An in situ incubation method for measuring the productivity and responses of under‐ice algae to ocean acidification and warming in polar marine habitats

During the Antarctic spring, algae grows under extensive areas of sea-ice and is a fundamental source of primary production. Understanding how under-ice (bottom-ice) algae will be affected by ocean warming and acidification is critically important in determining the probable future flow-on effects to the ecological communities this algae supports. To investigate this we designed and built a customised experimental system to assess the in situ responses of under-ice algae to changes in both seawater pH and temperature. We conducted two trials in 2013 followed by a successful 14-day incubation experiment in 2014 in the Ross Sea, Antarctica, using the system described here. Assessment of our main control parameters indicated we could reliably control and monitor both pH and temperature in transparent under-ice chambers. The “plug-and-play” nature of our novel system meant it was easy for divers to deploy and maintain in the very cold temperatures experienced under the sea-ice. Moreover, the system could be remotely sampled from a surface laboratory. This enabled robust monitoring and analyses of manipulated seawater conditions (e.g., pH and temperature), and of responses of the associated biological communities (e.g., fluxes in dissolved oxygen and nutrient levels).