The Baltic Sea is characterized by strong stratification, high nutrient loads from anthropogenic sources, and poor circulation. Consequently, an anoxic zone exists in the bottom waters of large regions of the Baltic Proper. Anoxic conditions have led to water column habitat loss, elimination of benthic fauna, disruption of food webs, and altered biogeochemical nutrient cycling. The onset of anoxic zone is concomitant to the depth of the halocline, which separates the fresher, oxygenated surface waters from the deeper anoxic waters. As a result, the interface of oxic-anoxic waters corresponds to a strong impedance contrast brought about primarily by a sharp gradient in salinity. The acoustic reflection from this impedance contrast was tracked utilizing a broadband (45-90 kHz) split-beam echo sounder in the western Baltic Proper. The broadband acoustic dataset provides the means to remotely observe the spatiotemporal variations of the oxic-anoxic interface. In this study, we capitalize on this to discern the mechanisms influencing the vertical distribution of oxygen in the water column and its effects on fine-scale fish distribution. The methodological development of high resolution anoxic layer tracking with the concurrent study of oceanographic and biological features using broadband acoustics opens up a new level of understanding of the fine-scale ecosystem interactions in the Baltic Sea.

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