Shine a light: Under-ice light and its ecological implications in a changing Arctic Ocean

Abstract

The Arctic marine ecosystem is shaped by the seasonality of the solar cycle, spanning from 24-h light at the sea surface in summer to 24-h darkness in winter. The amount of light available for under-ice ecosystems is the result of different physical and biological processes that affect its path through atmosphere, snow, sea ice and water. In this article, we review the present state of knowledge of the abiotic (clouds, sea ice, snow, suspended matter) and biotic (sea ice algae and phytoplankton) controls on the underwater light field. We focus on how the available light affects the seasonal cycle of primary production (sympagic and pelagic) and discuss the sensitivity of ecosystems to changes in the light field based on model simulations. Lastly, we discuss predicted future changes in under-ice light as a consequence of climate change and their potential ecological implications, with the aim of providing a guide for future research.

Keywords: Arctic Ocean; Arctic ecosystem; Light transmission; Phytoplankton; Primary production; Sea ice algae.

Fig. 1

Schematic of light transmission through atmosphere, sea ice and snow, and ocean (credit: Andrew Orkney)

Fig. 2

Effect of varying solar zenith angle (upper panel), and cloud cover (lower panel) on surface PAR irradiance. Data are presented as the ratio of light intensity relative to the equator at noon on midsummer’s day. The red box highlights solar zenith angles relevant for Arctic regions. All simulations done with Hydrolight Radiative Transfer Software that includes the RADTRAN sky model (Gregg and Carder 1990) and the cloud model from Kasten and Czeplak (1980)

Fig. 3

Example of sea ice algae forming a mat-like colony under the ice (left panel, credit: Carsten Wancke), and of Melosira arctica forming filaments affixed to the sea ice subsurface (right panel, credit: Oliver Müller)

Fig. 4

Under-ice PAR at the end of August 2012 for the case with a standard parameterization of the SSL (‘standard’ simulation, left hand side) and for ‘drainage’, when the SSL only exists if the sea ice is thicker than 50 cm (right hand side)

Fig. 5

Mean PAR (top row), mean sea ice algae chl a (middle row) and NPP of diatoms (bottom row) in latitude bands. The columns show on the left hand side: the standard parameterization of the SSL (‘standard’ simulation); in the middle: ‘drainage’—existence of an SSL only if the sea ice thickness > 50 cm; and on the right hand side: ‘drainage_2K_s’ with a doubled extinction coefficients for snow K_s. For the simulations ‘drainage’ and ‘drainage_2k_s’, NPP and PAR are shown as differences ‘drainage’—‘standard’ and ‘drainage_k_s’—‘drainage’, respectively