Search

Dredged materials from navigation channel maintenance represent a potentially valuable resource for wetland creation and restoration. In the northern Chesapeake Bay, fine-grained sediments from Baltimore Harbor approach channels are transported by barge southward for creation of wetlands on the site of an eroded island. High concentrations of ammonium, soluble reactive phosphorus, dissolved iron, and iron sulfide minerals in channel deposits are altered by the transport and drying of these materials prior to wetland development. The oxidation of iron sulfide minerals results in low pH, with the initiation of tidal inundation removing sulfuric acid from near-surface soil horizons and moderating the soil pH. Despite the loss of ammonium during dewatering and soil processing, the resultant soils retained high concentrations of dissolved and adsorbed ammonium. Iron-associated inorganic phosphorus represented a large pool of potentially labile phosphorus and along with the high ammonium, resulted in high nutrient concentrations for plant growth. Combined with results on plant growth presented elsewhere, these data suggest that fine-grained dredged materials from non-contaminated environments are well suited for the creation of tidal wetlands after placement and seasoning 1–2 years.

Tidal marsh restoration using dredged material is being undertaken in many coastal areas to replace lost habitat and ecosystem services due to tidal marsh loss. The fate of high levels of nitrogen (N) in fine-grained dredged material used as a substrate for marsh restoration is uncertain, but if exported tidally may cause subtidal habitat degradation. In this study, a mass balance was developed to characterize N fluxes in a two-year-old restored tidal marsh constructed with fine-grained dredged material at Poplar Island, MD, in Chesapeake Bay, and to evaluate the potential impact on the adjacent submersed aquatic vegetation (SAV) habitat. Denitrification and N accumulation in Spartina organic matter were identified as the major sinks (21.31 and 28.5 mg N m−2 d−1, respectively), while tidal export of TN was more modest (9.4 mg N m−2 d−1) and inorganic N export was low (1.59 mg N m−2 d−1). Internal cycling helped retain N within the marsh. Mineralization of N associated with labile organic matter in the dredged material was likely a large, but unquantified, source of N supporting robust plant growth and N exports. Exceedances of SAV water quality habitat requirements in the subtidal region adjacent to the marsh were driven by elevated Chesapeake Bay concentrations rather than enrichment by the marsh.

Introduction: The Chesapeake Bay was once renowned for expansive meadows of submerged aquatic vegetation (SAV). However, only 10% of the original meadows survive. Future restoration efforts will be complicated by accelerating climate change, including physiological stressors such as a predicted mean temperature increase of 2–6°C and a 50–160% increase in CO 2 concentrations. Outcomes: As the Chesapeake Bay begins to exhibit characteristics of a subtropical estuary, summer heat waves will become more frequent and severe. Warming alone would eventually eliminate eelgrass ( Zostera marina) from the region. It will favor native heat-tolerant species such as widgeon grass ( Ruppia maritima ) while facilitating colonization by non-native seagrasses (e.g., Halodule spp.). Intensifying human activity will also fuel coastal zone acidification and the resulting high CO 2 /low pH conditions may benefit SAV via a “CO 2 fertilization effect.” Discussion: Acidification is known to offset the effects of thermal stress and may have similar effects in estuaries, assuming water clarity is sufficient to support CO 2 -stimulated photosynthesis and plants are not overgrown by epiphytes. However, coastal zone acidification is variable, driven mostly by local biological processes that may or may not always counterbalance the effects of regional warming. This precarious equipoise between two forces – thermal stress and acidification – will be critically important because it may ultimately determine the fate of cool-water plants such as Zostera marina in the Chesapeake Bay. Conclusion: The combined impacts of warming, coastal zone acidification, water clarity, and overgrowth of competing algae will determine the fate of SAV communities in rapidly changing temperate estuaries.

Hyperspectral and sophisticated multispectral imagery are increasingly used to obtain detailed information on species composition, condition, biomass, and other characteristics of coastal marshes. However, how differing levels of tidal inundation affect the reflectance characteristics of emergent marsh vegetation remains not well documented. In 1994–1996, general reflectance spectra were collected for three common brackish marsh species with different canopy architectures (Schoenoplectus americanus, Spartina patens, and Spartina cynosuroides) in field experiments in which changing levels of tidal inundation were simulated in specially nonreflected enclosures. Reprocessing the data with software not then available has allowed greater delineation of variations in the data, especially in the red–near-infrared boundary, that previously could not be differentiated. Spectra for all three species showed significant reductions in near-infrared reflectance (900–1100 nm) with progressive substrate inundati...

