Temperature, stratification and barnacle larval settlement in two Californian sites

Barnacle settlement was monitored in two sites 100km apart along the coast of Alta and Baja California. Ill five periods of Observations completed between 1991 and 1996, Chthamalus spp., Pollicipcs polymerus, and Balanus glandula settlement was consistently higher in the northern site, La Jolla (LJ), than ill the southern site, La Salina (LS). For Chthamalus, the most abundant settler, settlement was higher in LJ in 58 Out of 60 paired dates, by a mean factor of 141. In 1996, time series of temperature in about 15m of water showed that the stratification was 72% higher, on average, and that the thermocline was shallower in LJ than in LS. Spectra Of temperature showed that internal motions of tidal and higher frequencies were more energetic and closer to the surface in LJ compared to LS. In LJ changes in settlement were positively correlated with changes in stratification. These results suggest that high-frequency internal motions are important in the onshore transport of larvae. low-frequency cooling events recorded in LJ apparently caused the energetic semidiurnal temperature variability to migrate from the bottom towards the surface, leading to the surface manifestation of the internal tide and surface internal tidal bores, which indicates that the surface nearshore bores occur in response to the shallowing of the thermocline. Tidal and higher frequency internal motions were more energetic when the thermocline was shallow during the low-frequency cooling events, than when it was deep and relatively weak during ordinary conditions. The major cooling event in LJ correlated with the local wind, suggesting local wind-driven upwelling. Oil the other hand, correlation of LS temperature with LJ temperature, winds, and sea level suggest propagation from the South. These results suggest that the low-frequency drops in temperature that modulate the nearshore internal tidal bores Lire Caused by a combination of the local wind and events that propagate poleward, possibly as coastally trapped waves. (C) 2002 Elsevier Science Ltd. All rights reserved.