Restoration of Estuarine Ecosystems

Estuaries rank among the most productive ecosystems on earth, but they are also subject to considerable ecological degradation associated with the loss and alteration of habitats and impairment of water quality due to multiple anthropogenic and natural drivers of change (Kennish, 2002; Kennish et al., 2008; Aubrey and Elliott, 2006). These impacts often result in diminished productivity and a serious reduction in ecosystem goods and services. More than 40% of the human population now lives within 100 km of the shore, and more than 70% resides in watersheds draining to the coast (Vitousek et al., 1997; Plutchak et al., 2010). Escalating population growth, urban and industrial development, modification of coastal watersheds and estuarine basins, pollution inputs, and overharvest of recreational and commercial species can threaten the system structure and function and the sustainability of the system resources. Estuarine ecosystem responses to human stressors, such as reduced biodiversity, altered biotic community structure, and changes in species distributions, are often exacerbated by the effects of stochastic natural drivers of change, such as hurricanes, northeasters, and storm surges.

Increasing human activities in the coastal zone create multiple stresses on estuaries that degrade water quality and damage habitats. Included here are the effects of eutrophication, wastewater inputs, chemical contaminants, freshwater diversions, draining and ditching of wetlands, hardened shorelines, sediment/turbidity influx, inlet stabilization, introduced species, and fisheries overexploitation. Single, cumulative, or synergistic processes linked to these factors can then lead to ecosystem dysfunction and serious depletion of resources (McLusky and Elliott, 2004; Adams, 2005; Borja et al., 2010). Some of the persistent drivers of change cause insidious decline of estuaries (e.g., eutrophication), whereas others result in acute impacts (e.g., large oil spills). Systems affected by anthropogenic stressors typically require extensive periods of time (up to 10–25 years) for complete recovery after the stressors are lifted (Borja et al., 2010). Restoration is a means by which estuarine habitat losses resulting from human activities and natural events can be mitigated and a system returned to a more improved pre-existing condition or state.

The term "restoration" as applied to estuarine ecosystems has varying connotations. Some investigators view restoration in the strictest sense as the return of an ecosystem to an exact pre-existing condition following degradation or disruption. However, recovery to the original natural system state is rarely achieved, and in most cases the attributes of the original system are not known which also complicates restoration assessment and success (Livingston, 2006). Other interpretations are less restrictive, defining ecological restoration as "an attempt to return a system to some historical state, although it is widely recognized that this is often difficult or even impossible to achieve" (Palmer, 2009). Attainment of a pristine condition subsequent to a restoration period is even less probable. NOAA (2008) defined restoration as "the process of re-establishing a self-sustaining habitat that closely resembles a natural condition in terms of structure and function." This differs from the definition of the USEPA (2003) which states "the return of a degraded ecosystem to a close approximation of its remaining natural potential." According to Elliott et al. (2007), a preferable definition of restoration is "the process of re-establishing, following degradation by human activities, a sustainable habitat or ecosystem with a natural (healthy) structure and functioning." To accomplish estuarine restoration, human-mediated actions are typically employed, although the process of unmanaged restoration is common.

Livingston (2006) noted that science and engineering methods are typically brought to bear to return aquatic systems damaged by human activities back to some level of natural productivity. Borja et al. (2010) conveyed that the altered natural system is not considered to be recovered unless secondary succession returns the ecosystem to a pre-existing condition or state. The trajectory of such ecosystem recovery, however, depends on the scales of time, space, and intensity of anthropogenic disturbance. Thus, the process of recovery may follow one of three pathways: (1) passive restoration through ecological succession; (2) re-direction through active ecological restoration; or (3) unattainable recovery (i.e., ecosystem collapse) (Borja et al., 2010).

Duarte et al. (2009) asserted that, because of multiple shifting baselines, the restoration of coastal ecosystems to an idealized past reference status after removal of human-induced pressure is highly unlikely. For example, in the case of eutrophication restoration, damaged ecosystems displayed convoluted trajectories of recovery not directly reversible to reference ecosystem conditions. The impacted systems did not return to the original conditions tracking reversal trajectories as a result of broad changes in environmental conditions which significantly affected the ecosystem dynamics and restoration targets. Reliable targets of restoration efforts must be set based on clear understanding of ecosystem responses to the shifting baselines; identifying unequivocally the reference status to which a damaged ecosystem is supposed to return subsequent to reduction of pressures remains a formidable and largely elusive task. Targets that ensure the maintenance of key ecosystem functions and that supply constant valuable ecosystem services to society may be the preferred estuarine restoration strategy (Duarte et al., 2009).

