Vertical distribution pattern of Sargassum horneri and its relationship with environmental factors around Gouqi Island

Abstract

Among environmental factors governing macroalgae distribution, temperature and daylength are considered to be the most geographically relevant worldwide. However, within a particular site, depth is the most useful predictor of phytobenthic community structure. Furthermore, factors such as light intensity, wave exposure, and sediment, which are also related to depth, can also affect the distribution pattern of macroalgae within these sites. Therefore, it is important to determine the relationship between these factors and their effects on the macroalgae distribution pattern. Furthermore, it will also be important to determine how these factors influence this distribution, as well as the mechanism by which they do so. This research will provide an important basis for the ecological protection and restoration of macroalgal beds. To this end, we investigated the spatial (vertical) and temporal pattern of the distribution of Sargassum horneri around Gouqi Island. Through field-based experiments, we tested the effects of environmental factors (light intensity, wave exposure, and sediment) on the vertical distribution and growth of S. horneri germlings. Adult S. horneri was dominant in the lower region of the intertidal zone at a depth of 121—240 cm, with the highest density reaching 86 ind/ m2. Although adult S. horneri is sensitive to density-dependent restriction, the observed density was suitable for the survival of the plant at this depth. The greatest length of adult S. horneri at this depth was 78.2 cm. Thus, most of the reproductive population of S. horneri is predominantly distributed at the bottom of intertidal zone. In the subtidal zone, at a depth below 480 cm, there was an absence of S. horneri. This suggests that light intensity plays an important role in growth of this plant. Currently, we cannot conclude if the lower light intensity actually results in the death of S. horneri, but it does restrict its growth. This is supported by our observations that the S. horneri germlings grew more slowly under lower light intensity. We found that an light intensity of 248 μmol m−2 s−1 was favorable for S. horneri during their rapid growth, and higher light intensity did not result in the death of a higher number of germlings. These results show that S. horneri flourish in highly light intensity zones. We also found that lower wave exposure reduced the risk of dislodging S. horneri from the substrate. The decreased disturbance of water flow in these regions created favorable conditions for the accumulation of sediment and periphyton on the substrate, which then promoted the early post-settlement stages of S. horneri. With increasing depth, sediment levels increased significantly on rocky substrates. Long-term coverage of the S. horneri germlings by deep sediment resulted in a marked increase in their death. Although there was little sediment at the bottom of intertidal zone, the higher light intensity and the increased water flow created a positive condition for the growth and survival of the S. horneri germlings. If there was little sediment in the subtidal zone, the distribution range of the macroalgae bed would be very broad, and the macroalgae would restore itself. Together, these observations suggest that sediment is likely the major factor that affects the vertical distribution of S. horneri.