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.

Interaction strength and extinction risk in a metacommunity

“…In coastal ecosystems, large-scale biological patterns have mostly been studied in relation to environmental variability over short temporal scales (Connolly et al 2001). The results presented here suggest that species interactions such as competition and facilitation can scale up through larval dispersal and lead to large-scale spatio-temporal patterns with specific scaling properties (e.g.…”

Recruitment of coastal fishes and oceanographic variability in central California

“…The west coast of North America is an eastern boundary ecosystem where spatial and temporal variation in larval recruitment can be influenced by the dynamics of wind-driven coastal upwelling (Parrish et al, 1981;Mooers and Robinson, 1984;Norton, 1987;Graham et al, 1992;Wing et al, 1995b;Connolly et al, 2001). Equatorward wind stress displaces coastal surface waters offshore via Ekman transport, which are replaced by colder, nutrient rich, subsurface waters (Huyer, 1983).…”

“…Relaxation or reversals in wind stress can lead to downwelling conditions where the warmer offshore surface waters and the coastal fronts are advected back onshore, delivering entrained larval organisms to the nearshore benthic habitat (Roughgarden et al, 1988;Farrell et al, 1991). Several studies have shown that variable recruitment of coastal organisms is related to upwelling dynamics in the California region (Parrish et al, 1981;Hollowed et al, 1987;Ebert and Russell, 1988;Roughgarden et al, 1988;Larson et al, 1994;Wing et al, 1995b;Connolly et al, 2001;Broitman et al, 2005;Laidig et al, 2007).…”

Delayed upwelling alters nearshore coastal ocean ecosystems in the northern California current

“…Decreased upwelling can have two effects: reduced nutrient supply to phytoplankton (8) and reduced offshore transport of phytoplankton and planktonic fish and invertebrate larvae that are crucial for replenishing coastal populations (9). Nutrient and phytoplankton reductions may decrease zooplankton (including larvae) survival, and thus decrease recruitment rates of planktotrophic larvae.…”