Harmful Algae 10 (2011) 310–318 Contents lists available at ScienceDirect Harmful Algae journal homepage: www.elsevier.com/locate/hal Stormwater nutrient inputs favor growth of non-native macroalgae (Rhodophyta) on O’ahu, Hawaiian Islands Brian E. Lapointe *, Bradley J. Bedford Center for Marine Ecosystem Health, Harbor Branch Oceanographic Institute at Florida Atlantic University, 5600 US 1 North, Ft. Pierce, FL 34946, United States ARTICLE INFO ABSTRACT Article history: In Hawaii, blooms of native and non-native macroalgae (limu) have become increasingly problematic in Received 27 August 2010 recent decades. Although the role of human vectors in introducing non-native macroalgae is well Received in revised form 23 November 2010 documented, the ecological role of nutrient pollution in facilitating blooms of these species is not. This Accepted 24 November 2010 study assessed the effects of stormwater discharges on the diversity, abundance, and nutrient content (C, Available online 1 December 2010 N, P and d15N) 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 Keywords: stormwater outfall (ASWO), whereas non-native species abundance increased. Limu tissue d15N values Hawaii at all three sites were within the range reported for sewage N. 15N, %N, and N:P ratios all increased with Limu d Native proximity to the ASWO, supporting the hypothesis that stormwater was a primary source of N Nitrogen enrichment in the study area. In contrast to N, limu %P showed little change among the sites, suggesting Non-native that the generally high N:P ratios, indicative of P-limitation, resulted from high N:P ratios from the Macroalgae upland watershed. Abundance and tissue %N of the non-native rhodophyte Acanthophora spicifera Phosphorus increased with proximity to the ASWO and were strongly correlated (r2 = 0.94) compared to native Stormwater 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. ß 2010 Elsevier B.V. All rights reserved. 1. Introduction aquaculture, has become extremely abundant, especially on Maui where large accumulations accumulate on beaches, interfering Non-native macroalgal invasions are a major driver of coastal with tourists’ use of beaches as a result of malodorous ecosystem change worldwide (UNEP, 2006; Williams and Smith, decomposition (Huisman et al., 2007). 2007). The geographically isolated Hawaiian Islands are especially Throughout the Hawaiian archipelago, there is growing concern vulnerable to biological invasions, which have resulted in about the displacement of native seaweeds, known as limu in the significant impacts on biodiversity (Staples and Cowie, 2001). Hawaiian language, by non-native species. Non-native invasive Biological invasions in the marine environment include introduc- limu compete with and displace native limu species important to tions of non-native seaweeds that date back to the 1950s when the Hawaiians for food, medicine, and religious purposes (Abbott, non-native rhodophyte Acanthophora spicifera was accidentally 1984). Russell (1992) documented how non-native A. spicifera and introduced to Pearl Harbor, O’ahu (Doty, 1961). Today, A. spicifera is H. musciformis displaced native populations of Laurencia nidifica considered the most pervasive non-native algal species in Hawaii and Hypnea cervicornis. In his seminal work on biological invasions, (Huisman et al., 2007). Since the 1950s, over 20 species of non- Elton (1958) emphasized the importance of human-mediated native macroalgae have been introduced to the Hawaiian Islands, vectors, especially physical transport. Humans have subsequently but only about five species have become established and form been recognized as the primary vector in the global epidemic of extensive blooms that alter coastal ecosystems (Russell, 1992; biotic invasions in aquatic ecosystems (Carlton and Geller, 1993). Rodgers and Cox, 1999; Smith et al., 2002). Recently, the non- Indeed, increasing evidence shows that human activities facilitate native rhodophyte Hypnea musciformis, imported from Florida for the physical spread of non-native species in Hawaiian coastal waters, despite recent progress in the prevention of non-native limu introductions (Staples and Cowie, 2001). * Corresponding author. Tel.: +1 772 242 2276; fax: +1 772 468 0757. Increased urbanization of upland watersheds is a major E-mail address: [email protected] (B.E. Lapointe). mechanism increasing nutrient pollution of coastal waters and 1568-9883/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hal.2010.11.004 B.E. Lapointe, B.J. Bedford / Harmful