To investigate the effect of different nutrient and temperature conditions on holopelagic Sargassum, six different manipulation experiments were performed in Puerto Morelos, Mexico in July 2022. Carbon fixation and nitrate uptake rates using stable isotopes (13C-bicarbonate and 15N-nitrate), and biomass elemental composition (particulate organic carbon, nitrogen and phosphorus) of Sargassum fluitans III were studied under different conditions in specialized ex-situ cultivation systems and bottle incubations. Experiment 1 tested the long-term effects (seven days), and Experiment 2 the short-term effects (two days) of different nitrate and phosphate concentrations. Experiment 4 investigated the temporal development of responses to elevated nutrient concentrations over the course of 36 hours. In Experiment 5, S. fluitans III was grown for one day under different temperatures. Experiment 6 tested the individual short-term (two days) effects of either elevated nitrate or elevated phosphate conditions. And in Experiment 8, different trace elements were provided through addition of olivine sand or volcanic ash.

The event location refers to where Sargassum was collected for the incubations. The experiments were performed ~85 m away under natural light conditions.