Tracking the spatiotemporal variability of the oxic-anoxic interface in the Baltic Sea with broadband acoustics

Anoxic zones, regions of the water column completely devoid of dissolved oxygen, occur in open oceans and coastal zones worldwide. The Baltic Sea is characterized by strong salinity-driven stratification, maintained by occasional water inflows from the Danish Straights and freshwater input from rivers. Between inflow events, the stratification interface between surface and deep waters hinders mixing and ventilation of deep water; consequently, the bottom waters of large regions of the Baltic are anoxic. The onset of the anoxic zone is closely coincident with the depth of the halocline and, as a result, the interface between oxic and anoxic waters corresponds to a strong impedance contrast. Here, we track acoustic scattering from the impedance contrast utilizing a broadband split-beam echosounder in the Western Gotland Basin and link it to a dissolved oxygen level of 2ml/l using ground truth stations. The broadband acoustic dataset provides the means to remotely observe the spatiotemporal variations in the oxic-anoxic interface, map out the extent of the anoxic zone with high resolution, and identify several mechanisms influencing the vertical distribution of oxygen in the water column. The method described here can be used to study other systems with applications in ongoing oceanographic monitoring programs.