Mark M. Brinson

CLIMATE CHANGE, HURRICANES AND TROPICAL STORMS, AND RISING SEA LEVEL IN COASTAL WETLANDS

Global climate change is expected to affect temperature and precipitation patterns, oceanic and atmospheric circulation, rate of rising sea level, and the frequency, intensity, timing, and distribution of hurricanes and tropical storms. The magnitude of these projected physical changes and their subsequent impacts on coastal wetlands will vary regionally. Coastal wetlands in the southeastern United States have naturally evolved under a regime of rising sea level and specific patterns of hurricane frequency, intensity, and timing. A review of known ecological effects of tropical storms and hurricanes indicates that storm timing, frequency, and intensity can alter coastal wetland hydrology, geomorphology, biotic structure, energetics, and nutrient cycling. Research conducted to examine the impacts of Hurricane Hugo on colonial waterbirds highlights the importance of long-term studies for identifying complex interactions that may otherwise be dismissed as stochastic processes. Rising sea level and even modest changes in the frequency, intensity, timing, and distribution of tropical storms and hurricanes are expected to have substantial impacts on coastal wetland patterns and processes. Persistence of coastal wetlands will be determined by the interactions of climate and anthropogenic effects, especially how humans respond to rising sea level and how further human encroachment on coastal wetlands affects resource exploitation, pollution, and water use. Long-term changes in the frequency, intensity, timing, and distribution of hurricanes and tropical storms will likely affect biotic functions (e.g., community structure, natural selection, extinction rates, and biodiversity) as well as underlying processes such as nutrient cycling and primary and secondary productivity. Reliable predictions of global-change impacts on coastal wetlands will require better understanding of the linkages among terrestrial, aquatic, wetland, atmospheric, oceanic, and human components. Developing this comprehensive understanding of the ecological ramifications of global change will necessitate close coordination among scientists from multiple disciplines and a balanced mixture of appropriate scientific approaches. For example, insights may be gained through the careful design and implementation of broad-scale comparative studies that incorporate salient patterns and processes, including treatment of anthropogenic influences. Well-designed, broad-scale comparative studies could serve as the scientific framework for developing relevant and focused long-term ecological research, monitoring programs, experiments, and modeling studies. Two conceptual models of broad-scale comparative research for assessing ecological responses to climate change are presented: utilizing space-for-time substitution coupled with long-term studies to assess impacts of rising sea level and disturbance on coastal wetlands, and utilizing the moisture-continuum model for assessing the effects of global change and associated shifts in moisture regimes on wetland ecosystems. Increased understanding of climate change will require concerted scientific efforts aimed at facilitating interdisciplinary research, enhancing data and information management, and developing new funding strategies.

Temperate freshwater wetlands: types, status, and threats

This review examines the status of temperate-zone freshwater wetlands and makes projections of how changes over the 2025 time horizon might affect their biodiversity. The six geographic regions addressed are temperate areas of North America, South America, northern Europe, northern Mediterranean, temperate Russia, Mongolia, north-east China, Korea and Japan, and southern Australia and New Zealand. Information from the recent technical literature, general accounts in books, and some first-hand experience provided the basis for describing major wetland types, their status and major threats. Loss of biodiversity is a consequence both of a reduction in area and deterioration in condition. The information base for either change is highly variable geographically. Many countries lack accurate inventories, and for those with inventories, classifications differ, thus making comparisons difficult. Factors responsible for losses and degradation include diversions and damming of river flows, disconnecting floodplain wetlands from flood flows, eutrophication, contamination, grazing, harvests of plants and animals, global warming, invasions of exotics, and the practices of filling, dyking and draining. In humid regions, drainage of depressions and flats has eliminated large areas of wetlands. In arid regions, irrigated agriculture directly competes with wetlands for water. Eutrophication is widespread, which, together with effects of invasive species, reduces biotic complexity. In northern Europe and the northern Mediterranean, losses have been ongoing for hundreds of years, while losses in North America accelerated during the 1950s through to the 1970s. In contrast, areas such as China appear to be on the cusp of expanding drainage projects and building impoundments that will eliminate and degrade freshwater wetlands. Generalizations and trends gleaned from this paper should be considered only as a starting point for developing world-scale data sets. One trend is that the more industrialized countries are likely to conserve their already impacted, remaining wetlands, while nations with less industrialization are now experiencing accelerated losses, and may continue to do so for the next several decades. Another observation is that countries with both protection and restoration programmes do not necessarily enjoy a net increase in area and improvement in condition. Consequently, both reductions in the rates of wetland loss and increases in the rates of restoration are needed in tandem to achieve overall improvements in wetland area and condition.

