Assessing Spring Bloom Timing in the Southern Ocean Using Biogeochemical Float Observations in Comparison to Satellite-Derived Estimates

Abstract

Phytoplankton are the base of the marine food web and influence oceanic carbon dioxide uptake. Phytoplankton bloom when there is sufficient light and nutrients. Changes in bloom timing affect the food web and potentially biogenic carbon uptake. In the Southern Ocean, studies using satellite ocean-color observations indicate long-term changes in bloom timing, but satellites miss phytoplankton growth below the surface. In this study, we estimate bloom initiation timing from chlorophyll (Chl) and phytoplankton carbon (Cp) derived from robotic profiling float measurements of chlorophyll fluorescence and particulate backscatter located south of 30°S in five distinct ocean biomes. We estimated bloom initiation from seasonal changes in mean Chl and Cp concentrations in the mixed layer depth (MLD), similar to what satellites observe, and from Chl and Cp integrated from the shallowest measurement or 7 m to 100 m (∑100 m) and 200 m (∑200 m). We found that bloom initiation estimates from Chl occur before estimates from Cp in permanently stratified and ice biomes. In permanently stratified biomes, MLD mean Chl and Cp (!ℎ#!"#, !$!"#) bloom initiation estimates occur earlier than depth-integrated Chl or Cp estimates (!ℎ#$%%, !%%) while the opposite is true in seasonally stratified biomes. Our results suggest that Chl can change without changes in Cp and that using float observations that include phytoplankton production in deeper waters offers useful insights into bloom initiation timing and emerging trends. If Chl was used to estimate timing alone, it might not be an accurate indicator of when phytoplankton production increases as the chlorophyll to carbon ratio in phytoplankton can vary.