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Viruses are recognized as ubiquitous components of marine ecosystems; however, there has been limited study of viral abundance and its ecological role in sediments. Viral abundance was determined in both the water column and sediments of a eutrophic (Brisbane River/Moreton Bay; 27Њ25ЈS, 153Њ5ЈE) and oligotrophic (Noosa River; 26Њ15ЈS, 153Њ0ЈE) estuary in subtropical Queensland, Australia. Viruses, bacteria, and microalgae from both water column and extracted sediment samples were enumerated using SYBR Green I staining and epifluorescence microscopy. Sediment viral abundance ranged from 10 7 to 10 9 particles cm Ϫ3 of sediment, bacterial abundance ranged from 10 7 to 10 8 cells cm Ϫ3 of sediment, and microalgal abundance ranged from 10 4 to 10 5 cells cm Ϫ3 sediment. Pelagic abundances for all microorganisms were 10-1,000-fold lower than sediment abundances. Correlations between viral abundances and suspended solids suggest that viruses sorbed to suspended material in the water column may settle out and contribute to the benthic viral population. Virus production was measured by a time course increase of viral abundance in seawater using a dilution technique. Virus production was highest in eutrophic waters of the Brisbane River, and addition of inorganic nutrients (NO ϩ NH ϩ PO ϩ SiO 3 ) stimulated viral productionrates at all stations by 14-52% above ambient, suggesting that inorganic nutrient availability may play a key role in aquatic viral abundance.The importance of estuaries in the flux of matter from terrestrial to marine environments and their proximity to major urban centers have made these ecosystems loci of current research. Estuaries often contain both salinity and eutrophication gradients as a result of nutrient-rich freshwater input from rivers that are diluted by oceanic exchange, particularly in temperate regions. Australian estuaries differ from those in many other geographic locations in that they are charac-

Action Learning draws its roots from different philosophies of learning and change, which in turn, influence its design and practice. This article identifies common factors and differences among three different 'schools' of practice (Scientific, Experiential and Critical Reflection). It then distinguishes Action Learning from the other action approaches in this volume.

During a 3-yr study in the Gulf of Mexico, we measured dinitrogen (N 2 ) fixation and nitrogen (N) release by Trichodesmium and compared these rates with water column N demand and the estimated N necessary to support blooms of Karenia brevis, a toxic dinoflagellate that severely affects the West Florida shelf. Net and gross N 2 fixation rates were compared in simultaneous incubations using 15 N 2 uptake and acetylene reduction, respectively. The difference between net and gross N 2 fixation is assumed to be an approximation of the rate of N release. Results demonstrate that Trichodesmium in the Gulf of Mexico are fixing N 2 at high rates and that an average of 52% of this recently fixed N 2 is rapidly released. Calculations suggest that observed densities of Trichodesmium can provide enough N to support moderately sized K. brevis blooms. Based on other studies where 15 N 2 and acetylene reduction were compared directly, it appears that N release from Trichodesmium is common but varies in magnitude among environments. In addition, carbon (C) and N-based doubling times for Trichodesmium vary among studies and environments. Comparing gross N 2 fixation and C fixation directly, C-based doubling times exceeded N-based doubling times, which suggests an imbalance in elemental turnover or a failure to fully quantify Trichodesmium N use.Blooms of the toxic dinoflagellate Karenia brevis occur in the oligotrophic waters of the eastern Gulf of Mexico where known nitrogen (N) sources are insufficient to support the observed biomass accumulations (Steidinger et al. 1998;Walsh and Steidinger 2001;Vargo et al. 2004). Estuarine transport can supply only an estimated 5% to 20% of the N required for the daily growth needs of a moderate population ( AcknowledgmentsWe thank the captains and crews of the R/V Walton Smith, R/ V Suncoaster, and R/V Pelican for help with sample collection. We also thank George Boneillo, Esther Cornfeld, Lloyd Godson, Ian Hewson, Sue Marasko, Ryan Morse, Sue Reynolds, Marta Sanderson, and Michelle Watson for their help in sample collection and analysis. We thank the Florida ECOHAB program for sharing data and ship time and Gabe Vargo for use of laboratory and facilities at the University of South Florida. We also thank two anonymous reviewers and Mary Scranton for their helpful comments.

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