The Role of Reference Wetlands in Functional Assessment and Mitigation

Compensatory mitigation for damages to wetlands in the United States occurs largely without explicit analysis and replacement of wetland functions. We offer an approach to standardize such analyses and strengthen the connection between ecological principles and policies for wetland resources. By establishing standards from reference wetlands chosen for their high level of sustainable functioning, gains and losses of functions can be quantified for wetlands used in compensatory mitigation. Advantages of a reference wetland approach include (1) making explicit the goals of compensatory mitigation through iden- tification of reference standards from data that typify sustainable conditions in a region, (2) providing templates to which restored and created wetlands can be designed, and (3) establishing a framework whereby a decline in functions resulting from adverse impacts or a recovery of functions following restoration can be estimated both for a single project and over a larger area accumulated over time. To establish reference standards, conditions inherent to highly functioning sites must be identified for classes of wetlands that share similar geomorphic settings. Ecological functions are then identified, and variables used to model the functions are employed in developing reference standards. Variables range from the highest levels of sustainable functioning to the complete absence of functions when a wetland ecosystem is displaced. An example given for wet pine flats in the North Carolina coastal plain illustrates how to determine the loss of a given function for an impacted wetland, how to calculate recovery (gains) in function through compensatory mitigation, and how to use the relationships between the two (loss vs. gain in function) to set minimum replacement ratios of restored to impacted area. In all cases, data from reference wetlands provide the benchmarks for making these estimates and for directing restoration or creation of wetlands toward the standards established for the wetland class. Programs to implement the use of reference wetlands require regional efforts that build upon the knowledge base of existing wetlands and their functioning.

Changes in the functioning of wetlands along environmental gradients

One of the prevalent gradients in wetlands is the continuum of depth and frequency of flooding. While much emphasis has been placed on the importance of hydrology as a driving force for wetlands, few other perspectives have emerged to demonstrate unifying patterns and principles. In contrast to the wetness continuum, the functioning of wetlands can be separated into two broad categories: (1) landscape-based transitions that occur within a wetland or group of similar wetland types and (2) resource-based transitions that allow comparisons of the flow of water and processing of nutrients among very different wetland types. Landscape-based continua include the transition from upstream to downstream in riverine wetlands and between aquatic and terrestrial ecosystems within a wetland. Along the upstream-downstream continuum, sources of flood-water delivery change dominance from ground-water discharge and overland runoff, as in low order streams, to dominance by overbank flooding, as in high order streams. With increasing size, properties related to the aquatic-to-terrestrial transition are replaced by properties related to wetland-atmospheric exchanges and by landscape maintenance, the latter not normally acknowledged as a wetland function. Resource-based continua include the extremes of (1) sources of water to wetlands (precipitation, overland flow, and ground water) and (2) the variation in inflows and outflows of nutrients and sediments. Emphasis on water source forces consideration of controls beyond the wetland’s boundaries. A broader view of biogeochemical functioning is gained by categorizing wetlands into groups based on the exchange of nutrients and sediments among landscape units rather than on serving as a sink or source for a particular element. Based on this analysis, the less frequently flooded or saturated portions of wetlands are no less functionally active than wetter portions; the functions are simply different. Efforts to classify wetlands according to their hydroperiod do little to reveal their fundamental properties.

DECOMPOSITION AND NUTRIENT EXCHANGE OF LITTER IN AN ALLUVIAL SWAMP FOREST

Weight loss from cellulose sheets was measured monthly at three sites (river, swamp floor, natural levee) of a North Carolina swamp forest dominated by Nyssa aquatica. Rates of loss were significantly different seasonally and between sites. Both temperature and moisture appeared to be important in controlling decomposition rates. For Nyssa leaves, dry weight decreased to 25% of original after 48 wk while twigs fell to 80Wo of original after 56 wk. Three modes of nutrient exchange were found: (1) accumulation of N, Ca, and Fe by both leaves and twigs, with stronger leaf accumula- tion of these elements; (2) strong leaf accumulation with rapid twig loss of P; and (3) losses by both leaves and twigs of K and Mg. Final atomic ratios of C:N and C:P were about 15:1 and 500:1, respectively, suggesting that P may be in short supply. The significance of nutrient accumulation is that there was little or no net release of P, N, Ca, and Fe from autumn leaf fall until tree growth in the spring. Conservation and recycling of nutrients appear to be tight even in swamps open to flooding during tree dormancy.

