||Despite a relatively short residence time of water in many shallow, semi-enclosed estuaries, phytoplankton blooms in nutrient enriched systems are a common phenomenon. This poses the question how is it possible to have phytoplankton populations bloom in response to local conditions of shallow estuaries, if the water residence times are similar to cell division times? To address this paradox we used the radium quartet as a tool to measure water mass age in coastal systems (Waquoit Bay, MA, USA) subject to different degrees of land-derived nitrogen load and hence differences in phytoplankton biomass. Recently, the radium quartet has been used as geochemical tracers to determine age of water masses. Based on a number of samples collected over the course of one year, the average radium-derived age (+/- stdev) of water in three sub-estuaries of Waquoit Bay (Childs River, Quashnet River, and Sage Lot Pond) was similar to 7 (+/- 4.7), 11 (+/- 62), and 17 (+/- 7.5) days, respectively. These values are significantly longer than previous estimates based on more traditional hydrodynamic methods. Furthermore, peak chlorophyll concentrations were associated with older water masses in the heavily freshwater-influenced sub-estuaries (Childs and Quashnet). Our results suggest that water age, temperature, and nutrients all play a role in controlling phytoplankton biomass however, water age was more important at the time of the year when temperature limits phytoplankton growth (late spring, early summer). We conclude that radium-derived age models, which are similar to artificial tracer-based approaches, may be the most appropriate method for studying the role of hydrodynamics on estuarine ecology. (C) 2013 Elsevier B.V. All rights reserved.