Globally, the eutrophication of coastal marine environments is a worsening problem that is accelerating the loss of biodiversity and ecosystem services. Coral reefs are among the most sensitive to this change, as chronic inputs of agricultural and wastewater effluents and atmospheric deposition disrupt their naturally oligotrophic state. Often, anthropogenic alteration of the coastal nitrogen pool can proceed undetected as rapid mixing with ocean waters can mask chronic and ephemeral nitrogen inputs. Monitoring nitrogen stable isotope values (d 15N) of benthic organisms provides a useful solution to this problem. Through a 7-yr monitoring effort in Quintana Roo, Mexico, we show that d 15N values of the common sea fan Gorgonia ventalina were more variable near a developed (Akumal) site than at an undeveloped (Mahahual) site. Beginning in 2007, the global recession decreased tourist visitations to Akumal, which corresponded with a pronounced 1.6 ‰ decline in sea fan d 15N through 2009, at which time d 15N values were similar to those from Mahahual. With the recovery of tourism, d 15N values increased to previous levels. Overall, 84 % of the observed variation in d 15N was explained by tourist visitations in the preceding year alone, indicating that variable nitrogen source contributions are correlated with sea fan d 15N values. We also found that annual precipitation accounted for some variation in d 15N, likely due to its role in groundwater flushing into the sea. Together, these factors accounted for 96 % of the variation in d 15N. Using a mixing model, we estimate that sewage can account for up to 42 % of nitrogen in sea fan biomass. These findings illustrate the high connectivity between land-based activities and coral reef productivity and the measurable impact of the tourism industry on the ecosystem it relies on.