Abstract

In September of 2004 Hurricane Ivan crossed over the continental slope and shelf off Mississippi in the Gulf of Mexico as a category 3-5 storm. The Naval Research Laboratory (Stennis Space Center) located an array of upward beaming Acoustic Doppler Current Meters (ADCP) directly under the path of the hurricane, measuring current profiles, temperature and pressure at the depth of the instruments, and amplitude of acoustic return (echo). In this project, we are evaluating the effects of the resulting intense currents, waves and mixing on ichthyoplankton in the water column due to passage of the hurricane. The ADCP examined here, was deployed from May-October 2004 in ~ 60 m of water depth near the edge of the continental shelf. Significant wave heights of ~ 18 - 20 m and current amplitudes exceeding 200 cm/s were experienced at this site. Echo amplitudes changed dramatically over the entire water column (~ 60 m). Echo amplitude grew rapidly upward from the bottom and downward from the sea surface. Prior to the hurricane passage, clear diurnal signals were evident in echo amplitude in the upper 25 m of the water column. After passage, echo amplitudes in the upper layer were heavily diminished and remained so for the next 6 weeks until termination of mooring deployment, although echo ampli-tudes from below the mixed layer returned to normal. There is a significant link between echo returns and the presence of ichthyoplankton. Hurricane Ivan had a large impact on an area noted for a strong summer production.

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