AQUATIC MICROBIAL ECOLOGY Aquat Microb Ecol Vol. 61: 45–56, 2010 doi: 10.3354/ame01434 Published online September 9 INTRODUCTION Heterotrophic protists are major members of the microbial community in aquatic and terrestrial envi- ronments that enhance nutrient cycling, can be the major source of mortality for bacteria and microalgae, and provide a critical link between lower trophic levels and higher-level consumers (e.g. Caron & Goldman 1990, Griffiths 1994, Calbet & Landry 2004, Adl & Gupta 2006). In aquatic systems, much of the research on the role of heterotrophic protists has focused on cil- iates and flagellates (e.g. Capriulo 1990, Strom 2000, Ichinotsuka et al. 2006). However, current evidence suggests that amoeboid protists deserve greater atten- tion given their abundance and diversity in a broad range of habitats. Amoeboid protists are ubiquitous, single-celled eukaryotes characterized by amoeboid motion and the presence of ≥1 locomotory pseudopods (Page 1983, 1988). Current classification systems, based on fine © Inter-Research 2010 · www.int-res.com *Email: amylesen@gmail.com Heterotrophic microplankton in the lower Hudson River Estuary: potential importance of naked, planktonic amebas for bacterivory and carbon flux Amy E. Lesen 1, * , Andrew R. Juhl 2 , O. Roger Anderson 2 1 Biology Department, Dillard University, 2601 Gentilly Blvd., New Orleans, Louisiana 70122, USA 2 Lamont-Doherty Earth Observatory, Division of Biology and Paleo Environment, PO Box 1000, 61 Route 9W, Palisades, New York 10964, USA ABSTRACT: The present study is the first to simultaneously document the contributions of bacteria, heterotrophic flagellates, ciliates, and naked, planktonic amebas to the carbon (C) budget of an estu- arine water column, and is also the first study of protistan bacterivory in the lower Hudson River Estu- ary (HRE). Observations were collected at a single near-shore location between June 2006 and May 2009. Bacterial counts and biomass varied approximately 1 order of magnitude on different dates, but were comparable to previous studies of the HRE and other estuaries. Of the 3 heterotrophic protist groups enumerated, heterotrophic nanoflagellates were the least variable and generally had the highest biomass (on average equaling 38% of the bacterial biomass). Counts and biomasses of cili- ates and amebas were highly variable, ranging over at least 3 orders of magnitude between sampling dates. Much of the variability in ameba abundance was consistent with previous observations of sea- sonality. Ciliate biomass averaged 8%, and ameba biomass averaged 15% of the bacterial biomass. Thus, at this location, the importance of amebas as micropredators may be comparable to that of the ciliates, a group generally receiving greater research attention. Ameba ingestion rates could not be measured directly but 3 indirect approaches for calculating ingestion rates produced mean values ranging from 1.2 to 2.5 ng C d –1 ng –1 ameba biomass. Each approach demonstrated that ameba C consumption at the study location was highly variable, but was at times high relative to the bacterial standing stock. Taken together, these data suggest that amebas may be more common and of greater importance in estuarine C-fluxes than generally appreciated. KEY WORDS: Amoeboid protists · Bacterivory · Bactivory · Grazing · Microzooplankton · Microbial ecology · Ameba · Amoeba Resale or republication not permitted without written consent of the publisher