The Subpolar North Atlantic Spring Bloom – What Did We Learn from the NAB 2008 Autonomous Experiment?

Abstract:

The subpolar North Atlantic bloom is one of the most remarkable features on the planet, with almost explosive ‘greening’ of the oceans. Over decades, investigators from countries bordering the North Atlantic have caught snippets of the bloom from research vessels or merchant ships transiting between continents. On 4 April 2008, Eric D’Asaro, Craig Lee, and I began a comprehensive study of the initiation and demise of the spring bloom using 2 types of autonomous platforms – a patch-following Lagrangian mixed-layer float and 4 float-following gliders. The 3 mo autonomous experiment integrated measurements from the float, gliders and ships, observations from satellites, and analyses from models. The diatom-dominated bloom began in mid April when the water column stabilized, not by solar warming, but rather by eddy-driven slumping of horizontal density gradients. The resulting bloom was patchy in both biomass and phytoplankton diversity, despite high, non-limiting concentrations of macronutrients. Magnitudes and relative proportions of net community productivity (NCP; determined from autonomous budgets of O2 and NO3) and net phytoplankton productivity (NPP; computed from ship-based photosynthetic parameters and float-based biomass and light) diverged as the bloom evolved, with higher fractions of particulate organic carbon (POC) consumed within the mixed layer as the bloom aged. Export productivity (EP; derived as the difference between NCP and accumulation rate of POC) was of similar magnitude during the May diatom bloom and the June picophytoplankton bloom. When silicic acid dropped to 1 µM, diatoms aggregated and sank; the deep flux event was dominated by resting spores of Chaetoceros. Although a short-lived event, it was ubiquitously observed by the 4 gliders and ship. An eddy-driven subduction event was likewise observed, indicating transport of otherwise non-sinking POC along isopycnals to depths of > 200 m. These striking export events reinforce the value of a persistent, distributed presence afforded by a network of autonomous platforms to catch and follow key, transient events. NAB 2008 demonstrated the power of autonomous platforms to decipher productivities and carbon fluxes at the patch scale, but left the enticing challenge of how to scale from the patch to the basin for another time.