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.

Lab on a Secchi disk: A prototype open‐source profiling package for low‐cost monitoring in aquatic environments

“…For example, low-cost electronic optical sensing packages can directly measure key apparent optical properties without biases caused by variations in vision among participants (e.g., see Burggraaff et al 2021). On the other hand, visual-based techniques like the Secchi depth and Forel-Ule measurements are less prone to failure (e.g., from electronic components failing), build on a long tradition in optical limnology and oceanography, meaning they have been rigorously tested over the years with longtime-series data available in some regions (Boyce et al 2010;Wernand et al 2013;Kahru et al 2023), and are useful for teaching the basic concepts (and history behind) optical limnology and oceanography.…”