Summer diatom blooms in the eastern North Pacific gyre investigated with a long-endurance autonomous vehicle

Date

2017-12-08

Authors

Anderson, Emily Elizabeth

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Abstract

Summertime phytoplankton blooms regularly develop in the North Pacific subtropical gyre, a region of open ocean that is far removed from any land-derived or Ekman upwelling nutrient sources. Limited direct sampling suggest these blooms are dominated by N₂-fixing diatom-cyanobacteria associations in the genera Rhizosolenia and Hemiaulus. The nitrogen fixing endosymbionts are hypothesized to be critical to the development of blooms in this nitrogen limited region. However, due to their remote location and unpredictable duration, prolonged in situ observations of the environment and biota are rare outside of the major time-series at Station ALOHA. In the summer of 2015, a proof-of–concept mission using the autonomous aquatic vehicle, Honey Badger (WaveGlider SV2), collected surface and near-surface (<20m) observations in the NPSG with hydrographic, meteorological, optical, and imaging sensors focusing on the abundance and distribution of these bloom forming symbioses. Hemiaulus and Rhizosolenia cell abundances were calculated using in-line digital holography for the entire mission from June-November. While the Honey Badger was not able to reach the 30°N subtropical front region where most of the summer diatom blooms have been observed, near-real time navigational control allowed it to transect two bloom regions identified by satellite chlorophyll-a concentrations. The two species did not co-occur in large numbers, rather the blooms were dominated by either Hemiaulus or Rhizosolenia. The 2-4 August 2015 bloom was comprised of 96% Hemiaulus diatoms and second bloom, 15-17 August, was dominated by Rhizosolenia diatoms (75%). The holograms also provide valuable insights into the occurrence of aggregated forms of Hemiaulus diatoms outside the identified Hemiaulus bloom area. The photosynthetic potential index (F[subscript v]:F[subscript m]) increased during both blooms. In addition, the diel pattern of F[subscript v]:F[subscript m] (nocturnal maximum; diurnal minimum) was consistent with macronutrient limitation throughout the mission with no evidence of Fe-limitation despite the presence of nitrogen fixing diatom-diazotroph assemblages. By the end of the 5-month mission, Honey Badger had covered about 3070 nautical miles (5690 km), taken 9139 holograms, skirted two major tropical storms (Guillermo and Ignacio), and reliably transmitted data to an ERDDAP data server in near real-time.

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