Surface chlorophyll variability in the Drake Passage region of the Southern Ocean

The Southern Ocean, the ocean surrounding the Antarctic continent, supports a diverse array of biological species and unique food webs. Given the important role of phytoplankton in Southern Ocean food webs and their influence on the global carbon reservoir, it is of interest to know whether their abundance has changed over time. Chlorophyll is a pigment present in all photosynthesizing phytoplankton that can be used to estimate the biomass and productivity of phytoplankton at a given time and location. This study analyzes surface chlorophyll variability in the Drake Passage region of the Southern Ocean using underway fluorometer derived chlorophyll from the ARSV Laurence M. Gould (2002-2015), along with satellite derived chlorophyll-a estimates from the MODIS sensor. We observe an austral fall bloom in the underway fluorometer record, ranging from 40%-100% of the maximum bloom concentration in a given year. This bloom is significant due to its timing and magnitude, and due to its absence from the recent relevant literature. We propose that recent studies failed to observe a fall bloom in this region due to reliance on satellite-derived estimates of chlorophyll for which there exist very few valid estimates in the Drake Passage during the austral fall. Analysis of MODIS estimates of chlorophyll also revealed that this sensor systematically underestimates chlorophyll concentration in the Drake Passage region. Lastly, a weak statistical relationship between surface ocean pCO2 and fluorometer-based chlorophyll was observed in the Drake Passage. We suspect that this weak relationship is due to the opposing effects of the thermally driven component of pCO2 and the component of pCO2 due to changes in total dissolved inorganic carbon