A Latitudinal Gradient in Recruitment of Intertidal Invertebrates in the Northeast Pacific Ocean

Abstract

Rates of propagule supply can be important determinants of spatial and temporal 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 region. To determine whether such a gradient exists, recruitment of intertidal barnacles and mussels was monitored at 17 sites across this region in 1996 and 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; correlations of recruitment with latitude were weak, and in all but one case, statistically insignificant. Nonetheless, trends in the data suggest that recruitment within central and northern California was highest between San Francisco and Monterey Bay, where larvae may be retained more nearshore than to the north or south. If so, apparently conflicting claims about latitudinal gradients in recruitment within California can be reconciled. The large-scale transition in recruitment rates supports the hypothesis that a marked shift in the intensity of upwelling near Cape Blanco in southern Oregon is a major cause of a coincident transition in community structure. Stronger upwelling (and thus offshore flow) to the south has been hypothesized to transport larvae further offshore and thereby reduce larval supply to nearshore benthic communities. This study confirms that the predicted differences in recruitment exist, and that these differences are large. Preliminary calculations indicate that regional differences in offshore flow are likely to make a larger contribution to the recruitment transitions than several other plausible causes. In addition, recruitment transitions are larger, more abrupt, and more consistent across species than corresponding shifts in percentage cover, which favor competitive dominants. This supports model predictions that competition for space is more intense where recruitment is high. However, the absence of 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 scales.