Growth or Just Mixing? Glider-Based Observations of Elevated Phytoplankton Biomass Proxies Over the Scotian Shelf Following Hurricane Arthur

The Intergovernmental Panel on Climate Change predicts an increased frequency of tropical cyclones in the future. Their effects on water column processes are not fully understood. A common interpretation of increased surface chlorophyll observed by satellite remote sensing after the passage of a storm attributes the increase to a bloom due to injection of nutrients from deep water. We present glider-based observations of optical properties of the water column after one storm that are inconsistent with this hypothesis. On July 5, 2014, Hurricane Arthur passed over the Northwest Atlantic and was downgraded to a post-tropical storm. The storm deepened the mixed layer by ~20 m and lowered sea surface temperature by 3°C. Near-surface chlorophyll and optical particulate backscattering (b_bp) showed an approximately 50% and 30% increase, respectively, as the mixed layer deepened. However, chlorophyll and b_bp averaged from 5 to 30 m showed no change during the onset of the storm nor for another 3 days following the storm. This suggests that the increase seen in the near-surface bio-optical measurements was due to the contributions of subsurface phytoplankton that mixed towards the surface rather than in-situ growth following entrainment of nutrient-rich water into the mixed layer. Interpretation of satellite remote-sensed surface data following storm-driven deep mixing should be undertaken with caution in the absence of in situ subsurface data.