Potential implications on the pelagic fish and zooplankton community of artificially induced deep-water releases of oil and gas during DeepSpill_2000 – an innovative acoustic approach.

During the DeepSpill_2000 field experiment (Johansen et al., 2001) oil and gas were artificially released on several occasions from a water depth of 844 meters at the Helland-Hansen site in the Norwegian sea. Prior to and during these releases a variety of sampling gear, echo sounders and other instrumentation were used to monitor and obtain information on the environmental conditions, oil and gas plumes and the biological activity in the experimental region. The main objective of the present work has been to provide a general description of the biological community of the experimental region, and to provide information on the variability of biological scatterers throughout the water column. By exploring and evaluating the short-term variability of the biological scattering structures, the organisms therein and concurrently examine scattering from the released oil and gas, new insights on how rising oil and gas might interact with the deep-water plankton and fish community of the experimental region could be gained. Acoustic scattering from oil and gas was easily observed during all experimental spill events and some situations are presented in this report. The magnitude of the acoustic scattering from organisms inhabiting the Deep Scattering Layer (DSL) between 300-500 m depth for an undisturbed situation and its natural variability is documented. It is shown that rising oil-gas plumes certainly influences the otherwise quite homogeneous DSL structure, although the precise mechanisms involved are still not revealed. When all scattering, from structures that with certainty can be attributed to the released oil and gas were excluded, the remaining acoustic scattering from the DSL seem to be nearly an order of magnitude higher compared to an undisturbed situation. The reason for this is still uncertain, but some tentative hypotheses are formulated. The most plausible explanation seem to be that a fraction of the released compounds and in particular gas bubbles, for various reasons have a prolonged residence time within the region of the DSL, hence contributing significantly to the increased scattering observed during the spills.