Critical military training and testing on lands along the nation’s coastal and estuarine shorelines are increasingly placed at risk because of encroachment pressures in surrounding areas, impairments due to other anthropogenic disturbances, and changes in climate and sea level. The U.S. Department of Defense (DoD) intends to enhance and sustain its training and testing assets and also optimize its stewardship of natural resources through the development and application of an ecosystem-based management approach on DoD installations. To accomplish this goal, particularly for installations in estuarine/coastal environments, the Strategic Environmental Research and Development Program (SERDP) launched the Defense Coastal/Estuarine Research Program (DCERP) as a 10-year effort at Marine Corps Base Camp Lejeune (MCBCL) in North Carolina. The results of the second 5 years of the program (DCERP2) are presented in the DCERP2 Final Report.
There were four overarching objectives of DCERP2. The first objective was to understand the effects of climate change impacts, including warming temperatures, variability in the hydrological cycle, storm events, and sea level rise on the coastal ecosystems at MCBCL from observations and measurements made over the 10-year program. The second objective was to understand the carbon cycle of the coastal and terrestrial ecosystems at MCBCL through a highly integrated sampling program. The third objective was to develop models, tools, and indicators to evaluate current and projected future ecosystem state changes and translate scientific findings into actionable information for installation managers. The last objective was to recommend adaptive management strategies to sustain ecosystem natural resources within the context of an active military installation.
DCERP2 was implemented in two phases. The first phase was a three-month planning period for identifying ecosystem processes and stressors, developing conceptual models to identify knowledge gaps, and working iteratively with DoD end-users to refine approaches to support MCBCL natural resource management. This effort resulted in the development of the DCERP2 Monitoring Plan and the DCERP2 Research Plan, which collectively served as the foundation for all DCERP2 activities. The DCERP Team approached and implemented the research and monitoring work by dividing the landscape into four distinct ecosystem modules: Aquatic/Estuarine, Coastal Wetlands, Coastal Barrier, and Terrestrial Modules. Because the effects of climate change have an overarching influence on all four ecosystem modules, climate change was treated as a fifth module (i.e., the Climate Change Module).
Implementation of the DCERP2 plans began in April 2013 and resulted in 13 research projects and five monitoring activities, as well as the enhancement of the DCERP Data and Information Management System (DIMS), which contains monitoring and research data from DCERP1 and DCERP2. DIMS provides optimized data storage and retrieval for integrated analysis, fostering information exchange among the various DCERP partners, other interested researchers, and stakeholders.
Climate Change Impacts on Ecosystem Processes
MCBCL encompasses 153,000 acres and occupies 80% of the shoreline of the New River Estuary (NRE), thus making the NRE a core consideration to the management of the installation. The remainder of MCBCL consists of terrestrial habitat (i.e., 90,000 acres), which is managed for training and is also a wildlife habitat. To understand the potential impacts of climate change, the DCERP2 Team selected the climate drivers that would have the most influence on the MCBCL ecosystems and the region of study. The climate drivers selected included temperature, precipitation, storminess, and sea level rise. The DCERP2 Team used empirical data to understand the present state of the ecosystems and their natural variability under current climate conditions and developed ecological process models to understand the potential climate impacts on the ecosystems in the future.
To study these future impacts, the research team developed an ensemble of 24 climate model projections that represent the worst-case scenario of increasing concentrations of greenhouse gases in the future. This ensemble showed agreement regarding the magnitude of future temperature changes (e.g., increases) as compared with the magnitude of changes in precipitation, which will increase, decrease, or remain the same. Understanding the future temperatures is important because temperatures affect the function of natural systems in fundamental ways, including determining the rates of chemical reactions, an organism’s metabolism, and the timing of critical life cycle events (phenology). In the terrestrial ecosystem, warming temperatures measured over the past 33 years were shown to have advanced egg-laying dates (5 days earlier) of red-cockaded woodpeckers (RCWs; Picoides borealis) at MCBCL and nearby at Fort Bragg, NC, and increased productivity at Fort Bragg. However, warmer temperatures are also associated with a lower coincidental survival rate of juvenile RCWs. Other considerations related to higher future temperatures may further complicate RCW management, such as the ability to maintain a regular prescribed fire regime, without which the RCW’s longleaf pine (Pinus palustris) habitat cannot be sustained.
