Detecting changes in marine ecosystems and ocean colour*

Monitoring changes in marine phytoplankton is important as they form the foundation of the marine food web and are crucial in the carbon cycle. Climate change is affecting marine phytoplankton by altering their nutrient, temperature, light and chemical environments. Often Chlorophyll-a (Chl-a) is used to track changes in phytoplankton, since there are global, regular satellite-derived estimates, and such studies suggesting complex, but as yet limited, patterns of long-term change over the last two decades. However, satellite sensors do not measure Chl-a directly. Instead, Chl-a is estimated from remote sensing reflectance (R_RS): the ratio of upwelling radiance to the downwelling irradiance at the ocean’s surface. We use a unique ocean physics, biogeochemistry and ecosystem model that explicitly includes a representation of the ocean’s optical properties to explore how climate change signals are manifested in Chl-a, phytoplankton communities, and ocean colour over the course of the 21^st century. We show that R_RS in the blue-green spectrum is likely to have a stronger and earlier climate change-driven signal than Chl-a. This is because R_RS integrates not only changes to Chl-a, but also alterations in other optically important constituents. Phytoplankton community structure, which strongly affects ocean optics, is likely to show one of the clearest and most rapid signatures of changes to the base of the marine ecosystem.