There has been a long history associated with restoration of damaged estuarine environments (Kennish, 2000; Zedler, 2000, 2001; Livingston, 2006; ), and restoration efforts have increased over the past several decades in concert with accelerating drivers of change linked to activities of an expanding coastal human population. As noted by Madgwick and Jones (2002) and Elliott et al. (2007), restoration is generally needed to overcome reductions in the following elements of an ecosystem: (1) habitat fragmentation; (2) habitat and species diversity; (3) population size, dynamics and range of species; and (4) goods and services. Restoration of estuaries may take various forms such as revegetating salt marsh, mangrove, and seagrass habitats, repopulating shellfish beds, removing contaminated bottom sediments and hardened shoreline structures, installing oyster reef substrate, re-establishing freshwater inflow, removing invasive species, and improving water quality of land runoff.

Tidal marsh restoration has been frequently attempted, as exemplified by ongoing work along the shores of the Delaware Estuary. Here, various measures are being taken to stabilize the eroding shorelines of tidal marshes using natural intertidal reef communities comprised of shellfish (e.g., mussels), coconut fiber biologs and mats, bagged culch, and wooden stakes positioned at high and mid intertidal zones (Figures 1-6). Delaware Estuary tidal marshes are experiencing heavy inundation and erosion due to sea level rise, land subsidence, wave action, boat wakes, and other factors leading to rapid retreat in some areas. The long-term goal is to protect the shoreline and restore damaged marsh habitat (Partnership for the Delaware Estuary, 2008).

The success of restoration is assessed by how close the altered ecosystem returns to a pre-existing natural condition or state in terms of constituent flora and fauna, and habitats. There has been much greater success in the past at re-establishing structural components of damaged estuaries, while the restoration of ecological function and site fidelity has lagged behind. Re-establishing ecological function is critical to system sustainability.

Palmer (2009) contends that despite great management efforts and significant investments to restore estuaries and watersheds, many continue to be degraded. This has been documented worldwide for many different estuarine systems in which human impacts have significantly depleted important biotic species, destroyed wetland and seagrass habitat, degraded water quality, and accelerated species invasions (Lotze et al., 2006). According to Palmer (2009), ecological processes must be integrated more fully into restoration protocols to achieve greater sustainability, shifting away from largely engineering-driven designs which have not yielded consistently high rates of restoration success. Ecological science, therefore, must play a greater role in shaping estuarine restoration. The problem has been exacerbated by the lack of long-term quantitative data to accurately assess the outcome of restoration projects. Information compiled on restoration sites is also often biased by subjective qualitative observations rather than careful quantitative measurements (Elliott et al., 2007).

Elliott et al. (2007) examined the restoration of impacted biogenic reefs, salt marsh, seagrass, and upper estuarine water quality. They concluded that, although recovery techniques employed today are worthwhile, they rarely replace the habitat lost through human-mediated degradation. As noted by Elliott et al. (2007), these techniques are most successful in semi-enclosed coastal bays or lagoons, estuaries, and fringing habitats than in open coastal and marine habitats. Restoration targets are usually set by relating an impacted system to a historic reference habitat or biotic community, or by relating the impacted system to a nearby undisturbed reference location. The reference sites are also used for evaluating restoration trajectories (Steyer et al, 2003). Typically, restoration actions are not implemented on the scale of the entire estuary, but at a much smaller spatial scale amounting to tens or hundreds of meters of seafloor or shoreline, as in the case of oyster reef, seagrass beds, biogenic reefs, and salt marsh remediation (Grizzle et al., 2008). This can be a significant limitation since the cause of local as well as broader impacts may be coupled to ecosystem-wide drivers of change, such as eutrophication (Kennish et al., 2007; 2010).

Stochastic events (e.g., hurricanes, tsunamis, and earthquakes) can reset the restoration endpoint, thereby significantly altering the trajectories of recovery. In some cases, such natural disturbances can restructure the system in a way that it can never return to the reference condition. This should become evident when examining internal feedbacks over time and carefully monitoring the level of system degradation and rate of recovery (Zedler, 2000).