Litterfall, Stemflow, and Throughfall Nutrient Fluxes in an Alluvial Swamp Forest

Nutrient deposition to the forest floor of an alluvial swamp in the North Carolina Coastal Plain was measured and compared with other wetland and upland forests. For the alluvial forest, annual litterfall was 6428 kg dry mass/ha of which 63% was Nyssa aquatic leaves. Nutrient flux to the forest floor in kilograms per hectare per year for litterfall and aqueous sources (stemflow plus throughfall), respectively, was 2779 and 91.5 for organic carbon, 72.77 and 10.31 for N, 5.38 and 1.55 for P, 7.19 and 9.21 for S, 21.1 and 11.96 for K, 45.1 and 15.31 for Ca, and 17.0 and 7.60 for Mg. Most of these values are near the upper range or higher than those reported for mature upland temperate forests and still-water swamps. The particularly high values for nitrogen and phosphorus in the alluvial forest may be a consequence of fluvial sources, whereas nutrient sources for upland forests and still-water swamps are restricted to atmospheric inputs and weathering.

Multiple states in the sea-level induced transition from terrestrial forest to estuary

In this paper we provide a conceptual model to examine changes in ecosystem state during the transition from terrestrial forest to shallow estuarine environments for coastal mainland marshes at the Virginia Coast Reserve (VCR), United States of America. Ecosystem states are characterized by plant community dominants and soil/sediment characteristics. The five states considered are upland or wetland forest, organic high marsh, intertidal mineral low marsh, autotrophic benthic with or without submersed aquatic vascular plants, and heterotrophic benthic (estuarine bottom). Transitions between states are described from the perspective of a fixed forest location undergoing transition from one ecosystem state to another. Rising sea level is acknowledged as the master variable that forces the process of change overall. Each state is hypothesized to have self-maintaining properties and thus is resistant to change from rising sea level; alternatively, transitions between states are facilitated by disturbance or exposure to acute stress. For change to occur, resistance must be overcome by events that are more abrupt than rising sea level and that appear as accentuated pulsings, which result in another self-maintaining and resistnnt state. Such events facilitate plant species replacement and alter sediment conditions. Mechanisms responsible for causing a state to cross a threshold are unique for each transition type and include brackish-water intrusion (osmotic stress and sulfide toxicity), tidal creek encroachment (redistribution of sediments), erosive currents and waves (resuspension of sediments, which increases light extinction), and increasing water depth (leads to greater bottom shading). Field experiments relevant to scales at which pulsings occur are not abundant in coastal marshes.

Response of Wetlands to Rising Sea Level in the Lower Coastal Plain of North Carolina

Most of the coastal wetlands of the South Atlantic region of the United States are expected to diminish in size in response to the opposing forces of increasing human population growth and accelerating rates of rising sea level. We evaluated several models that project the response of coastal wetlands to rising sea level and concluded that current models appear unsuited for wetlands of the Albemarle-Pamlico peninsula of North Carolina. We came to this conclusion after we examined the distribution of wetlands, elevation contours, estimates of surface slope, soil types, and peat deposits on the peninsula. Most of the data were obtained from U.S. Geological Survey topographic quadrangle maps, U.S. Fish and Wildlife Service National Wetlands Inventory maps, U.S. Soil Conservation Service soil surveys, and inventories of peat deposits. Some unusual features of this pen- insula are low elevation (56% of total area <1.5 m), extensive coverage by wetlands (53%) and hydric soils (90%), negligible slopes of the land surface, virtual absence of tides, and lack of abundant sources of sediment. In the process of reconstructing how past rises in sea level most likely led to present conditions, it became apparent that vertical accretion of peat in situ is largely responsible for landscape features in areas where elevations are lowest. Were it not for these deposits, the land surface area of the peninsula would be decreasing relative to sea level. This situation contrasts sharply with areas in the eastern United States fringed by tidal marshes, which are undergoing overland migration at a rate dictated by landward slope and the rate of rising sea level. If the rate of sea level rise accelerates, it is doubtful if vertical accretion rates of peat can prevent submergence of extensive areas of wetlands in the Albemarle-Pamlico peninsula. Land use and drainage in the lowest elevations of the peninsula are currently being affected by sea level. Future land management of the peninsula will be constrained by potential landscape changes as a result of rising sea level.

