COS 194-4 - Interoceanic comparison of benthic-pelagic coupling along rocky coasts of Nova Scotia and Oregon

Abstract: Coastal marine habitats are connected via flows of propagules, nutrients, and energy – collectively termed ecological subsidies. Upwelling controls the delivery of marine subsidies to the nearshore ocean, and surf zone hydrodynamics affect their transport to shore. However, the relative importance of these processes remains understudied in many regions. To evaluate the combined influence of upwelling and surf zone hydrodynamics on subsidy delivery from the nearshore ocean to wave-exposed rocky shores on the Atlantic and Pacific Oceans, we compared benthic-pelagic coupling in Nova Scotia and Oregon. Filter-feeder larvae and their food supply of phytoplankton and detritus are crucial marine subsidies to rocky intertidal habitats. Thus, we sampled barnacle recruitment at 19 rocky intertidal sites from 2014 to 2018. We used NASA satellite imagery to measure concentrations of phytoplankton and particulate organic carbon (POC) in the nearshore ocean adjacent to our sites. Finally, we measured intertidal topography, surf zone width, and upwelling magnitude and frequency to characterize the oceanographic regimes. We asked the following: (Q1) What are the seasonal, annual, local, and regional patterns in barnacle recruitment, POC, and phytoplankton? (Q2) Do upwelling and/or surf zone hydrodynamics regulate the delivery of these marine subsidies?

(Q1) On wave-exposed rocky shores in Nova Scotia, barnacle recruitment was limited to May and June and mean annual recruitment was similar from 2014 to 2018. In contrast, wave-exposed rocky shores in Oregon experienced barnacle recruitment from April through October, peaking in summer. Barnacle recruitment was greatest in 2015 in Oregon, and sites on the central Oregon coast had the most barnacles. Between coasts, barnacle recruitment in Oregon was an order of magnitude greater than in Nova Scotia. Phytoplankton concentrations mirrored the results of barnacle recruitment, but POC concentrations were equivalent between coasts.

(Q2) Upwelling is relatively more important than surf zone hydrodynamics in delivering marine subsidies to rocky coasts. In both regions, we found no relationship between barnacle recruitment and either intertidal slope or surf zone width. Instead, barnacle recruitment was associated with intermittent upwelling in Oregon. Furthermore, upwelling magnitude was positively correlated with phytoplankton and POC concentrations in Oregon, but not in Nova Scotia. These findings indicate that although Nova Scotia and Oregon both experience cold-temperate conditions, the magnitude of marine subsidy delivery is region-specific. Overall, our comparative study shows that while upwelling and surf zone hydrodynamics are important drivers of marine subsidy delivery to some rocky shores, additional factors may explain their delivery elsewhere.