An integrated multiple driver mesocosm experiment reveals the effect of global change on planktonic food web structure

We conducted a mesocosm experiment with an integrated multiple driver design to assess the impact of future global change scenarios on plankton, a key component of marine food webs. The experimental treatments were based on the RCP 6.0 and 8.5 scenarios developed by the IPCC, which were Extended (ERCP) to integrate the future predicted changing nutrient inputs into coastal waters. The mesocosm experiment was conducted over three weeks in late-summer (August-September) 2018. Seawater containing a natural plankton community was collected from the coastal North Sea. At the onset of the experiment, CO2 saturated seawater was added to the ERCP scenario mesocosms to adjust pCO2 and pH levels for each scenario. To create a realistic environment, we also manipulated the atmospheric pCO2 in the enclosed mesocosm tanks throughout the experiment. Seawater temperature was adjusted daily according to the current North Sea temperature measured at the Helgoland Roads for the Ambient, and 1.5°C and 3.0°C warmer for the ERCP 6.0 and ERCP 8.5 scenarios, respectively. Dissolved nutrient concentrations were determined at the onset of the experiment and adjusted to reach the desired N:P ratios. Samples were taken in an interval of 1-3 days depending on the phytoplankton bloom development, and community composition, except for the large mesozooplankton, was monitored throughout the experiment period.