MEPS 427:105-115 (2011)  -  DOI: https://doi.org/10.3354/meps09057

ABSTRACT: On 4 sample dates in summer 2007, vertically stratified zooplankton samples and oceanographic data were collected at 7 stations located from 0.5 to 28 km from shore near Coos Bay, Oregon, USA. Two of the sample dates (27 June and 14 August) were characterized by upwelling conditions and the other two (3 and 18 July) by downwelling or relaxation. We identified larval and thread-­drifting Mytilus californianus and M. trossulus/galloprovincialis. Thread-drifting M. californianus and M. trossulus/galloprovincialis ranged in size from 306 to 627 µm and from 305 to 832 µm, respectively. On all sample dates, larval and thread-drifting mytilids were rare in surface waters (0 to 10 m depth), abundant below these depths at the 3 inshore stations (0.7 to 4.5 km offshore) and rare or absent at all depths at more seaward stations. Thread drifters were caught within 10s of meters of the bottom. There was no relationship between either the strength of the alongshore currents or wave energy and the abundance of thread drifters. Thread drifters were significantly more abundant during downwelling/relaxation events than during upwelling events. The higher abundances during downwelling may be due to favorable conditions for thread drifting within the benthic boundary; ­density instability generated by the downwelling circulation interacting with the cross-shelf density gradient creates ­vertical currents, which may aid lift-off of thread drifters from the bottom.

KEY WORDS: Mytilis · Thread drifting · Upwelling · Downwelling