Stormwater nutrient inputs favor growth of non-native macroalgae (Rhodophyta) on O’ahu, Hawaiian Islands

In Hawaii, blooms of native and non-native macroalgae (limu) have become increasingly problematic in recent decades. Although the role of human vectors in introducing non-native macroalgae is well documented, the ecological role of nutrient pollution in facilitating blooms of these species is not. This study assessed the effects of stormwater discharges on the diversity, abundance, and nutrient content (C, N, P and δ15N) of native and non-native limu at three sites in the intertidal zone at Ewa Beach, O’ahu. The results showed that native limu species diversity and abundance decreased with proximity to a stormwater outfall (ASWO), whereas non-native species abundance increased. Limu tissue δ15N values at all three sites were within the range reported for sewage N. δ15N, %N, and N:P ratios all increased with proximity to the ASWO, supporting the hypothesis that stormwater was a primary source of N enrichment in the study area. In contrast to N, limu %P showed little change among the sites, suggesting that the generally high N:P ratios, indicative of P-limitation, resulted from high N:P ratios from the upland watershed. Abundance and tissue %N of the non-native rhodophyte Acanthophora spicifera increased with proximity to the ASWO and were strongly correlated (r2 = 0.94) compared to native rhodophytes, indicating that stormwater N enrichment provided this invader a competitive advantage (lower C:N ratio) over native limu. These results indicate that the spread of non-native macroalgae in oligotrophic coral reef regions can be facilitated by anthropogenic nutrients in stormwater runoff, thereby threatening native species and ecosystem services.

Research highlights▶ Abundance and diversity of native seaweeds (limu) decreased with increasing proximity to stormwater discharges at Ewa Beach, Oahu, Hawaiian Islands. ▶ Abundance of two non-native red limu, Acanthophora spicifera and Hypnea musciformis, both increased with increasing proximity to stormwater discharges. ▶ δ15N, %N, and N:P ratios of limu tissue increased with increasing proximity to the stormwater outfall, supporting the hypothesis that stormwater discharges were a primary source of nutrient enrichment. ▶ Tissue N of Acanthophora spicifera increased more than that of native limu with increasing proximity to the stormwater outfall, indicating stormwater nutrient enrichment provides this pervasive invader with a competitive advantage over native limu. ▶ This study supports the hypothesis that nutrient enrichment from stormwater runoff is an important ecological factor facilitating invasions of non-native limu in the Hawaiian Islands.