Cathleen Wigand

Effects of different submersed macrophytes on sediment biogeochemistry

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.

Sediment chemistry associated with native and non-native emergent macrophytes of a Hudson River marsh ecosystem

In tidal freshwater marshes of the Hudson River, coverage byPhragmites australis andLythrum salicaria has increased greatly over the past twenty years, althoughTypha angustifolia is still the predominant vegetation. Prior to any attempts at marsh restoration via removal of exotic/invasive plant species, we wanted to describe the current relationship between these plants and sediment nutrient pools. Extant stands (n=3 of each) ofT. angustifolia, L. salicaria, andP. australis were sampled with porewater equilibrators in the spring and summer of 1995 and summer 1996 to measure porewater ammonium, nitrate, and phosphate. Porewater pools of phosphate were significantly lower (p<0.05) in stands ofL. salicaria in summer, with concentrations only half those measured in stands ofP. australis andT. angustifolia. Porewater ammonium did not differ among plant communities, and nitrate was undetectable in sediments associated with all three communities. Sequestration of nutrients in above-ground biomass differed significantly among plant species, indicating differential demand on sediment nutrient pools. There were significant decreases in porewater ammonium from spring to summer. Growing season estimates of nitrogen incorporation into above-ground plant tissue are more than adequate to explain the removal of ammonium from porewater for all plant communities. Similarly, plant uptake of porewater phosphate was several times greater than springtime standing stocks of dissolved inorganic P. Concentrations of porewater phosphate remained high in the summer, indicating rapid replenishment from other sediment phosphorus pools. Depletion of porewater ammonium in the summer and low N:P in plant tissues suggest N limitation of these marsh plants. Our data suggest that marsh management practices intended to shift the relative vegetation coverage towards native and non-invasive species should consider the subtle but ecologically significant effects on nutrient cycling.

Vascular Plants as Engineers of Oxygen in Aquatic Systems

Abstract The impact of organisms on oxygen is one of the most dramatic examples of ecosystem engineering on Earth. In aquatic systems, which have much lower oxygen concentrations than the atmosphere, vascular aquatic plants can affect oxygen concentrations significantly not only on long time scales but also on time scales of less than a day. Aquatic plants are generally thought of as adding oxygen to aquatic systems through photosynthesis, but the impact of vascular aquatic plants on oxygen varies greatly with plant morphology. Floating-leaved plants that vent oxygen to the atmosphere can strongly deplete oxygen. In some ecosystems where floating-leaved plants have replaced submersed vegetation, oxygen concentrations have been substantially reduced. These oxygen changes can have cascading impacts on nutrient and trace gas chemistry and on the suitability of plant beds as habitat for invertebrates and fishes.

Relationships of nitrogen loadings, residential development, and physical characteristics with plant structure in New England salt marshes

We examined the vascular plant species richness and the extent, density, and height ofSpartina species of ten Narragansett Bay, Rhode Island (United States) fringe salt marshes which had a wide range of residential land development and N-loadings associated with their watersheds. Significant inverse relationships of tallS. alterniflora with species richness and with the extent and density ofS. patens and shortS. alterniflora were observed. Extent and density ofS. patens and extent of shortS. alterniflora were positively and significantly related with plant species richness. Marsh elevation and area did not significantly correlate with plant structure. Flood tide height significantly and inversely correlated withS. patens, but did not significantly relate toS. alterniflora or plant species richness. Marsh width significantly and positively correlated with plant species richness andS. patens and inversely correlated with tallS. alterniflora. Significant inverse relationships were observed for N-load, % residential development, and slope withS. patens, shortS. alterniflora, and species richness, and significant positive relationships with tallS. alterniflora. The marsh slope and width were significantly correlated with N-load and residential development that made it difficult to determine to what extent anthropogenic stressors were contributing to the variation in the plant structure among the marshes. At five marhes with similar slopes, there were significant inverse relationships of N-load withS. patens (density and extent) and a positive relationship with tallS. alterniflora (extent). Although there were no significant relationships of slope with the plant metrics among the five sites, other physical factors, such as the flood tide height and marsh width, significantly correlated with the extent and density ofSpartina species. Significant relationships of N-load with plant structure (albeit confounded by the effect of the physical characteristics) support the hypothesis of competitive displacement of dominant marsh plants under elevated nitrogen. It is likely that the varying plant structure in New England marshes is a response to a combination of natural factors and multiple anthropogenic stressors (e.g., eutrophication and sea level rise).

