Assessing growth of pelagic sargassum in the Tropical Atlantic

Pelagic sargassum species, historically endemic to the North Atlantic, have now been identified in the Tropical Atlantic. For over a decade, massive blooms of pelagic sargassum have dispersed seasonally from this ‘new’ Tropical Atlantic source region, negatively impacting the coasts of Caribbean and West African countries. Significant strides have been made towards adaptation to this new ‘hazard’, yet many knowledge gaps remain that constrain improvements to these efforts. The key objective of this study was to add to the knowledge of pelagic sargassum growth rates in the Tropical Atlantic to better inform forecasting and innovative applications. Growth rates of the three commonly occurring morphotypes from the Tropical Atlantic source region, S. fluitans III, S. natans I and S. natans VIII, were studied in-situ using novel 'growth mesocosms' in neritic waters of Barbados under cool and warm ambient conditions. Overall, results reveal a significant difference in growth rates among all pelagic sargassum morphotypes, with S. fluitans III on average doubling its biomass in 13 days, S. natans I in 22 days and S. natans VIII in 31 days. Furthermore, growth rates of S. fluitans III decreased significantly with an increase in ambient temperature from the cool (mean 27.6 °C) to warm (mean 29.6 °C) water period. During both periods S. fluitans III grew significantly faster than the other two morphotypes. However, S. natans I only grew significantly faster than its conspecific morphotype, S. natans VIII, during the cool period. These findings highlight the different growth responses among sargassum morphotypes under varying conditions and add to the considerable variability in growth rates reported by recent similar studies conducted elsewhere in the Tropical Atlantic. As such, it is clear that further research will be needed to unravel the complex mechanisms affecting pelagic sargassum growth rates before they can confidently inform improvements in management applications.