Differing responses to abiotic stresses and increased nutrient availability may play a role in the invasion and spread of introduced Phragmites australis Cav. (Trin.) ex. Steud. and the decline of native P.a. americanus Saltonstall, P.M. Peterson & Soreng in North America. We present results from an outdoor experiment where native and introduced P. australis seedlings were grown under two nutrient treatments. Both subspecies responded positively to increased nutrients but introduced plants clearly outperformed natives, growing taller, producing more stems, and had three to four times higher biomass. The biomass of introduced P. australis growing in low nutrients was similar to that of the native in high nutrients. Aboveground:belowground biomass ratios were nearly 1.25 for both native and introduced plants across treatments and reflect the high investment P. australis seedlings place on shoot production in their first year of growth. Our results also demonstrate that introduced P. australis can have explosive growth over a single growing season, even when established from seed. This implies that management of young, newly established populations may be prudent where introduced P. australis is considered undesirable, irregardless of whether eutrophication is an issue.

We constructed a numerical simulation model of an enclosed, submersed macrophyte ecosystem to examine the effects of nutrient supply rates, grazer densities and initial conditions on macrophytes and epiphytic algae. The model included an internal, non-structural plant nutrient pool that controlled root and leaf nutrient uptake and was calibrated to literature values and mesocosm experiments. Simulations were run to examine how initial conditions, nutrient supply rates and the presence of grazers may affect macrophyte–algal competition experiments. Simulations revealed that the outcome of an experiment could be largely controlled by the initial conditions or by biomass changes during the experiment. For example, high initial macrophyte biomass reduced light and nutrients available for algae, which prevented the overgrowth of algae, even at high nutrient addition rates. Epiphytic algae biomass increased with water exchange rate regardless of inflow nutrient concentration. Submersed plants grew best at lower exchange rates that allowed nutrient concentrations to be drawn down, slowing algal growth. Simulations showed that the effect of grazers on epiphyte biomass was greatest at intermediate nutrient addition rates. When grazers were absent, macrophyte biomass could be highest with low or high inflow nutrient concentrations, depending on the exchange rate. Model analysis also revealed that it is essential to consider nutrient loading rate per unit macrophyte biomass, not just nutrient concentration, when quantifying the effects of eutrophication on submersed macrophytes in shallow water. These results show the utility of using a simulation model together with ecosystem experiments. They also show how water exchange rates, which can depend on bed size, could affect eutrophication responses of submersed macrophyte ecosystems.

We developed an algorithm for calculating habitat suitability for seagrasses and related submerged aquatic vegetation (SAV) at coastal sites where monitoring data are available for five water quality variables that govern light availability at the leaf surface. We developed independent estimates of the minimum light required for SAV survival both as a percentage of surface light passing though the water column to the depth of SAV growth (PLW min) and as a percentage of light reaching reaching leaves through the epiphyte layer (PLL min). Value were computed by applying, as inputs to this algorithm, statistically dervived values for water quality variables that correspond to thresholds for SAV presence in Chesapeake Bay. These estimates ofPLW min andPLL min compared well with the values established from a literature review. Calcultations account for tidal range, and total light attenuation is partitioned into water column and epiphyte contributions. Water column attenuation is further partitioned into effects of chlorophylla (chla), total suspended solids (TSS) and other substances. We used this algorithm to predict potential SAV presence throughout the Bay where calculated light available at plant leaves exceededPLL min. Predictions closely matched results of aerial photographic monitoring surveys of SAV distribution. Correspondence between predictions and observations was particularly strong in the mesohaline and polythaline regions, which contain 75–80% of all potential SAV sites in this estuary. The method also allows for independent assessment of effects of physical and chemical factors other than light in limiting SAV growth and survival. Although this algorithm was developed with data from Chesapeake Bay, its general structure allows it to be calibrated and used as a quantitative tool for applying water quality data to define suitability of specific sites as habitats for SAV survival in diverse coastal environments worldwide.

Photosynthetically available radiation (PAR; 400–700 nm, E m−2 d−1) is the fraction of the total solar energy (Mjoules m−2 d−1) that is used by organisms for photosynthesis and vision. We present a statistical summary of a 17-yr time series of PAR data (1982–1998) collected near Chesapeake Bay as well as a second set of data on PAR and total solar energy gathered over a shorter time span (1997–1998). The time series data (5,126 daily totals) varied between 1–67 E m−2 d−1 and were used to estimate the minimum and maximum values of PAR as a function of day of the year. In monthly frequency distributions of the PAR data, three modes were observed corresponding to sunny, partly cloudy, and overcast days. The second set of PAR and total solar energy data were used to examine the ratio of PAR to total solar energy, which was 2.04 E Mjoule−1 for PAR between 10 and 70 E m−2 d−1. On overcast days, the ratio increased to as high as 3 E Mjoule−1 as PAR increased in importance as a fraction of the total solar energy. These values were consistent with others in the literature, and the relationships reported here can be used to predict the climatology of PAR and total solar energy within the Chesapeake region. The PAR data were also combined with reported minimum values of PAR for net primary production in the surface mixed layer of the water column of aquatic systems to estimate the combinations of mixed layer depth and diffuse attenuation coefficient (number of optical depths) under which light limitation of phytoplankton primary production is expected to occur.

