How was the deep scattering layers (DSLs) influenced by the Deepwater Horizon Spill? --- Evidences from 10-year NTL oil/gas ADCP backscattering data collected at the spill site

There are suspicions that the 2010 DWH oil spill might have affected the biomass in the deep scattering layers (DSLs), at least during the period in which the spill was active and oil dispersants were used. The acoustic backscattering intensity (ABI) data from acoustic Doppler current profilers (ADCPs) have been shown to detect and monitor the spatial and temporal evolution of DSLs in many oceans. Since 2005 with the issue of a Notice of Lessees and Operators (NTL), namely, NTL No. 2005-G5, large amounts of continuous ADCP data have been collected by oil/gas companies in the Northern Gulf at more than 100 stations and made publically available via the National Data Buoyancy Center (NDBC) website. NTL ADCPs data have also been collected prior to, during and after the DWH spill at the spill site. The ADCP with station # 42872 was mounted on the DWH rig and collected ABI data from 2005 until the rig sank in April 2010. ADCPs with station # 42916 and 42868 were then moved into the spill region and collected ABI data during and after the spill. The deep scattering layers were well resolved by those 38 kHz with vertical range of 1000m. The SSL provides key food for many large sea-animals, including whales, dolphins, billfishes and giant tunas and therefore have important roles in the ecosystem of the deep Gulf.

By carefully applying calibrations and corrections, the ABI data can be converted to biologically meaningful mean volume backscattering strength (MVBS) and areal backscattering strength (ABS). This is an effective and powerful way to study the pelagic communality dynamics in the deep scattering layers and to investigate greater details that were previously inaccessible. Utilizing the NTL data collected during the past 10 years around the DWH site, we investigate the spill influence on deep scattering layers by comparing the biomass pre- and post BP spill and comparing biomass variations in areas with and without oil contamination. Preliminary results have shown that there is a clear decrease trend of relative biomass in the deep scattering layer in 2010 after the spill. We also find extremely dense scattering patches at the depth of DSLs, which appear only during the spill and are likely formed by spill materials. Statistical analysis on the layer depth, intensity, and thickness and their variations over time are also investigated.