Paradigm lost? Cross-shelf distributions of intertidal invertebrate larvae are unaffected by upwelling or downwelling

Abstract

We tested the hypothesis that larvae of intertidal invertebrates are swept offshore dur- ing upwelling and shoreward during downwelling. During summer 2007, vertically stratified zoo- plankton samples and oceanographic data were collected at 7 stations located from 0.7 to 27 km from shore near Coos Bay, Oregon, USA. Half the sample dates (27 June and 14 August) were character- ized by upwelling conditions (lines of constant temperature and salinity tilted upward, and a band of cold surface water was against the coast) and the other half (3 and 18 July) were characterized by downwelling or relaxation (lines of constant temperature and salinity were flat, and warm surface waters were in contact with the shore). We identified and staged larvae of Neotrypaea californiensis, Balanus glandula, B. nubilus, Chthamalus dalli, Pollicipes polymerus, and Semibalanus cariosus/B. crenatus and identified (to species or taxa level) Mytilus californianus, M. trossulus, Hiatella arctica, Dendraster excentricus, and pinnotherid and pagurid zoea. On all sample dates, all taxa and larval stages were rare in surface waters (0 to 10 m depth) and, with one exception (B. nubilus cyprids), were abundant at the 3 inshore stations (0.7 to 4.5 km offshore) and very rare or absent at seaward stations. The average distance offshore of all taxa and larval stages ranged from 0.9 to 4 km from shore and did not vary with upwelling and downwelling. Upwelling and downwelling had no effect on the cross-shelf distribution of the larvae of intertidal invertebrates; the hypothesis that upwelling carries larvae offshore and downwelling carries them back onshore was not supported.