Age Tracers and Residence Time in the Hudson River Estuary

The Hudson River is one of the most nutrient loaded rivers in the country, however phytoplankton bloom do not occur, possibly as a result of how quickly water moves though the Hudson River estuary. Slower water residence times may then allow for significant phytoplankton growth. Water age and residence time, which are compliments of one another under stead-state conditions, are important factors in determining where phytoplankton move and how long they spend within a favorable portion of the estuary. This research involved introducing a freshwater and saltwater age tracer into the Regional Ocean Modeling System (ROMS) for the Hudson River estuary domain to observe the distribution of ages within the spring-neap tidal cycle and across different river discharge rates. These discharge rates represented average (500 m³/s), relatively high (1000 m³/s), and relatively low (200 m³/s) river flow conditions for the Hudson River. Saltwater age followed a distribution similar to salinity, while freshwater age distribution mostly represented river transit time. Under steady state conditions, combined freshwater and saltwater age may be used to calculate a rough estimate of estuary residence time. The results show that the residence time of the full estuary appears to be at greater than the doubling time of phytoplankton for all discharge rates and by over five days for even the relatively high discharge case. This leads to the conclusion that other estuary factors, including light availability and salinity, may be more important for limiting phytoplankton growth than residence time.