David M. Burdick

Quantifying eelgrass habitat loss in relation to housing development and nitrogen loading in Waquoit Bay, Massachusetts

Change analysis of eelgrass distribution in Waquoit Bay demonstrated a rapid decline of eelgrass habitat between 1987 and 1992. Aerial photography and ground-truth assessments of eelgrass distribution in the Waquoit Bay National Estuarine Research Reserve documented progressive loss in eelgrass acreage and fragmentation of eelgrass beds that we relate to the degree of housing development and associated nitrogen loading, largelyvia groundwater, within various sub-basins of the estuary. The sub-basins with greater housing density and higher nitrogen loading rates showed more rapid rates of eelgrass decline. In eelgrass mesocosm studies at the Jackson Estuarine Laboratory, excessive nitrogen loading stimulated proliferation of algal competitors (epiphytes, macroalgae, and phytoplankton) that shade and thereby stress eelgrass. We saw domination by each of these three algal competitors in our field observations of eelgrass decline in Waquoit Bay. Our study is the first to relate housing development and nitrogen loading rates to eelgrass habitat loss. These results for the Waquoit Bay watershed provide supporting evidence for management to limit development that results in groundwater nitrogen loading and to initiate remedial action in order to reverse trends in eelgrass habitat loss.

Ecological responses to tidal restorations of two northern New England salt marshes

Efforts are underway to restore tidal flow in New England salt marshes that were negatively impacted by tidal restrictions. We evaluated a planned tidal restoration at Mill Brook Marsh (New Hampshire) and at Drakes Island Marsh (Maine) where partial tidal restoration inadvertently occurred. Salt marsh functions were evaluated in both marshes to determine the impacts from tidal restriction and the responses following restoration. Physical and biological indicators of salt marsh functions (tidal range, surface elevations, soil water levels and salinities, plant cover, and fish use) were measured and compared to those from nonimpounded reference sites. Common impacts from tidal restrictions at both sites were: loss of tidal flooding, declines in surface elevation, reduced soil salinity, replacement of salt marsh vegetation by fresh and brackish plants, and loss of fish use of the marsh.Water levels, soil salinities and fish use increased immediately following tidal restoration. Salt-intolerant vegetation was killed within months. After two years, mildly salt-tolerant vegetation had been largely replaced in Mill Brook Marsh by several species characteristic of both high and low salt marshes. Eight years after the unplanned, partial tidal restoration at Drakes Island Marsh, the vegetation was dominated bySpartina alterniflora, a characteristic species of low marsh habitat.Hydrologic restoration that allowed for unrestricted saltwater exchange at Mill Brook restored salt marsh functions relatively quickly in comparison to the partial tidal restoration at Drakes Island, where full tidal exchange was not achieved. The irregular tidal regime at Drakes Island resulted in vegetation cover and patterns dissimilar to those of the high marsh used as a reference. The proper hydrologic regime (flooding height, duration and frequency) is essential to promote the rapid recovery of salt marsh functions. We predict that functional recovery will be relatively quick at Mill Brook, but believe that the habitat at Drakes Island will not become equivalent to that of the reference marsh unless the hydrology is further modified.

Variation in soil salinity associated with expansion of Phragmites australis in salt marshes

Salinity is a well-known stressor of Phragmites australis (common reed), leading to reduced success in brackish and salt marshes. Although saline, many remaining salt marshes in New England are changing in structure and function due to tidal restrictions and rapid proliferation of P. australis. The poor reputation of this native plant (its dominance is used as an indicator of marsh degradation) has stimulated management and research using natural stressors for control. Our field study associated natural variability in soil salinity levels over time and space with vigor and spread rates of P. australis. Over 2 years, salinity was measured 15 times from three depth intervals (5–20, 35–50, and 65–80 cm) at five stations established in six colonies of P. australis. Our results indicated that salinity in tidal marshes varied temporally due to the extent of tidal flooding (salinity was greater during spring tides compared with neap tides) and regional freshwater runoff (salinity was lower in the spring). If the growing season is split into early (May–July) and late (August–October) periods, interesting patterns emerged (salinity increased with depth early, but decreased with depth late). Shoot height, cover, and expansion rate of the six colonies were measured twice over 3 years. In general, the stands of P. australis were expanding into salt marsh at 0.35 m per year, and increasing in cover (8% per year), even though the canopy height decreased at all but two of the sites over the study period. Salinity was lower in marshes where tides were artificially restricted (11–16 ppt compared with 19–24 ppt for the natural marshes), and one of these sites exhibited rapid P. australis expansion. At sites with natural hydrology, P. australis appeared to be expanding more slowly, shading out marsh species, and perhaps avoiding salinity stress by accessing natural sources of fresher water at different soil depths during different seasons.

