Vol. 92: 77-87, 1993 I MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published January 26 Spatial distribution of viruses, bacteria and chlorophyll a in neritic, oceanic and estuarine environments William P. Cochlan*, Johan Wikner **, Grieg F. Steward, David C. Smith, Farooq Azam Marine Biology Research Division, 0202, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0202. USA ABSTRACT: The spatial distribution of viruses was investigated in the coastal and oceanic waters of the Southern California Bight, USA, and the brackish waters of the Gulf of Bothnia, Sweden, using the direct harvesting technique and transmission electron microscopy. The vertical and horizontal distribu- tions of viruses were examined in relation to bacterial abundance and chlorophyll a. Total virus abun- dances ranged from 0.3 to 52 X log 1-l; higher concentrations of viruses were found in the upper 50 m of the water column and in coastal environments. Viruses with capsid diameters less than 60 nm dom- inated the virus community, were morphologically characterized as bacteriophages and were respon- sible for most of the observed spatial variability. Bacteria abundance alone explained 67 % of the spa- tial variability in virus numbers, thereby suggesting that bacteria constituted the major host organisms for viruses in these physically dlverse habitats. INTRODUCTION Viruses often numerically dominate the microbial community in marine and freshwater environments with estimates of virus abundance generally ranging from 10' to 10" I-' (e.g. Bergh et al. 1989, Proctor & Fuhrman 1990, Hara et al. 1991, Wommack et al. 1992). The coupled dynamics of virus abundance with bacte- ria and phytoplankton over a spring bloom period have been demonstrated, and marked increases in virus numbers observed within an enclosed seawater sam- ple (Bersheirn et al. 1990, Bratbak et al. 1990). These observations, together with the presence of phage- infected bacteria in seawater samples (Proctor & Present adresses: ' Hancock Institute for Marine Studies, University of Southern California, Los Angeles, California, 90089-0373, USA ' ' Umea Marine Scientific Center. Box 3124, S-903 04. UmeA. Sweden Fuhrman 1990) and numerous isolation of viruses with marine bacteria and algae as hosts (e.g. Cannon 1987, Moebus 1987, Cottrell & Suttle 1991, Suttle et al. 1991, Van Etten et al. 1991), strongly suggest that the major- ity of viruses observed in seawater by transmission electron microscopy (TEM) are indigenous to the aquatic environment, and constitute an active compo- nent of the food web. Given the potential importance of viruses as agents of mortality for phytoplankton and bacteria, and as vectors of genetic information, there is a need for knowledge of their spatial distribution in relation to the distribution of bacteria and phytoplankton. Quantita- tive distribution data may demonstrate the carrying capacity for viruses in different aquatic environments, and thereby indicate the likelihood for potential virus proliferation. Few systematic studies of total virus dis- tribution have been reported (Hara et al. 1991, Wommack et al. 1992) and only 1 with detailed depth profiles (Hara et al. 1991). Both studies were in bays 0 Inter-Research 1993