Identifying nitrogen sources for macroalgal growth in variously polluted coastal areas of southern Vietnam

Abstract

Levels of δ¹⁵N and molar C/N ratios were determined for the first time in 73 species of green, brown and red macroalgae at nine sites from differently polluted coastal areas of southern Vietnam in order to identify sources of dissolved inorganic nitrogen (DIN) used by seaweeds for their growth and development. The study sites were provisionally divided into two groups: open coast localities with presumably clear seawater and relatively isolated localities (bays and lagoons) polluted by waste from mariculture and sewage from cities and large villages. The algae collected at “pristine” and polluted sites had on average δ¹⁵N signatures of 3.3±0.9‰ and 8.6±1.1‰, respectively. Macroalgae from the west shore of Phu Quoc Island, O Lan Lagoon and Nha Trang Bay assimilated DIN from wastewater enriched with the heavy nitrogen isotope. Among the macroalgae investigated, bioindicators of DIN source were identified. These indicator algae belong mainly to the genera Ulva, Padina, Turbinaria, Gracilaria and Hydropuntia. Analysis of molar C/N ratios in algal tissue showed that thalli collected at “pristine” sites suffered limitation in nitrogen (on average, the C/N ratios=21.6±13.3). In most of the polluted areas, macroalgae were also N limited (on average, the C/N ratios=15.3±8.2).