Amphipode Themisto libellula from the Arctic Ocean. Credit: Alfred Wegener Institute / Barbara Niehoff
For example, due to less and thinner sea ice, sunlight can penetrate much farther below the surface. As a result, under certain conditions, the primary production – i.e., the growth – of microalgae in the water and ice can increase substantially. How these changing light conditions are affecting higher trophic levels in the food chain – like zooplankton, which feed in part on microalgae – remains poorly understood.
Both the daily migration at lower latitudes and the seasonal migration in the polar regions are predominantly dictated by sunlight. The tiny organisms usually prefer twilight conditions. They like to stay below a certain light intensity , which is usually quite low and lies well into the twilight range.
What the experts found: Due to the steadily declining ice thickness, the critical irradiance level would drop to greater depths earlier and earlier in the year and wouldn’t return to the surface layer until later and later in the year. Since the zooplankton fundamentally remain in waters below this critical level, their movements would mirror this change.