Restoration of the ecological structure and function of a damaged estuarine system requires considerable time depending on the type and severity of the human pressure, the kind of habitat, the geographic location of the impacted system, and other factors. Craft et al. (2002) indicated that a time span of 12–15 years is needed for a restored site to exhibit similar structural features as a reference site. Borja et al. (2010) revealed more variable time scales of ecosystem recovery. In a review of 51 long-term projects, they showed that the time span of recovery for biotic elements and substrate after removal of human pressure effects ranged from 10–25 years, a range of recovery times similar to that reported by Jones and Schmitz (2009) for brackish and marine systems (i.e., 10–20 years). The most protracted recovery times were logged for large oil spills, as well as persistent, extensive stressors (e.g., wastewater discharges, mine tailings, and land reclamation (up to 25 years). Recovery of biotic communities follows the physico-chemical restoration of the system and influx of organisms (Borja et al., 2010). Some estuaries, however, have not shown significant ecological improvement despite many years of extensive restoration and conservation efforts (Lotze et al., 2006). For example, Williams et al. (2010) determined that ecological health-related water quality indicators (i.e., chlorophyll a, dissolved oxygen, and Secchi depth) and biotic indicators (i.e., phytoplankton and benthic indices) have generally not improved for Chesapeake Bay over the past 25 years. In fact, two indicators (chlorophyll a and water clarity) have substantially worsened since 1986 for that system even though substantial oyster bed and seagrass restoration efforts have been conducted, and nutrient load reductions have been pursued.

Borja et al. (2010) noted that the Nervioń Estuary in Basque Country, northern Spain has yet to recover from more than a century of urban and industrial development. Although substantial efforts have been made to reverse the ecosystem decline over the past 30 years through restoration and other actions, recovery has not been successful. While recovery of ecosystem structure may have been achieved, ecosystem functioning has not. It is unclear at present if the endpoint of ecosystem recovery can be realized.

Legislative action has been taken in different countries which incorporates restoration of degraded estuarine and marine habitats as a major target (Borja et al., 2008). The Clean Water Act in the U.S. as well as the Water Framework Directive and the Marine Strategy Framework Directive in Europe provide examples. The Estuary Restoration Act (ERA), which was passed into law in the U.S. in 2000 and later amended in 2007, established a national estuary restoration strategy consisting of a coordinated and integrated federal approach to restore damaged estuarine habitat that also involves non-federal partners. Legislation action has led to ecosystem-based management of aquatic systems including the development of new measures to assess ecosystem status and to plan for designing effective restoration (Latimer et al., 2003; Hering et al. 2010). In concert with this strategy has been the protocol to base management decisions on ecological effects of natural and anthropogenic pressures and ecosystem responses (Borja et al., 2010).

The restoration of estuaries is fraught with pitfalls because of the vagaries of environmental conditions, difficulties differentiating and addressing human and natural-induced impacts, and the financial cost to achieve successful habitat and ecosystem recovery. Because of these factors, the restoration of estuaries has lagged behind efforts to restore terrestrial and freshwater systems, both of which have a much longer history. The most successful restoration activities in estuaries have involved sheltered and fringing habitats (e.g., salt marshes, mangroves, and beaches) and structural elements (e.g., seagrasses and biogenic reefs) (Elliott et al., 2007).

Alongi (1998) reported a seagrass restoration success rate of about 45% based on a literature review. The most successful seagrass restoration methods have included: (1) transplantation of 10- to 20-cm wide plugs of sediment, roots, rhizomes, and blades to the restoration sites; (2) transplantation of shallow plugs consisting of sods, turf, and grids to damaged sites; and (3) hand-planting of seeds. Of these methods, the use of plugs has been most successful, although in recent years, seeding has proven highly successful in the Virginia coastal bays in the U.S. For example, Orth et al. (2006, 2010) documented a substantial increase in eelgrass beds in these bays subsequent to nearly a decade of restoration efforts involving the broadcasting of seeds. The transplanting of eelgrass in this region, however, has yielded less promising results, with little success observed beyond 1 to 2 years (Orth et al., 2010).

Mangrove restoration has been most successfully achieved in Southeast Asia (e.g., Vietnam, Indonesia, Malaysia, and Thailand) by outplanting young or mature trees, as well as propagules. While outplanting of trees increases the probability of restoration success, it is far more expensive than the use of propagules. Achieving structural success at a mangrove outplanting site is less problematic than achieving functional success relative to reference locations.

The most extensive wetlands restoration projects in the U.S. have involved salt marsh systems. Typically, spatially localized tracts of salt marsh vegetation are restored in degraded areas. It is difficult to address the dynamic nature of salt marsh habitats, and thus it often takes years for the constructed marsh to approach the structural and functional characteristics of natural marshes in the same area. For example, the hydrological, chemical, geomorphological, and soil characteristics of the salt marsh habitat are highly variable and therefore extremely difficult to replicate. The ecological conditions of the restored marsh are so variable that more than a decade of time may be required for comparable biotic communities to re-establish. More research and restoration activities must be conducted in both degraded and reference systems to improve restoration success.