[1] The timing or phenology of the annual cycle of phytoplankton biomass can be monitored to better understand the underpinnings of the marine ecosystem and assess its response to environmental change. Ten-year, global maps of the mean date of bloom onset, peak concentration and termination of bloom duration were constructed by extracting these phenological metrics from Generalized Linear Models (GLM) fit to time series of 1 Â 1 daily estimates of SeaWiFS chlorophyll concentrations dating from September 1997 to December 2007 as well as to MODIS chlorophyll concentrations from July 2002 to July 2010. The fitted models quantitatively define the annual cycle of phytoplankton throughout the global ocean and from which a baseline of phenological characteristics was extracted. The analysis revealed regionally consistent patterns in the shape and timing of the annual cycle of chlorophyll concentration that are broadly consistent with other published studies. The results showed that a single bloom predominates over the global ocean with secondary, autumn blooms being limited in both location and spatial extent. Bloom duration tended to be zonally consistent, but meridionally complex and did not become progressively shorter with increasing latitude as is sometimes depicted. Both the shape of the annual cycle and the phenological climatologies can be used in future studies to detect significant departures over time.

Climate change is expected to affect the timing and magnitude of numerous environmental conditions, including temperature, wind, and precipitation. Amongst other repercussions, such alterations will engender a response in marine ecosystem productivity manifested by changes in the timing and magnitude of phytoplankton biomass and primary productivity. Several investigations have examined the change in magnitude in chlorophyll concentration in relation to changing environmental conditions, but little has been done to examine the change in the timing of the annual cycle of phytoplankton biomass. In order to establish a baseline from which to assess any future changes in the phenology of phytoplankton biomass, we constructed nine‐year climatologies of phytoplankton bloom onset, maturity, start of bloom decay, and termination in the central North Atlantic. This was accomplished by extracting annual values of these phenological markers from Generalized Linear Models fit to pentad (five‐day) estimates of SeaWiFS chlorophyll concentrations dating from 1998 to 2006. This novel modeling approach, which produced results consistent with known patterns of phytoplankton bloom dynamics in this region, provides a statistically robust approach to detect and account for changes in the annual cycle of phytoplankton biomass.