Development of an epiphyte indicator of nutrient enrichment: threshold values for seagrass epiphyte load.

•Quantitative threshold ranges for an epiphyte load indicator of coastal nutrient enrichment are derived from statistical distributions of submerged aquatic vegetation (SAV) responses.•The threshold epiphyte loads are associated with −25 and −50% levels of decrease in SAV response metrics such as biomass, shoot density, percent cover, production and growth.•Given multiple sources of variability in the seagrass response data, threshold ranges corresponding to low, moderate, high, and very high levels of epiphytes are proposed rather than single, critical point values.