Physical and biogeochemical drivers of multi-year isoscape in the California upwelling system

Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) are commonly employed to reconstruct past change in marine ecosystems and nutrient cycling. However, multiple biogeochemical and physical drivers govern spatiotemporal variability of these isotopic signals, particularly in dynamic coastal systems, complicating interpretation. Here, we coupled a modern multi-year (2010-2019) δ¹³C and δ¹⁵N isoscape record from intertidal mussels (Mytilus californianus) with high-resolution ocean model output and satellite chlorophyll-a observations in the California Current System (32°-43° N) to identify major drivers of isotopic variability. Our results show that spatial variations in δ¹³C are largely related to primary production, whereas spatial δ¹⁵N variability is driven by water mass mixing. Major isotopic change was also related to ocean climate variability; however, these effects vary regionally. In northern and central California, δ¹⁵N values are predominantly a function of nitrate utilization, whereas in southern California, δ¹⁵N varies due to shifts in water mass composition. In all regions, δ¹³C values are driven by productivity, with the largest changes occurring in southern California. Our findings provide novel insight into regional differences in predominant drivers of isotopic variability, and links to modern ocean climate variability. These findings offer crucial information needed for robust interpretations of California Current palaeoceanographic δ¹³C and δ¹⁵N records.