Growth rates of pelagic Sargassum species in the Mexican Caribbean

The recent recurring blooms of pelagic Sargassum spp. (sargasso) in the tropical Atlantic forming the Great Atlantic Sargasso Belt, are composed of distinct algal species and morphological forms, for which little understanding of basic development exists. Growth rate measurements across a range of environmental conditions in semi-controlled settings may aid in understanding the blooms. However, measuring the growth of this pelagic seaweed has proved to be challenging, as it tends to lose vigor in standard culturing systems. This work describes in-situ and ex-situ culture systems, that mimicked the pelagic condition by keeping the algae in motion. Growth rates measured in both systems were comparable, and in the same order of magnitude as those measured for sargasso in the Sargasso Sea. The morphotype VIII of the species S. natans, which became abundant with the blooms, had a higher in-situ growth rate (doubling time 17.9 – 26.3 d) than S. fluitans III (doubling time 25.6 – 83.3 d) under the studied conditions and temperatures (27–29 °C). Fertilizer addition to the in-situ trials did not result in higher growth rates, even though tissue content analysis showed that both N and P were taken up by the algae. In the ex-situ trials, S. natans VIII responded unfavorably to higher temperature (31 °C), whereas S. fluitans III did not. This preliminary study showed that both in-situ and ex-situ culture are feasible to determine growth rates of sargasso, paving the way for future (semi-) controlled studies on bloom dynamics in relation to abiotic and biotic conditions.