Development and application of an algae multi-metric index to inform ecologically relevant nitrogen reduction targets

Nutrient enrichment can impair the biotic integrity of steams and rivers. Nutrients derived from land use practices have been identified as sources of water quality impairment in several Central California Coast watersheds resulting in excess algal growth. We developed an algae-based multi-metric index to assess the biotic integrity of streams and rivers in this region because algae often respond directly to changes in nutrient levels. Additionally, we apply the algal index of biotic integrity to the development of an effects-based nitrogen reduction target. Eleven individual metrics based on diatom autecologies, community structure, ecological guilds, tolerance, and intolerance were incorporated into the index. All algal production metrics failed reproducibility criteria for inclusion in the index. The index of biotic integrity was highly correlated with human disturbance (r = − 0.6213) and was significantly different between classes of least-, intermediate-, and most-disturbed sites. Piecewise linear regression showed a steep negative relationship between nitrate–N and the index with a breakpoint of 0.505 mg/L nitrate–N, above which the negative trend became insignificant. This change in the relationship between nitrate and the index of biotic integrity suggests that this breakpoint can aid the development of a reasonable effects-based criterion for nitrate–N in this region.