Abstract:Qinzhou Bay, which is the biggest in Guangxi, is one of the most important areas for developing a marine economy as it is species-rich. In recent years, as human impact has become more serious, problems associated with the geological environment have become more complicated, increasing pressure on the local environment. Because an examination of the benthos is one way of monitoring environmental conditions, ecological surveys of the macrobenthos in Qinzhou Bay were undertaken during four cruises during 2008 and 2009. A total of 94 macrobenthic species were identified, including 36 species of mollusk, 22 species of polychaete, 21 species of crustacean, and six species of echinoderm, as well as nine other species. The greatest number of species was found in the inner bay, followed by the open bay, outside the bay, and neck bay, respectively. The Mollusca was the dominant group in the benthos, followed by the Polychaeta, although the dominant species varied with season. In spring, the macrobenthos was dominated by Periglypta lacerata, Scalopidia spinosipes and Perinereis cultrifera. By contrast, P. lacerata, S. spinosipes and Euclymene annandalei were the most abundant species in summer, Periglypta chemnitzi and Balanus reticulatus in autumn, and Umbonium costatum and P. lacerata in winter. The dominant species, therefore, were similar in spring and summer, but changed in autumn and winter. Periglypta lacerata was the only dominant species throughout most of the year. The average total density and average total biomass of macrobenthos were 439 ind./m2 and 115.14 g/m2, respectively. Compared with data from the last 20 years, the average abundance of individuals has increased, while biomass has decreased. The average abundances and biomasses were greatest in winter, followed by autumn, and least in spring and summer. An examination of the horizontal distribution of species revealed that the average abundances and biomasses varied with no obvious trends. The average abundances and biomasses were different in different areas, being highest in the open bay, followed by the outer bay, entrance to the bay and finally inner bay. The species richness index is 4.01(D), the diversity index 1.80 (H') and the evenness index 0.73 (J'), which are low. The average species richness index has the same trend in different areas: greatest in the inner bay, least at the entrance to the bay, and similar in the open bay and outer bay. The richness, diversity and evenness indices are significantly negatively correlated with water depth. The species richness and evenness indices are significantly negatively correlated with sediment pH in summer, whereas density is positively correlated with sulfide in autumn, and biomass is positively correlated with sulfide and organics in winter. An abundance/biomass comparison curve method was used to determine the levels of disturbance of the macrobenthic communities. In spring, the macrobenthic community was only slightly disturbed, whereas in the other three seasons the macrobenthic communities were moderately disturbed. However, disturbance was greater in autumn and winter when compared with that in summer.