Brian L. Howes

Transport of groundwater-borne nutrients from watersheds and their effects on coastal waters

Anthropogenic activities on coastal watersheds increase nutrient concentrations of groundwater. As groundwater travels downslope it transports these nutrients toward the adjoining coastal water. The resulting nutrient loading rates can be significant because nutrient concentrations in coastal groundwaters may be several orders of magnitude greater than those of receiving coastal waters. Groundwater-borne nutrients are most subject to active biogeochemical transformations as they course through the upper 1 m or so of bottom sediments. There conditions favor anaerobic processes such as denitrification, as well as other mechanisms that either sequester or release nutrients. The relative importance of advective vs. regenerative pathways of nutrient supply may result in widely different rates of release of nutrients from sediments. The relative activity of denitrifiers also may alter the ratio of N to P released to overlying waters, and hence affect which nutrient limits growth of producers. The consequences of nutrient (particularly nitrate) loading include somewhat elevated nutrient concentrations in the watercolumn, increased growth of macroalgae and phytoplankton, reduction of seagrass beds, and reductions of the associated fauna. The decline in animals occurs because of habitat changes and because of the increased frequency of anoxic events prompted by the characteristically high respiration rates found in enriched waters.

Denitrification in nitrate-contaminated groundwater: Occurrence in steep vertical geochemical gradients

A relatively narrow vertical zone (5–6 m thick) of NO3− containing groundwater was identified using multilevel sampling devices in a sand and gravel aquifer on Cape Cod, MA, USA. The aquifer has been chronically contaminated by surface disposal of treated sewage 0.3 km upgradient from the study area. The NO3− zone was anoxic and contained high concentrations of N2O (16.5 μM), suggesting that it was a zone of active denitrification. Denitrifying activity was confirmed with direct measurement using acetylene block incubations with aquifer core material; the peak rate was 2.4 nmol N reduced (g sed)−1 day−1. Concentrations of dissolved inorganic carbon and N2 were close to atmospheric equilibrium in uncontaminated groundwater, but were more than 2 times higher within the contaminant plume. Excess CO2 and N2 suggested in situ formation with a stoichiometry of C and N mineralized via denitrification of 0.8 (C/N). Denitrification within the aquifer resulted in an increase in the natural δ15N of NO3− (from +13.6 to +42.0%.) and the N2 produced, with an isotopic enrichment factor, ϵ, of −13.9%.. Vertical profiles of NH4+ and δ15N of NH4+ indicated that dissimilatory reduction of NO3− to NH4+ was also occurring but mass balance calculations indicated that denitrification was the predominant process. These results demonstrate that a combination approach using field mass balance, stable isotope analysis, and laboratory incubations yields useful insight as to the significance of denitrification in aquifer sediments and that closely spaced vertical sampling is necessary to adequately quantify the processes controlling C and N transport and transformation within these environments.

Annual Carbon Mineralization and Belowground Production of Spartina Alterniflora in a New England Salt Marsh

The annual rates and depth distribution of organic carbon mineralization to CO₂ were determined in sediments supporting stands of short Spartina alterniflora. Carbon dioxide production was estimated by two independent techniques. We constructed a CO₂ budget based on measurements of CO₂ emission from the sediment to the atmosphere, export of dissolved inorganic carbon in porewater exchange, and changes in the porewater pool of dissolved inorganic carbon throughout the year. We also measured CO₂ production in salt marsh sediments by monitoring changes in total inorganic carbon in cores. The estimates obtained by the two methods were similar, giving a total annual CO₂ production of between 67 and 70 mol°m— ²°yr— ¹. We also measured the losses of organic matter as methane (O.1—0.3 mol°m— ²°yr— ¹) and dissolved organic carbon (0—3 mol°m— ²°yr— ¹) and burial (7.4 mol°m— ²°yr— ¹) in order to construct a carbon budget for the sediments. These data, when combined with the estimates of carbon mineralization, gave an estimate for the organic carbon loading to the sediments of 68—78 mol°m— ²°yr— ¹. About 95% of the annual carbon input either decomposes to CO₂ in situ or is buried, and <5% is exported from the sediment. We estimated that belowground C production in short S. alterniflora in this Massachusetts marsh is 58—75 mol°m— ²°yr— ¹.