Negative connotations of invasive plants worldwide have implicated them as the bearers of unfavorable ecosystem change. We contrasted 5-yr-old and 20-yr-oldPhragmites populations with pre-invasion areas occupied byTypha spp. andPanicum virgatum in an oligohaline tidal marsh of Chesapeake Bay. Peak live biomass was 3 times greater, while standing dead and litter was twice as great in the 20-yr-oldPhragmites. It is this abundance of concentrated litter on the marsh surface of maturePhragmites populations that we implicate as encouraging the trapping of organic and mineral matter. The rate of vertical accretion in 20-yr-oldPhragmites populations is 3–4 mm yr−1 above the adjacent populations. By integrating the constant initial concentration and constant rate of supply models on individual210Pb cores, we estimate thatPhragmites populations require a minimum of 7-yr post-colonization to enhance rates of accretion in this system. In ligh of the considerable loss of marsh habitat from relative sea-level rise, this finding contests the view that invasion creates strictly undesirable change at the ecosystem level.

The common reed, Phragmites australis, has spread throughout Gulf and Atlantic Coast marshes of the U.S. in the past thirty years. In the Chesapeake Bay area, natural resource managers are uncertain as to the current distribution or recent colonization rate of this low-salinity marsh grass. A geographic information system (GIS) was used to determine the distribution and expansion rate of P. australis within four brackish and three tidal freshwater marshes in the upper Chesapeake Bay region. Vegetation patterns were mapped by interpreting aerial photographs from the 1930s, 1970s, 1980s, and 1990s. The aerial extent of P. australis stands was measured by digitizing vegetation boundaries, correcting for distortion, and analyzing the data using a GIS. A geometic growth formula documented an intrinsic rate of increase for each P. australis stand. In addition, aboveground biomass was sampled from monotypic stands of P. australis. Results show that P. australis is present in all seven marshes; however, it is most pervasive in the three tidal freshwater marshes. Based on interpretations of photographs, P. australis was present prior to 1938 in these three marshes. In each successive time period, there was a net increase of P. australis in all seven marshes that were examined. In the past ten years, however, the rate of increase has declined or stabilized in each marsh in which P. australis was well-established prior to 1985. This slowdown is especially prevalent in the three tidal freshwater marshes. In contrast, the three brackish marshes most recently colonized by P. australis showed high annual intrinsic rates of increase (0.06 to 0.19 yr−1 or more). Also, the highest biomass estimates were found in two of these three marshes.

Major processes controlling sedimentological characteristics and accretionary rates and patterns were evaluated in two of the most common tidal marsh settings (estuarine tributaries and estuarine embayments) in Chesapeake Bay, one of the largest microtidal estuaries in the world. Textural analyses of 29 vibracores show spatial and temporal trends in grain size and organic matter which reflect differences in depositional settings within the marshes and changes in the controlling processes, as well as broader events in the surrounding watersheds. Spatially, the surficial sediments normally follow expected patterns, with coarsest sediments and lowest organic matter contents found in bay margin or channel margin locations, while the finest, organic-rich sediments occur in interior and submerged upland marshes. However, temporally, the marshes do not always show the expected increasing dominance of organic input over time (indicated by increasing % loss on ignition with decreasing depth), indicative of the marsh building process. Sea-level rise, tidal channel migrations, and anthropogenic effects likely contribute to decreases in organic matter contents in the upper sediment column. Furthermore, accretion rates and stratigraphic characteristics can be affected by these same processes, illustrating the importance of understanding temporal changes in marsh depositional environments over various time scales before sedimentologic and accretionary patterns are evaluated. Examination of the vibracores reveals that the marsh is composed of a complex stratigraphy with at least four major stratigraphic sequences occurring: (1) emerging or developing marsh sequences characteristic of channel margin and interior marshes; (2) submerging or mineral matter enriched marshes, also characteristic of channel and interior marsh areas; (3) high-energy marsh sequences characteristic of bay margin environments; and (4) submerged upland marsh sequences. The sedimentological properties of the stratigraphic sequences are controlled by overwash processes, tidal channel migrations, duration of tidal flooding, various anthropogenic effects, and sea-level rise. Accretion rates and patterns over the last ∼200 years, determined from pollen histories, differ between the marshes along the estuarine tributary (Nanticoke River) and those found in the tidal embayment (Monie Bay). Marshes in the upper-estuarine tributary have a larger riverine sediment input, and trapping of the high sediment loads in the upper estuary often yields marsh accretion rates (maximum of 0.74 cm/yr) which are greater than relative sea-level rise (∼0.4 cm/yr), resulting in stable marshes. By comparison, estuarine processes limit penetration of high sediment loads to the lower reaches of marshes along the tributary and, as a consequence, accretion rates are generally lower than the rate of relative sea-level rise, and marsh loss is prevalent. Overall, accretion rates in the marshes in the embayment are close to or less than the local sea-level rise and do not have as distinctive spatial patterns as the tributary marshes. Although marsh loss has yet to be identified as a significant process here, decreases in organic content of the upper sediment column may signal the early stages of submergence due to sea-level rise.

