Ocean pH time-series and drivers of variability along the northern Channel Islands, California, USA

Abstract

Eastern boundary current systems (EBCSs) experience dynamic fluctuations in seawater pH due to coastal upwelling and primary production. The lack of high-resolution pH observations in EBCSs limits the ability to relate field pH exposures to performance of coastal marine species under future ocean change (acidification, warming). This 3-yr study describes spatio-temporal pH variability across the northern Channel Islands, along a persistent temperature gradient (1-4°C) within the eastern boundary California Current System. pH and Conductivity, Temperature, Depth, and Oxygen sensors were deployed on island piers in eelgrass and kelp habitat and on a subtidal mooring. Due to event-scale primary production, the temperature gradient across the islands did not manifest in a pH gradient. We resolved spatial pH variability on diel (ΔpHT 0.05-0.2: photosynthesis), event-scale (ΔpHT<0.1-0.2: upwelling, phytoplankton blooms, wind relaxation), and seasonal (ΔpHT 0.06: warming) time frames. In the kelp forest, summer mean pHT (8.01-8.02) and magnitude of diel pHT cycles (ΔpHT 0.12-0.10) were comparable year-to-year, despite 2.1°C warming from 2012 to 2014. Compared to nearby mainland sites, the northern Channel Islands experienced few low pH events. The majority of pHT observations were>7.9. The lowest pH observations (>1 SD below mean pHT) occurred under either warm (respiration during warm nights) or cold (advection of upwelled water) temperatures. We emphasize the importance of incorporating site-specific environmental variability in studies of ocean change biology, particularly in the design of multistressor experiments.