Seasonal and spatial variations of macrobenthic community in intertidal Sargassum field off Naozhou Island, South China Sea

Seasonal and spatial variations in the macrobenthic community in an intertidal gulfweed field off Naozhou Island were investigated from April 2011 to January 2012. We identified 407 macrobenthic organism species: 64 macroalgae (15.72% of total species); 123 Arthropoda (30.22%); 107 Mollusca (26.29%); 67 Annelida (16.46%); 15 Cnidaria (3.69%); 14 Echinodermata (3.44%); 8 Porifera (1.97%); 4 Chordata (0.98%); 2 Urochorda (0.49%); and 1 each of Echiura, Sipunculida and Angiospermae (0.25%). Arthropods, mollusks, annelids, and macroalgae were four main groups in the benthic organism community and accounted for 88.70% of the total species. In terms of abundance of biomass, there were 15 dominant species, including 8 macroalgal, 5 mollusk and 2 arthropod species. The species diversity indices ranged from 1.21 to 4.42 with an average of 2.80; the annual average of evenness was 0.47. Species richness indices varied from 2.97 to 7.24 (average 4.97); and Simpson dominance indices varied from 0.20 to 0.92 in annual average of 0.75. Macroalgal distribution showed clear vertical zonation and seasonal changes. The number of macroalgal species found in the high tide zone, upper middle tide zone, middle tide zone, lower middle tide zone, low tide zone, and below low tide zone were 1, 8, 10, 16, 22, and 32, respectively. Biomass among the benthic organisms, macroalgae and crustaceans had significant positive correlation ( P <0.05) with the coefficients of 0.802, 0.894, and 0.592, respectively. Macroalgal biomass showed a clearly negative correlation with the concentration of dissolved inorganic nitrogen (DIN) ( R =0.486, P <0.05). During the winter and spring, when macroalgae were growing vigorously, DIN reached the lowest level in seawater and displayed a clear weak correlation between macroalgal growth and the nutrient concentration.