Porewater phosphate levels in submersed macrophyte grassbeds varied among years in the upper Chesapeake Bay (Maryland, USA) coincident with macrophyte species variation during these same years (1990, 1993, 1995). When native, deep-rooted Vallisneria americana Michx. was a codominant in the grassbed, the porewater phosphate concentrations were significantly lower (P < 0.001) than concentrations when the exotic, shallow-rooted species Hydrilla verticillata (L.f.) Royle and Myriophyllum spicatum L. were codominants. There were significant relationships (P < 0.001) between solid-phase inorganic phosphorus and reactive metals (Fe, Mn) in both native and exotic grassbeds. However, the slopes of the regression relationships between years were significantly different (P < 0.001), suggesting greater retention of inorganic phosphorus in sediments when V. americana was a codominant at the site. In addition, significant relationships between reactive manganese and iron in the sediments were observed, but the coefficient of determination was statistically greater (P < 0.001) when V. americana was a codominant at the site. Furthermore, plant cores of V. americana and H. verticillata had noticeably different sediment redox profiles, with the oxidation-reduction status of V. americana sediments being more oxidized in the root zone (i.e., +125 mV vs −5 mV at 4 cm depth). These data suggest that macrophyte species composition can alter sediment biogeochemistry resulting in varying porewater phosphate and solid-phase phosphorus and metal levels. Possible explanations for these biogeochemical differences may be attributed to morphological differences among macrophyte species (i.e., root/shoot ratio, canopy type, growth form) and differences in root oxygenation capabilities.

The introduced submersed aquatic plant, Hydrilla verticillata, is currently spreading in the Chesapeake Bay, and is colonizing previously unvegetated areas and displacing native flora. To examine the effects of this invasion, we compared faunal densities between Hydrilla beds and unvegetated sand areas. Transplants of Hydrilla and sampling from natural Hydrilla and unvegetated patches indicate that this plant enhances the abundances of a variety of benthic fauna. Among two clam species transplanted into Hydrilla patches and adjacent unvegetated areas, one demonstrated enhanced survival within Hydrilla patches while the other exhibited reduced tissue growth. Similarly, the growth form of Hydrilla may have been affected by clam presence. Dissolved oxygen, sediment characteristics, and sediment resuspension also varied between Hydrilla and unvegetated areas. The spread of Hydrilla in the Chesapeake Bay clearly has significant, though complex, effects on the bottom community. Such effects emphasize the potential importance of introduced plants in altering community characteristics.

Estuaries throughout the world are experiencing water quality problems as the result of human population growth in coastal areas. By establishing the habitat requirements of critical submerged aquatic vegetation, water quality can be evaluated and restoration goals can be made. This study used submerged vegetation in Chesapeake Bay to examine the habitat and health of the Bay. Both natural distributions and transplant survival in different studies were analyzed. The five habitat requirements used were light attenuation, total suspended solids, chlorophyll, dissolved inorganic nitrogen, and dissolved inorganic phosphorus. Water-quality conditions supporting vegetation growth to one meter depth was used. This study represents the first attempt at linking habitat requirements of a living resource to water quality standards in an estuarine system. It allows for predictive capability without detailed knowledge of the precise nature of vegetation/water quality interactions.

Presence of vesicular-arbuscular mycorrhizal fungi was found toenhance phosphate uptake in the submersed plant Vallisneriaamericana compared with plants treated with a fungicidal medium(i.e., Captan). Incorporation of 33P-orthophosphate into roottissue in short-term incubations was over 85% greater for plantswith active mycorrhizae. In addition, we measured a fine-scale irongradient and elevated concentrations of solid-phase phosphate in theextensive sheath surrounding the roots. The coupling of fungalsymbionts with phosphorus storage in the sheath may be an importantmechanism of phosphate assimilation in submersed aquatic macrophytes.Contrary to the effect on phosphate uptake, we did not find that15NH4 assimilation by Vallisneria americanaroots wasenhanced by the presence of the mycorrhizal association.

The Multiscale Experimental Ecosystem Research Center has conducted a series of mesocosm experiments to quantify the effects of scale—in terms of time, depth, radius, exchange rate, and ecological complexity—on biogeochemical processes and trophic dynamics in a variety of coastal habitats. The results indicate that scale effects can be categorized as (a) fundamental effects, which are evident in both natural and experimental ecosystems, and (b) artifacts of enclosure, which are solely attributable to the artificial environment in mesocosms. We conclude that multiscale experiments increase researchers' understanding of scale in nature and improve their ability to design scale-sensitive experiments, the results of which can be systematically compared with each other and extrapolated to nature.