Nitrogen transport and transformations in a shallow aquifer receiving wastewater discharge: A mass balance approach

Nitrogen transport and transformations were followed over the initial 3 years of development of a plume of wastewater-contaminated groundwater in Cape Cod, Massachusetts. Ammonification and nitrification in the unsaturated zone and ammonium sorption in the saturated zone were predominant, while loss of fixed nitrogen through denitrification was minor. The major effect of transport was the oxidation of discharged organic and inorganic forms to nitrate, which was the dominant nitrogen form in transit to receiving systems. Ammonification and nitrification in the unsaturated zone transformed 16–19% and 50–70%, respectively, of the total nitrogen mass discharged to the land surface during the study but did not attenuate the nitrogen loading. Nitrification in the unsaturated zone also contributed to pH decrease of 2 standard units and to an N2O increase (46–660 µg N/L in the plume). Other processes in the unsaturated zone had little net effect: Ammonium sorption removed <1% of the total discharged nitrogen mass; filtering of particulate organic nitrogen was less than 3%; ammonium and nitrate assimilation was less than 6%; and ammonia volatilization was less than 0.25%. In the saturated zone a central zone of anoxic groundwater (DO ≤ 0.05 mg/L) was first detected 17 months after effluent discharge to the aquifer began, which expanded at about the groundwater-flow velocity. Although nitrate was dominant at the water table, the low, carbon-limited rates of denitrification in the anoxic zone (3.0–9.6 (ng N/cm3)/d) reduced only about 2% of the recharged nitrogen mass to N2. In contrast, ammonium sorption in the saturated zone removed about 16% of the recharged nitrogen mass from the groundwater. Ammonium sorption was primarily limited to anoxic zone, where nitrification was prevented, and was best described by a Langmuir isotherm in which effluent ionic concentrations were simulated. The initial nitrogen load discharged from the groundwater system may depend largely on the growth and stability of the sorbed ammonium pool, which in turn depends on effluent-loading practices, subsurface microbial processes, and saturation of available exchange sites.

Biogeochemistry of dimethylsulfide in a seasonally stratified coastal salt pond

Abstract Dimethylsulfide (DMS) is the major volatile reduced organic sulfur compound in the water column of coastal Salt Pond, Cape Cod, MA. DMS concentration and vertical distributions vary seasonally in response to changing biogeochemical processes in the pond. When the pond is thermally stratified in summer, maximum DMS concentrations of up to 60 nmol/1 were found in the oxygen-deficient metalimnion. DMS concentrations in the epilimnion (typically 5–10 nmol/1) were always an order of magnitude higher than in the hypolimnion (

Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides, elemental sulfur, and pyrite☆

Rates of sulfate reduction, oxygen uptake and carbon dioxide production in sediments from a short Spartina alterniflora zone of Great Sippewissett Marsh were measured simultaneously during late summer. Surface sediments (0–2 cm) were dominated by aerobic metabolism which accounted for about 45% of the total carbon dioxide production over 0–15 cm. Rates of sulfate reduction agreed well with rates of total carbon dioxide production below 2 cm depth indicating that sulfate reduction was the primary pathway for sub-surface carbon metabolism. Sulfate reduction rates were determined using a radiotracer technique coupled with a chromous chloride digestion and carbon disulfide extraction of the sediment to determine the extent of formation of radiolabelled elemental sulfur and pyrite during shortterm (48 hr) incubations. In the surface 10 cm of the marsh sediments investigated, about 50% of the reduced radiosulfur was recovered as dissolved or acid volatile sulfides, 37% as carbon disulfide extractable sulfur, and only about 13% was recovered in a fraction operationally defined as pyrite. Correlations between the extent of sulfate depletion in the marsh sediments and the concentrations of dissolved and acid volatile sulfides supported the results of the radiotracer work. Our data suggest that sulfides and elemental sulfur may be major short-term end-products of sulfate reduction in salt marshes.

Salt Marsh development studies at Waquoit Bay, Massachusetts: Influence of geomorphology on long-term plant community structure

Stochastic events relating to beach formation and inlet dynamics have been the major factors influencing the development of the Waquoit Bay tidal marshes. This results from the physical structure of the Waquoit Bay system where tidal exchange is limited to one or two small inlets and is in contrast to marsh development in nearby Barnstable Marsh where direct unrestricted exchange with Cape Cod Bay has smoothed the effects of stochastic events on vegetation development. We contend that vegetation development in salt marshes where connections to adjacent waters are restricted will be dominated by abiotic factors (e.g. storms, sedimentation rates, etc.) while those marshes directly linked to open bodies of water and where alterations to hydrodynamic factors are gradual, autecological processes (e.g. interspecific competition) will dominate long-term plant community development. The results from the five marsh systems within the Waquoit Bay complex suggest that once a vegetation change occurs the new community tended to persist for long periods of time (100s–1000s years). Stability of the ‘new’ community appeared to depend upon the stability of the physical structure of the system and/or time between perturbations necessary to allow the slower autecological processes to have a discernable effect. In order for the plant community to persist as long as observed, the vegetation must also be exerting an influence on the processes of development. Increased production of roots and rhizomes and growth characteristics (density of culms) are some of the factors which help to maintain long-term species dominance. It is clear from this investigation that the structure of the plant community at any one point in time is dependent upon numerous factors including historical developmental influences. To properly assess changes to the present plant community or determine recent rates of accretion, historic developmental trends must be considered. The factors that have influenced the development of marsh in the past will be important in understanding and formulating predictive models in the future.