These complicated temperature interactions can also be observed in estuarine water quality. Under the Estuarine Simulation Model (ESM) scenarios that increased temperatures from +1°C (+1.8°F) to +5°C (+9°F) above current temperatures, hypoxia (i.e., low dissolved oxygen concentrations caused by decomposition of organic matter) in the NRE would increase, subsequently stressing the fish and shellfish living in these waters. In contrast, the ESM predicted water quality improvements such as decreases in chlorophyll a concentrations and days with chlorophyll a concentrations exceeding the state standard of greater than 40 μg/L. These temperature responses are also subject to modulation by changes in freshwater inflow and loadings due to inter-annual precipitation variability. However, the future impacts of precipitation at MCBCL are difficult to determine for all ecosystems because down-scaled climate projections indicated either a slight decrease or increase in the amount and intensity of rainfall.
Although there will be interactions with temperature and precipitation in the future, the effects of sea level rise on MCBCL’s coastal marshes and the coastal barrier, Onslow Island, are more significant. Marsh resilience will depend upon the rate of sea level rise in combination with marsh plant productivity, suspended sediment inputs that regulate vertical accretion, and the slope of the adjacent uplands for marsh migration. The DCERP2 Team developed the Geospatial Marsh Model, which predicted that marshes throughout the estuarine gradient gain area under the lowest sea level rise scenario (0.3 m by 2100) through a combination of expansion via marsh migration upslope and enhanced vertical accretion. Above the medium sea level rise scenario (1.3 m by 2100), the model predicted that both middle and lower NRE marshes drown because of limited sediment supply and reduced ability to continue to migrate landward. The marsh that demonstrated the highest resilience is located along the Intracoastal Waterway. This marsh that has the highest sediment supply and the lowest surrounding slope was only predicted to drown at the highest sea level rise rate (greater than 1.8 m by 2100).
Over the past 70 years, measured changes to the coastal barrier indicated that storms (defined here by decadal hurricane frequency) and sea level rise, not military training use, had the largest influence on beach position and overwash. The DCERP2 Beach Morphology Model, which used the lowest (0.3 m) and highest (2.5 m) sea level rise by 2100, projected a substantial loss of infrastructure on the southern portion of the barrier island regardless of sea level rise rate by 2035 and a complete loss of usable beach by 2065. Conversely, the same model scenarios indicated that the northern portion of Onslow Island will remain stable and likely grow seaward, thereby allowing MCBCL managers to consider moving the amphibious assault training areas to the more stable northern end of Onslow Beach.
Carbon Cycles in Coastal Ecosystems
Quantifying carbon cycling in the estuarine/coastal landscape within MCBCL’s boundaries hinged upon measuring intra-ecosystem carbon inventories and inter-ecosystem fluxes at appropriate spatial and temporal scales. Coastal carbon fluxes are highly variable in space and time and are challenging to measure; thus, few coastal carbon budgets are currently available, and most of these have relied on limited observational scales. The DCERP2 Team conducted novel research that captured daily, seasonal, annual, and inter-annual variabilities in carbon fluxes and exchanges across the shallow and pelagic areas of the estuary from the head of the estuary to its discharge into the coastal ocean to determine the main drivers of the carbon cycle for the NRE. The team also measured carbon fluxes before and after a major storm event and determined that significant fluxes resulted from the storm’s passage that were not considered in most other carbon studies.
The NRE carbon budget quantified flows and tracked the complex processes that control whether carbon supplied from the watershed or formed in the estuary is stored internally, exported into the ocean, or emitted into the atmosphere. The NRE was in near metabolic balance annually, with only minor exchanges of carbon dioxide between the atmosphere and estuary. Depending upon river discharge, the NRE was either slightly net heterotrophic (more respiration than photosynthesis) or slightly autotrophic (more photosynthesis than respiration). This small variation around net neutral metabolism was maintained by counter-balancing multiple reactions and exchanges between the deeper estuarine channel and shallow shoals and between the upper and lower portions of the estuary. Daily variations in carbon fluxes were as large as those on seasonal time scales, which have been ignored in many other estuarine carbon studies, thus demonstrating the need to perform high-resolution temporal and spatial measurements to understand the mechanisms driving the observed carbon fluxes.