Dramatic losses of tidal wetlands in the Mississippi Delta and a few areas along the U.S. Atlantic coast have raised concerns about whether these marshes will survive if global sea level continues to rise due to greenhouse warming [Stevenson et al., 1986]. Original greenhouse warming sea-level scenarios projected global sea levels several meters or more higher than present by 2100 [Barth and Titus, 1984], which would result in the disappearance of all coastal marshes, as the scarcity of marsh deposits from the rapid transgression during the middle Holocene testifies [Rampino and Sanders, 1981]. However, more recent estimates of global sealevel change suggest that some coastal marshes could survive [Douglas et al., 2000].

Degraded water quality due to water column availability of nitrogen and phosphorus to algal species has been identified as the primary cause of the decline of submersed aquatic vegetation in Chesapeake Bay and its subestuaries. Determining the relative impacts of various nutrient delivery pathways on estuarine water quality is critical for developing effective strategies for reducing anthropogenic nutrient inputs to estuarine waters. This study investigated temporal and spatial patterns of nutrient inputs along an 80-km transect in the Choptank River, a coastal plain tributary and subestuary of Chesapeake Bay, from 1986 through 1991. The study period encompassed a wide range in freshwater discharge conditions that resulted in major changes in estuarine water quality. Watershed nitrogen loads to the Choptank River estuary are dominated by diffuse-source inputs, and are highly correlated to freshwater discharge volume. in years of below-average freshwater discharge, reduced nitrogen availability results in improved water quality throughout most of the Choptank River. Diffuse-source inputs are highly enriched in nitrogen relative to phosphorus, but point-source inputs of phosphorus from sewage treatment plants in the upper estuary reduce this imbalance, particularly during summer periods of low freshwater discharge. Diffuse-source nitrogen inputs result primarily from the discharge of groundwater contaminated by nitrate. Contamination is attributable to agricultural practices in the drainage basin where agricultural land use predominates. Groundwater discharge provides base flow to perennial streams in the upper regions of the watershed and seeps directly into tidal waters. Diffuse-source phosphorus inputs are highly episodic, occurring primarily via overland flow during storm events. Major reductions in diffuse-source nitrogen inputs under current landuse conditions will require modification of agricultural practices in the drainage basin to reduce entry rates of nitrate into shallow groundwater. Rates of subsurface nitrate delivery to tidal waters are generally lower from poorly-drained versus well-drained regions of the watershed, suggesting greater potential reductions of diffuse-source nitrogen loads per unit effort in the well-drained region of the watershed. Reductions in diffuse-source phosphorus loads will require long-term management of phosphorus levels in upper soil horizons. *** DIRECT SUPPORT *** A01BY074 00021

Atypical fungal vesicles and arbuscules were found within the roots of the submersed macrophyteVallisneria americana collected at the tidal fresh headwaters of the Chesapeake Bay (Susquehanna flats) in July 1991 and 1992, suggesting the presence of a myocrrhizal association. In order to determine whether the presence of the fungus facilitates phosphorus uptake and plant growth,V. americana cores were placed in separate pots in an aquatic greenhouse and were given one of the following treatments: control, fungicide (Captan) application, or fungicide plus phosphate enrichment. Fungicide addition resulted in significantly decreased shoot elongation rates and chlorophylla production; phosphate enrichment plus fungicide restored plant growth to control levels. Low nitrogen in plant tissues of fungicide treatment groups suggests nitrogen uptake may also be promoted by the fungal association. A second laboratory experiment withV. americana grown from turions demonstrated the negative effects of the fungicide are only evident on plant growth when fungal infection is present, indicating the fungicide was not directly toxic to the macrophyte, but acted by disrupting a mycorrhizal relationship. This study supports the hypothesis that mycorrhizae are important in nutrient acquisition and growth ofVallisneria in an estuarine environment.

The decline of submersed aquatic vegetation (SAV) in tributaries of the Chesapeake Bay has been associated with increasing anthropogenic inputs, and restoration of the bay remains a major goal of the present multi-state “Bay Cleanup” effort. In order to determine SAV response to water quality, we quantified the water column parameters associated with success of transplants and natural regrowth over a three-year period along an estuarine gradient in the Choptank River, a major tributary on the eastern shore of Chesapeake Bay. The improvement in water quality due to low precipitation and low nonpoint source loadings during 1985–1988 provided a natural experiment in which SAV was able to persist upstream where it had not been for almost a decade. Mean water quality parameters were examined during the growing season (May–October) at 14 sites spanning the estuarine gradient and arrayed to show correspondence with the occurrence of SAV. Regrowth of SAV in the Choptank is associated with mean dissolved inorganic nitrogen

