Marine Ecology Progress Series 488:145

This authors' personal copy may not be publicly or systematically copied or distributed, or posted on the Open Web, except with written permission of the copyright holder(s). It may be distributed to interested individuals on request. Vol. 488: 145–155, 2013 MARINE ECOLOGY PROGRESS SERIES Published August 15 doi: 10.3354/meps10404 Mar Ecol Prog Ser FREE ACCESS Effects of shoreline erosion on salt-marsh floral zonation Ryan M. Moody1,*, Just Cebrian1,3, Sara M. Kerner2, Kenneth L. Heck, Jr.1,3, Sean P. Powers1,3, Carl Ferraro4 1Dauphin Island Sea Lab, 101 Bienville Blvd., Dauphin Island, Alabama 36528, USA 2Department of Biological Sciences, Florida Institute of Technology, 150 West University Blvd., Melbourne, Florida 32901, USA 3Department of Marine Sciences, University of South Alabama, LSCB 25, Mobile, Alabama 36688, USA 4State Lands Division Coastal Section, Alabama Department of Conservation and Natural Resources, 3111 5 Rivers Blvd, Spanish Fort, Alabama 36527, USA ABSTRACT: The loss of salt-marsh habitat is proceeding at an alarming rate worldwide, resulting in the loss of ecosystem function and reduced exchange with adjacent habitats. The immediate result of shoreline erosion is the loss of fringing vegetation, but it is unclear how, and at what temporal scale, intertidal floral zones respond to shoreline loss. Using a transect-based approach over a 3 yr period, we compared community composition among 3 intertidal salt marshes in coastal Alabama that differ in the intensity of wave-generated disturbance. Each study marsh exhibited net annual rates of shoreline retreat, but differed in its exposure to wave-induced disturbance and erosional state. We also examined whether wave attenuation by oyster break waters reduced annual rates of shoreline erosion, providing a means to test whether the intensity of shoreline erosion affects inter-annual patterns of marsh zonation. Community composition differed across sites, but not between breakwater treatments and site-specific rates of shoreline loss, suggesting that differences in community composition are not due to differences in erosion intensity, but to other contrasts across sites. The cohesive nature of the floral communities at each site suggests that marshes maintain their zonation structure by migrating inland as the shoreline recedes. The inland migration of marsh zones will, however, ultimately be limited by the lack of area available for marsh colonization. The preservation of coastal salt marshes is, therefore, vitally contingent on effective shoreline management strategies. KEY WORDS: Community · Erosion · Flora · Gulf of Mexico · Living shorelines · Salt marsh Resale or republication not permitted without written consent of the publisher INTRODUCTION turbance often sets the stage for successional processes and, as a result, plays a key role in generating Disturbance is a fundamental force that acts on diversity in marine ecosystems (Connell 1978, Petrai - every level of community organization over a variety tis et al. 1989, Paine et al. 1998, Sousa 2001, Frater- of spatial and temporal scales (Pickett & White 1985, rigo & Rusak 2008). White & Jentsch 2001). The frequency, intensity, and Salt-marsh floral communities are organized into duration of disturbance influences community struc- distinct zones along landward-to-seaward gradients ture through its effects on biotic interactions, includ- (Pomeroy & Wiegert 1981, Bertness & Ellison 1987, ing predation, competition, and facilitation (Paine Levine et al. 1998, Emery et al. 2001). Species inhab- 1974, Bruno et al. 2003). By opening substrate for iting the stressful, low intertidal zone are typically colonization and limiting competitive exclusion, dis- able to withstand long periods of inundation, anoxic *Email: [email protected] © Inter-Research 2013 · www.int-res.com 146 Mar Ecol Prog Ser 488: 145–155, 2013 sediments, and considerable daily fluctuations in net loss of fringing salt marshes, and annual rates of temperature and salinity (Niering & Warren 1980, shoreline loss in excess of 2 to 3 m yr−1 have been copy Mendelssohn et al. 1981, Pomeroy & Wiegert 1981). documented for decades along the Atlantic and Disturbances such as ice-scouring, wave-induced northern GOM (Phillips 1986, Smith 1990). European erosion, catastrophic storm events, wrack deposition, marshes have also experienced increased rates of and persistent drought conditions also influence the shoreline erosion in recent decades as a result of Author structure of marsh communities by generating het- increased wave height, tidal fluctuations, and sedi- erogeneous patchworks of successional change ment loss due to bioturbation (Wolters et al. 2005, (Bertness & Ellison 1987, Levine et al. 1998, Bruno & Möller 2006). Kennedy 2000, Silliman et