The current understanding of Arctic ecosystems is deeply rooted in the classical view of a bottom-up controlled system with strong physical forcing and seasonality in primary-production regimes. Consequently, the Arctic polar night is commonly disregarded as a time of year when biological activities are reduced to a minimum due to a reduced food supply. Here, based upon a multidisciplinary ecosystem-scale study from the polar night at 79°N, we present an entirely different view. Instead of an ecosystem that has entered a resting state, we document a system with high activity levels and biological interactions across most trophic levels. In some habitats, biological diversity and presence of juvenile stages were elevated in winter months compared to the more productive and sunlit periods. Ultimately, our results suggest a different perspective regarding ecosystem function that will be of importance for future environmental management and decision making, especially at a time when Arctic regions are experiencing accelerated environmental change [1].

Bivalve closure responses to detect contaminants have often been studied in ecotoxicology as an aquatic pollution biosensor. We present a new laboratory procedure to estimate its potential and limits for various contaminants and animal susceptible to stress. The study was performed in the Asiatic clam Corbicula fluminea and applied to cadmium. To take into account the rate of spontaneous closures, we integrated stress problems associated with fixation by a valve in common apparatus and the spontaneous rhythm associated with circadian activity to focus on conditions with the lowest probability of spontaneous closing. Moreover, we developed an original system by impedance valvometry, using light-weight impedance electrodes, to study free-ranging animals in low-stress conditions and a new analytical approach to describe valve closure behavior as a function of response time and concentration of contaminant. In C. fluminea, we show that cadmium concentrations above 50 microg/L can be detected within less than 1 h, concentrations down to 16 microg/L require 5 h of integration time, and values lower than 16 microg/L cannot be distinguished from background noise. Our procedure improved by a factor of six the cadmium sensitivity threshold reported in the literature. Problems of field applications are discussed.