Ecological subsidies to rocky intertidal communities: Linear or non-linear changes along a consistent geographic upwelling transition?

Through bottom–up inputs and larval transport, benthic–pelagic links can have an important effect on benthic community structure. Recent work on community structure of northeast Pacific rocky shores has focused on latitudinal differences in recruitment of intertidal invertebrates as a driver of variation in community structure. Recruitment differences are associated with a transition in upwelling near Cape Blanco in southern Oregon. Here we examine the transition in recruitment along an unstudied gap on the northern California and Oregon coasts, document a latitudinal gradient in bottom–up factors, and examine if major coastal promontories associated with upwelling plumes potentially separate benthic–pelagic coupling into regions. We monitored the recruitment of intertidal invertebrates, chlorophyll a concentrations in coastal waters, and the growth rates of mussels at numerous sites along the northern California and Oregon coasts. The transition in recruitment of intertidal invertebrates from north to south changed from very high levels north of Cape Blanco, to intermediate levels between Capes Blanco and Mendocino, to very low levels south of Cape Mendocino. The specific shape of the recruitment cline varied among species. Chlorophyll a concentrations and mussel growth rates were higher north of Cape Blanco than south of Cape Blanco, indicating that bottom–up factors may also drive regional differences in rocky shore community structure. Distinctive timing between regions of recruitment and plankton pulses suggests that benthic–pelagic coupling may be somewhat independent between these regions, which are separated by major coastal promontories. Our results highlight the large variability in spatially coupled ecosystems along the northern California and Oregon coasts that drive the latitudinal gradient in rocky shore community structure in the northeast Pacific.

► Do key processes vary linearly or non-linearly across 330 km of rocky coastline?
► Recruitment varied non-linearly across the gap.
► Abundance of phytoplankton varied linearly across the gap.
► Mussel growth varied non-linearly across the gap.
► Intertidal communities along this gap vary in complex, mostly non-linear ways.