Topographically generated fronts, very nearshore oceanography and the distribution and settlement of mussel larvae and barnacle cyprids

Abstract

Flow patterns adjacent to shore may prevent or aid shoreward migration of benthic invertebrate larvae. We hypothesized that a front at the mouth of Sunset Bay, Oregon, prevents shoreward dispersal of larvae, significantly altering settlement of mussel larvae and barnacle cyprids. Settlement was measured at three sets of moorings (three moorings per site) distributed across the front at Sunset Bay. From 6 July to 4 September 2000, samples were collected roughly every other day. Concurrently, we made vertical zooplankton tows adjacent to each mooring site and collected physical oceanographic data. During upwelling-favorable winds, the front was always present at the bay mouth, separating significantly cooler, saltier and denser offshore water from that within the bay. During downwelling winds, the front broke down and we found no significant difference in the surface physical oceanographic parameters across the bay mouth. During upwelling, the concentration of mussel larvae was higher seaward of the front than landward, but there was no significant difference in concentration during downwelling, suggesting that the front may act as a barrier to the shoreward dispersal of mussels. Mussel settlement was too low and sporadic to allow statistical analysis. There was no difference in cyprid concentrations across the bay mouth whether the front was present or not. Cyprid settlement was, however, nearly an order of magnitude lower at moorings seaward of the front than at those landward. A significant cross-correlation was found between settlement at the offshore mooring and tidal range (r=0.464, lag=0 days) and between settlement at the mid and inner moorings and downwelling winds (r=0.532 mid bay, r=0.532 inner bay, lag=0 days). Seaward of the front, settlement varied with tidal range, while landward of the front, most settlement occurred as brief pulses during downwelling winds, periods when the front was not present. We found large differences in the distribution of cyprids, and mussel larvae and cyprid settlement relative to the front; larval distributions and settlement varied with upwelling versus downwelling winds and was due to differences in the very nearshore (i.e. within 100‐1000 m of shore) coastal oceanography.