Light in the Polar Night

Abstract

How much light is available for biological processes during Polar Night? This question appears simple enough. But the reality is that conventional light sensors for measuring visible light ({\textasciitilde}350 to {\textasciitilde}700 nm) have not been sensitive enough to answer it. Beyond this technical challenge, ``light'' is a general term that must be qualified in terms of ``light climate'' before it has meaning for biological systems. In this chapter, we provide an answer to the question posed above and explore aspects of light climate during Polar Night with relevance to biology, specifically, how Polar Night is defined by solar elevation, atmospheric light in Polar Night and its propagation underwater, bioluminescence in Polar Night and the concept of Polar Night as a deep-sea analogue, light pollution, and future perspectives. This chapter focuses on the quantity and quality of light present during Polar Night, while subsequent chapters in this volume focus on specific biological effects of this light for algae (Chap. ``Marine Micro- and Macroalgae in the Polar Night''), zooplankton (Chaps. ``Zooplankton in the Polar Night'' and ``Biological Clocks and Rhythms in Polar Organisms''), and fish (Chap. ``Fish Ecology in the Polar Night'').