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

Abstract

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. 0.5mm thickness), and 0% with a heavy dusting (approx. 0.7mm thickness). For the germlings, 1.0% of survived when covered by medium (approx. 0.7mm thickness) sediment coverage, whereas high coverage (approx. 1.8mm 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.