The balance between grazing and viral lysis of both photosynthetic and heterotrophic microbes determines the food resources that are available, and how much of each resource is present, for higher trophic levels. Mobile Bay is the fourth largest estuary in the USA, based on river discharge, and is home to a variety of nursery habitats for larvae and juveniles of economically important species which depend for their food on production of organisms of microbial size. To characterize the patterns of growth and removal rates of this fraction of organisms in Mobile Bay, we carried out modified dilution experiments in summer and winter at 3 stations along a salinity and nutrient gradient. Rates of growth, grazing and viral lysis for prokaryotes and dominant phytoplankton (cyanobacteria, diatoms and dinoflagellates) were determined for each experiment. Grazing and viral lysis of prokaryotes did not appear to be related to season or location, and both processes were detected within the bay as well as on the shelf. For phytoplankton, grazing was the dominant removal process inshore in the summer, while viral lysis of phytoplankton was detected in the fall and in winter. In winter, an apparent increase in dinoflagellates, and in the overall biomass of phytoplankton, was concurrent with increased viral lysis, suggesting that recycling by viruses may be an important source of nutrients in coastal waters.KEY WORDS: Grazing · Viral-mediated mortality · Phytoplankton · Prokaryotes · Estuarine · Microbial food web Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 65: [143][144][145][146][147][148][149][150][151][152][153][154][155][156][157] 2011 Landry et al. 1995) for several years in a range of environments. In many instances, growth and grazing appear to be nearly balanced (Juhl & Murrell 2005, Palomares-García et al. 2006, Putland & Iverson 2007, Paterson et al. 2008, suggesting a strong top-down control of phytoplankton abundance. Viral lysis has been measured in fewer systems, but in many studies, lysis also appears to be a factor controlling the abundance of prokaryotes (Wilhelm et al. 2002, Boras et al. 2009, Karuza et al. 2010) and, in a few instances, phytoplankton (Baudoux et al. 2006(Baudoux et al. , 2008. There are fewer studies in which both grazing and viral lysis have been measured, but often grazing appears to be the dominant process in controlling host abundances (Guixa-Boixereu et al. 1999, Pedrós-Alió et al. 2000, Evans et al. 2003, Kimmance et al. 2007, Baudoux et al. 2008. It has been suggested that, as grazing is less specific and occurs at higher rates, grazing controls the abundance of prey while viral lysis -being a species-or strain-specific interaction -controls community diversity (Weinbauer 2004, Longnecker et al. 2010.When both grazing and viral lysis have been estimated, it is often observed that grazing increases both host productivity and viral lysis, with the grazers and viruses acting synergistically and producing results that are no...