Reduced snow and increased nutrients show enhanced ice-associated photoautotrophic growth using a modified experimental under-ice design

Under-ice photoautotrophs in lakes are generally considered to be limited by light rather than nutrients. Despite reduced light intensity under the ice, there is increasing evidence that suggests some lakes support high levels of photoautotrophs. We explored how snow cover (i.e., light) and nutrients (i.e., nitrogen and phosphorus) influence ice-associated photoautotroph growth in a Minnesota, USA lake. Using a novel under-ice approach, we deployed nutrient diffusing substrates (single or combined nutrient amendments) under two different light scenarios (snow covered, reduced light; snow removed, increased light) near the water-ice interface to mimic a range of conditions ice-associated photoautotrophs may be exposed to. Natural snow cover reduced light compared with snow removal, particularly early in the experiment before snow began to melt. When comparing photoautotroph chlorophyll a (Chl a) between snow treatments, we found a significant snow effect with higher concentrations in the snow removed treatment. We also found a significant nutrient effect, for all nutrient treatments, on Chl a concentrations in both snow conditions. The effect of any nutrient treatment on Chl a concentrations was similar. Our results suggest that ice-associated photoautotrophs were able to grow in all snow conditions, but snow removal resulted in higher growth and nutrient availability also mediated responses. Thus, both light and nutrient conditions in the winter may strongly affect ice-associated photoautotroph dynamics.

Automated summary

This study investigated the impact of snow cover and nutrients on the growth of under-ice photoautotrophs in a lake. The results showed that both snow cover and nutrients significantly affected the chlorophyll a concentrations of photoautotrophs, with higher growth observed when snow was removed. Therefore, both light and nutrient conditions in winter may play a crucial role in under-ice photoautotroph dynamics.