We examined the hypothesis that nutrients from land sources are enhancing the food supply for fish in coastal reefs of northwestern Cuba. Spatial variation in stable isotope ratios (δ¹³C and δ¹⁵N) was investigated at two sites heavily impacted by mixed pollution sources in Havana City, Cuba, and two reference sites. Significant differences in isotope signatures were found among sites for territorial species, e.g., δ¹⁵N was consistently enriched at impacted sites. The most plausible explanation is enriched nitrogen and carbon sources in the food web derived from organic pollution of the coastal zone by human wastewaters. This supports earlier findings that fish were larger in the vicinity of the harbor because of nutrient additions due to sewage that can increase ecosystem productivity and enhance growth of fish. It is also evidence of the direct land-to-water linkage in coastal zone food webs and illustrates how additional nutrients may affect marine food web functions.