Ecology, 96(11), 2015, pp. 2960–2972 Ó 2015 by the Ecological Society of America Nutrient enrichment intensifies hurricane impact in scrub mangrove ecosystems in the Indian River Lagoon, Florida, USA ILKA C. FELLER, 1,4 EMILY M. DANGREMOND, 1 DONNA J. DEVLIN, 2 CATHERINE E. LOVELOCK, 3 C. EDWARD PROFFITT, 2 AND WILFRID RODRIGUEZ 1 1 Smithsonian Environmental Research Center, Smithsonian Institution, 647 Contees Wharf Road, Edgewater, Maryland 21037 USA 2 Department of Biological Sciences, Charles E. Schmidt College of Science, Florida Atlantic University, c/o Harbor Branch Oceanographic Institution, 5775 Old Dixie Highway, Fort Pierce, Florida 34946 USA 3 Centre for Marine Studies/School of Integrative Biology, University of Queensland, St Lucia, Queensland 4072 Australia Abstract. Mangroves are an ecological assemblage of trees and shrubs adapted to grow in intertidal environments along tropical, subtropical, and warm temperate coasts. Despite repeated demonstrations of their ecologic and economic value, multiple stressors including nutrient over-enrichment threaten these and other coastal wetlands globally. These ecosystems will be further stressed if tropical storm intensity and frequency increase in response to global climate changes. These stressors will likely interact, but the outcome of that interaction is uncertain. Here, we examined potential interaction between nutrient over-enrichment and the September 2004 hurricanes. Hurricanes Frances and Jeanne made landfall along Florida’s Indian River Lagoon and caused extensive damage to a long-term fertilization experiment in a mangrove forest, which previously revealed that productivity was nitrogen (N) limited across the forest and, in particular, that N enrichment dramatically increased growth rates and aboveground biomass of stunted Avicennia germinans trees in the interior scrub zone. During the hurricanes, these trees experienced significant defoliation with three to four times greater reduction in leaf area index (LAI) than control trees. Over the long term, the þ N scrub trees took four years to recover compared to two years for controls. In the adjacent fringe and transition zones, LAI was reduced by .70%, but with no differences based on zone or fertilization treatment. Despite continued delayed mortality for at least five years after the storms, LAI in the fringe and transition returned to pre-hurricane conditions in two years. Thus, nutrient over-enrichment of the coastal zone will increase the productivity of scrub mangroves, which dominate much of the mangrove landscape in Florida and the Caribbean; however, that benefit is offset by a decrease in their resistance and resilience to hurricane damage that has the potential to destabilize the system. Key words: coarse woody debris; delayed mortality; disturbance; Florida; growth rate; hurricane; Indian River Lagoon; leaf area index; mangrove forest; nutrient enrichment; resilience; resistance. INTRODUCTION Hurricanes are a frequent but aperiodic natural form of disturbance along coastal regions around the world. In Florida and throughout much of the Caribbean, these episodic storms have direct and indirect effects on the dynamics of mangrove forests, which dominate coastal landscapes in this region (Lugo 2008, Smith et al. 2009, Jiang et al. 2014). The initial damage to mangroves and subsequent recovery time depend not only on hurricane intensity (e.g., wind speed, forward speed, and storm surge), but also on forest structure, including stand density, tree height, and biomass accumulation (Roth 1992, Doyle and Girod 1997, Piou et al. 2006, Ross et al. 2006). In mangroves, such structural characteristics also vary in undisturbed forests along environmental gradi- ents in tidal elevation, salinity, and nutrient availability (Lugo and Snedaker 1974, Sherman et al. 2001, Ball 2002, Feller et al. 2003a, b, Castaneda-Moya et al. 2013). During recovery, these characteristics can be influenced by huge allochthonous subsidies of nutrients delivered to mangrove forests by hurricanes (Scatena et al. 1996, Silver et al. 1996, Davis et al. 2004, Lovelock et al. 2011). The mangrove forests that line the Atlantic coast of Florida were hit by more than 100 hurricanes between 1851 and 2004 (Blake et al. 2005), but multiple storms hitting the same area of coast in a given season are extremely rare events (Liu 2007). However, two of the four hurricanes that hit Florida in 2004 made landfall within ;3 km of each other along the Indian River Lagoon (IRL) and followed nearly identical paths across the state. Hurricane Frances, a Category 2 storm (Saffir- Simpson Hurricane Wind Scale [NHC 2012]), came ashore near Stuart on 5 September (Bevan 2014) with 167 km/h maximum sustained winds. It was among the Manuscript received 1 October 2014; revised 17 April 2015; accepted 21 April 2015; final version received 13 May 2015. Corresponding Editor: R. A. Dahlgren. 4 E-mail: felleri@si.edu 2960