Algae 10 (2011) 310–318 311 may facilitate invasions of non-native limu in Hawaiian coastal nitrogen sources on coral reefs, particularly that from human waters. Anthropogenic nutrient pollution of coastal waters has sewage, which can also be a significant source of nitrogen been widely recognized as a common factor linking an array of enrichment in urban stormwater runoff (Wanielista and Yousef, problems, including harmful algal blooms, dead zones, seagrass 1993; Dillon and Chanton, 2008). and coral reef die-offs, declining fisheries, and marine mammal and seabird deaths (ECOHAB, 1997; NRC, 2000; Howarth et al., 2. Materials and methods 2000; MEA, 2005; HARRNESS, 2005; UNEP, 2006). In Hawaiian coastal waters, Soegiarto (1972) and Johannes (1975) first 2.1. Selection of sampling sites suggested a linkage between nutrient pollution from sewage and the expansion of the non-native rhodophyte A. spicifera in To test the hypothesis that stormwater nutrient pollution Kaneohe Bay, O’ahu. Blooms of the native invasive chlorophyte affects the relative abundance of non-native versus native limu in Dictyosphaeria cavernosa, which overgrew and killed corals in intertidal communities, three study sites were chosen along a Kaneohe Bay, O’ahu, were also linked to nutrient enrichment from gradient of exposure to stormwater discharge in the Ewa Beach sewage (Banner, 1974; Smith et al., 1981). Following sewage area on O’ahu (Fig. 1). The urbanized Ewa Beach area had a diversion from Kaneohe Bay in the late 1970s, nutrient concentra- stormwater drainage system constructed when this neighborhood tions and D. cavernosa biomass both decreased (Hunter and Evans, was developed in the 1970s. Currently, several stormwater outfalls 1995), demonstrating the ecological importance of nutrient discharge into the intertidal zone where longshore currents in the enrichment to Kaneohe Bay. Since then, however, blooms of nearshore area generally flow from the urbanized Ewa Beach area non-native limu have expanded throughout the Hawaiian Islands westwardly towards One’Ula State Beach Park. The easternmost (Russell, 1992; Rodgers and Cox, 1999; Smith et al., 2002), sampling site (Amio) in our study, located near the Amio Street especially in coastal waters adjacent to urbanized watersheds. stormwater outfall (ASWO), was chosen to be representative of Although sewage has been identified as a significant nitrogen direct stormwater impacts (Fig. 2a). A second site near Papipi Road source supporting blooms of native and non-native limu (Dailer (Papipi), 625 m west of Amio, was chosen as a site less impacted et al., 2010), studies have not addressed the importance of nutrient by the ASWO discharges to the east (Fig. 2b). The third site at Kaloi enrichment (nitrogen, N and phosphorus, P) from urban storm- Gulch (Kaloi) in One’Ula State Beach Park, west of Papipi and water runoff to the relative abundance of native and non-native 975 m from Amio, was selected as a reference site least impacted limu, such as A. spicifera, in Hawaiian coastal waters. Urban by stormwater discharges to the east (Fig. 2c). The three intertidal stormwater runoff can contain relatively high concentrations of sites were sampled during low (minus) tides, March 3–7, 2008. ammonium, nitrate, total N, soluble reactive P, and total P, which combined with the high volumes of stormwater following rain 2.2. Sampling for taxonomic composition of limu communities events, can account for considerable nutrient loads to coastal waters (Wanielista and Yousef, 1993). Four separate 30 m survey transects were established end-to- We posed the hypothesis that nutrients from stormwater runoff end in the low intertidal zone at each site using Keson fiberglass would affect the nutrition and relative abundance of native and survey tapes. The four transects were independent, had no spatial non-native limu. To test this hypothesis, we studied intertidal limu overlap, and resulted in a total surveyed length of 120 m at each communities at three locations in Ewa Beach, O’ahu. The study was site. Qualitative collections of conspicuous limu species were multi-faceted, and involved measuring the following variables in sampled along the four transects at each site; specimens were intertidal communities at the three sites: macroalgal species identified according to Abbott (1999), Abbott and Huisman (2004), presence, percent cover of abundant taxa, tissue C:N:P contents to and Huisman et al. (2007). gauge the degree of N versus P limitation (Atkinson and Smith, Limu communities along the