Seasonal change of multifrequency backscatter in three Baltic Sea habitats

Abstract

This study investigated the seasonality of acoustic backscatter intensities, exploring three habitats in the southwestern Baltic Sea: 1) a mussel-covered reef, 2) coarse sand and gravel, and 3) seagrass meadows. Backscatter information of different, partly calibrated frequencies (200, 400, 550, and 700 kHz) was collected in three seasons (May, August, and October). The acoustic data were supported by point samples and video profiles for grain size and benthic community analysis. Angular response curves helped to quantify the seasonal backscatter response of the different frequencies. The multifrequency and multiseasonal backscatter maps distinguish the three habitats and reveal variable seasonal differences in acoustic backscatter, but not all changes in the benthic community can be recognized in the acoustic data. 1) The high-backscatter response of the mussel-covered reef shows little seasonal differences and was frequency independent. 2) The ecologically valuable coarse sand and gravel areas show small-scale seasonal alterations in the sediment composition and morphology, mainly caused by changes in local hydrodynamics. Higher frequencies were found best suited to identify coarse sand and gravel. 3) Seagrass meadows seasonality is dominated by growth of seagrass blades, increasing the backscatter response compared to bare sand. The use of multiple frequencies is beneficial as the low frequency is sensitive to changes in the shallow subsurface and benthic features such as seagrass rhizomes, while the higher frequency highlights changes related to coarser sediment.

Benthic habitat mapping: A review of three decades of mapping biological patterns on the seafloor

“…Recently, the ability to ping at multiple acoustic frequencies simultaneously has enabled socalled "multispectral" backscatter mapping using multibeam sonars (Brown et al, 2019), which has potential to increase the resolvability of seabed substrate properties (Feldens et al, 2018;Gaida et al, 2018;Janowski et al, 2018;. Multifrequency surveys may now be conducted using single beam (e.g., Cutter & Demer, 2014;Mopin et al, 2022), sidescan (e.g., Tamsett et al, 2016;Fakiris et al, 2019), multibeam (e.g., Gaida et al, 2020;Menandro et al, 2022;Schulze et al, 2022), and synthetic aperture (Barclay et al, 2005;Rymansaib et al, 2019) side scan sonars. A summary of remote sensing technologies and sensors used to collect geospatial data for benthic habitat mapping is provided in Table 1.…”

Bottom and Suspended Sediment Backscatter Measurements in a Flume—Towards Quantitative Bed and Water Column Properties

“…Acoustic systems have long been in use for measuring water depths and mapping subaqueous morphology. Moreover, measuring acoustic backscatter (i.e., echo intensities depending on the target intrinsic capability to return incoming acoustic energy after scattering as a function of time or range [1]), single-beam (SBESs) and multibeam (MBESs) multi-frequency and broadband echosounders are the state-of-the-art technique for investigating both seabed interface [2][3][4][5][6][7][8][9] and water column [10] characteristics. For example, today's current applications of seabed interface characterization are the high-resolution mapping of seabed sediments for geoscience applications (the study of sedimentary processes and geological layering), hydrography, habitat characterization and monitoring [9] and ecology.…”

“…Moreover, measuring acoustic backscatter (i.e., echo intensities depending on the target intrinsic capability to return incoming acoustic energy after scattering as a function of time or range [1]), single-beam (SBESs) and multibeam (MBESs) multi-frequency and broadband echosounders are the state-of-the-art technique for investigating both seabed interface [2][3][4][5][6][7][8][9] and water column [10] characteristics. For example, today's current applications of seabed interface characterization are the high-resolution mapping of seabed sediments for geoscience applications (the study of sedimentary processes and geological layering), hydrography, habitat characterization and monitoring [9] and ecology. Water column applications mainly focus on detecting biotic targets of the pelagic ecosystem, ranging from zooplankton to fishes and marine mammals [11][12][13] and on monitoring abiotic targets, such as suspended sediment plumes [14,15] and gas seepage [16], and fluid expulsion [17] from the seabed.…”

“…Consequently, the geoscientific interpretation of BS data requires reference data. Most reference values available today come from either modeling (e.g., theoretical angular response curves of generic sediment types; [24]) or from at-sea experiments over complex, natural beds (e.g., sediment heterogeneity, interface roughness, layering, shell content, vegetation, and sediment disturbance by living organisms [8]), further complicated by the influence of seasonality [9], survey conditions [5] and the difficulty of adequate validation by ground-truthing [8]. Moreover, BS levels for coarser sediments were found to be ambiguous [25].…”

Temporal Variability of High Frequency Seafloor Acoustic Backscatter From a Sandy Site in the Gulf of Maine

“…Biological activity (bioturbation) can both create and destroy roughness elements (e.g smoothing sharp features, reducing bedform heights, deposit feeding) 3,9 . Seasonal changes occurring on time spans of days to months can be caused by environmental factors such as the presence of sea grass 13 . This work builds upon previous studies conducted by Catoire 4 and Hare et al 7 .…”