RIBBED MUSSEL NITROGEN ISOTOPE SIGNATURES REFLECT NITROGEN SOURCES IN COASTAL SALT MARSHES

The stable nitrogen isotope ratio in tissue of the ribbed mussel (Geukensia demissa) was investigated as an indicator of the source of nitrogen inputs to coastal salt marshes. Initially, mussels were fed a diet of 15N-enriched algae in the laboratory to determine how the tissue nitrogen isotope ratio (δ15N) changed with time. Steady-state times were calculated and found to be size dependent, ranging from 206 to 397 d. This indicated that mussels are long-term integrators of δ15N from their diet and may reflect nitrogen inputs to a marsh. Next, indigenous mussels were collected from 10 marshes with similar hydrology and geomorphology in Narragansett Bay, Rhode Island, USA, and mussel δ15N values were evaluated as indicators of nitrogen source. Significant positive correlations were observed between δ15N in mussels and the fraction of residential development in the marsh watersheds. In contrast, mussel isotope ratios showed significant negative correlations with the fraction of combined agricultural and recreational land use. These correlations suggested that the mussel nitrogen isotope signature is influenced by nitrogen derived from human activities in the adjoining marsh watershed. A more detailed examination of these relationships indicated that land use practices in close proximity to marshes and estuarine characteristics may also influence the observed nitrogen isotope signature. A simple, empirical model based on the 10 watersheds was developed to predict mussel δ15N from land use characteristics. The predictive ability of the model was tested with data from 12 additional marshes having similar geomorphology as the original 10, but differing in hydrology and mode of nutrient input. The model showed that ribbed mussel nitrogen isotope signatures may provide information on the source of nitrogen to coastal areas. This could be of use in developing general policies or strategies for monitoring and assessing coastal eutrophication. In addition, the isotopic ratio of mussels is useful as a proxy for watershed land use practices when assessing ecological responses to nutrient enrichment in coastal marshes.

Below the disappearing marshes of an urban estuary: historic nitrogen trends and soil structure

Marshes in the urban Jamaica Bay Estuary, New York, USA are disappearing at an average rate of 13 ha/yr, and multiple stressors (e.g., wastewater inputs, dredging activities, groundwater removal, and global warming) may be contributing to marsh losses. Among these stressors, wastewater nutrients are suspected to be an important contributing cause of marsh deterioration. We used census data, radiometric dating, stable nitrogen isotopes, and soil surveys to examine the temporal relationships between human population growth and soil nitrogen; and we evaluated soil structure with computer-aided tomography, surface elevation and sediment accretion trends, carbon dioxide emissions, and soil shear strength to examine differences among disappearing (Black Bank and Big Egg) and stable marshes (JoCo). Radiometric dating and nitrogen isotope analyses suggested a rapid increase in human wastewater nutrients beginning in the late 1840s, and a tapering off beginning in the 1930s when wastewater treatment plants (WWTPs) were first installed. Current WWTPs nutrient loads to Jamaica Bay are approximately 13 995 kg N/d and 2767 kg P/d. At Black Bank, the biomass and abundance of roots and rhizomes and percentage of organic matter on soil were significantly lower, rhizomes larger in diameter, carbon dioxide emission rates and peat particle density significantly greater, and soil strength significantly lower compared to the stable JoCo Marsh, suggesting Black Bank has elevated decomposition rates, more decomposed peat, and highly waterlogged peat. Despite these differences, the rates of accretion and surface elevation change were similar for both marshes, and the rates of elevation change approximated the long-term relative rate of sea level rise estimated from tide gauge data at nearby Sandy Hook, New Jersey. We hypothesize that Black Bank marsh kept pace with sea level rise by the accretion of material on the marsh surface, and the maintenance of soil volume through production of larger diameter rhizomes and swelling (dilation) of waterlogged peat. JoCo Marsh kept pace with sea-level rise through surface accretion and soil organic matter accumulation. Understanding the effects of multiple stressors, including nutrient enrichment, on soil structure, organic matter accumulation, and elevation change will better inform management decisions aimed at maintaining and restoring coastal marshes.

Soil respiration rates in coastal marshes subject to increasing watershed nitrogen loads in southern New England, USA.

