Disentangling spatial and temporal patterns from multifrequency active acoustic data reveals pelagic structuring in the eastern sub-Antarctic region

A significant portion of mid-trophic level organisms, including micronekton and macrozooplankton, inhabit the (meso-)pelagic zone (0-1000 m). These organisms play a crucial role in the marine ecosystem and the cycling of biochemical compounds. However, uncertainties remain regarding their global biomass, diversity and spatiotemporal distribution. Previous research has shown that backscatter intensity measured with echosounders, which serves as a proxy for pelagic organism density, can be used for biogeography, revealing distinct echobiomes in marine ecosystems. However, most published studies on acoustic-based biogeography have primarily focused on the 38 kHz, leaving the large-scale and regional distribution of multi-
frequency backscatters relatively unknown. As the detection of organisms is frequency dependent, analyzing a single frequency might not adequately capture changes in pelagic community composition. Here, to investigate the distribution of pelagic fauna in the eastern sub-Antarctic pelagic region through acoustic data, we propose considering the joint variation of multiple acoustic frequencies. We analyzed multifrequency acoustic data collected at 18, 38, 70, 120, and 200 kHz during the GEOTRACES SWINGS GS02 - THEMISTO cruise (2021), crossing the subtropical Southern Indian Ocean and the subantarctic Southern Ocean. Our findings indicate that while the 38 kHz exhibited spatial patterns consistent with previous literature, other frequencies did not display such pronounced responses to various oceanic areas and fronts. These results will contribute to a better understanding of the distribution of pelagic fauna in these diverse biogeochemical regions and of the response of pelagic biomass to climate change. As the effects of climate change begin to emerge from decade-long oceanographic databases, we strongly encourage continued acoustic monitoring of this region.