samples, ranged from 0.10 to 0.79 mg C m −3 h −3 , with population generation times of 9.7 to 26.5 h. There was a significant linear relation ( r = 0.95) between HNAN biomass and HNAN productivity. We calculated that HNAN may graze at least 30% to 50% of daily bacterioplankton production in Georgia coastal waters.