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A simple 19th-century tool is still useful to ocean scientists in the age of satellites, new research shows. The research is published in the journal Frontiers in Marine Science. A Secchi disk - historically called a "dinner plate" by sailors - is used in the open ocean to measure concentrations of microscopic algae called phytoplankton. Sailors lower the white disk into the water and record the depth at which it disappears. In the new study, a research team including the University of Exeter, Plymouth Marine Laboratory, Vrije Universiteit (Netherlands) and the Italian Institute of Marine Sciences (ISMAR) compared the performance of Secchi disks with satellites and high-performance chromatography. Secchi disks performed almost as well as modern methods at monitoring phytoplankton abundance - meaning Secchi measurements going back more than a century can help scientists understand long-term changes in the ocean. "Phytoplankton produce half the world's oxygen and form the base of ocean