Vertical Transport Timescale of Surface‐Produced Particulate Material in the Chesapeake Bay.

Accumulation and remineralization of surface‐produced particulate organic matter (POM) in the water column and seabed link closely to hypoxia and the health of aquatic ecosystems. The POM retention time provides a key timescale to interpret biochemical reaction processes. In this study, we investigated the spatiotemporal variations in the vertical particulate age (VPA) of surface‐produced POM, which is the mean time elapsed since the particulates last contact the surface, by incorporating major physical processes including sinking, resuspension, and deposition in the Chesapeake Bay. It was found that the vertical transport time for the particulates (i.e., VPA) is much longer than the dissolved counterparts as the former consists of new material from the surface and the resuspended aged material that has elongated resting on the seabed after deposition. The VPA is sensitive to settling velocity, especially in low‐frequent resuspension environments, and varies over 2 orders of magnitude with settling velocity from 0 to 10 m/day. Slow‐sinking material can remain in suspension and seldom settle to the seabed, thus mainly contribute to pelagic processes, while the fast‐sinking material connects closely with benthic processes. The seasonality of VPA decreases as the settling velocity increases. No significant difference in VPA was found between wet and dry years, yet the episodic strong flood events entrain old materials from the depositional lateral shoals to increase VPA in the channel. The transport age bridges cross disciplinaries by providing the fourth‐dimensional age information as a common currency to compare the physical transport timescale with the timescales for biochemical reactions. Plain Language Summary: The Chesapeake Bay is a highly productive estuary, characterized by spring phytoplankton blooms and subsequent accumulations of particulate organic matter (POM) in the bottom layer, which fuels summertime hypoxia. The retention time of POM provides an important timescale to interpret biochemical reactions in estuaries. In this study, we applied the vertical particulate age (VPA), the average time elapsed since the POM leaving the surface, to estimate the downward‐transport time. The VPA accounts for all possible trajectories, including direct sinking and interactions with the seabed via resuspension and deposition. It was found that the VPA is much longer than the vertical transport time for dissolved material due to the elongated resting of particulates on the seabed and contributions from the resuspended old material. The VPA is sensitive to the settling velocity and increases 2 orders of magnitude with the settling velocity from 0 to 10 m/day in less dynamic environments. The slow‐sinking material can remain in suspension while the fast‐sinking material mostly stays on the seabed. No significant difference in the VPA was found between wet and dry years except during the episodic freshwater pulse, which brought aged materials from the depositional shoals to increase the VPA in the channel. Key Points: The vertical particulate age (VPA) explains the time lag between the springtime algae blooms and the summertime hypoxia in the bayLong resting of particulates on the seabed and resuspension of the aged seabed material largely elongate the VPA in the water columnThe VPA is sensitive to settling velocity and episodic freshwater pulse. The latter entrains old material from shoals to the deep channel [ABSTRACT FROM AUTHOR]