Assessing the influence of sewage outfalls on seagrass meadows using nitrogen isotopes

Untreated sewage discharged increases the nutrient loads and changes ecosystem functions. It increases the values of the nitrogen isotopic signature (delta N-15) of primary producers such as seagrasses. Itaparica Island (Bahia, Brazil) has undergone extensive urbanization over 50 years. Most of the island has no sewage treatment, and a bridge's construction could increase its population ten times. We evaluated the effects of sewage inputs on the delta N-15 of seagrass (Halodule wrightii) across Itaparica Island in 14 areas of the island with different degrees of urbanization. Average values of delta N-15 ranged from -3.95 parts per thousand (+/- 1.04 SD) to 2.73 parts per thousand (+/- 1.61). The highest human occupation site also has the highest mean value of delta N-15, and seagrass shoot density. The significant correlation (p < 0.05) between delta N-15 values and shoot density may indicate a possible anthropogenic pressure impacting meadow abundance. Despite a positive correlation, increased anthropogenic nutrient supply can support algae growth and harm seagrass ecosystems.