Can stormwater be detected by algae in an urban reef in Hawai‘i?

Nitrogen (N) enrichment of tropical reefs can result in the dominance of invasive algae. The invasive alga Acanthophora spicifera and the native alga Laurencia nidifica are part of a diverse reef assemblage in ‘Ewa Beach, O‘ahu. Their N contents and δ15N values were investigated to determine if N was enriched and to evaluate potential nitrogenous sources near and removed from storm-drain outlets. δ15N values of algae (3.8–17.7‰) were within and above the range for algae around the island (1.9–11.9‰). Elevated algae N isotope values (δ15N > +7‰, [N] > 1.6%) and seawater nitrate + nitrite levels (0.59–7.93 μM) indicated a mixed, high nutrient environment. The overlap in δ15N values with multiple nitrogenous sources precluded identification. However, spatial and temporal patterns did not support stormwater as the dominant, nitrogenous source. Patterns were congruent with algal incorporation of terrestrial derived N, subjected to a high degree of biogeochemical cycling.