A latitudinal gradient in recruitment of intertidal invertebrates in the northeast Pacific Ocean

Rates of propagule supply can be important determinants of spatial and temp oral patterns in community structure. In the northeast Pacific Ocean. large -scale differences in the structure of intertidal invertebrate communities have been attributed to a latitudinal gradient in recruitment in this regio n. To determine whether such a gradient exists, recruitment of intertidal b arnacles and mussels was monitored at 17 sites across this region in 1996 a nd 1997. A latitudinal gradient in recruitment was detected in this study. This gradient was approximately a stepcline: annual recruitment, on average , was 1-2 orders of magnitude higher in central and northern Oregon than in central and northern California. In contrast to the regional differences, large-scale gradients in recruitment within California were small; correlat ions of recruitment with latitude were weak, and in all but one case, stati stically insignificant. Nonetheless, trends in the data suggest that recrui tment within central and northern California was highest between San Franci sco and Monterey Bay, where larvae may be retained more nearshore than to t he north or south. If so, apparently conflicting claims about latitudinal g radients in recruitment within California can be reconciled. The large-scal e transition in recruitment rates supports the hypothesis that a marked shi ft in the intensity of upwelling near Cape Blanco in southern Oregon is a m ajor cause of a coincident transition in community structure. Stronger upwe lling (and thus offshore flow) to the south has been hypothesized to transp ort larvae further offshore and thereby reduce larval supply to nearshore b enthic communities. This study confirms that the predicted differences in recruitment exist, an d that these differences are large. Preliminary calculations indicate that regional differences in offshore flow are likely to make a larger contribut ion to the recruitment transitions than several other plausible causes. In addition, recruitment transitions are larger, more abrupt, and more consist ent across species than corresponding shifts in percentage cover, which fav or competitive dominants. This supports model predictions that competition for space is more intense where recruitment is high. However, the absence o f strong, large-scale recruitment gradients within California suggests that mesoscale processes are relatively more important than latitudinal trends in upwelling as determinants of community structure patterns at smaller sca les.