Control of denitrification in a septage-treating artificial wetland: the dual role of particulate organic carbon

We examined the factors controlling organic carbon (C) cycling and its control of nitrogen (N) removal via denitrification in an aerated artificial wetland treating highly concentrated wastewater to nutrient-removal standards. Processing of organic material by the septage-treating wetland affected the biological reactivity (half-life, or t1/2) of organic C pools through microbial degradation and gravity fractionation of the influent septage. Primary sedimentation fractionated the initial septage material (t1/2 = 8.4d) into recalcitrant waste solids (t1/2 = 16.7d) and highly labile supernatant (t1/2 = 5.0d), allowing this reactive fraction to be further degraded during treatment in aerobic wetland tanks until a less labile material (t1/2 = 7.3d) remained. Organic C contributions from in situ fixation by nitrifying bacteria or algae in these tanks were small, about 1% of the C degradation rate. In the aerated tanks, denitrification was correlated with particulate organic C loading rates, although the average C required (0.35 mg C L(-1)h(-1)) to support denitrification was only 12% of the total C respiration rate (2.9 mg C L(-1)h(-1)). Additions of plant litter (2.5g C L(-1)) to the aerated tanks under normal operating conditions doubled denitrification rates to 0.58 mg N L(-1)h(-1), and reduced effluent nitrate levels by half, from 12.7 to 6.4 mg N L(-1). However, C degradation within the plant litter (0.15mg C L(-1)h(-1)) was sufficient to have accounted for only 35% of the additional denitrification. Evidence from laboratory and full-scale plant litter additions as well as process monitoring indicates that the stimulation of denitrification is due to the respiration-driven formation of anaerobic microsites within particulate organic C. In this aerated highly C-loaded septage-treating wetland, anaerobic microsite, rather than C substrate availability limits denitrification.

Salt Marsh Values: Retrospection from the end of the Century

Two of the greatest problems in coastal waters are eutrophicaton and rapid decline in populations of important fish species. Salt marshes are important in combating both these problems. A paradigm for salt marsh function: marshes import inorganic nutrients and export organic nutrients and, as a result, grow fish. As ground and tidal water flow through salt-water wetlands, plants, bacteria and algae produce or transform the organic matter of the food chain that supports fish and shellfish populations. While salt marshes modify the principal plant nutrients, N and P, some of the pathways result in removal of nutrients from biologically active systems. Nitrogen is removed primarily either by being trapped in refractory organic matter that contributes to marsh maintenance through accretion or through loss to the atmosphere (as N2) by denitrification. Salt marshes along the Atlantic coast of the United States have changed during the past century; the number of hectares has declined and the nutrient loading per hectare has increased. We examine data on the correlation between fish catch and various marsh features from Long Island, New York in 1880. We review research on the ways salt marshes reduce both the level and rate of eutrophication of coastal waters by intercepting nitrate in discharging groundwater. Finally, we consider how these functions have changed with the decrease in area of salt marshes along the Atlantic coast from Georgia to Maine.

Quantifying Dissolved Nitrogen Flux Through a Coastal Watershed

Available nitrogen loading models, commonly used to estimate subsurface fluxes of dissolved nitrogen to coastal waters, have not been quantitatively or systematically compared; nor have they generally been field-verified at regional scales. We employed three published loading models, a site-specific model based upon water use data, and both Darcian and non-Darcian field approaches to obtain estimates of steady state, dissolved nitrogen flux through a permeable Massachusetts watershed. The two field approaches, based on independent data, yielded similar results. Results of the published loading models agreed closely with each other, but exceeded the mean of the field approaches (130 ± 12 mol N m−1 aquifer width yr−1) by 60%, on average. The Water Use loading model agreed closely with the field results (within 4%), largely because it did not require estimates of occupancy rate, which was found to be the major source of error to the published models. The observed, median concentration of total dissolved nitrogen (TDN) in groundwater increased from 1.9 to 313 μM during transport through the subbasin, confirming loading model predictions that >99% of the TDN flux is anthropogenic. In contrast to the watershed inputs, downgradient TDN was dominantly nitrate (98%), indicating near-complete nitrification during transport. Significant transverse horizontal and vertical variations were found in the groundwater TDN distribution at scales of meters and tenths of meters, respectively, consistent with a large number of discrete nitrogen sources at the ground surface, and low transverse macrodispersivities in the aquifer. Loading models, if properly verified by field measurements at the stream tube scale, hold promise for characterizing the effects of land use on subsurface nitrogen flux through coastal watersheds.