Mean soil respiration rates (carbon dioxide efflux from bare soils) among salt marshes in Narragansett Bay, RI ranged from 1.7–7.8 μmol m−2 s−1 inSpartina patens in high marsh zones and 1.7–6.0 μmol m−2 s−1 inS. alterniflora in low marsh zones. The soil respiration rates significantly increased along a gradient of increasing watershed nitrogen (N) loads (S. alterniflora, R2 = 0.95, P = 0.0008;S. patens, R2 = 0.70, P = 0.02). As the soil respiration increased, the percent carbon (C) and N in the soil surface layer decreased in theS. alterniflora, suggesting that in part, the increased soil respiration rates are contributing to the increased turnover of labile organic matter. In contrast, there were no apparent relationships between the soil respiration rates in the high marsh and the soil C and N contents of the surface layer. However, there was a broad-scale pattern and significant inverse relationship between the high marsh soil respiration rates and the landscape belowground biomass ofS. patens. As more and more salt marsh systems are subjected to increasing nutrient loads, decomposition rates of soil organic matter may increase in marsh soils leading to higher turnover rates of C and N.

Outline of A New Approach to Evaluate Ecological Integrity of Salt Marshes

The integrity of coastal salt marshes can be determined from the extent to which they provide key ecosystem services: food and habitat for fish and wildlife, good water quality, erosion and flood control, and recreation and cultural use. An outline of a new approach for linking ecosystem services with metrics of structure and function to evaluate the ecological integrity of salt marshes is described. One main objective of the approach is to determine whether differences in structure and function can be detected among salt marshes with similar geomorphology and hydrology but different degrees of anthropogenic stress. The approach is currently being applied to salt marshes of Narragansett Bay, RI, USA. Stable nitrogen isotopic ratios of the marsh biota reflected the nitrogen sources from the adjacent watersheds and were significantly correlated with percent residential land use. Results show that plant zonation significantly ( r = –0.82; p < 0.05) relates with percent residential land use and is potentially a sensitive indicator of anthropogenic disturbance of New England salt marshes. We are currently examining species diversity, denitrification rates, and susceptibility to erosion among the sites for additional indicators of salt marsh condition. Our results to date suggest that this approach will provide the methods needed for managers to systematically monitor and evaluate the integrity of salt marshes.

Contributions of Organic and Inorganic Matter to Sediment Volume and Accretion in Tidal Wetlands at Steady State

Abstract A mixing model derived from first principles describes the bulk density (BD) of intertidal wetland sediments as a function of loss on ignition (LOI). The model assumes that the bulk volume of sediment equates to the sum of self‐packing volumes of organic and mineral components or BD = 1/[LOI/k1 + (1‐LOI)/k2], where k1 and k2 are the self‐packing densities of the pure organic and inorganic components, respectively. The model explained 78% of the variability in total BD when fitted to 5075 measurements drawn from 33 wetlands distributed around the conterminous United States. The values of k1 and k2 were estimated to be 0.085 ± 0.0007 g cm−3 and 1.99 ± 0.028 g cm−3, respectively. Based on the fitted organic density (k1) and constrained by primary production, the model suggests that the maximum steady state accretion arising from the sequestration of refractory organic matter is ≤ 0.3 cm yr−1. Thus, tidal peatlands are unlikely to indefinitely survive a higher rate of sea‐level rise in the absence of a significant source of mineral sediment. Application of k2 to a mineral sediment load typical of East and eastern Gulf Coast estuaries gives a vertical accretion rate from inorganic sediment of 0.2 cm yr−1. Total steady state accretion is the sum of the parts and therefore should not be greater than 0.5 cm yr−1 under the assumptions of the model. Accretion rates could deviate from this value depending on variation in plant productivity, root:shoot ratio, suspended sediment concentration, sediment‐capture efficiency, and episodic events.

A RAPID, NON-DESTRUCTIVE METHOD FOR ESTIMATING ABOVEGROUND BIOMASS OF SALT MARSH GRASSES

Understanding the primary productivity of salt marshes requires accurate estimates of biomass. Unfortunately, these estimates vary enough within and among salt marshes to require large numbers of replicates if the averages are to be statistically meaningful. Large numbers of replicates are rarely taken, however, because they involve too much labor. Here, we present data on a fast, non-destructive method for measuring aboveground biomass of Spartina alterniflora and Phragmites australis that uses only the average height of the five tallest shoots and the total density of shoots over 10 cm tall. Collecting the data takes only a few minutes per replicate, and calculated values for biomass compare favorably with destructive measurements on harvested samples.