Although the concept that marshes act as major sediment sinks may be accurate when viewing them over the last few millennia, tidal transport studies suggest considerable variability with most marshes presently exporting material on an annual basis. High-salinity marshes along the mid-Atlantic coast of the United States appear to be losing 1–2 kg m−2 yr−1 while submerged upland marshes on the Delmarva Peninsula are eroding at rates of up to 14 kg m−2 yr−1. By comparison, at least one deltaic marsh on the Dutch coast along with several estuarine marshes appear to be accumulating sediment. In order to assess the trapping ability of marshes in large estuaries, we constructed a sediment budget for Chesapeake Bay which included a variety of wetland types. Calculations indicate that estuarine marshes trap 5–11% of the annual Chesapeake Bay sediment input, or about one half that previously estimated. It appears that most sedimentation in estuaries, and perhaps other coastal systems, occurs in subtidal flats below the limit of emergent marsh vegetation. As mud flats become vegetated and estuarine infilling proceeds, there may be a tendency for tidal currents to become ebb-dominated which promotes a net export of particulates. The extent to which major storm events are capable of returning enough material to balance the long-term accretionary budget of tidal marshes and keep them abreast of rising sea level is open to question. Differences in tidal dynamics, seasonal changes in sea levels and higher temperatures may help explain why, in the U.S., southern marshes are more susceptible to export and eventual erosion than northern marshes. We hypothesize that another factor, the recent reductions of terrigenous sediment inputs from the southern river systems of the U.S., may also be critical. Sediment starvation may have led to undernourishment of wetland systems of the coastal zone over the last half century which may be reflected in the net export measured in the tidal marshes in this region. Furthermore, we postulate that changes in sediment inputs are more important than eustatic sea level rise in causing the past losses of marshes which are now undergoing mass erosion. Thus, future wetland survival will depend as much as particulate inputs to the coastal zone as on the prospects of a global rise in sea level, and more efforts should be made to quantify the sediment budgets of tidal marshes.

Characteristics of sediment transport and deposition were studied in a Maryland brackish marsh system which has experienced extensive erosion (2300 ha) over the last four decades. Ebb-tide domination of the marsh system contributes to a net output of 720,000 MT of sediment per year (13.8 kg m−2 yr−1). This export allows very little inorganic sediment to reach the interior marsh despite the sporadic high concentrations of total suspended solids in the tidal channels (10–1400 mg l−1). Vertical accretion in these marshes is primarily by peat deposition instead of the accumulation of mineral sediment. 210Pb dating suggests that accretion rates ranging from 1.7 to 3.6 mm yr−1 in the area of active marsh loss probably have not kept pace with sea-level rise since 1940 (3.9 mm yr−1). The reliance on organic sedimentation in the past has resulted in the development of a thin (0.1–0.5 m) consolidated marsh mat over 1–4 m of unconsolidated organic ooze. Erosion typically follows a sequence of incipient deterioration of the mat, possibly associated with heavy grazing activity by muskrats, and subsequent development of interior ponds. Once formed, the ponds slowly enlarge by undifferentiated rotational slumping. When they reach about a hectare in size they rapidly elongate in a NW-SE direction by wind-generated wave undercutting of the mat during major NW-tracking storms. Several hypotheses are presented to explain fundamental reasons why this marsh type is not accreting rapidly enough to keep abreast of apparent sea-level rise.

Worldwide, there arc 500-700 species of submersed angiosperms adapted to freshwater and estuarine environments compared with 50 species adapted to marine waters. In their evolution from freshwater ancestors, seagrasses have undergone extensive anatomical changes (e.g. reduction in floral and leaf structures, reduction of xylem tissue with a lacuna1 gas transport system), as well as physiological adaptations (bicarbonate utilization in photosynthesis). Seagrasses appear to have more annual production than do their freshwater counterparts because they develop greater standing crops and have the capacity to store photosynthetic products in extensive rhizome systems in the sediments. For example, maximum productivity of > 10 g C rnd2 d-* has been reported for tropical seagrass species (Cymodocea nodosa and Thalassia testudinum), but the maximum productivity of temperate freshwater species such as Myriophyllum or tropical freshwater species such as Hydrilla is usually 50% of their diets. Also, sea urchins (Diadema antilarum) consume plant material, creating bare halos around tropical patch reefs in the Caribbean Sea. It is difficult to generalize regarding brackish submersed aquatics in estuaries because their coverage is variable due to light limitation and algal overgrowth from eutrophication. Freshwater macrophytes seem rarely grazed by fish (except via exotic introductions of Tilapia or carp), but waterfowl use is often significant at the end of the growing season. Thus, trophic relations in freshwater macrophyte beds may be qualitatively different and much more pulsed than in seagrass systems, with more r-selection in lakes and more K-selection in marine environments.

The rate of acetylene reduction was determined for Myriophyllum spicatum, Potamogeton perfoliatus, Ruppia maritima and Elodea canadensis , in a brackish water tributary of Chesapeake Bay. Ruppia had the highest nitrogen fixation rate on a dry weight basis [66 ng-at. N(g wt h) −1 ]. However the fixation per square meter of creek bottom was highest [16·3 μg-at. N(m 2 h) −1 ] for Myriophyllum due to its greater biomass. Generally, our low rates are more comparable to those associated with seagrasses in North Carolina, than to many higher rates reported from tropical waters. We postulate that low nitrogen fixation rates are a consequence of the availability of nitrogen either in the water column or sediments of temperate estuaries such as the Chesapeake Bay, and the North Carolina Sounds.

