The effects of sediment on Sargassum horneri in the early life stages on rocky subtidal reefs

• Field investigation showed S. horneri predominated at sites with less sediment and intermediate water motion.
• Lab experiment showed small changes in the abundance of deposited sediment can prevent zygote settlement.
• Relatively small increases in settling sediment can affect the survival of already settled zygotes/germlings.

Sediment plays an important role in controlling the distribution of macroalgae. We examined the distribution of Sargassum horneri in the field considering the effect of sedimentation, and conducted sediment manipulation experiments to test the hypothesis that sedimentation affects the early settlement stages of S. horneri. Field samplings were conducted on rocky subtidal platforms off Gouqi Island, in Zhoushan Archipelago in the Eastern China Sea. Water depth and motion were also measured. Sediment traps and a suction pump were used to collect in situ sediment. In the laboratory, two experiments were conducted to test the effects of sediment cover on zygote attachment and germling survival, respectively. Attachment rate of S. horneri zygotes was reduced from 81.3% of the control group to 3.6% with a medium dusting of sediment (approx. 0.5 mm thickness), and 0% with a heavy dusting (approx. 0.7 mm thickness). For the germlings, 1.0% of survived when covered by medium (approx. 0.7 mm thickness) sediment coverage, whereas high coverage (approx. 1.8 mm thickness) caused 100% mortality. Our study showed: 1) in the field, distribution of S. horneri was negatively correlated with sediment amount, and S. horneri was always found abundant in sites with less sediment and intermediate water motion; 2) in laboratory-based experiments, small changes in the abundance of deposited sediment prevented zygote settlement; and 3) relatively small increases in settling sediment affected the survival of already settled zygotes/germlings. In conclusion, we found that sediment was an important driver of S. horneri distribution, especially during the early settlement stages.