Does horizontal mixing explain phytoplankton dynamics?

T he modern era of biological oceanography arguably began in 1978 with the successful launch of the Coastal Zone Color Scanner on the Nimbus 7 satellite. Although limited by today’s standards, the Coastal Zone Color Scanner provided the first glimpse of the complex, beautiful, and difficult-to-sample interactions between single-celled phytoplankton and the turbulent mixing of the surface ocean. In the intervening decades, oceanographers have made tremendous advances, with more and better ocean color sensors such as the Sea-viewing Wide Field-of-view Sensor, Moderate Resolution Imaging Spectroradiometer, and Medium Resolution Imaging Spectrometer. The PNAS report by d’Ovidio et al. (1) shows us how much further we need to go and provides a glimpse into the true complexity of the surface ocean and the mechanisms driving this ecological landscape. Three decades of satellite ocean color measurements have led to fundamental observations and discoveries about the role of the microscopic unicellular algae in regulating the biogeochemistry of our planet. We routinely make maps of both terrestrial and ocean plant and algae biomass, using the photosynthetic pigment chlorophyll as a proxy, and link this to the biogeochemical cycling of carbon through estimates of net primary production (2). Although there is certainly room for improvement in these estimates (3), these nearly synoptic measurements show the oceans respond to basin-scale decadal oscillations, such as El Niño and the Atlantic and Pacific Decadal Oscillations (4), and have helped to identify both the short(decadal) and long-term (centuries) response of the oceans to climate change (5, 6). Despite the many advances in ocean color, we still rely almost exclusively on estimates of bulk biomass as chlorophyll. Unlike the most simple microscope, chlorophyll alone tells us almost nothing about the individual types of phytoplankton, or the role these groups play in the ocean ecosystem. The report by d’Ovidio et al. (1) begins to unpack this black box by applying a new ocean color method that partitions the bulk optical signal retrieved from satellites into major phytoplankton functional types using a numerical algorithm called PHYSAT (7). PHYSAT takes advantage of the subtle biooptical differences between major groups of phytoplankton, or functional types, to identify the dominant types of phytoplankton from ocean color. Currently, these functional types include diatoms, coccolithophores, nanoeukaryotes, Phaeocystis, Synechococcus-like cyanobacteria, and Prochlorococcus. Other researchers have developed similar approaches for both phytoplankton size (8) and additional phytoplankton groups such as the nitrogen-fixing cyanobacteria Trichodesmium (9), demonstrating that this approach could be extended to at least a few more phytoplankton functional types. d’Ovidio et al. (1) apply PHYSAT to a region of intense physical mixing, where the Brazil and Malvinas currents interact in the southern hemisphere. They combine the results of PHYSAT with altimetry data from a suite of satellite sensors to estimate sea surface height, from which -128 -128