A nitric acid digestion method is presented for rapid and inexpensive preparation of 14 C-labelled macrophyte tissue for liquid scintillation counting. No significant difference was found when the method was compared to combustion in oxygen. Results from time-series experiments with labelled sucrose as well as plant materials indicated little loss of labelled organics as volatile compounds over a 16 h period, and complete recovery of the label was attained.

The rates of production, leaf grazing, and stem decomposition associated with Hibiscus moscheutos were quantified to contrast the ecology of this brackish herbaceous perennial with fresh and salt marsh species. Average net aerial primary production (NAPP) during 1978 and 1979 was estimated to be slightly over 1200 g°m—2°yr—1, which was calculated by summing: peak standing crop (49%); leaf mortality and litter production (36%); and losses to herbivores (15%). Three species of herbivorous insects were found to be important grazers: Atomacera decepta, Chionodes hibiscella, and Althaeus hibisci. Herbivore consumption was far greater than previously reported in salt marsh systems and rivals that of terrestrial communities. The root: shoot ratio (2.3) is comparable to many fresh marsh species but lower than most high—salinity species. The decomposition rate of Hibiscus stem material was estimated to be on the order of 7—8 yr, or up to five times slower than stems and leaves of other typical brackish marsh species (Typha, Scirpus, and Phragmites). The export potential of detritus from decaying stems is apparently minor because of this slow rate of decomposition combined with the diminished tidal action typical of Hibiscus—dominated marshes. We hypothesize that this unusual marsh dominant differs from Spartina in salt marsh systems by nourishing a more robust grazing food chain and by not exporting significant quantities of detrital material.

The role of physical interfaces in estuarine nitrogen (N) dynamics is discussed. We consider here: four N-transformation processes (uptake, regeneration, de nitrification, nitrification) and five interfaces (water mass fronts and transitions, watershed-estuarine boundaries, the pycnocline of stratified estuaries, the sediment-water boundary, the redox discontinuity layer). Seven examples are presented. First, we show that phytoplankton production and, in turn, NH + 4 recycling can be stimulated at interfaces where two water masses meet, with one being relatively clear and the other nutrient-rich. Second, data are provided to illustrate that N recycling rates tend to exceed (by 2–8 fold) inputs of “new” N entering across watershed-estuary boundaries, although annual net primary production is more a function of the latter. Third, we argue that periodic occurrences of high NH + 4 concentrations in strongly stratified water columns reflect active NH + 4 oxidation at the estuarine pynocline. Fourth, evidence is given to indicate that intensive remineralization of NH + 4 occurs in the uppermost flocculent layer of sediments, and that fluxes estimated from diagenic modeling would tend to overlook this. Next, we show that denitrification, which is concentrated near the sediment redox discontinuity layer (RDL), may be a major component of estuarine N budgets (50–60% of NH + 4 recycling). Sixth, we indicate that de nitrification can be fueled either by sediment nitrification just above the RDL or via NO − 3 diffusion from overlying waters. Seventh, recent experimental results are considered to demonstrate the effects of macrophytic roots enhancing nitrification (and possibly denitrification) by transporting O 2 and deepening the oxidized zone of sediments. Finally, we propose some generic properties of estuarine interfaces which may account for their importance in N cycling.

Physical, chemical and biological influences of submersed vascular plants (dominated by Potamogeton perfoliatus and Ruppia maritima) on their surrounding environment are summarized for portions of upper Chesapeake Bay. Rates of accretion of organic matter in these ecosystems were high owing to the combined effects of vascular plant and associated algal production and the trapping of particulate organics of phytoplanktonic origin. Time-series observations of seston along transects traversing vegetated bottoms indicated significantly less turbid water over the plant beds, due both to increased deposition and to decreased resuspension of fine-grain sediments. Submersed plants provided a preferred habitat for many animal populations, and abundance of fishes (predominantly juveniles) was significantly greater in these plant beds than in adjacent unveqetated areas. Recent declines in several species of migrating waterfowl which feed directly on plant material were highly correlated with contemporaneous decreases in plant distribution. Rapid uptake of dissolved inorganic nitrogen (N) and phosphorus (P) was demonstrated for these communities, with subsequent incorporation into plant material via both growth and facultative increases in percent N and P composition. Upon senescence and death, submersed vascular plants decayed at moderate rates, with relatively slow releases of nutrients and low dissolved oxygen (O2) demand compared to algae (micro and macro) and to marsh grass. Thus, organic carbon (C) from these submersed plants is transferred to microbial food-chains, with minimal secondary effects of O2 depletion and nutrient enrichment. Part of the influence of these plant communities on the upper Bay is summarized in terms of three materials budgets for 1960, where these plants contributed 33% to the organic C budget, while acting as a seasonal sink for 210 and 7% of the total sediment and nitrogen inputs (respectively) to tne estuary.

