Evaluating historic and modern optical techniques for monitoring phytoplankton biomass in the Atlantic Ocean

Traditional measurements of the Secchi depth ( z SD ) and Forel-Ule colour were collected alongside modern radiometric ocean clarity colour, in-situ chlorophyll-a concentration (Chl-a), on four Atlantic Meridional Transect (AMT) cruises. These data used to evaluate historic optical techniques for monitoring Chl-a, remote-sensing algorithms. Historic broadly consistent with current understanding, inversely related beam diffuse attenuation, positively ratio blue green reflectance euphotic depth. The relationship between AMT was found be in closer agreement historical relationships when using infinite depth, rather than water above disk at half . Over range 0.03-2.95 mg m -3 , Chl-a tightly correlated these variables, explaining highest amount variance (89%), closely followed by (85%) (71-81%, depending scale used). Existing algorithms that predict from variables evaluated, perform well, albeit some systematic differences. Remote sensing good over values (8.5 - 51.8 m, r 2 >0.77, unbiased root mean square differences around 4.5 m), but a slight positive bias (2.0 5.4 m). agreed well >0.68, <1). We investigated impact environmental conditions wind speed estimation propose path forward include effect theory. discuss benefits challenges collecting future directions research. Our dataset is made publicly available support research community working topic.