al. 2005). Living shoreline initiatives have been proposed as In the western Atlantic, a large tidal range supports effective and environmentally friendly alternatives to expansive meadows of smooth cordgrass, Spartina bulkheads and seawalls for protecting shorelines and alterniflora. In contrast, tidal ranges in the northern curbing the erosion trends observed world-wide Gulf of Mexico (GOM) are narrow (typically 0.4 to (NRC 2007). Living shorelines are structures com- 0.6 m), permitting the black needlerush Juncus roe - posed of biogenic materials, such as oyster clutch, me rianus to dominate the mid and high intertidal that are deployed seaward of eroding shorelines in zones. Consequently, S. alterniflora is often relega- an effort to revert that trend, stabilize the shoreline, ted to fringing bands and isolated patches at the sea- and create healthy coastal habitat. In the northern ward edge (Stout 1984, Levine et al. 1998). S. alterni- GOM, restored oyster reefs are a favored model for flora and J. roemarianus are the typical dominant living shoreline approaches (Meyer & Townsend floral species in Gulf marshes, and their zonation pat- 2000, Peterson et al. 2003, Coen et al. 2007, Scyphers terns are driven by the inability of J. roemerianus to et al. 2011). Indeed, subtidal oyster reefs naturally thrive in the stressful conditions of the low intertidal. occur adjacent to U.S. intertidal marshes and, in Fringing marshes exposed to moderate- and high- addition to their role as biogenic habitat for finfish energy wave conditions in the GOM are often char- and shellfish communities, they further serve to acterized by steeper shoreline slopes. This, in combi- attenuate erosive wave action, stabilize sediments, nation with the narrow tidal range in the region, and reduce marsh retreat (Dame & Patten 1981, often leads to compressed patterns of floral zonation Meyer et al. 1997). within narrow vegetation bands. Little is known about how shoreline erosion and Due to their location at the land-sea interface, salt retreat affects the distribution and zonation of marsh marshes mitigate coastal erosion and buffer adjacent communities in marshes of the west Atlantic and uplands from storm events, filter land-derived nutri- northern GOM. Addressing this gap is important to ent pollution before it enters coastal waters, and our understanding of the resilience of salt-marsh provide habitat for many ecologically and economi- habitats in the face of both acute and chronic stres- cally important finfish and shellfish species (Kneib & sors. Specifically, it will provide insight into the main- Wagner 1994, Peterson & Turner 1994, Minello et al. tenance of floral diversity, effects on primary produc- 2003). Marshes trap and bind sediments, which tion and the accumulation of organic material, the allows for the accumulation of below-ground bio- capacity for sediment filtration and trapping, and mass and marsh maintenance (Morgan et al. 2009). shoreline protection (Morgan et al. 2009). The accretion of sediment ultimately facilitates a rise In this paper, we examine patterns of marsh zona- in the elevation of the marsh surface, and a gradual tion across a range of shoreline erosion intensity. seaward progression of the marsh edge (Gleason et Three sites with net annual rates of shoreline retreat, al. 1979, Pomeroy & Wiegert 1981). These processes but differing in their erosional state and exposure to have been slowed, and in many cases reversed, as a wave-induced disturbance, were studied. We also result of anthropogenic coastal hydrological modifi- examined whether the deployment of oyster reefs cations, coastal subsidence and uplift, natural shifts across the degrading shorelines reduced annual in wind and current regimes, and accelerated sea- rates of shoreline erosion, thereby permitting us to level rise (reviewed by Kennish 2001, Gedan et al. experimentally test whether the intensity of shoreline 2009, Prahalad et al. 2011). Indeed, loss of fine sedi- erosion affects patterns of marsh distribution and ments and subsequent shoreline erosion is one of the zonation from year to year. In particular, our study most serious problems that marshes face in many was designed to identify key changes in floral zona- areas of the world (Syvitski et al. 2005). For instance, tion in response to wave-induced shoreline erosion rates of shoreline loss exceeding 0.3 m yr−1 result in a and the potential for floral zones to remain intact Moody et al.: Shoreline erosion and floral zonation 147 through upland migration along the intertidal gradi- Roland & Douglass 2005). The 0.6 km long shoreline ent. We hypothesized that shorelines with low rates of SPaP faces south and is located 15 km due north of copy of shoreline loss would retain their

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