Multidecadal changes in ocean transparency: Decrease in a coastal upwelling region and increase offshore

Abstract

Detection of the effects of climate change on ocean ecosystems is often limited by the short duration of available time series. Here, we use ocean transparency measurements (the Secchi disk depth, ZSD) in the California Current Ecosystem since 1949 and combine them with satellite estimates. Historic in situ measurements of ZSD were irregular in space and time and are difficult to interpret in time series due to biases introduced by changing locations and timing. We normalize historic ZSD measurements with satellite-derived mean climatology and create a merged in situ—satellite time series of ZSD for the last ~ 73 yr. Although interannual variability in ZSD is dominated by El Niño Southern Oscillation-related variability (~ 50% of the total variance in many areas), a secular trend of decreasing transparency that is correlated with increasing productivity is detected 0–300 km from the coast in an area affected by coastal upwelling north of 27°N. In contrast, increasing transparency (correlated with decreasing productivity) is detected offshore (> 1000 km from the coast). In addition to those general trends, transparency is also increasing in coastal area off Baja California south of 27°N.