Lightfoot, M. (2023). Light sensitivity of the Arctic copepod Metridia longa during Midnight Sun and Polar Night. M.Sc. Thesis, University of Delaware, Newark, Delaware, USA. 67 pp.
481624
Lightfoot, M.
2023
Light sensitivity of the Arctic copepod Metridia longa during Midnight Sun and Polar Night.
M.Sc. Thesis, University of Delaware, Newark, Delaware, USA.
The Arctic is defined by a seasonal light regime extending between 24-hour light (Midnight Sun) and 24-hour dark (Polar Night). Light itself acts as an important cue for marine zooplankton, dictating their orientation/navigation and vertical migration, prey detection and predator avoidance, and population dynamics/reproductive strategies. Changes in the spectral, intensity, and duration components of downwelling irradiance during Midnight Sun and Polar Night contribute to the annual Arctic light climate, and to what characteristics of light are used for zooplankton visual processes. Additionally, as the Arctic region is warming at an alarming rate, the loss of annual sea ice and snow coverage is projected to increase incoming illumination into the water column, impacting zooplankton visual systems, trophic dynamics, and predator-prey interactions.
Given the marked differences in annual light climate at high latitudes, the objective of this study was to determine the behavioral responses of zooplankton species to spectral and irradiance light stimuli during Midnight Sun and Polar Night. The copepod species Metridia longa copepods were selected as the target species for this research due to their biomass in Atlantic-Arctic waters, vertical migration behaviors, and bioluminescent capabilities. While Polar Night twilight peak emissions are blue dominant (? max = 455 nm) there is a shift towards green light (? max = 550 nm) availability during Midnight Sun due to spring phytoplankton blooms and suspended particulate matter in the water column. Therefore, it was hypothesized that copepods will exhibit a broad spectral response sensitivity in the blue-green wavelengths in order to compensate for the annual shifts in available spectra. Additionally, it is expected that M. longa copepods will have a heightened irradiance response during Polar Night compared to Midnight Sun. Zooplankton visual systems are often more sensitive in low light conditions, thus it is hypothesized that irradiance responses will be increased during Polar Night.
Sampling took place in the Barents Sea in May 2022 (Midnight Sun) and January 2023 (Polar Night), with additional sampling in Kongsfjorden during Polar Night in January 2023. Light response/sensitivity was assessed on an individual basis in this study to provide insight into the individual variability in light sensitivity among M. longa copepods. This was accomplished using an adjustable, high throughput apparatus to test 64 individual zooplankton swimming behavior individually and simultaneously to spectral and irradiance light stimuli.
Results suggest a consistent blue-green dominant spectral response between Midnight Sun and Polar Night to wavelengths ranging from 400 to 550nm, along with an increased irradiance response by an order of magnitude among Polar Night copepods compared to those tested during Midnight Sun. Additionally, irradiance response among M. longa copepods may vary with developmental stage, but are not dependent on location or temperature during Polar Night. From these results, it was determined that M. longa female copepods can detect light down to 119 m during Midnight Sun in the Barents Sea and 55 m in Kongsfjorden during Polar Night. These irradiance thresholds correspond to reported depths of zooplankton across seasons as they move vertically with isolumes. In a changing Arctic, loss of sea ice and snow attenuating incoming irradiance is predicted to increase the pelagic visual lightscape. Since light sensitivity was not temperature dependent in this study, thresholds of detectable light in the water column should not vary with climate change-related increases in water temperature. This suggests that zooplankton like M. longa may be forced deeper in the water column to maintain themselves in darkness. Future work should assess M. longa response sensitivities under a more climate-focused lens, and incorporate additional components like cameras and other sensor positions to the apparatus created for this study to provide a full picture to zooplankton behavior in response to light at high latitudes
