A Broadband Acoustic Study of the Spatiotemporal Variability of the Baltic Sea Anoxic Zone

Elizabeth Frances Weidner1, Christian Stranne2, Thomas C Weber3, Jonas Hentati-Sundberg4, Martin Jakobsson2 and Larry Mayer5, (1)Center for Coastal and Ocean Mapping Joint Hydrographic Center, Durham, NH, United States, (2)Stockholm University, Department of Geological Sciences, Stockholm, Sweden, (3)University of New Hampshire Main Campus, Mechanical Engineering, Durham, NH, United States, (4)Swedish University of Agricultural Sciences, Aquatic Resources, Sweden, (5)University of New Hampshire, Center for Coastal and Ocean Mapping, Durham, United States
Abstract:
Anoxic zones in the Baltic Sea are associated with water column habitat
loss, elimination of benthic fauna, disruption of marine food webs, and
altered nutrient cycling. In the Baltic Proper, anoxic conditions are
commonly constrained to bottom water regions (depth >50 meters) due to
the mainly 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 use 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 the spatial variations of the anoxic zone across the Baltic
Proper with horizontal resolution on the order of meters to 10's of
meters, much higher than ever previously 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 controlling said
distribution.
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