Preliminary explorations of environmental tolerances and growth rates of holopelagic Sargassum morphotypes

To predict spatial and temporal dynamics of macroalgal blooms, including the Great Atlantic Sargassum Belt, understanding the environmental tolerances and growth rates of different species under varying conditions is essential. A series of preliminary experiments were conducted exposing three common holopelagic Sargassum morphotypes to different temperature (21.6–30.6 °C) and salinity (26.2–40.0 psu) conditions. Sargassum tolerance was assessed by two methods: a Health Metric calculated daily from changes in a specimen’s blade and vesicle count and coloration patterns, and growth rates determined from pre- and post-treatment wet weights. Morphotypes exhibited different responses to treatment conditions. Health Metric values and growth rates for S. fluitans III were not significantly impacted by tested temperatures or salinities. S. natans I tolerated a wide range of temperatures but only moderate salinities. While S. natans VIII grew well and maintained strong Health Metrics across examined salinities, it was less tolerant of cold temperatures and exhibited overall lower growth rates than other morphotypes. Given inconsistent findings among recent growth rate studies, including this one, additional experiments of longer duration that continue to explore temperature and salinity effects and use specimens from across holopelagic Sargassum’s geographic range are necessary to understand growth ecology and parameterize models.