The 2008 North Atlantic Spring Bloom Experiment III: Bloom dynamics

As part of the 2008 North Atlantic Spring Bloom Experiment (NAB08), 2 Lagrangian Floats and 4 Seagliders were deployed in early April before the bloom began. Mixed-layer depth exceeded 150m; chlorophyll concentrations were low and dissolved nutrient concentrations were high; CTD profiles of chlorophyll fluorescence and optical backscattering (proxies for phytoplankton and particle concentrations, respectively) were low and uniform throughout the mixed layer. By mid April autonomously-sensed chlorophyll fluorescence and particle backscattering began to exponentially increase, with a concomitant increase in dissolved oxygen. Phytoplankton biomass and oxygen concentrations were still increasing in early May when the R/V Knorr arrived on station for a 3-week process cruise. Satellite imagery indicated that the bloom was spatially patchy in early May. Large diatom cells, including chain-forming species, dominated the phytoplankton community in early May. By mid-May, community dominance shifted to dinoflagellates and picoeukaryotes as nutrient concentrations decreased. Discrete spikes in chlorophyll fluorescence, optical backscatter, and beam transmission began to appear below the mixed layer, first extending to 250m and then progressively deeper (deepest depth sampled was 900m, by Seagliders). The deep optical spikes were associated with sinking organic particles; the largest catch of sinking cells, based on material collected by PELAGRA floating sediments traps, coincided with a horizontally and vertically wide-spread distribution of optical spikes observed both with the ship's CTD and by Seagliders. Autonomous measurements continued until the end of June, with brief ship-based sampling in mid and late June. Phytoplankton and particle biomass oscillated but remained higher than pre-bloom conditions, as did concentrations of dissolved oxygen. The frequency of subsurface optical spikes diminished, suggesting that the major carbon flux event occurred in mid May. These observations show the power of coordinated autonomous and ship-based sampling, using complementary bio-optical and biogeochemical measurements to measure coupled biophysical processes controlling ocean carbon fluxes.