The four naturally occurring Ra isotopes serve as tools to quantify submarine groundwater discharge (SGD) in coastal areas. In constructing the Ra mass balance for a given system, Ra sources and sinks must be accurately determined, then measured or eliminated as negligible. SGD &mdash a Ra source &mdash is commonly determined by difference. In Long Island Sound (LIS), a significant Ra source is diffusive flux from muddy sediments. This study investigates the seasonality of <super>223,224</super>Ra diffusive fluxes linked to seasonal Mn redox cycling correlated with hypoxia (dissolved oxygen &le 3.0 mg l<super>-1</super>) in LIS. The three components of this study &mdash water column and nearshore concentrations and laboratory flux core incubations &mdash demonstrate a coupling of Ra and Mn cycling in LIS, both temporally and spatially. The overall relationship between Mn and Ra in LIS suggests that seasonal redox cycling of Mn is an important factor controlling Ra distributions in the Sound. Solid phase Mn<super>4+</super> (under oxic conditions) appears to restrict the diffusion of Ra from muddy sediment, whereas dissolved phase Mn<super>2+</super> (under hypoxic conditions) is associated with higher Ra concentrations in LIS and `pulses' of Ra in laboratory flux core incubations. Therefore, the diffusive flux of Ra to a given coastal sea should be evaluated seasonally to more accurately quantify SGD.