The DCERP2 Team determined that the flux of carbon across the estuary to the coastal salt marshes and the barrier island boundaries was relatively minor. Overall, the annual net carbon balance in the marshes was driven by carbon exchanges with the atmosphere and short-term sediment deposition. Lateral fluxes of carbon between the estuary and marsh, both through drainage of the marsh platform during ebbing tides and via porewater advection, were small when compared with the atmospheric exchanges. Most marshes at MCBCL fixed more carbon than they respired; however, the small aerial extent of coastal wetlands relative to the total area of MCBCL limits their contribution to the larger landscape carbon budget. The other ecosystem services, such as fisheries habitat and attenuating wave energy, provided by these marshes at MCBCL may be a more important role than carbon storage. The carbon flux measurement of the coastal barrier showed that storms drove the transition of the island from being a carbon sink to a carbon source. This transition occurs because storms increased carbon loss through shoreface erosion of peat deposits buried beneath the island and reduced carbon storage through washover sand deposition that buries backbarrier marshes. Future changes in the magnitude and frequency of storms could accelerate carbon loss through increased erosion.
In contrast to the estuarine ecosystems, the terrestrial portion of the MCBCL landscape represents the largest carbon management potential in both size and quantities of carbon that could be stored. Results of forest carbon modeling showed that above-ground live carbon storage for three prevalent pine species on MCBCL was highest in longleaf pine, moderate in loblolly pine (Pinus taeda), and lowest in pond pine (Pinus serotina) forests. Longleaf pine is the forest species of greatest focus and management aboard MCBCL and at several other DoD installations in the Southeastern US. Longleaf pine represents the greatest opportunity for carbon storage into the future under management scenarios designed to maintain or restore longleaf habitat, primarily through using prescribed fire.
Models, Tools, and Indicators to Assess Ecosystem State Change and Recommend Adaptive Management Strategies
The DCERP2 Team developed ecosystem models, tools, and environmental indicators to assist installation managers in making more informed management decisions. The DCERP2 Team identified several indicators of changes in ecosystem state that can serve as useful targets for managers and inform decisions about land-use changes, point source discharges, forest management practices, and marsh mitigation activities. The communication of information to DoD managers and other end-users was an ongoing process and involved maps, user guides, easy-to-interpret model outputs, annual reports, workshops, and one-on-one meetings. These outreach efforts and products were designed to share DoD–relevant information and provide MCBCL with adaptive management strategies. All DCERP products were collected throughout the program in DIMS, which is currently available to all registered users at https://dcerp.rti.org. The DIMS data portal fulfills the SERDP’s data management goal for DCERP by providing an accessible Web-based platform for distributing all DCERP data, tools, models, and other products to all three target audiences: researchers, DoD installation managers, and local stakeholders, including the public.
The research, monitoring, and modeling efforts conducted as part of DCERP2 resulted in a greater understanding of MCBCL’s diverse ecosystems and their interactions with respect to the carbon cycle, management of carbon, and plausible future climate conditions. In addition, the research results provided an understanding of which on- and off-installation activities are currently affecting these ecosystems and what management actions could be implemented to best sustain the military’s training mission and natural resource assets of MCBCL. The DCERP2 Team recommended that MCBCL continue long-term monitoring in a scaled-back manner in several of the ecosystems of study. Long-term monitoring data provide information about the status of critical indicators such as chlorophyll a concentration, thus allowing shifts in baseline conditions or increases in historic variation to be detected. These shifts in baseline conditions or historic variability may suggest changes to the way in which an ecosystem functions. The knowledge gained from DCERP2 will provide benefits to other DoD installations in similar coastal settings and to the scientific community, other coastal managers, and the public at large.