Marsh systems are often considered sites of major sediment accumulation, where vertical accretion rates are equal to or greater than sea level rise. However, the degree of apparent sea level rise (land subsidence plus change in eustatic sea level) along the coast of the United States is variable, ranging from over 10 mm y-1 decline along the coast of southeastern Alaska to almost 10 mm y-1 rise along the northeastern Maine and Louisiana coasts. This variability in sea level rise accounts for some of the discrepancy in vertical accretion rates in marshes. We reviewed 15 areas dated with Lead-210 or Cesium-137, however, and at least four are clearly not keeping pace with sea level rise. Among these are Blackwater marsh in Chesapeake Bay, and the backmarsh areas of Barataria Bay, Four-league Bay and Lake Calcasieu in Louisiana. There is surprisingly strong correlation (r = 0.83) between mean tidal range and accretionary balance for 15 marshes where we obtained data. Based on overall sedimentary processes, tidal wetlands can be classified into at least six major categories: emerging coastal, submerging coastal, estuarine, submerged upland, floating, and tidal freshwater marshes. Acceleration of present rates of sea level rise could cause major alterations in each of these six types and reduce their carbon export potential to surrounding coastal waters.

Two irregularly flooded brackish marshes of the Chesapeake Bay estuarine system were studied to detect patterns of tidal nutrient exchange and utilization. A 5.7 ha marsh on the Choptank River was monitored monthly from October 1974 to August 1975 for dissolved inorganic nitrogen and phosphorus. During these diurnal samplings, nutrient exchange with the estuary was eliminated by a tidal gate and total aquatic oxygen metabolism was measured. During winter, photosynthesis exceeded respiration, and total nitrogen increased to 80 μg-at liter−1 and declined through spring to 8 μg-at liter−1. Phosphate varied erratically from 0.8 to 4.3 μg-at liter−1. Tidal import and export of the above nutrients were also monitored monthly from January to June 1975. During winter, there appears to be a net flow of inorganic nitrogen and phosphorus to the estuary. In spring this pattern is reversed for both nutrients. Two years tidal flow data from a larger 127 ha marsh on the Patuxent River revealed that dissolved inorganic nitrogen is also taken up in May, June and July. However, when the dissolved organic fraction is also consisered, net nitrogen flow was always to the estuary in every month sampled. It is estimated that the dissolved nitrogen flux from the marsh was 4.14 g m−2 yr−1, or about 18-82% of the estimated nitrogen of the vegetation. Also, the net annual flux of dissolved phosphorus was 0.19 g m−2 yr−1 to the estuary or 8-23% of the phosphorus in standing crop. Data from both marshes suggest that, in contrast to previously studied high salinity marshes, brackish marshes act more as sources of nutrients to the estuary. However, these marshes may be trapping some nutrients, either from upland areas surrounding them, or possibly from infrequent massive river deposition.

Marsh loss in the Nanticoke estuary dates from the 1920s. Since 1938 overall rate of loss has averaged 49.6 ha annually, with rates increasing down estuary. Most losses have occurred in submerged upland marshes; tidal freshwater marshes are stable. Interior ponding has been the primary mechanism of loss. Rising water levels are postulated to be the underlying force behind marsh losses in this estuary. R ECENT predictions of accelerated rates of sea-level rise in the next century because of anthropogenic greenhouse warming of the atmosphere have renewed concern about increased coastal erosion and land loss. The immediate threat is greatest for coastal marshes, where vertical accretion rates are outstripped by local sea-level rise. The ominous consequences faced by coastal marshes from a rapidly rising sea level are best exemplified in the Mississippi delta, where local rates of submergence, largely from subsidence, are approximately 1 cm annually. Particularly evident is the rapidity of the primary mechanism of marsh loss, which consists of interior-pond formation, coalescence, and enlargement. Elsewhere along the Gulf and Atlantic coasts of the United States, documentation of rates and processes of marsh loss is more sketchy. Most of the reported losses are local, involving creek-bank erosion, formation of rotten spots, and shoreline-cliff retreat. To date, extensive marsh loss from interior ponding has been reported only for the Blackwater Wildlife Refuge on the Eastern Shore of Maryland (Stevenson, Ward, and Kearney 1986), where approximately one-half of the upland marshes have been lost since 1938. The extent of interior ponding and other mechanisms of marsh loss in other marsh types of Chesapeake Bay is unknown. The research reported in this article was undertaken to assess the broad importance of interior ponding and other processes in marsh types of the Nanticoke River, a representative marsh-dominated estuary of Chesapeake Bay. The Nanticoke River is one of the largest estuarine tributaries on the Eastern Shore. The stream meanders almost