Developing success criteria for restored eelgrass, salt marsh and mud flat habitats

Abstract When estuarine habitats are restored, it is crucial to determine their success or failure. How can we tell, bringing a minimum of preconceived notions to the task and using a valid scientific process, if the functions and values of habitats have been recreated and returned to the estuary? In the New Hampshire Port Mitigation Project (1993–1995), we formulated literature-based success criteria (SC), but could not quantitatively defend their scientific validity. We are now using the project as a laboratory for developing and testing rigorous SC. We developed indicators of chosen habitat functions, then created statistical representations of natural, local reference sites for comparison to the functional development of restored habitats. An explanation of the steps in developing and testing the method are followed by a test application of our SC methodology using the data from eelgrass transplant sites (6.2 acres) created for the New Hampshire Port Mitigation Project. At the same time, we are developing a methodology for a more generic model of SC that we are applying to the restorations of salt marsh and mud flat for the Port Mitigation. We believe the methodology to develop and apply SC is transferable to other locations and habitats not only because of its objective foundation but because it is based on data collected locally.

Determinants of expansion forPhragmites australis, common reed, in natural and impacted coastal marshes

The rapid spread ofPhragmites australis in the coastal marshes of the Northeastern United States has been dramatic and noteworthy in that this native species appears to have gained competitive advantage across a broad range of habitats, from tidal salt marshes to freshwater wetlands. Concomitant with the spread has been a variety of human activities associated with coastal development as well as the displacement of nativeP. australis with aggressive European genotypes. This paper reviews the impacts caused by pure stands ofP. australis on the structure and functions of tidal marshes. To assess the determinants ofP. australis expansion, the physiological tolerance and competitive abilities of this species were examined using a field experiment.P. australis was planted in open tubes paired withSpartina alterniflora, Spartina patens, Juncus gerardii, Lythrum salicaria, andTypha angustifolia in low, medium, and high elevations at mesohaline (14‰), intermediate (18‰), and salt (23‰) marsh locations. Assessment of the physiological tolerance ofP. australis to conditions in tidal brackish and salt marshes indicated this plant is well suited to colonize creek banks as well as upper marsh edges. The competitive ability ofP. australis indicated it was a robust competitor relative to typical salt marsh plants. These results were not surprising since they agreed with field observations by other researchers and fit within current competition models throught to structure plant distribution within tidal marshes. Aspects ofP. australis expansion indicate superior competitive abilities based on attributes that fall outside the typical salt marsh or plant competition models. The alignment of some attributes with human impacts to coastal marshes provides a partial explanation of how this plant competes so well. To curb the spread of this invasive genotype, careful attention needs to be paid to human activities that affect certain marsh functions. Current infestations in tidal marshes should serve as a sentinel to indicate where human actions are likely promoting the invasion (e.g., through hydrologic impacts) and improved management is needed to sustain native plant assemblages (e.g., prohibit filling along margins).

Landscape conservation in a forested wetland watershed.

M ore than one-half of the 40 million ha of wetlands in the coterminus United States is forested (Frayer et al. 1983). Most of these wetlands (57%; Abernethy and Turner 1987) are in the southeastern United States. They are characterized as permanently, semipermanently, or intermittently flooded and are dominated by cypress (Taxodium spp.), tupelo (Nyssa spp.), and oak (Quercus spp.). The broad Mississippi River alluvial floodplain, which extends from the Gulf of Mexico to southern Illinois, historically supported the largest United States expanses of forested wetlands, but since the 1950s these areas have been rapidly converted to the production of cotton, corn, and soybeans (OTA 1984). Brinson et al. (1981) estimated the loss of riparian forest at more than 70% since presettlement days. Abernethy and Turner (1987) calcu-

Phragmites australis management in the United States: 40 years of methods and outcomes

We reviewed all available studies on Phragmites australis management in the United States. Our results show that there is a heavy emphasis on herbicides to manage Phragmites, relative to other methods, and a lack of information on what types of plant communities establish once Phragmites is removed. Our model of Phragmites establishment and reproduction describes the invasion as a symptom of watershed-scale land use and disturbance. We advocate more holistic approaches to control and management that focus on improving water quality and minimizing human disturbance to deter future invasion and improve resilience of native plant communities.

