73 ABSTRACT The subtropical estuaries of Florida Bay and Moreton Bay are shallow, at equivalent latitudes, and both subjected to increasing eutrophication. However, these are highly contrasting systems. Similar studies on nutrient availability, phytoplank- ton composition, and rates of enzyme activities were conducted on a seasonal basis but in different years in these two systems. Florida Bay, a lagoonal estuary, supports picocyanobacterial blooms, which expanded in magnitude and duration upon an injection of phosphorus (P) and organic matter during a major disturbance event of 2005. Moreton Bay, an estuary at the transition between river- dominated and lagoonal, supports proportionately more diatoms and dinoflagellate blooms, and in response to a major storm, diatoms increased in abundance. Community composition and nutrient uptake pathways were, in both cases, related to the dissolved nutrient N:P ratios. Keywords: diatoms, dinoflagellates, cyanobacteria, enzyme activities, nutrient ratios, storm events INTRODUCTION Relationships between nutrient availability and plankton dynamics are of critical importance to the fundamental understanding of aquatic ecosystem function and to the management of these ecosys- tems, especially those that have been, or continue to be, impacted by excessive nutrient loading. The status and trends in nutrient loading and phyto- plankton response often guide both short-term and long-term management decisions with respect to water quality. Temperate coastal and estuarine systems have been well studied with regard to nu- trient-phytoplankton dynamics, especially in Europe and North America (e.g., Baltic Sea, Nehring et al. 1984, Rönnberg and Bonnsdorf 2004; Kattegatt, Richardson and Jørgensen 1996; Chesapeake Bay, Fisher et al. 1992, Cooper and 1 Corresponding author glibert@umces.edu COMPARATIVE NUTRIENT AND PHYTOPLANKTON DYNAMICS IN TWO SUBTROPICAL ESTUARIES: FLORIDA BAY, USA, AND MORETON BAY, AUSTRALIA Patricia M. Glibert 1 University of Maryland Center for Environmental Science, Horn Point Laboratory, P.O. Box 775, Cambridge, MD 21613 Cynthia A. Heil Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, 100 8 th Ave. S., St. Petersburg, FL 33701 Jeffrey Alexander University of Maryland Center for Environmental Science, Horn Point Laboratory, P.O. Box 775, Cambridge, MD 21613 Susan Murasko Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, 100 8 th Ave. S., St. Petersburg, FL 33701 Judith M. O’Neil University of Maryland Center for Environmental Science, Horn Point Laboratory, P.O. Box 775, Cambridge, MD 21613 Contributions in Marine Science December 2009 Vol 38: 73-89 Published by the Marine Science Institute, The University of Texas at Austin, 750 Channelview Drive, Port Aransas, Texas 78373-5015