Strategies for assessing the cumulative effects of wetland alteration on water quality

Assessment of cumulative impacts on wetlands can benefit by recognizing three fundamental wetland categories: basin, riverine, and fringe. The geomorphological settings of these categories have relevance for water quality.Basin, or depressional, wetlands are located in headwater areas, and capture runoff from small areas. Thus, they are normally sources of water with low elemental concentration. Although basin wetlands normally possess a high capacity for assimilating nutrients, there may be little opportunity for this to happen if the catchment area is small and little water flows through them.Riverine wetlands, in contrast, interface extensively with uplands. It has been demonstrated that both the capacity and the opportunity for altering water quality are high in riverine wetlands.Fringe wetlands are very small in comparison with the large bodies of water that flush them. Biogeochemical influences tend to be local, rather than having a measurable effect on the larger body of water. Consequently, the function of these wetlands for critical habitat may warrant protection from high nutrient levels and toxins, rather than expecting them to assume an assimilatory role.The relative proportion of these wetland types within a watershed, and their status relative to past impacts can be used to develop strategies for wetland protection. Past impacts on wetlands, however, are not likely to be clearly revealed in water quality records from monitoring studies, either because records are too short or because too many variables other than wetland impacts affect water quality. It is suggested that hydrologic records be used to reconstruct historical hydroperiods in wetlands for comparison with current, altered conditions. Changes in hydroperiod imply changes in wetland function, especially for biogeochemical processes in sediments. Hydroperiod is potentially a more sensitive index of wetland function than surface areas obtained from aerial photographs. Identification of forested wetlands through photointerpretation relies on vegetation that may remain intact for decades after drainage. Finally, the depositional environment of wetlands is a landscape characteristic that has not been carefully evaluated nor fully appreciated. Impacts that reverse depositional tendencies also may accelerate rates of change, causing wetlands to be large net exporters rather than modest net importers. Increases in rates as well as direction can cause stocks of materials, accumulated over centuries in wetland sediments, to be lost within decades, resulting in nutrient loading to downstream aquatic ecosystems.

Application of a Geomorphic and Temporal Perspective to Wetland Management in North America

The failure of managed wetlands to provide a broad suite of ecosystem services (e.g., carbon storage, wildlife habitat, ground-water recharge, storm-water retention) valuable to society is primarily the result of a lack of consideration of ecosystem processes that maintain productive wetland ecosystems or physical and social forces that restrict a manager’s ability to apply actions that allow those processes to occur. Therefore, we outline a course of action that considers restoration of ecosystem processes in those systems where off-site land use or physical alterations restrict local management. Upon considering a wetland system, or examining a particular management regime, there are several factors that will allow successful restoration of wetland services. An initial step is examination of the political/social factors that have structured the current ecological condition and whether those realities can be addressed. Most successful restorations of wetland ecosystem services involve cooperation among multiple agencies, acquisition of funds from non-traditional sources, seeking of scientific advice on ecosystem processes, and cultivation of good working relationships among biologists, managers, and maintenance staff. Beyond that, in on-site wetland situations, management should examine the existing hydrogeomorphic situation and processes (e.g., climatic variation, tides, riverine flood-pulse events) responsible for maintenance of ecosystem services within a given temporal framework appropriate for that wetland’s hydrologic pattern. We discuss these processes for five major wetland types (depressional, lacustrine, estuarine, riverine, and man-made impoundments) and then provide two case histories in which this approach was applied: Seney National Wildlife Refuge with a restored fen system and Bosque del Apache National Wildlife Refuge where riverine processes have been simulated to restore native habitat. With adequate partnerships and administrative and political support, managers faced with degraded and/or disconnected wetland processes will be able to restore ecosystem services for society in our highly altered landscape by considering wetlands in their given hydrogeomorphic setting and temporal stage.