Impacts of Seawalls on Saltmarsh Plant Communities in the Great Bay Estuary, New Hampshire USA

Seawalls are often built along naturally dynamic coastlines, including the upland edge of salt marshes, in order to prevent erosion or to extend properties seaward. The impacts of seawalls on fringing salt marshes were studied at five pairs of walled and natural marshes in the Great Bay Estuary of New Hampshire, USA. Marsh plant species and communities showed no difference in front of walls when compared with similar elevations at paired controls. However, seawalls eliminated the vegetative transition zone at the upper border. Not only did the plant community of the transition zone have high plant diversity relative to the low marsh, but it varied greatly from site to site in the estuary. The effects of seawall presence on other marsh processes, including sediment movement, wrack accumulation, groundwater flow, and vegetation distribution and growth, were examined. Although no statistically significant effects of seawalls were found, variation in the indicators of these processes were largely controlled by wave exposure, site-specific geomorphology and land use, and distance of the sampling station from the upland. Trends indicated there was more sediment movement close to seawalls at high energy sites and less fine grain sediment near seawalls. Both trends are consistent with an increase in energy from wave reflection. The distribution of seawalls bordering salt marshes was mapped for Great and Little Bays and their rivers. Throughout the study area, 3.54% of the marshes were bounded by shoreline armoring (5876xa0m of seawalls along 165.8xa0km of marsh shoreline). Localized areas with high population densities had up to 43% of marshes bounded by seawalls. Coastal managers should consider limiting seawall construction to preserve plant diversity at the upper borders of salt marshes and prevent marsh habitat loss due to transgression associated with sea level rise.

Human facilitation of Phragmites australis invasions in tidal marshes: a review and synthesis.

Efforts to manage or prevent Phragmites australis invasion in salt and brackish marshes are complicated by the lack of a general causal role for specific human activities. The pattern of invasion within a marsh differs among sites, and each may have different causal histories. A review of the literature finds three establishment/invasion patterns: (1) from stands established on ditch- or creek-bank levees toward interior portions of high marshes, (2) from stands along upland borders toward high marsh interiors, and (3) centroid spread from high marsh stands established in ostensibly random locations. Each invasion pattern seems to have different anthropogenic precursors, therefore preventing generalizations about the role of any one human activity in all sites. However, historical and experimental evidence suggests that regardless of invasion pattern, establishment is much more likely at sites where rhizomes are buried in well-drained, low salinity marsh areas. Any human activity that buries large rhizomes, increases drainage, or lowers salinity increases chances of establishing invasive clones. To integrate these patterns and improve our understanding of the rapid spread of Phragmites, recent evidence has been synthesized into a dichotomous flow chart which poses questions about current site conditions and the potential for proposed activities to change site conditions that may facilitate invasion. This simple framework could help managers assess susceptibility and take preventative measures in coastal marshes before invasion occurs or before removal becomes very expensive.

EFFECTS OF STRESSORS ON INVASIVE AND HALOPHYTIC PLANTS OF NEW ENGLAND SALT MARSHES: A FRAMEWORK FOR PREDICTING RESPONSE TO TIDAL RESTORATION

Salt marsh restoration practices based on the reintroduction of tides to hydrologically-altered wetlands may be hindered by a lack of specific knowledge regarding plant community response to environmental change. Since saltmarsh plant communities are controlled by physical stress tolerance and competition, we conducted a field experiment that measured effects of saltwater flooding and competitive interactions on plants as a guide for predicting habitat response to tidal restoration. Six plant species of New England salt marshes were studied: halophytes Spartina alterniflora, Spartina patens, and Juncus gerardii and brackish invasive species Phragmites australis, Typha angustifolia, and Lythrum salicaria. Plant shoots were transplanted across a gradient of three flooding and three salinity regimes and arranged into pair-wise competitive combinations. After one growing season, saltwater flooding was found to decrease transplant survival, biomass production, and/or relative growth for all species. Reduction in halophyte growth was largely due to increased flood duration; brackish species were most reduced by increased salinity. Interspecific competition also influenced species growth, although the short duration of the study may have weakened these effects. Transplants paired with S. alterniflora had reduced growth, but combinations with Juncus produced increased growth. Standardized factors of stress tolerance and relative competitive strength were derived for the six study species as a framework for understanding community-level changes in marshes. As an aid to resource managers, experimental results can be used to predict plant community response to existing and potential alterations in saltmarsh tidal hydrology.