Variation in the macroalgae community structure during summer in the artificial reef zones of Shuangdao Bay, Weihai, Shandong Province, China

Artificial reefs provide a substrate for macroalgae to form artificial algal beds. We surveyed the sessile macroalgae community in four artificial reef zones(ARZ) of Jiaming, Beihai, Shengkun, and Tonxin in Shuangdao Bay in June and August, 2013. These reefs were constructed in 2006, 2006, 2010, 2008. We documented the composition of the macroalgal communities and the interactions between the communities and environmental factors. Ten transects(50 cm × 50 cm) were set randomly in each ARZ. All the macroalgae in a transect were collected by a SCUBA diver. Additionally, underwater videos were taken to provide additional information. Environmental factors(temperature, light intensity, pH, DO, salinity, depth, and transparency) were recorded at the time of collection. The samples of algae were identified, measured, and weighed in the laboratory. We identified 9 species, including 5 species of Rhodophyta, 2 species of Phaeophyta, and 2 species of Chlorophyta. Rhodophyta was the most abundant genera, accounting for 93.69% of the biomass in June and 85.26% in August. The macroalgal communities differed significantly between the surveys in June and August, with a dissimilarity score of 79.65%, based on analysis of similarities(ANOSIM). Over 90% of the dissimilarity was contributed by five species, Chrysymenia wrightii, Heterosiphonia japonica, Gracilaria textorii, Sargassum muticum, and Ulva pertusa. The macroalgal community composition was similar at Jiaming and Tongxin(R = 0.059 in June and 0.217 in August, P0.1, ANOSIM). The biomass increased from 164.99 g/m2(June) to 243.829 g/m2(August), whereas the percentage cover decreased from 64.53%–91.20%(June) to 52.53%–72.53%(August), except in the Shengkun ARZ. The dominant species was H. japonica in June and C. wrightii in August. Multidimensional scaling(MDS) analysis and clusters based on Bray-Curtis similarities revealed that the ARZs could be divided into four clusters, one containing Jiaming, Beihai, and Tongxin in June, one containing these three ARZs in August, one containing Shengkun in June, and one containing Shengkun in August. Based on principal component analysis(PCA) and redundancy analysis(RDA), water temperature, salinity, and benthic light intensity have the most influence on macroalgae community composition. Temperature was the most important factor, accounting for 45.30% of the variation in the communities. The communities in June consisted primarily of cold and warm temperate species whereas those in August consisted primarily of subtropical and warm temperate species. The relationship between salinity and community structure may be a function of the large amount of rainfall that occurred in August, whereas the effect of salinity might be negligible in other circumstances over a year. The transparency depth was ca. 2.5 m in June, but less than 2.0 m in August. Thus, light may be a limiting factor for the distribution of macroalgae. The ARZ in Shengkun had the lowest biomass, potentially a function of low light levels due to its depth. Additionally, blooms of fouling organisms(Spongiidae, Ciona intestinalis and Obelia sp.) in August affected the percentage rate. The appearance and abundance of Phaeophyta is representative of the maturity of the macroalgal communities. However, Phaeophyta was only found at the Beihai ARZ, a site that was deployed ca. seven years previously. Given the poor underwater light conditions, we speculate that the macroalgal communities in Shuangdao Bay require 7 years to mature.