Winter vertical migration of Arctic zooplankton
In recent years, evidence has been found of Diel Vertical Migration (DVM) in zooplankton during the Polar Night in the Arctic Ocean. However, the drivers of this behaviour during an apparent lack of illumination and food are poorly understood, as is its spatial extent across the Arctic Ocean. A novel dataset comprising 58 deployments of moored Acoustic Doppler Current Profilers is used in this study to observe the vertical migratory behaviour of zooplankton on a pan-Arctic scale. Methods of circadian rhythm analysis are applied to detect synchronous activity. South of 75°N, DVM continues throughout winter (albeit with reduced vertical amplitude when compared to other times of the year). DVM is seen to cease for a short period of time (up to 50 days) at latitudes between 75° and 82°N. The duration for which DVM ceases is controlled primarily by latitude (and therefore the altitude of the sun), but is modified by sea-ice presence and other environmental parameters. A DVM pattern is not seen at 90°N at any time of the year, but ⇠365 day periodicity is detected in the surface backscatter levels at this latitude. During the Polar Night, the moon controls the vertical positioning of zooplankton across the Arctic. Aggregations at depth coincide with an avoidance of the surface for several (<6) days over the full moon. The deepest aggregation is seen at 110 m, indicating a depth limit of moonlight perception. A new type of migratory behaviour is described: Lunar Vertical Migration (LVM) which exists as LVM-day (24.8 hour periodicity) and LVM-month (29.5 day periodicity) on a pan-Arctic scale. The strongest LVM-day is seen at 50 m depth. Sea-ice and cloud are found to modify both of these lunar responses. The results presented by this thesis show continuous activity throughout winter, and challenge assumptions of a quiescent Polar Night.
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