Nitrogen stable isotope ratios and molar C:�N ratios were determined for 18 species of green, brown and red macroalgae and for 2 species of cyanobacteria collected at the Luhuitou coral reef ( Sanya Bay , Hainan Island , China ). Concentrations of dissolved inorganic nitrogen (DIN) and orthophosphates in seawater at the Luhuitou and the Xiaodonghai reefs were measured during different seasons. Composition of macroalgal and cyanobacterial communities was also investigated at these sites.

High values of macroalgae (> 8) confirmed that wastes from the marine animal farms and sewages were the main sources of dissolved inorganic nitrogen (DIN) in Sanya Bay . The main source of nitrogen for cyanobacteria was determined as molecular nitrogen dissolved in seawater. Concentrations of orthophosphates and DIN in seawater above the reefs averaged 0.15 mM and 2.35 mM, respectively, exceeding the threshold concentrations (algal blooms) in 1.5-2.5-fold. It was shown that in the intertidal zone, mainly mono- and bidominant communities of crust algae and algal turf represent macroalgal communities. In the upper subtidal zone, both communities of algal turf and those of frondose and fleshy algae (Sargassum spp., Padina spp., Turbinaria ornata, Lobophoravariegata, etc.) were common. The algae, especially, in the upper-subtidal zone, were tightly overgrown by epiphytes. The invasive species and algal blossom were not recorded.

We conclude that the main sources of DIN in the waters of Sanya Bay are human sewage or/and effluents from marine animal farms. In spite of high level of pollution by DIN in Sanya Bay , the molar C: N ratios in algal thalli were still at high level (on average >17) that give the ground to consider that algae of Luhuitou reef are limited in nitrogen. The latter may be explained by low photosynthetic rates in macroalgae over the difficulty in CO₂ and bicarbonate diffusion to the chloroplasts under abundance of sediments and epiphytes at the surface of algal thalli.

dissolved inorganic nitrogen, macroalgal communities, nitrogen uptake, orthophosphates,

�C: N ratio.