Title:

Studying the spatiotemporal variability of the anoxic zone in the Baltic sea using broadband acoustics

Anoxic zones in the Baltic Sea are associated with water column habitat loss, disruption of marine food webs, and altered nutrient cycling. In the Gotland basin, anoxic conditions are constrained to bottom water regions (depth >50 meters) due mainly to salinity-controlled stratification and poor circulation. The interface between the oxygenated surface waters and anoxic bottom waters is characterized by a strong gradient in density. We used a broadband (45-90 kHz) split-beam echosounder and concurrent CTD profiles to show the impedance contrast brought about by the density gradient corresponds to the upper limit of the anoxic zone. This provides the means to observe near-continuous spatial variations of the anoxic zone with horizontal resolution on order of meters to 10’s of meters, much higher than ever reported. Additionally, the nature of the acoustic scattering at the oxic-anoxic interface affords us important quantitative information on the vertical distribution of dissolved oxygen in the water column and the processes that control it.

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